# What are the smallest star clusters affected by Galaxy Rotation Curve?

We all know that Milky Way exhibits Galaxy Rotation Curve which makes it rotate very differently that Newtonian mechanics would expect. But Milky Way is a pretty large galaxy, and the one we only see inside, so maybe it's not that good of an research object.

What are the smallest galaxies or star clusters whose rotation is clearly non-Newtonian? Are dwarf galaxies' rotation curves affected? Are globular clusters inside galaxies such as ours affected, or is their movement decisively Newtonian?

Yes, dwarf galaxy rotation curves are affected -- in fact, they tend to require relatively more dark matter than is required to explain the rotation curves of giant galaxies like the Milky Way. The smallest known systems are so-called ultrafaint dwarf galaxies (UFDs), which can have as little as $$sim 10^{4}$$ solar masses worth of baryons (in the form of stars), but apparently have as much as 100 times this in the form of dark matter -- versus the roughly 10:1 (DM:baryons) case for galaxies like the Milky Way. This means that some of these systems have fewer stars than typical globular clusters.

But globular clusters, on the other hand, do not show signs of dark matter/non-Newtonian motion: you can explain the motions of their stars as being due to the normal, Newtonian gravity of all the baryonic mass (i.e., the stars, plus a plausible amount of stellar remnants like neutron stars and stellar-mass black holes).

## Dwarf spheroidal galaxy

A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and to systems that are companions to the Andromeda Galaxy (M31). While similar to dwarf elliptical galaxies in appearance and properties such as little to no gas or dust or recent star formation, they are approximately spheroidal in shape and generally have lower luminosity.

Despite the radii of dSphs being much larger than those of globular clusters, they are much more difficult to find due to their low luminosities and surface brightnesses. Dwarf spheroidal galaxies have a large range of luminosities, and known dwarf spheroidal galaxies span several orders of magnitude of luminosity. [1] Their luminosities are so low that Ursa Minor, Carina, and Draco, the known dwarf spheroidal galaxies with the lowest luminosities, have mass-to-light ratios (M/L) greater than that of the Milky Way. [2] Dwarf spheroidals also have little to no gas with no obvious signs of recent star formation. [3] [4] When it comes to the Local Group, dSphs are primarily found near the Milky Way and M31. [5] [6]

The first dwarf spheroidal galaxies discovered were Sculptor and Fornax in 1938. [2] The Sloan Digital Sky Survey has resulted in the discovery of 11 more dSph galaxies as of 2007 [7] By 2015, many more ultra-faint dSphs were discovered, all satellites of the Milky Way. [8] Nine potentially new dSphs were discovered in the Dark Energy Survey in 2015. [9] Each dSph is named after constellations they are discovered in, such as the Sagittarius dwarf spheroidal galaxy, all of which consist of stars generally much older than 1–2 Gyr that formed over the span of many gigayears. [2]

For example, 98% of the stars in the Carina dwarf spheroidal galaxy are older than 2 Gyr, formed over the course of three bursts around 3, 7 and 13 Gyr ago. [2] The stars in Carina have also been found to be metal-poor. [10] This is unlike star clusters because, while star clusters have stars which formed more or less the same time, dwarf spheroidal galaxies experience multiple bursts of star formation. [2]

Because of the faintness of the lowest-luminosity dwarf spheroidal galaxies and the nature of the stars contained within them, some astronomers suggest that dwarf spheroidal galaxies and globular clusters may not be clearly separate and distinct types of objects. [11] Other recent studies, however, have found a distinction in that the total amount of mass inferred from the motions of stars in dwarf spheroidals is many times that which can be accounted for by the mass of the stars themselves. Studies reveal that dwarf spheroidal galaxies have a dynamical mass of around 10 7 M , which is very large despite the low luminosity of dSph galaxies. [1]

Although at fainter luminosities of dwarf spheroidal galaxies, it is not universally agreed upon how to differentiate between a dwarf spheroidal galaxy and a star cluster however, many astronomers decide this depending on the object's dynamics: If it seems to have more dark matter, then it is likely that it is a dwarf spheroidal galaxy rather than an enormous, faint star cluster. In the current predominantly accepted Lambda cold dark matter cosmological model, the presence of dark matter is often cited as a reason to classify dwarf spheroidal galaxies as a different class of object from globular clusters, which show little to no signs of dark matter. Because of the extremely large amounts of dark matter in dwarf spheroidal galaxies, they may deserve the title "most dark matter-dominated galaxies." [12]

Further evidence of the prevalence of dark matter in dSphs includes the case of Fornax dwarf spheroidal galaxy, which can be assumed to be in dynamic equilibrium to estimate mass and amount of dark matter, since the gravitational effects of the Milky Way are small. [13] Unlike the Fornax galaxy, there is evidence that the UMa2, a dwarf spheroidal galaxy in the Ursa Major constellation, experiences strong tidal disturbances from the Milky Way. [9]

A topic of research is how much the internal dynamics of dwarf spheroidal galaxies are affected by the gravitational tidal dynamics of the galaxy they are orbiting. In other words, dwarf spheroidal galaxies could be prevented from achieving equilibrium due to the gravitational field of the Milky Way or other galaxy that they orbit. [2] For example, the Sextans dwarf spheroidal galaxy has a velocity dispersion of 7.9±1.3 km/s, which is a velocity dispersion that could not be explained solely by its stellar mass according to the Virial Theorem. Similar to Sextans, previous studies of Hercules dwarf spheroidal galaxy reveal that its orbital path does not correspond to the mass contained in Hercules. [14] Furthermore, there is evidence that the UMa2, a dwarf spheroidal galaxy in the Ursa Major constellation, experiences strong tidal disturbances from the Milky Way. [9]

## Galaxy cluster

A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity [1] with typical masses ranging from 10 14 –10 15 solar masses. They are the largest known gravitationally bound structures in the universe and were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. [2] One of the key features of clusters is the intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with star clusters, such as galactic clusters—also known as open clusters—which are structures of stars within galaxies, or with globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. The galaxy groups and clusters can themselves cluster together to form superclusters.

Notable galaxy clusters in the relatively nearby Universe include the Virgo Cluster, Fornax Cluster, Hercules Cluster, and the Coma Cluster. A very large aggregation of galaxies known as the Great Attractor, dominated by the Norma Cluster, is massive enough to affect the local expansion of the Universe. Notable galaxy clusters in the distant, high-redshift Universe include SPT-CL J0546-5345 and SPT-CL J2106-5844, the most massive galaxy clusters found in the early Universe. In the last few decades, they are also found to be relevant sites of particle acceleration, a feature that has been discovered by observing non-thermal diffuse radio emissions, such as radio halos and radio relics. Using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have also been found in many galaxy clusters.

## ROTATION CURVES OF SPIRAL GALAXIES

A ROTATION CURVE is a graph of the ROTATION SPEED at different distances from the Center of Rotation.

Solid Body Rotation: Wheel, Record, CD, etc.
Particles farther from the center must have a higher speed in order to go around once in the same time period as particles close to the center.
Keplerian Rotation: Planets around the Sun.
Closer planets have the highest speeds while the most distant planet has the slowest. This is because almost all the mass of the solar system is in the Sun.

### MILKY WAY ROTATION CURVE

1) Very steep "solid body" inside 4,000 LY.
2) FLAT ROTATION (Constant speed) outside 4,000 LY.

A Flat Rotation Curve is observed in almost ALL spiral galaxies!

Destruction of Galactic Star Clusters
In galactic clusters, the stars closer to the center of the MW will move ahead of stars that are further away from the center of the MW. This stretches out the cluster and it will eventually come apart.

Since the Milky Way (and all other spiral galaxies!) have a FLAT ROTATION CURVE, stars farther from the center feel a gravitational pull from more matter than stars close to the center. (If these stars felt the same mass, more distant stars would be slower like the planets in the solar system). There is more and more mass yet less and less gas and stars! Therefore, there must be some other matter we call "DARK MATTER".

### DARK MATTER

The "Dark" of "Dark Matter" means astronomers have not been able to detect ANY Electro-Magnetic Radiation (such as X-rays, visible light or radio waves) from it. It is only known to be there due to its gravitational pull.

## What are the smallest star clusters affected by Galaxy Rotation Curve? - Astronomy

We present HST WFPC2 observations, supplemented by ground-based Hα data, of the star-cluster populations in two pairs of interacting galaxies selected for being in very different kinds of encounters seen at different stages. Dynamical information and n-body simulations provide the details of encounter geometry, mass ratio, and timing. In NGC 5752/4 we are seeing a weak encounter, well past closest approach, after about 2.5×10 8 yr. The large spiral NGC 5754 has a normal population of disk clusters, while the fainter companion NGC 5752 exhibits a rich population of luminous clusters with a flatter luminosity function. The strong, ongoing encounter in NGC 6621/2, seen about 1.0×10 8 yr past closest approach between roughly equal-mass galaxies, has produced an extensive population of luminous clusters, particularly young and luminous in a small region between the two nuclei. This region is dynamically interesting, with such a strong perturbation in the velocity field that the rotation curve reverses sign. From these results, in comparison with other strongly interacting systems discussed in the literature, cluster formation requires a threshold level of perturbation, with stage of the interaction a less important factor. The location of the most active star formation in NGC 6621/2 draws attention to a possible role for the Toomre stability threshold in shaping star formation in interacting galaxies. The rich cluster populations in NGC 5752 and NGC 6621 show that direct contact between gas-rich galaxy disks is not a requirement to form luminous clusters and that they can be triggered by processes happening within a single galaxy disk (albeit triggered by external perturbations).

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

## Blue Lurkers and Blue Stragglers: Rapidly-Rotating Stars and their Fountain of Youth

This guest post was written by Catherine Manea, a first year graduate student at UT Austin. Catherine’s research involves tracing the chemical and dynamical evolution of the Galaxy using planetary nebulae and stars. In her free time, Catherine enjoys walking her dog and hiking the Austin trails.

Authors: Emily Leiner, Robert D. Mathieu, Andrew Vanderburg, Natalie M. Gosnell, and Jeffrey C. Smith

First Author’s Institution: University of Wisconsin-Madison

Status: Published in The Astrophysical Journal, open access on arXiv

Star clusters are comoving, gravitationally-bound groups of stars that were born more or less around the same time. Therefore, all stars in a cluster have about the same age. However, many clusters are home to blue straggler stars (BSSs) that, curiously, appear much younger (i.e. bluer, hence the “blue” part) than their fellow cluster members. Today’s paper investigates blue lurkers, the low-luminosity end of the BSS distribution, and how to identify them in star clusters via their anomalously short rotation periods.

### Young, Blue Stars Refusing to Evolve?

To understand blue lurkers, we must first understand BSSs. BSSs have perplexed astronomers since 1953, when Allan Sandage created the first color-magnitude diagram (CMD) of M3, one of the largest and brightest globular clusters located in the Galactic halo. CMDs plot the absolute magnitude versus color of a stellar population, and the location of the main sequence “turn-off” is used to determine the age of the cluster. Massive, highly luminous blue stars leave the main sequence sooner than lower-mass, redder stars. Therefore, the fewer blue stars still reside on the main-sequence, the older the cluster. Figure 1 presents the CMD of M67, the cluster hosting the blue lurkers investigated by today’s paper. The main-sequence turn-off is located at the noticeable “hook” in the CMD. Note the peculiar population of blue stars to the left of the turn-off—those are BSSs.

Figure 1: Color-magnitude diagram (CMD) of M67 showing the location of blue straggler stars (BSSs). These peculiar stars lie past the main-sequence turnoff (light-yellow region) and should have evolved off the main sequence already. They appear younger than the rest of the population. Figure taken from first author Emily Leiner’s website.

Upon plotting the CMD of M3, Sandage noticed a group of stars that didn’t seem to fit in. They were bluer than the main-sequence turn-off and should have evolved off of the main sequence already. They appeared to be younger than the rest of their neighbors. This phenomenon is observed in many old globular clusters. It is natural to question whether these seemingly young stars are stowaways, infiltrators from other regions of the galaxy that have somehow found themselves hanging out with an older crowd of stars. However, that is not the case these stars were born at the same time as their cluster neighbors. Instead, they have seemingly aged backwards by conspiring with another star through a binary interaction.

### What’s their Secret to Youth?

Three main formation pathways explain the strange position of BSSs on the CMD: binary mass-transfer, binary mergers, and collisions. Highly dense portions of globular clusters see the most instances of BSSs, which supports the theory that these binary interactions create BSSs. In such interactions, two intermediate-mass stars combine or transfer mass to form a more massive star. More massive stars tend to be bluer, so this post-interaction object climbs up the main sequence. If the initial stars were massive enough, their combined final product surpasses the main sequence turn-off and ends up in the extremely blue BSS regime.

### Blue Lurkers: the Shy Sibling of Blue Stragglers

Today’s paper investigates the meeker sibling of BSSs, “blue lurkers,” and methods to identify them in clusters. Blue lurkers are a lower-mass subset of BSSs: they are post-interaction objects that have moved up the main-sequence due to some sort of binary interaction. However, since they are the product of two low-mass stars interacting to form an intermediate-mass star, they lack the obvious, extremely blue CMD position held by BSSs. BSSs lie past the main-sequence turn-off in CMDs because their progenitors were two intermediate-mass stars that, when combined, produce a very massive star. Blue lurkers, on the other hand, are intermediate mass stars created from the interaction of two low-mass stars, so their CMD position is less-blue, overlapping with that of normal main-sequence stars. This is shown in Figure 2.

### Binary Interactions Speed Up Stellar Rotation

Blue lurkers are hard to differentiate from regular stars because their position on the CMD blends in with that of main-sequence stars. The authors get around this by using the anomalously high stellar rotation rates shared by BSSs and blue lurkers to locate blue lurkers hiding in the main-sequence.

Stellar rotation is an age diagnostic because sun-like stars share spin-down trends. Upon formation, regardless of initial angular momentum, stars converge to a common initial rotation rate that slows predictably as the star ages. Blue stragglers and blue lurkers rotate much faster than single stars in the same region of the CMD because they’ve experienced some sort of interaction, such as binary mass transfer, a merger, or a collision, that has spun them up. Thus, by studying rotation rates in a sample of stars with similar masses and ages, one can point out stars that have experienced an interaction from their unusually high rotation rate. This is exactly what the authors did in today’s paper: they looked for anomalously-fast rotators that lie in the turn-off region of M67’s CMD to locate blue lurkers.

Typical sun-like stars in M67 have rotation rates of 20-30 days, so we expect all main sequence stars in the cluster to share this rotation rate. However, the authors discovered 11 M67 members with rotation periods shorter than 15 days that lie in the main sequence turn-off. These stars have the rotation rate of a much younger star, contradicting their position on the CMD. Given that they are confirmed members of M67, their unusually fast rotation must indicate that they have experienced a binary interaction, causing them to gain angular momentum and mass and appear bluer. These rapid rotators are formationally similar to BSSs except that they are lower in mass and thus don’t have an obvious, extreme blue position when plotted in a CMD.

### Measuring Stellar Rotation

So how did the authors find these rapid rotators? They employed two methods: first, they looked for periodicity in the K2 lightcurves of M67 members. To search for periodicity, they created Lomb-Scargle periodograms of the lightcurves (Figure 3). Lomb-Scargle periodograms break down the light curve of an object into its component periods (think Fourier transform!).

Dark starspots co-rotating with the surface of a star cause periodic dimming of a star’s observed light and thus can be used to trace rotation. The most powerful period in the periodogram can be attributed to the rotation rate—around 3 days for the star in Figure 3—though other physical processes, such as planetary transits, binary eclipsing, and stellar pulsations can also cause the star’s light to change periodically with time. The authors found nine M67 members with anomalously rapid rotation rates using this method.

In addition to using K2 light curves, the authors looked for rapid rotation signatures in the spectra of M67 members. Rapidly rotating stars exhibit broader spectral lines due to the respective blue- and red-shifting of emitted light from approaching and receding edges of the star. Spectral line widths translate to vsini measurements, which signify the radial component of a star’s rotation speed relative to the observer. Vsini measurements were used in addition to K2 light curves to catch any additional rapid rotators that the light curve method might have missed. Two additional rapid rotators were found through this method, for a total of 11 rapid rotators at M67’s main sequence turnoff.

These 11 rapidly rotating single stars were found out of 400 main-sequence stars in M67, indicating that

3% of main-sequence stars are blue lurkers. This result has important implications for the study of binarity in stellar clusters: binary surveys look for current binary systems but may fail to account for systems that were binaries but merged into singular stars, producing these blue lurkers and BSSs. Using rotation rates in tandem with direct imaging and spectral surveys can fill this gap.

Finally, the authors look to the future: their method of using rotation rates to identify post-interaction stars will be even more useful with the upcoming TESS and PLATO missions that will yield a wealth of new data in the coming decade.

## What are the smallest star clusters affected by Galaxy Rotation Curve? - Astronomy

D any one of the above is possible

E none of the above is true

*A decrease in the mass of the Sun

B increase in the Sun's gravitational attraction on the planets

C increase in the Sun's rotation rate

*C use wire mesh instead of mirrors

D were found to be useless since no astronomical objects emit radio waves

E can only be used during daylight hours

*A collect more photons from faint objects

B help us see radiation that cannot get through the Earth's atmosphere

D separate light into its spectrum

E a blackbody has no spectrum

A at different temperatures

D using different systems of measurement

E not moving at all, relative to one another

A areas obscured by higher layers

B ashes of nuclear burning brought to the surface by convection

C holes in the photosphere that allow us to see deeper regions

*D regions which are cooler and darker than surrounding material

D the Sun isn't generating energy it's just cooling

E endothermic chemical reactions

C ultraviolet, x-ray, gamma ray

D all of the above have wavelengths greater than visible light

E none of the above have wavelengths greater than visible light

E none of the above use a lens as the objective

A temperature and velocity

*B temperature and peak wavelength

E Doppler shift and wavelength

A temperature and velocity

B temperature and peak wavelength

E Doppler shift and wavelength

D all of the above spectra

E none of the above spectra

A core, chromosphere, radiative layer, photosphere, convective layer, corona

B core, convective layer, radiative layer, photosphere, chromosphere, corona

*C core, radiative layer, convective layer, photosphere, chromosphere, corona

D core, photosphere, radiative layer, convective layer, chromosphere, corona

E core, radiative layer, photosphere, convective layer, chromosphere, corona

A[ x 4 He à 4 x 1 H + energy

B^ x 1 He à 1 x 4 H + energy

C^ x 1 H + energy à 1 x 4 He

*D^ x 1 H à 1 x 4 He + energy

E none of the above summarize the core reactions

*A construct an HR diagram for the cluster

B count the number of cepheids in the cluster

C measure the Doppler shifts of many stars in the cluster

D wait for a supernova in the cluster and measure its light curve

E check the radial and transverse velocities

B red-shifted optical spectra

C spheres of orbiting photons

A exploding as a supernova

*B the exhaustion of hydrogen in its core

C the onset of helium burning

D the onset of the CNO cycle

A an existing star is broken apart

*B an interstellar cloud collapses by its own gravity

C fresh material falls onto the core of a previously-dead star

D nuclear reactions start inside a large planet

E new stars don't form anymore

The following table gives the name, absolute magnitude, apparent magnitude, and spectral type for five stars. Answer the following six questions using this table and the appropriate letter corresponding to the star that is best described by the line given. Note that stars/letters may be used more than once.

E there exist no such stars

*A a nearby cloud of gas to collapse to form a new star

B a nearby cloud of gas to form a planetary nebula

C other nearby stars to become neutron stars or black holes

D other nearby stars to explode as supernovae

*A all stars spend most of their lives there

B most stars have been born very recently

C other stars are not plotted on the HR diagram

D other stars are very faint and hard to see

E other stars prevent them leaving the diagonal

A has been observed only on Jupiter.

B is caused by the planet's magnetic field.

*C is caused by rising and sinking gases.

D is more obvious on Saturn than Jupiter.

E explains the formation of Cassini's division.

A Pluto has a large iron-nickel core.

*B Pluto is about 50% water and 50% rocky material.

C Pluto should have a magnetic field about one-third as strong as Earth's.

D Pluto is still geologically active.

E Pluto probably has a small ring system that hasn't yet been detected.

A trails behind the head along the orbital path.

D points perpendicular to the orbital path.

*E points away from the sun.

II. The tremendous distances between stars

III. The finite speed at which objects can travel

*A the number of intelligent civilizations in our galaxy.

B the number of stars in our galaxy.

C the number of people on the Earth.

D the lifespan of a civilization.

E the lifespan of a species.

FINAL Examination ESSAY Question

Choose ONE of the following themes for your essay. The essay counts for 50 out of the 300 points that are associated with this final. Don't skimp.

A) Describe the various methods that astronomers use to determine the distances to astronomical objects ranging from the planets, to the stars, to other star clusters, to other galaxies and to other clusters of galaxies.

B) Describe the various steps involved in the birth, main sequence life, and ultimate death of a one-solar mass star.

C) Describe the electromagnetic spectrum and how astronomers make use of all aspects of received photons and their interactions with matter to determine such characteristics as chemical composition, motion, luminosity, temperature, etc.

D) Discuss the various theories, and the evidence, of the birth and current nature of the universe, and how astronomers hope to determine from among the possible destinies, the ultimate destiny of the universe.

## The Rotation Curve of the Milky Way

Now that we have a concept of the size, stellar populations, and an overall understanding of the Milky Way as a galaxy, let us consider another property that we can determine for the Milky Way: its mass. In most instances, when we intend to calculate the mass of an astronomical object, we return to Newton's version of Kepler's third law:

P 2 = ( 4 π 2 x a 3 ) / G ( m 1 + m 2 )

The Sun is orbiting around the Galactic center, so in principle, if we can measure the Sun's distance from the Galactic Center and its orbital period, this means we can estimate the sum of the masses of the Sun and the Galaxy (at least the portion of the Galaxy that is interior to the Sun's orbit). Since we anticipate the Galaxy's mass to far exceed the Sun's mass, we can take the value that we calculate to be the Galaxy's mass. So, what is the answer? How massive is our galaxy?

The distance from the Sun to the Galactic Center can be measured using a few different techniques, but it is a difficult measurement to make. It is still the case that researchers disagree about the exact value, but it is approximately 8 kpc (that is, 8,000 parsecs). There is a related, but also difficult measurement to make, and that is the velocity of the Sun with respect to the Galactic Center. It is approximately 200 km/sec, which allows us to estimate the period of the Sun's orbit around the Galactic Center in the following way:

1. Assume the Sun is following a circular orbit with radius 8,000 parsecs.
2. Calculate the circumference of the Sun's orbit: c = 2 π r = ( 2 π ) * ( 8000 pc ) * ( 3.1 x 10 13 km / pc ) = 1.6 x 10 18 km .
3. Calculate the period of the orbit by taking the circumference and dividing by the velocity: P = 1.6 x 10 18 km / 200 km/sec = 8.0 x 10 15 sec ≈ 250 million years .

If you take the semi-major axis of the Sun's orbit to be 8 kiloparsecs and the orbital period to be 250 million years, you can determine that the Milky Way's mass interior to the Sun's orbit is approximately 10 11 solar masses, or 100 billion times the mass of the Sun.

Now, let us compare and contrast motions in the Solar System of the planets and motions in the Galaxy of the stars. What we did above to calculate the period of the Sun's orbit was to use the equation:

orbital period (P) = orbit circumference (2πr) / orbital velocity (v)

We can rearrange this equation and calculate orbital velocity for any object given its period and semi-major axis. If we apply this to the planets in the Solar System, you find that as you get more distant from the Sun, the orbital velocity of the object is slower. Below is a two-dimensional plot that I created for the orbital velocities of the planets (and Pluto) as a function of their distance from the Sun. Each point is labeled with the first letter of the object's name (e.g., V = Venus). This type of plot (orbital velocity as a function of distance from the center) is referred to as a rotation curve.

The behavior of the planets in the Solar System as exhibited in this plot is often referred to as Keplerian Rotation. Clearly, the Milky Way Galaxy is more complicated than the Solar System. There are at least 100 billion objects, gas clouds, and dust, and there is not one single dominant mass in the center. However, astronomers expected that as you got more distant from the center of the Galaxy, the velocities of the stars should fall off in a manner similar to the Keplerian rotation exhibited by the planets in the Solar System. However, astronomers have observed that there is a significant difference between the predicted shape of the Milky Way's rotation curve and what is actually measured. See the image below.

The solid line labeled B is a schematic rotation curve similar to what is measured for the Milky Way. The dashed line labeled A is the predicted rotation curve displaying Keplerian rotation. What the rotation curve B tells us is that our model of the Milky Way so far is missing something. In order for objects far from the center of the Galaxy to be moving faster than predicted, there must be significant additional mass far from the Galactic Center exerting gravitational pulls on those stars. This means that the Milky Way must include a component that is very massive and much larger than the visible disk of the Galaxy. We do not see any component in visible light or any other part of the electromagnetic spectrum, so this massive halo must be dark. Today, we refer to this as the "dark matter halo" of the Galaxy, and we will discuss dark matter more in our lesson on cosmology.

Returning to the image of the Milky Way that we studied before, the wire frame halo is actually meant to represent the extent of the dark matter halo. In the image below, compare the scale of the disk to the scale of the dark matter halo.

## Title: THE STAR CLUSTER SYSTEM IN THE NEARBY STARBURST GALAXY M82

We present a photometric study of star clusters in the nearby starburst galaxy M82 based on the UBVI-, YJ- and H-band Hubble Space Telescope images. We find 1105 star clusters with V < 23 mag. Of those, 1070 are located in the disk region, while 35 star clusters are in the halo region. The star clusters in the disk are composed of a dominant blue population with a color peak at (B - V) Almost-Equal-To 0.45, and a weaker red population. The luminosity function of the disk clusters shows a power-law distribution with a power-law index = -2.04 <+->0.03, and the scale height of their distribution is h = 9.''64 <+->0.''40 (164 <+->7 pc), similar to that of the stellar thin disk of M82. We have derived the ages of 630 star clusters using the spectral energy distribution fit method by comparing UBVI(YJ)H-band photometric data with the simple stellar population models. The age distribution of the disk clusters shows that the most dominant cluster population has ages ranging from 100 Myr to 1 Gyr, with a peak at about 500 Myr. This suggests that M82 has undergone a disk-wide star formation about 500 Myrmore » ago, probably through the interaction with M81. The brightest star clusters in the nuclear region are much brighter than those in other regions, indicating that more massive star clusters are formed in the denser environments. On the other hand, the colors of the halo clusters are similar to those of globular clusters in the Milky Way, and their ages are estimated to be older than 1 Gyr. These are probably genuine old globular clusters in M82. « less

## Contents

The word galaxy was borrowed via French and Medieval Latin from the Greek term for the Milky Way, galaxías (kúklos) γαλαξίας ( κύκλος ) [15] [16] 'milky (circle)', named after its appearance as a milky band of light in the sky. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so the baby will drink her divine milk and thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way. [17] [18]

In the astronomical literature, the capitalized word "Galaxy" is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term Milky Way can be traced back to a story by Chaucer c. 1380 :

"See yonder, lo, the Galaxyë
Which men clepeth the Milky Wey,
For hit is whyt."

Galaxies were initially discovered telescopically and were known as spiral nebulae. Most 18th to 19th century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought of as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called island universes, but this term quickly fell into disuse, as the word universe implied the entirety of existence. Instead, they became known simply as galaxies. [19]

Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC (New General Catalogue), the IC (Index Catalogue), the CGCG (Catalogue of Galaxies and of Clusters of Galaxies), the MCG (Morphological Catalogue of Galaxies) and UGC (Uppsala General Catalogue of Galaxies). All the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a spiral galaxy having the number 109 in the catalogue of Messier, and also having the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, and PGC 37617.

The realization that we live in a galaxy that is one among many galaxies, parallels major discoveries that were made about the Milky Way and other nebulae.

### Milky Way

The Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars. [20] Aristotle (384–322 BCE), however, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World that is continuous with the heavenly motions." [21] The Neoplatonist philosopher Olympiodorus the Younger (c. 495 –570 CE) was critical of this view, arguing that if the Milky Way is sublunary (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have parallax, which it does not. In his view, the Milky Way is celestial. [22]

According to Mohani Mohamed, the Arabian astronomer Alhazen (965–1037) made the first attempt at observing and measuring the Milky Way's parallax, [23] and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." [24] The Persian astronomer al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars." [25] The Andalusian astronomer Ibn Bâjjah ("Avempace", d. 1138) proposed that the Milky Way is made up of many stars that almost touch one another and appear to be a continuous image due to the effect of refraction from sublunary material, [21] [26] citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects are near. [21] In the 14th century, the Syrian-born Ibn Qayyim proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars." [27]

Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars. [28] [29] In 1750 the English astronomer Thomas Wright, in his An Original Theory or New Hypothesis of the Universe, speculated (correctly) that the galaxy might be a rotating body of a huge number of stars held together by gravitational forces, akin to the Solar System but on a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk. [30] [31] In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way. [32]

The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center. [33] [34] Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center. [31] Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our host galaxy, the Milky Way, emerged. [35]

### Distinction from other nebulae

A few galaxies outside the Milky Way are visible on a dark night to the unaided eye, including the Andromeda Galaxy, Large Magellanic Cloud, the Small Magellanic Cloud, and the Triangulum Galaxy. In the 10th century, the Persian astronomer Al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud". [36] In 964, Al-Sufi probably mentioned the Large Magellanic Cloud in his Book of Fixed Stars (referring to "Al Bakr of the southern Arabs", [37] since at a declination of about 70° south it was not visible where he lived) it was not well known to Europeans until Magellan's voyage in the 16th century. [38] [37] The Andromeda Galaxy was later independently noted by Simon Marius in 1612. [36] In 1734, philosopher Emanuel Swedenborg in his Principia speculated that there may be galaxies outside our own that are formed into galactic clusters that are minuscule parts of the universe that extends far beyond what we can see. These views "are remarkably close to the present-day views of the cosmos." [39] In 1745, Pierre Louis Maupertuis conjectured that some nebula-like objects are collections of stars with unique properties, including a glow exceeding the light its stars produce on their own, and repeated Johannes Hevelius's view that the bright spots are massive and flattened due to their rotation. [40] In 1750, Thomas Wright speculated (correctly) that the Milky Way is a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways. [31] [41]

Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae. [31] In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture. [42]

In 1912, Vesto Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high Doppler shifts, indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us. [43] [44]

In 1917, Heber Curtis observed nova S Andromedae within the "Great Andromeda Nebula" (as the Andromeda Galaxy, Messier object M31, was then known). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies. [45]

In 1920 a debate took place between Harlow Shapley and Heber Curtis (the Great Debate), concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift. [46]

In 1922, the Estonian astronomer Ernst Öpik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object. [47] Using the new 100 inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. [48] In 1936 Hubble produced a classification of galactic morphology that is used to this day. [49]

### Modern research

In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas [50] and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in our galaxy. These observations led to the hypothesis of a rotating bar structure in the center of our galaxy. [51] With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter. [52] [53]

Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other things, Hubble data helped establish that the missing dark matter in our galaxy cannot solely consist of inherently faint and small stars. [55] The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion ( 1.25 × 10 11 ) galaxies in the observable universe. [56] Improved technology in detecting the spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allow detection of other galaxies which are not detected by Hubble. Particularly, galaxy surveys in the Zone of Avoidance (the region of the sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies. [57]

A 2016 study published in The Astrophysical Journal and led by Christopher Conselice of the University of Nottingham used 20 years of Hubble images to estimate that the observable universe contains at least two trillion ( 2 × 10 12 ) galaxies. [8] [9] However later observations with the New Horizons space probe from outside the zodiacal light reduced this to roughly 200 billion ( 2 × 10 11 ). [58] [59]

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as star formation rate in starburst galaxies and activity in the cores of active galaxies. [5]

### Ellipticals

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently, these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead, they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller globular clusters. [60]

The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction of galaxies, resulting in a collision and merger. They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy clusters. [61]

#### Shell galaxy

A shell galaxy is a type of elliptical galaxy where the stars in the galaxy's halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. The shell-like structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy. As the two galaxy centers approach, the centers start to oscillate around a center point, the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy NGC 3923 has over twenty shells. [62]

### Spirals

Spiral galaxies resemble spiraling pinwheels. Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter which extends beyond the visible component, as demonstrated by the universal rotation curve concept. [63]

Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) which indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region. [64] A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms. [65] The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense. [66]

In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity. The spiral arms are thought to be areas of high-density matter, or "density waves". [67] As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars. [68]

#### Barred spiral galaxy

A majority of spiral galaxies, including our own Milky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure. [69] In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter (a, b or c) which indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a tidal interaction with another galaxy. [70] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms. [71]

Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy [72] about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×10 11 ) [73] stars and has a total mass of about six hundred billion (6×10 11 ) times the mass of the Sun. [74]

#### Super-luminous spiral

Recently, researchers described galaxies called super-luminous spirals. They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way's 100,000 light-year diameter). With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-infrared light. They are thought to have an increased star formation rate around 30 times faster than the Milky Way. [75] [76]

### Other morphologies

are galactic formations that develop unusual properties due to tidal interactions with other galaxies.
• A ring galaxy has a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy. [77] Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation. [78]
• An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme.
• Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted. [80] Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds.

### Dwarfs

Despite the prominence of large elliptical and spiral galaxies, most galaxies are dwarf galaxies. These galaxies are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, containing only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across. [81]

Many dwarf galaxies may orbit a single larger galaxy the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered. [82] Dwarf galaxies may also be classified as elliptical, spiral, or irregular. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead.

A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million solar masses, regardless of whether the galaxy has thousands or millions of stars. This has led to the suggestion that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale. [83]

### Interacting

Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust. [84] [85] Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies will usually not collide, but the gas and dust within the two forms will interact, sometimes triggering star formation. A collision can severely distort the shape of the galaxies, forming bars, rings or tail-like structures. [84] [85]

At the extreme of interactions are galactic mergers. In this case the relative momentum of the two galaxies is insufficient to allow the galaxies to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to morphology, as compared to the original galaxies. If one of the merging galaxies is much more massive than the other merging galaxy then the result is known as cannibalism. The more massive larger galaxy will remain relatively undisturbed by the merger, while the smaller galaxy is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy. [84] [85]

### Starburst

Stars are created within galaxies from a reserve of cold gas that forms into giant molecular clouds. Some galaxies have been observed to form stars at an exceptional rate, which is known as a starburst. If they continue to do so, then they would consume their reserve of gas in a time span less than the lifespan of the galaxy. Hence starburst activity usually lasts only about ten million years, a relatively brief period in the history of a galaxy. Starburst galaxies were more common during the early history of the universe, [87] and, at present, still contribute an estimated 15% to the total star production rate. [88]

Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to create H II regions. [89] These massive stars produce supernova explosions, resulting in expanding remnants that interact powerfully with the surrounding gas. These outbursts trigger a chain reaction of star building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the starburst activity end. [87]

Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is M82, which experienced a close encounter with the larger M81. Irregular galaxies often exhibit spaced knots of starburst activity. [90]

### Active galaxy

A portion of the observable galaxies are classified as active galaxies if the galaxy contains an active galactic nucleus (AGN). A significant portion of the total energy output from the galaxy is emitted by the active galactic nucleus, instead of the stars, dust and interstellar medium of the galaxy. There are multiple classification and naming schemes for AGNs, but ones in the lower ranges of luminosity are called Seyfert galaxies, while those with luminosities much greater than that of the host galaxy are known as quasi-stellar objects or quasars. AGNs emit radiation throughout the electromagnetic spectrum from radio wavelengths to X-rays, though some of the radiation may be absorbed by dust or gas associated with the AGN itself or with the host galaxy.

The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassive black hole (SMBH) at the core region of the galaxy. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc. [91] The luminosity of an AGN depends on the mass of the SMBH and the rate at which matter falls onto it. In about 10% of these galaxies, a diametrically opposed pair of energetic jets ejects particles from the galaxy core at velocities close to the speed of light. The mechanism for producing these jets is not well understood. [92]

#### Blazars

Blazars are believed to be an active galaxy with a relativistic jet that is pointed in the direction of Earth. A radio galaxy emits radio frequencies from relativistic jets. A unified model of these types of active galaxies explains their differences based on the viewing angle of the observer. [92]

#### LINERS

Possibly related to active galactic nuclei (as well as starburst regions) are low-ionization nuclear emission-line regions (LINERs). The emission from LINER-type galaxies is dominated by weakly ionized elements. The excitation sources for the weakly ionized lines include post-AGB stars, AGN, and shocks. [93] Approximately one-third of nearby galaxies are classified as containing LINER nuclei. [91] [93] [94]

#### Seyfert galaxy

Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses but unlike quasars, their host galaxies are clearly detectable. Seyfert galaxies account for about 10% of all galaxies. Seen in visible light, most Seyfert galaxies look like normal spiral galaxies, but when studied under other wavelengths, the luminosity of their cores is equivalent to the luminosity of whole galaxies the size of the Milky Way.

#### Quasar

Quasars (/ˈkweɪzɑr/) or quasi-stellar radio sources are the most energetic and distant members of active galactic nuclei. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared to be similar to stars rather than extended sources similar to galaxies. Their luminosity can be 100 times that of the Milky Way.

### Luminous infrared galaxy

Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of electromagnetic power output, above 10 11 L☉ (solar luminosities). In most cases, most of the energy comes from large numbers of young stars, which heat surrounding dust, which then reradiates the energy in the infrared. Luminosity high enough to be a LIRG requires a star formation rate of at least 18 M☉ yr −1 . Ultra-luminous infrared galaxies (ULIRGs) are at least ten times more luminous still and form stars at rates >180 M☉ yr −1 . Many LIRGs also emit radiation from an AGN. Infrared galaxies emit more energy in the infrared than at all other wavelengths combined with peak emission typically at wavelengths of 60 to 100 microns. LIRGs are uncommon in the local Universe but were much more common when the Universe was younger.

### Magnetic fields

Galaxies have magnetic fields of their own. [95] They are strong enough to be dynamically important: they drive mass inflow into the centers of galaxies, they modify the formation of spiral arms and they can affect the rotation of gas in the outer regions of galaxies. Magnetic fields provide the transport of angular momentum required for the collapse of gas clouds and hence the formation of new stars.

The typical average equipartition strength for spiral galaxies is about 10 μG (microGauss) or 1 nT (nanoTesla). For comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss or 30 μT (microTesla). Radio-faint galaxies like M 31 and M33, our Milky Way's neighbors, have weaker fields (about 5 μG), while gas-rich galaxies with high star-formation rates, like M 51, M 83 and NGC 6946, have 15 μG on average. In prominent spiral arms, the field strength can be up to 25 μG, in regions where cold gas and dust are also concentrated. The strongest total equipartition fields (50–100 μG) were found in starburst galaxies, for example in M 82 and the Antennae, and in nuclear starburst regions, for example in the centers of NGC 1097 and of other barred galaxies. [95]

Galactic formation and evolution is an active area of research in astrophysics.

### Formation

Current cosmological models of the early universe are based on the Big Bang theory. About 300,000 years after this event, atoms of hydrogen and helium began to form, in an event called recombination. Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. As a result, this period has been called the "dark ages". It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos. [97] [98] These primordial structures would eventually become the galaxies we see today.

#### Early galaxy formation

Evidence for the appearance of galaxies very early in the Universe's history was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and earliest-to-form galaxy seen at that time. [99] While some scientists have claimed other objects (such as Abell 1835 IR1916) have higher redshifts (and therefore are seen in an earlier stage of the universe's evolution), IOK-1's age and composition have been more reliably established. In December 2012, astronomers reported that UDFj-39546284 is the most distant object known and has a redshift value of 11.9. The object, estimated to have existed around 380 million years [100] after the Big Bang (which was about 13.8 billion years ago), [101] is about 13.42 billion light travel distance years away. The existence of galaxies so soon after the Big Bang suggests that protogalaxies must have grown in the so-called "dark ages". [97] As of May 5, 2015, the galaxy EGS-zs8-1 is the most distant and earliest galaxy measured, forming 670 million years after the Big Bang. The light from EGS-zs8-1 has taken 13 billion years to reach Earth, and is now 30 billion light-years away, because of the expansion of the universe during 13 billion years. [102] [103] [104] [105] [106]

The detailed process by which the earliest galaxies formed is an open question in astrophysics. Theories can be divided into two categories: top-down and bottom-up. In top-down correlations (such as the Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years. [108] In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy. [109] Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared within them. These were composed almost entirely of hydrogen and helium and may have been more massive than 100 times the Sun's mass. If so, these huge stars would have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium. [110] This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel. [111]

In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60 . Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it. [112] [113]

### Evolution

Within a billion years of a galaxy's formation, key structures begin to appear. Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added. [114] During this early epoch, galaxies undergo a major burst of star formation. [115]

During the following two billion years, the accumulated matter settles into a galactic disc. [116] A galaxy will continue to absorb infalling material from high-velocity clouds and dwarf galaxies throughout its life. [117] This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formation of planets. [118]

The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology. [120] Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails. Examples of these formations can be seen in NGC 4676 [121] or the Antennae Galaxies. [122]

The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km/s, and—depending upon the lateral movements—the two might collide in about five to six billion years. Although the Milky Way has never collided with a galaxy as large as Andromeda before, evidence of past collisions of the Milky Way with smaller dwarf galaxies is increasing. [123]

Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common. Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked about ten billion years ago. [124]

### Future trends

Spiral galaxies, like the Milky Way, produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms. [125] Elliptical galaxies are largely devoid of this gas, and so form few new stars. [126] The supply of star-forming material is finite once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end. [127] [128]

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (10 13 –10 14 years), as the smallest, longest-lived stars in our universe, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed of compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars, and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions. [127] [129]

Deep sky surveys show that galaxies are often found in groups and clusters. Solitary galaxies that have not significantly interacted with another galaxy of comparable mass during the past billion years are relatively scarce. Only about five percent of the galaxies surveyed have been found to be truly isolated however, these isolated formations may have interacted and even merged with other galaxies in the past, and may still be orbited by smaller, satellite galaxies. Isolated galaxies [note 2] can produce stars at a higher rate than normal, as their gas is not being stripped by other nearby galaxies. [130]

On the largest scale, the universe is continually expanding, resulting in an average increase in the separation between individual galaxies (see Hubble's law). Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This on-going merger process (as well as an influx of infalling gas) heats the inter-galactic gas within a cluster to very high temperatures, reaching 30–100 megakelvins. [131] About 70–80% of the mass in a cluster is in the form of dark matter, with 10–30% consisting of this heated gas and the remaining few percent of the matter in the form of galaxies. [132]

Most galaxies are gravitationally bound to a number of other galaxies. These form a fractal-like hierarchical distribution of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster, and these formations contain a majority of the galaxies (as well as most of the baryonic mass) in the universe. [133] [134] To remain gravitationally bound to such a group, each member galaxy must have a sufficiently low velocity to prevent it from escaping (see Virial theorem). If there is insufficient kinetic energy, however, the group may evolve into a smaller number of galaxies through mergers. [135]

The largest structures in the universe are larger than expected. Are these actual structures or random density fluctuations?

Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity. [136] Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as the brightest cluster galaxy, which, over time, tidally destroys its satellite galaxies and adds their mass to its own. [137]

Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids. [138] Above this scale, the universe appears to be the same in all directions (isotropic and homogeneous)., [139] though this notion has been challenged in recent years by numerous findings of large-scale structures that appear to be exceeding this scale. The Hercules-Corona Borealis Great Wall, currently the largest structure in the universe found so far, is 10 billion light-years (three gigaparsecs) in length. [140] [141] [142]

The Milky Way galaxy is a member of an association named the Local Group, a relatively small group of galaxies that has a diameter of approximately one megaparsec. The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group many of the other member galaxies are dwarf companions of these two. [143] The Local Group itself is a part of a cloud-like structure within the Virgo Supercluster, a large, extended structure of groups and clusters of galaxies centered on the Virgo Cluster. [144] And the Virgo Supercluster itself is a part of the Pisces-Cetus Supercluster Complex, a giant galaxy filament.

The peak radiation of most stars lies in the visible spectrum, so the observation of the stars that form galaxies has been a major component of optical astronomy. It is also a favorable portion of the spectrum for observing ionized H II regions, and for examining the distribution of dusty arms.

The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared, which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail. [147] Infrared is also used to observe distant, red-shifted galaxies that were formed much earlier. Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy.

The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies. The Earth's atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. (The ionosphere blocks signals below this range.) [148] Large radio interferometers have been used to map the active jets emitted from active nuclei. Radio telescopes can also be used to observe neutral hydrogen (via 21 cm radiation), including, potentially, the non-ionized matter in the early universe that later collapsed to form galaxies. [149]

Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole. [150] The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of supermassive black holes at the cores of galaxies was confirmed through X-ray astronomy. [151]

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