# Are metallicities of molecular clouds lower in the outskirts of the galaxy?

(this question was originally posted in an answer by user PSR-1937-21 to another post. I find it an interesting one, but since they don't seem to be active anymore, I'm posting it to see if somebody knows the answer.)

From how mass is usually distributed, it seems reasonable that metallicity would be lower in the outskirts of the galaxy. If the metallicity is lower, would that allow for larger stars to form? (see for instance "Why is metallicity important in the death of stars?"). Maybe there is a larger Jeans mass due to warmer clouds, the result of less efficient radiation of thermal energy?

## Low-metallicity globular star cluster challenges formation models

On the outskirts of the nearby Andromeda Galaxy, researchers have unexpectedly discovered a globular cluster (GC) - a massive congregation of relic stars - with a very low abundance of chemical elements heavier than hydrogen and helium (known as its metallicity), according to a new study. The GC, designated RBC EXT8, has 800 times lower abundance of these elements than the Sun, below a previously-observed limit, challenging the notion that massive GCs could not have formed at such low metallicities. GCs are dense, gravitationally bound collections of thousands to millions of ancient stars that orbit in the fringes of large galaxies many GCs formed early in the history of the Universe. Because they contain some of the oldest stars in a galaxy, GCs provide astronomers with a record of early galaxy formation and evolution. The most metal-poor GCs have abundances about 300 times lower than the Sun and no GCs with metallicities below that value were previously known. This was thought to indicate a limit to metal content - a metallicity floor - that was required for GC formation several mechanisms have been proposed to explain this limit. Søren Larsen and colleagues report the discovery of an extremely metal-deficient GC in the Andromeda Galaxy. Spectral analysis of RBC EXT8 shows that its metallicity is nearly three times lower than the most metal-poor clusters previously known, challenging the need for a metallicity floor. "Our finding shows that massive globular clusters could form in the early Universe out of gas that had only received a small 'sprinkling' of elements other than hydrogen and helium. This is surprising because this kind of pristine gas was thought to be associated with proto-galactic building blocks too small to form such massive star clusters," said Larsen.

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## Are metallicities of molecular clouds lower in the outskirts of the galaxy? - Astronomy

The discovery of extragalactic objects with very low heavy element abundance was made by Searle and Sargent (1972) who reported the properties of two intriguing galaxies, IZw18 and IIZw40. They emphasised that they could be genuinely young galaxies in the process of formation, because of their extreme metal under-abundance, more than 10 times less than solar, and even more extreme than that of H II regions found in the outskirts of spiral galaxies. At the time of this discovery the general wisdom that most galaxies (in particular the ellipticals) had been formed over a short period during a dynamical free fall time of few 10 7 years (Eggen et al. 1962) started to be challenged (e.g. Searle and Zinn 1978). It is also during the 70s that the first hierarchical models of galaxy formation were constructed (Press and Schecter 1974). Because dwarf galaxies condense from smaller perturbations than giants, the Cold Dark Matter models (CDM) predict that low mass galaxies could still be forming at present epoch. The discovery by Searle and Sargent (1972) has been an impressive stroke, since one of these two galaxies (IZw18) is still in the book of records, as we shall later elaborate on. These two objects gave rise to many systematic searches for more objects in a quest for local genuine young galaxies or unevolved galaxies'', depending upon the alternative viewpoints that some galaxies could be caught in the process of formation or that they simply were the result of a very mild evolution over the Hubble time. These galaxies had the advantage of being gas-rich, with spectra dominated by strong emission lines (see Fig. 6) favouring their detection. Many techniques have been employed to find them, sometimes at large distances despite their intrinsic low luminosity.

The last nearly three decades have brought a wealth of data from numerous studies on dwarf galaxies, including information on their chemical composition. It became clear that metal-poor'' would be analogous of low mass'' galaxies (Lequeux et al. 1979). For this reason, our review will largely focus on dwarf galaxies, but we shall address the question of the existence of large and massive proto-galaxies - essentially devoid of metals - at large redshift in the last section. The dwarf galaxies are not only interesting for understanding the process of galaxy formation. For the gas-rich ones with active star formation, one motivation to study them has been the hope to better understand the processes of massive star formation in low metallicity gas. The fact that they are dwarfs means that spiral waves can not be sustained. They turn out to be test cases for chemical evolutionary models and offer the possibility to approach the primordial helium abundance with a minimum of extrapolation to early conditions. Many galaxies of different kinds can be identified as metal-poor and it is an interesting question to find out about the connections that bridge them together. Finally, a lot of new studies concentrate on the impact of massive star formation onto the ISM in star-bursting dwarf objects, that in turn can lead to constrain the supernovae rate, IMF, the metal dependence of the winds in Wolf-Rayet (WR) stars etc. In fact there are many issues in astronomy where it is essential to understand dwarf galaxies (actively forming stars or not).

A definition of what is a metal-poor galaxy is indeed necessary at this point. To define the metallicity (Z, i.e. the relative abundance of elements other than hydrogen and helium) of a galaxy requires some words of caution because in a given galaxy, depending where one looks, this quantity may vary substantially. For example in our Galaxy, the bulge, the solar neighbourhood and the halo differ in metallicity. The most metal-poor halo stars have heavy element abundances 10 -4 times that of the Sun (Cayrel 1996) while stars in the Galactic centre may be three times more metal rich than the Sun. On the other hand, the large ionised complexes in the ISM show a narrower range down to only 1/10 the solar value. Thus metallicity depends on what one looks at: stars or gas, and if one considers gas - what phase: neutral atomic, molecular or ionised? Moreover the metallicity of stars is found to depend on their age, and depends on which elements are investigated that a star or nebula is deficient of a certain element does not automatically mean that the overall chemical abundances are low.

Thus, one must be careful in defining what metallicity means for a given galaxy before comparing observations and looking for trends. Another complication stems from the different techniques in use for determining chemical abundances. Metallicities of Local Group dwarf spheroidals (dSph) have largely been investigated through photometry of their resolved stellar populations, which are dominated by old stars, and are found to be metal-poor, as measured by [Fe/H]. In general dSph galaxies contain little or no gas, and no H II regions. Dwarf irregular (dI) galaxies on the other hand usually have plenty of gas and ongoing star formation as witnessed by the presence of H II regions. It is relatively easy to derive metallicities, especially oxygen (O) of the ionised gas in H II regions explaining why most investigations are based on such. Hence the metallicity'' of dSphs and dIs measures quantities that are not directly comparable. Some galaxies have no H II regions and very low surface brightness, and the only available spectroscopic method hitherto, has been to study individual bright stars which are within reach in nearby galaxies only. In more distant galaxies, individual stars are not observable and the metallicity has to be inferred from either absorption line spectra of an integrated stellar population (not necessarily homogeneous in age or composition) or from the nebular emission lines, if any. Again, the different methods in general measure different elements.

So what useful definitions of metallicity'' do we have at hand? The possibility of using the metallicity of the H II regions has the advantage of providing an up to date'' metallicity, while stars reflect the metallicity of the cloud from where they were born, perhaps a Hubble time ago. Nebular abundances show large spatial abundance variations and gradients in some galaxies, e.g. giant spiral galaxies, like the Milky Way. Although measurable abundance inhomogeneities could be expected, most dwarf galaxies seem rather well mixed. A possible problem with metal-poor H II regions is self-pollution of fresh metals by winds from young massive stars, so that the abundance inferred from the nebular emission lines may not be representative of that in the local ISM (Kunth & Sargent 1986). On the other hand, the metallicity of the stars in a galaxy depends on which stellar population is studied. Thus it is not surprising, in particular for galaxies which have experienced continuous star formation, that stars have different abundances. Integrated spectra of galaxies provide a luminosity-weighted average of the metallicity. This average metallicity will change with time due to the photometric evolution of the stellar population, even if no new stars are formed.

A good, well defined, metallicity indicator would be the fraction of baryonic matter (presumably having initially primordial composition) that has been converted into heavier elements by means of stellar nucleosynthesis. This material may have been returned to the ISM or may still be locked up in stars. Such a definition would indicate that our main interest goes beyond the element abundances themselves from the fact that they provide information about the history of a galaxy. The relative abundances and gas mass fractions might unveil different histories among galaxies with the same metallicities. However, it is clear from he above discussion that such a definition of metallicity remains impractical, since not directly measurable. However, we would like to keep this ideal definition in mind for the rest of the discussion.

Now we wish to go back to the question - What is a metal-poor galaxy? Under the assumption that various tracers give a plausible picture about the ideal metallicity'' we can now try to compare different kinds of galaxies. One of the most fundamental parameters for a galaxy is of course its mass. This mass, which may consist of stars, gas, dust, baryonic- and non-baryonic dark matter, is more difficult to measure, than e.g. the luminosity, but to the first approximation, mass and luminosity correlate. Based on their luminosity and size, galaxies can be divided into dwarfs and giants. It has been found that the metallicity of a galaxy in the local Universe correlates positively with its luminosity (although with a large scatter), thus also reflecting a positive correlation with mass. The reason for this behaviour is a fundamental issue to understand. One explanation could be that dwarfs evolve more slowly because of smaller mass densities, which to the first order fits with the observation that dwarfs, except dwarf elliptical/spheroidals, are more gas-rich than giants. Another possible explanation is that dwarfs have weaker gravitational potential hence are more susceptible to loose metal enriched material from supernova driven winds. We will elaborate more on this later.

A natural reference for element abundances and the ratio between them, could be the Sun. Thus a starting point could be that metal-poor'' means anything which has sub-solar abundances and vice versa for metal rich'', which implies that basically all local galaxies fainter than our Galaxy are metal-poor. High redshift neutral gas clouds, which may be the building blocks of today's galaxies, are observed to have metallicities down to 0.001 Z . Thus, there is a large range of metallicity to explore, and it is meaningful to distinguish between metal-poor, very metal-poor and extremely metal-poor. Since this review is called the most metal-poor galaxies'' it is natural that we focus on the latter two subclasses. What do we see locally? Among dSph we find metallicities extending down to 1/100 Z , while the LMC and SMC are at roughly 1/3 Z and 1/8 Z respectively. Dwarf irregulars have sub-solar abundances, ranging down to 1/40 Z . In addition there are many blue compact galaxies (BCGs) in the range 1/10 to 1/50 Z , with, as we shall show later, IZw18 at the extreme.

A more workable definition could use the minimum enrichment one predicts for a single burst event, using the instantaneous recycling closed box model. Kunth and Sargent (1986) found that such minimal expected metallicity increment in a pristine H II region would be higher than or equal to the metallicity of IZw18. Similarly one finds that even converting only on the order of two percent of pristine gas in a galaxy to stars, results in a metallicity of 1/50 Z , i.e. the metallicity of IZw18. There are of course a lot of uncertainties that go into these calculations, but they can be a useful guide. Another, more practical guide, is to consider the O/H distribution of star forming dwarf galaxies studied over the last 30 years. This shows a peak around 1/10 Z and drops sharply below that value. Moreover, for most models: of stellar winds, evolutionary tracks, WR-stars, star formation etc., a critical dependence on metallicity is seen around 1/10 Z . This is why we have adopted throughout this review the working hypothesis that galaxies with metallicity below 1/10 Z will be considered as very metal deficient. Therefore galaxies like the Magellanic clouds will not be our main interest in this paper. Moreover, such a limit means that this review will be biased towards dwarf galaxies. In particular we will focus on blue compact galaxies (BCGs). The reason is partly due to selection effects: since blue compact galaxies have bright emission lines and high surface brightness, it is fairly easy to discover them and derive their metal content. Thus, there exists a lot of high quality data on BCGs, but we should keep in mind that very metal-poor galaxies may be as common among other types of dwarf galaxies.

Metallicities can be studied at great distances under special conditions. Observations of high redshift QSOs and radio galaxies (e.g. Dunlop et al. 1994), reveal the presence of dust and metal rich gas, suggesting that prior stellar nucleosynthesis has already taken place. High redshift QSO absorption line systems show a wide range of metallicities, from one thousandth solar up to 1/3 solar. Thus, while the average metallicity of the Universe certainly must have increased since the early epochs, the situation is more complex than a simple picture where high redshift means metal-poor, and low redshift metal-rich. Objects with high and low metallicities are found at all redshifts. Surely we expect objects that in the local Universe appear as metal deficient to be even more deficient at high redshift, if we could observe their precursors. Also the ancestors of the local metal rich galaxy population, i.e. the giant spirals and ellipticals, should have started out with very low abundances unless they were gradually built up by merging smaller galaxies. Currently, both the theoretical and observational pictures, tell that the latter is an important mechanism. Dwarf galaxies, the survivors who form the local metal-poor galaxy population, may thus be the principal building blocks of the Universe on large scales.

The structure of the rest of paper is as follows: In Section 2 we discuss how metallicities are measured and in Sect. 3 the physical mechanisms that control the metallicity of a galaxy. In Section 4 we review the physics of metal-poor galaxies in the local Universe, while in Sect. 5 turning to some key objects like IZw18. In Sect. 6 we discuss survey techniques, and the distribution in space and luminosity of metal-poor galaxies. In Sect. 7 we examine observed trends in the metal-poor galaxy population and various possible evolutionary links. In Sect. 8 we focus on cosmology and the high redshift Universe, and in Sect. 9 we conclude.

## Monstrous Cosmic Gas Cloud Set To Ignite The Milky Way

When we think about our galaxy, most people think about the stars in the sky, the grand, sweeping spiral arms, the disk-like plane of our Milky Way filled with dust, and the bulge in the galactic center. All of this combines to make up our run-of-the-mill home, complete with some 400 billion stars not so different from our own. And our Milky Way, visible from anyplace on Earth during a dark, moonless night, is just one of hundreds of billions of galaxies similar to it in our Universe.

Yet this one is not only our own, it contains much more than what's visible to us. In particular, the galaxy has a huge massive halo beyond the disk. It isn't just full of dark matter, either, but various incarnations of normal matter, including more than a hundred globular clusters (collections of hundreds of thousands of stars, all bound together within just a few tens of light years) and -- very importantly -- gigantic clouds of molecular gas, moving at high speeds throughout the outskirts of our galaxy. These clouds can collapse and form stars, they can pass through the plane of our galaxy and trigger new episodes of star formation, or they can gravitationally interact with other masses, including:

• infalling dwarf galaxies or tidal debris,
• globular clusters,
• other molecular clouds,

The gravitational interactions are particularly interesting, because whenever you have three bodies interacting, two often become more tightly bound while the third gets a "kick," potentially ejecting it. This is how we use planets to assist spacecrafts in their journey towards the outer Solar System, and the same principle can allow gas clouds to be ejected from our own galaxy. In one very particular, peculiar case, however, a gas cloud in our own galaxy almost got kicked out, but not quite.

There are hundreds of high-velocity gas clouds moving at hundreds of km/s through the outskirts of our galaxy, mostly in stable orbits that keep them out of the galactic plane. They're typically irregularly shaped, thousands of light years across, and contain many millions of times the mass of our Sun. However, one such cloud, known as the "Smith Cloud" (above), is very different from all the others. It's much more distant, and it's moving towards us incredibly rapidly: at about 310 km/s, or around 700,000 miles per hour. And when I say it's moving towards us, it's projected to collide at extreme velocities with the disk of our galaxy in about 30 million years: a long time for a human, but a very short time for our galaxy. (For comparison, it takes our Sun about 200 million years to complete a single orbit around our galactic center.) Here's how we found this out.

We used chemical abundance measurements (heavy element enrichment levels) and orbital calculations to conclude it has a Galactic origin. The Hubble data show that the Smith Cloud is enriched in sulfur to Milky Way-like levels. If it came from outside the Galaxy, it would have much lower levels of sulfur enrichment.

In addition, Fox was able to use the orbital dynamics of the cloud -- thanks to the Hubble data -- to conclude that this cloud came from our own galaxy, was "kicked" somehow to almost escape (but not quite), and is now taking the plunge under the influence of gravity back towards our Milky Way's disk!

"[T] he Cloud's orbit tracks back to the Galactic disk about 70 million years ago," Fox continued. In 30 million years, the cloud will collide with the galactic plane, triggering an incredible star formation event. The amount of gas present in this cloud -- more than 11,000 light years wide -- should trigger over two million new stars in our galaxy. What causes a molecular cloud to do this? Fox isn't sure:

However, the origin of the cloud's high velocity is certainly a matter of debate. It could potentially be a dark matter halo that passed through the Galactic disk, accumulated gas, and continued on its journey.

No matter what the answer turns out to be, this is one of the most interesting discoveries we've made so far about space, and it's not only right in our own backyard, it's going to get much more interesting in our very near-term cosmic future.

## Big galaxies steal star-forming gas from their smaller neighbours

An artist's impression showing the increasing effect of ram-pressure stripping in removing gas from galaxies, sending them to an early death. Credit: ICRAR, NASA, ESA, the Hubble Heritage Team (STScI/AURA)

Large galaxies are known to strip the gas that occupies the space between the stars of smaller satellite galaxies.

In research published today, astronomers have discovered that these small satellite galaxies also contain less 'molecular' gas at their centres.

Molecular gas is found in giant clouds in the centres of galaxies and is the building material for new stars. Large galaxies are therefore stealing the material that their smaller counterparts need to form new stars.

Lead author Dr. Adam Stevens is an astrophysicist based at UWA working for the International Centre for Radio Astronomy Research (ICRAR) and affiliated to the ARC Centre of Excellence in All Sky Astrophysics in 3 Dimensions (ASTRO 3-D).

Dr. Stevens said the study provides new systematic evidence that small galaxies everywhere lose some of their molecular gas when they get close to a larger galaxy and its surrounding hot gas halo.

"Gas is the lifeblood of a galaxy," he said.

"Continuing to acquire gas is how galaxies grow and form stars. Without it, galaxies stagnate.

"We've known for a long time that big galaxies strip 'atomic' gas from the outskirts of small galaxies.

"But, until now, it hadn't been tested with molecular gas in the same detail."

Two viewing angles of a galaxy undergoing ram-pressure stripping in the IllustrisTNG simulation. Each column shows matter of a different form in the galaxy and its immediate surroundings. From left to right: (1) atomic gas (2) molecular gas (3) all gas (4) stars and (5) dark matter. Credit: Adam Stevens/ICRAR

ICRAR-UWA astronomer Associate Professor Barbara Catinella said galaxies don't typically live in isolation.

"Most galaxies have friends," she says.

"And when a galaxy moves through the hot intergalactic medium or galaxy halo, some of the cold gas in the galaxy is stripped away.

"This fast-acting process is known as ram pressure stripping."

The research was a global collaboration involving scientists from the University of Maryland, Max Planck Institute for Astronomy, University of Heidelberg, Harvard-Smithsonian Center for Astrophysics, University of Bologna and Massachusetts Institute of Technology.

Molecular gas is very difficult to detect directly.

The research team took a state-of-the-art cosmological simulation and made direct predictions for the amount of atomic and molecular gas that should be observed by specific surveys on the Arecibo telescope in Puerto Rico and the IRAM 30-meter telescope in Spain.

They then took the actual observations from the telescopes and compared them to their original predictions.

The two were remarkably close.

Associate Professor Catinella, who led the Arecibo survey of atomic gas, says the IRAM 30-meter telescope observed the molecular gas in more than 500 galaxies.

"These are the deepest observations and largest sample of atomic and molecular gas in the local Universe," she says.

"That's why it was the best sample to do this analysis."

The team's finding fits with previous evidence that suggests satellite galaxies have lower star formation rates.

Dr. Stevens said stripped gas initially goes into the space around the larger galaxy.

"That may end up eventually raining down onto the bigger galaxy, or it might end up just staying out in its surroundings," he said.

But in most cases, the little galaxy is doomed to merge with the larger one anyway.

"Often they only survive for one to two billion years and then they'll end up merging with the central one," Dr. Stevens said.

"So it affects how much gas they've got by the time they merge, which then will affect the evolution of the big system as well.

"Once galaxies get big enough, they start to rely on getting more matter from the cannibalism of smaller galaxies."

## 'Warm Neptune' Has Unexpectedly Primitive Atmosphere

A study combining observations from NASA's Hubble and Spitzer space telescopes reveals that the distant planet HAT-P-26b has a primitive atmosphere composed almost entirely of hydrogen and helium.

A study combining observations from NASA's Hubble and Spitzer space telescopes reveals that the distant planet HAT-P-26b has a primitive atmosphere composed almost entirely of hydrogen and helium. Located about 437 light-years away, HAT-P-26b orbits a star roughly twice as old as the sun.

The analysis is one of the most detailed studies to date of a "warm Neptune," or a planet that is Neptune-sized and close to its star. The researchers determined that HAT-P-26b's atmosphere is relatively clear of clouds and has a strong water signature, although the planet is not a water world. This is the best measurement of water to date on an exoplanet of this size.

The discovery of an atmosphere with this composition on this exoplanet has implications for how scientists think about the birth and development of planetary systems. Compared to Neptune and Uranus, the planets in our solar system with about the same mass, HAT-P-26b likely formed either closer to its host star or later in the development of its planetary system, or both.

"Astronomers have just begun to investigate the atmospheres of these distant Neptune-mass planets, and almost right away, we found an example that goes against the trend in our solar system," said Hannah Wakeford, a postdoctoral researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study published in the May 12, 2017, issue of Science. "This kind of unexpected result is why I really love exploring the atmospheres of alien planets."

To study HAT-P-26b's atmosphere, the researchers used data from transits -- occasions when the planet passed in front of its host star. During a transit, a fraction of the starlight gets filtered through the planet's atmosphere, which absorbs some wavelengths of light but not others. By looking at how the signatures of the starlight change as a result of this filtering, researchers can work backward to figure out the chemical composition of the atmosphere.

In this case, the team pooled data from four transits measured by Hubble and two seen by Spitzer. Together, those observations covered a wide range of wavelengths from yellow light through the near-infrared region.

"To have so much information about a warm Neptune is still rare, so analyzing these data sets simultaneously is an achievement in and of itself," said co-author Tiffany Kataria of NASA's Jet Propulsion Laboratory in Pasadena, California.

Because the study provided a precise measurement of water, the researchers were able to use the water signature to estimate HAT-P-26b's metallicity. Astronomers calculate the metallicity, an indication of how rich the planet is in all elements heavier than hydrogen and helium, because it gives them clues about how a planet formed.

To compare planets by their metallicities, scientists use the sun as a point of reference, almost like describing how much caffeine beverages have by comparing them to a cup of coffee. Jupiter has a metallicity about 2 to 5 times that of the sun. For Saturn, it's about 10 times as much as the sun. These relatively low values mean that the two gas giants are made almost entirely of hydrogen and helium.

The ice giants Neptune and Uranus are smaller than the gas giants but richer in the heavier elements, with metallicities of about 100 times that of the sun. So, for the four outer planets in our solar system, the trend is that the metallicities are lower for the bigger planets.

Scientists think this happened because, as the solar system was taking shape, Neptune and Uranus formed in a region toward the outskirts of the enormous disk of dust, gas and debris that swirled around the immature sun. Summing up the complicated process of planetary formation in a nutshell: Neptune and Uranus would have been bombarded with a lot of icy debris that was rich in heavier elements. Jupiter and Saturn, which formed in a warmer part of the disk, would have encountered less of the icy debris.

Two planets beyond our solar system also fit this trend. One is the Neptune-mass planet HAT-P-11b. The other is WASP-43b, a gas giant twice as massive as Jupiter.

But Wakeford and her colleagues found that HAT-P-26b bucks the trend. They determined its metallicity is only about 4.8 times that of the sun, much closer to the value for Jupiter than for Neptune.

"This analysis shows that there is a lot more diversity in the atmospheres of these exoplanets than we were expecting, which is providing insight into how planets can form and evolve differently than in our solar system," said David K. Sing of the University of Exeter and the second author of the paper. "I would say that has been a theme in the studies of exoplanets: Researchers keep finding surprising diversity."

JPL manages the Spitzer Space Telescope for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington.

## A Distorted Galaxy And Its Cloaked Clouds of Gas

I find all galaxies to be beautiful, from huge, symmetric elliptical puffballs to glorious, grand design spirals.

But man, J082354.96 is seriously messed up. It’s still beautiful, though:

Wow. It’s quite the train wreck, and can definitely be labeled as “peculiar”. That’s an actual galaxy type, along with elliptical, disk (or spiral, like our Milky Way), and irregular. That last is for galaxies with no overall shape peculiars have a definite shape, just a weird one.

J08 is about 650 million light years away, and clearly has something going on to give it this weird, drawn out, and oddly pleasing curvy hooked shape. To any astronomer’s eye, it’s obviously undergone an interaction: a cosmic collision with or nearby pass of another galaxy. That will commonly elongate a galaxy like this, and even cause those curls at the ends. As two galaxies collide (and sometimes merge), the huge collective gravities of each stretch the other out like taffy, and an off-center collision can cause vast arcs of gas and stars to be drawn out.

Interestingly, it’s not clear to me where the other galaxy is that did this. It’s possible they merged completely, forming J08 as we see it now, disturbed and weird but probably beginning to settle down after the eons-long encounter. If they didn’t merge, though, it’s difficult to say what happened to the other galaxy just from examining this image alone. There are a couple of galaxies near J08 in the full picture, but without knowing their distance they could be located much closer or farther from Earth than J08 itself, completely unrelated to it.

This galaxy was observed by Hubble to find out what it looks like in ultraviolet light, as part of a study the structure of these galaxies. UV is strongly emitted by hot, massive, blue stars, which don’t live very long. As it happens, J08 is a starburst galaxy, cranking stars out at a high rate. A lot of those stars are the massive and hot kind, and they light up the gas and dust around them—these are strung out along the galaxy, which you can see as those blue regions in the Hubble picture. This is actually pretty typical after a big galaxy collision gas clouds collide, collapse, and form stars at a furious rate.

However, there’s more going on here. The UV light seen in the Hubble image above is pretty much emitted by stars and warm gas. But if you look farther into the ultraviolet, a new feature comes up, a very special color of UV strongly emitted by hydrogen gas. When you hit a hydrogen atom with enough energy, its sole electron will jump from one energy level to the next, like a person hopping up a step on a staircase. In this case, the electron jumps up from the bottom energy level to the next one up. After a time, it’ll plop back down and emit a UV photon at 121.6 nanometers wavelength (way outside what the human eye can see). This light is so special it has its own name: Lyman Alpha, or Lyα.

The astronomers studying J08 used the orbiting GALEX observatory to take a look at the Lyα being emitted in the galaxy. They processed the data to remove a lot of unwanted light interfering with the Lyα, and what they found is interesting:

In the upper image (a combination of several observations from Hubble and GALEX), red shows light from warm gas clouds, green from the massive stars, and the Lyα (normally invisible to the human eye) is colored blue. As you can see, quite a bit of Lyα appears to be coming from the outskirts of the galaxy. It’s coming from the interior as well, but that’s overwhelmed by the other light and hard to see here. The point is that the Lyα emission is also coming from parts of the galaxy well beyond where we see visible light being emitted. J08 is an extreme example (the galaxy itself is stretched out) but they found similar results in about a dozen other galaxies they looked at as well.

It turns out that many galaxies are surrounded by a thin halo of hydrogen gas, but it’s very hard to detect because it’s spread out. It doesn’t emit optical light we can see, and it’s too cold to emit UV light on its own. But those massive hot stars are sending out light at all colors of UV, including Lyα, and the gas on the outskirts absorbs and re-emits it, betraying its presence. That’s why we see Lyα coming from the outer parts of the galaxy. The actual mechanism occurring is more complicated than this—isn’t it always?—but that’s the basics of it. J08 and the others were chosen because they’re relatively nearby, and their structures could be picked out by Hubble. Once we understand how and where Lyα is emitted by them, we can use that to better understand more distant galaxies where we can’t see the structure directly.

This is important to know because Lyα is used to determine how many hot, massive stars are born in these kinds of galaxies. It also reveals the structures of these galaxies, including the location of gas that is otherwise invisible. It’s also used in other ways, like finding the distances to galaxies and vast gas clouds, and even what conditions were like in the early Universe. All that from a simple quirk in the simplest atom of them all.

I find it fascinating that the Universe is so accommodating to our inquisitive nature. It leaves clues everywhere about itself, and all you need to learn about it is a bit of math and physics, technology, and above all curiosity. With those features in combination, the entire cosmos can be revealed.

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## Galaxies

The ultraluminous infrared galaxy Markarian 273, a major merger of two
gas-rich spiral galaxies Dave Sanders and Joshua Barnes are studying the multiwavelength properties of a complete sample of far-infrared selected galaxies in the local universe, as part of the Great Observatories All-Sky LIRGs Survey, in order to understand the origin and evolution of the most luminous infrared systems, with infrared luminosities 10&ndash1000 times the total bolometric lumi­nosity of our Milky Way.

These &ldquoluminous infrared galaxies&rdquo (LIRGs) appear to be triggered through mergers of massive gas-rich spiral galaxies, an event which leads to powerful starbursts and the growth of supermassive black holes. The end stages of the merger process lead to qua­sarlike luminosities, including the final stage marked by binary active galaxy nucleus. The eventual merger of the two supermassive black holes is accompanied by a massive &ldquoblow-out&rdquo phase, ex­pelling as much as several billion solar masses of gas and dust into the intergalactic medium, leaving a massive gas-poor elliptical as the merger remnant. This exotic process of galaxy transformation, although relatively rare in the local universe, is now believed to be one of the dominant processes of galactic evolution in the early universe, when the space density of LIRGs was

10,0000 times larger than observed locally, and coinciding with the peak epoch in the formation of quasars and superstarbursts.

1. Interactions are correlated with high-IR luminosities and mergers with very high IR luminosities.
2. Starbursts are the dominant underlying energy source in interacting galaxies.
3. The starburst initial mass function is biased against high-mass stars.
4. While optical and 2 µm spectroscopy showed no evidence for buried AGNs in a sample of 30 LIRGs, mid-IR spectroscopy revealed high excitation coronal lines in

Thus interactions and mergers trigger bursts of star for­mation with a &ldquobottom-heavy&rdquo initial mass function, and the starburst dominates the bolometric luminosity: the mergers are making elliptical galaxies.

### Galaxy Collisions

Joshua Barnes uses N-body methods to simulate galactic collisions and other aspects of galactic dynamics. One area of ongoing effort is improving existing techniques for force calculation, construction of initial conditions, and simulation including star formation and recycling of interstellar material. A second area of emphasis is developing accurate models of well-observed interacting galaxies. Ultimately, one objective of this research is to test dark-matter models and prescriptions for star formation by comparing detailed models of specific interacting galaxies with observations.

Computer-generated model of NGC 4676 overlaid on maps of the actual HI and stellar distributions

### Chemical Abundances of Normal, Nearby Spiral Galaxies

Fabio Bresolin is studying the chemical abundances of yourg stars and HII regions in spiral galaxies. Among the most interesting results is the measurement of the metallicity of HII regions in outer spiral disks, where the star formation rate is about 2 orders of magnitude lower than in the inner, optically bright disks. The spectroscopic investigation of the faint nebulae in these &ldquoextended&rdquo disks has been carried out with 8 m class telescopes on Mauna Kea and in Chile.

For the four cases studied to date he has found that the metallicity of the outer disks, rather than following an exponential decline with distance from the center (as is typically found for the inner disks), flattens out to a virtually constant value. Moreover, the metallicity measured in the outer disks is rather high (about 1/3 solar), contrary to the expectations for galactic regions that are considered to be unevolved relative to the inner regions.

The O/H abundance ratio in NGC 3621 as a function of distance from the galaxy's center.

### The Mass-Metallicity Relationship for Galaxies

The mass-metallicity relationship of galaxies is a key to understanding the physics of galaxy formation and evolution in an expanding universe dominated by dark matter and dark energy. Unfortunately, the standard technique to determine the metallicities of star-forming galaxies— using emission line spectra of HII regions — is subject to large systematic uncertainties that are poorly understood.

Rolf Kudritzki has pioneered an alternative approach, namely to use the Keck teleacope to obtain low-resolution spectra of individual red and blue supergiant stars in external galaxies. These objects are the brightest stars in the universe with absolute magnitudes in the range -9 to -11.

Metallicities of individual supergiant stars (blue) as a function of galactocentric radius in the giant spiral galaxy M81.The red points are planetary nebulae. The lower metallicities of the planetary nebulae are probably accounted for by the fact that they were formed billions of years earlier than the blue supergant stars.

### Extragalactic Planetary Nebulae

Planetary nebulae (PNs) are easy to de­tect in early-type galaxies at distances smaller than 25 Mpc. Once detected, the strong emission lines in PN spectra are well suited for accurate radial velocity measurements. PNs are valuable test particles for studying angular momentum content and dark matter existence and its distribution in elliptical galaxies, which are hard observational problems.

Roberto Mendez has been using the Subaru telescope on Mauna Kea to discover and measure the velocities of more than one thousand PNs in galaxies like NGC 4697, NGC 821, and NGC 4649. The figure shows radial velocities of PNs in the flattened, almost edge-on elliptical NGC 4697, plotted as a function of their coordinates along the major axis of the galaxy. The slight asymmetry in the distribution is because of the rotation of the PN system, which is significant inside, but becomes undetectable in the outskirts. The marked outward decrease in the velocity dispersion can be interpreted either as a relative lack of dark matter in the halo of NGC 4697, or as the consequence of radial anisotropy in the PN velocity distribution.

## Contents

Astronomers use several different methods to describe and approximate metal abundances, depending on the available tools and the object of interest. Some methods include determining the fraction of mass that is attributed to gas versus metals, or measuring the ratios of the number of atoms of two different elements as compared to the ratios found in the Sun.

### Mass fraction Edit

Stellar composition is often simply defined by the parameters X, Y and Z. Here X is the mass fraction of hydrogen, Y is the mass fraction of helium, and Z is the mass fraction of all the remaining chemical elements. Thus

For the surface of the Sun, these parameters are measured to have the following values: [5]

Description Solar value
Hydrogen mass fraction X sun = 0.7381 >=0.7381>
Helium mass fraction Y sun = 0.2485 >=0.2485>
Metallicity Z sun = 0.0134 >=0.0134>

Due to the effects of stellar evolution, neither the initial composition nor the present day bulk composition of the Sun is the same as its present-day surface composition.

### Chemical abundance ratios Edit

The overall stellar metallicity is conventionally defined using the total hydrogen content, since its abundance is considered to be relatively constant in the Universe, or the iron content of the star, which has an abundance that is generally linearly increasing in the Universe. [6] Iron is also relatively easy to measure with spectral observations in the star's spectrum given the large number of iron lines in the star's spectra (even though oxygen is the most abundant heavy element – see metallicities in HII regions below). The abundance ratio is the common logarithm of the ratio of a star's iron abundance compared to that of the Sun and is calculated thus: [7]

where N Fe >> and N H >> are the number of iron and hydrogen atoms per unit of volume respectively. The unit often used for metallicity is the dex, contraction of "decimal exponent". By this formulation, stars with a higher metallicity than the Sun have a positive common logarithm, whereas those more dominated by hydrogen have a corresponding negative value. For example, stars with a [Fe/H] value of +1 have 10 times the metallicity of the Sun (10 1 ) conversely, those with a [Fe/H] value of −1 have 1 ⁄ 10 , while those with a [Fe/H] value of 0 have the same metallicity as the Sun, and so on. [8] Young Population I stars have significantly higher iron-to-hydrogen ratios than older Population II stars. Primordial Population III stars are estimated to have metallicity less than −6, a millionth of the abundance of iron in the Sun. [9] [10] The same notation is used to express variations in abundances between other individual elements as compared to solar proportions. For example, the notation "[O/Fe]" represents the difference in the logarithm of the star's oxygen abundance versus its iron content compared to that of the Sun. In general, a given stellar nucleosynthetic process alters the proportions of only a few elements or isotopes, so a star or gas sample with certain [/Fe] values may well be indicative of an associated, studied nuclear process.

### Photometric colors Edit

Astronomers can estimate metallicities through measured and calibrated systems that correlate photometric measurements and spectroscopic measurements (see also Spectrophotometry). For example, the Johnson UVB filters can be used to detect an ultraviolet (UV) excess in stars, [11] where a smaller UV excess indicates a larger presence of metals that absorb the UV radiation, thereby making the star appear "redder". [12] [13] [14] The UV excess, δ(U−B), is defined as the difference between a star's U and B band magnitudes, compared to the difference between U and B band magnitudes of metal-rich stars in the Hyades cluster. [15] Unfortunately, δ(U−B) is sensitive to both metallicity and temperature: if two stars are equally metal-rich, but one is cooler than the other, they will likely have different δ(U−B) values [15] (see also Blanketing effect [16] [17] ). To help mitigate this degeneracy, a star's B−V color can be used as an indicator for temperature. Furthermore, the UV excess and B−V color can be corrected to relate the δ(U−B) value to iron abundances. [18] [19] [20]

Other photometric systems that can be used to determine metallicities of certain astrophysical objects include the Strӧmgren system, [21] [22] the Geneva system, [23] [24] the Washington system, [25] [26] and the DDO system. [27] [28]

### Stars Edit

At a given mass and age, a metal-poor star will be slightly warmer. Population II stars' metallicities are roughly 1/1000 to 1/10 of the Sun's ([Z/H] = −3.0 to −1.0 ), but the group appears cooler than Population I overall, as heavy Population II stars have long since died. Above 40 solar masses, metallicity influences how a star will die: outside the pair-instability window, lower metallicity stars will collapse directly to a black hole, while higher metallicity stars undergo a Type Ib/c supernova and may leave a neutron star.

#### Relationship between stellar metallicity and planets Edit

A star's metallicity measurement is one parameter that helps determine whether a star may have a giant planet, as there is a direct correlation between metallicity and the presence of a giant planet. Measurements have demonstrated the connection between a star's metallicity and gas giant planets, like Jupiter and Saturn. The more metals in a star and thus its planetary system and proplyd, the more likely the system may have gas giant planets. Current models show that the metallicity along with the correct planetary system temperature and distance from the star are key to planet and planetesimal formation. For two stars that have equal age and mass but different metallicity, the less metallic star is bluer. Among stars of the same color, less metallic stars emit more ultraviolet radiation. The Sun, with 8 planets and 5 known dwarf planets, is used as the reference, with a [Fe/H] of 0.00. [29] [30] [31] [32] [33]

### HII regions Edit

Young, massive and hot stars (typically of spectral types O and B) in H II regions emit UV photons that ionize ground-state hydrogen atoms, knocking electrons and protons free this process is known as photoionization. The free electrons can strike other atoms nearby, exciting bound metallic electrons into a metastable state, which eventually decay back into a ground state, emitting photons with energies that correspond to forbidden lines. Through these transitions, astronomers have developed several observational methods to estimate metal abundances in HII regions, where the stronger the forbidden lines in spectroscopic observations, the higher the metallicity. [34] [35] These methods are dependent on one or more of the following: the variety of asymmetrical densities inside HII regions, the varied temperatures of the embedded stars, and/or the electron density within the ionized region. [36] [37] [38] [39]

Theoretically, to determine the total abundance of a single element in an HII region, all transition lines should be observed and summed. However, this can be observationally difficult due to variation in line strength. [40] [41] Some of the most common forbidden lines used to determine metal abundances in HII regions are from oxygen (e.g. [O II] λ = (3727, 7318, 7324) Å, and [O III] λ = (4363, 4959, 5007) Å), nitrogen (e.g. [NII] λ = (5755, 6548, 6584) Å), and sulfur (e.g. [SII] λ = (6717,6731) Å and [SIII] λ = (6312, 9069, 9531) Å) in the optical spectrum, and the [OIII] λ = (52, 88) μm and [NIII] λ = 57 μm lines in the infrared spectrum. Oxygen has some of the stronger, more abundant lines in HII regions, making it a main target for metallicity estimates within these objects. To calculate metal abundances in HII regions using oxygen flux measurements, astronomers often use the R23 method, in which

R 23 = [ O II ] 3727 Å + [ O III ] 4959 Å + 5007 Å H β , =>]_<3727

where O III 3727 Å + O III 4959 Å + 5007 Å >_<3727

mathrm >> is the sum of the fluxes from oxygen emission lines measured at the rest frame λ = (3727, 4959 and 5007) Å wavelengths, divided by the flux from the Hβ emission line at the rest frame λ = 4861 Å wavelength. [42] This ratio is well defined through models and observational studies, [43] [44] [45] but caution should be taken, as the ratio is often degenerate, providing both a low and high metallicity solution, which can be broken with additional line measurements. [46] Similarly, other strong forbidden line ratios can be used, e.g. for sulfur, where [47]

S 23 = [ S II ] 6716 Å + 6731 Å + [ S III ] 9069 Å + 9532 Å H β . =>]_<6716

Metal abundances within HII regions are typically less than 1%, with the percentage decreasing on average with distance from the Galactic Center. [40] [48] [49] [50] [51]