# How can we infer the mass of SMBH in galaxies that are not active anymore?

I know it is possible to infer the mass of a supermassive black hole (SMBH) by many methods, i.e., stellar orbits for out Galaxy, Iron line profile from the accretion disk, and probably other methods (perhaps from the spectrum of the radiation disk itself, that can be related with the central mass, if supposed thermal in origin). What I don't know is: how can we infer the mass of SMBH in galaxies that are not active anymore?

I can think of two methods.

Both rely on the dynamics of material surrounding the SMBH, which is affected up to a distance of the order of the "sphere of influence". This is the region where the BH dominates the dynamics as compared to the enclosed mass of the galaxy. The sphere of influence is:

$$R_{mathrm{infl}} equiv frac{G M_{mathrm{SMBH}}}{sigma_{mathrm{bulge}}^2} >> R_{mathrm{horizon}} approx R_{mathrm{Schwartzschild}} equiv frac{G M_{mathrm{SMBH}}}{c^2}$$

While typically $sigma_{mathrm{bulge}} approx 250 km s^{-1}$, it is well known that $c approx 300000 km s^{-1}$. This means that the influence of a BH can be felt much further away than its event horizon, which is where the accretion takes place. In fact the ratio between the two distances is about 1000000.

Exploiting this fact, astronomers have been using two methods to probe extragalactic*, quiescent SMBH:

• The first method is to observe CO lines (radio astronomy) to trace gas circling the BH. The gas does not need to be near the event horizon, which is much smaller than the sphere of influence. In fact CO observations rely on the gas being relatively dense but cold. Essentially the speed at which the CO gas will rotate is a (quadrature) sum of the declining component due to the stellar mass, plus the Keplerian component due to SMBH mass.
• The second method is completely analogous, but relies on measuring the unresolved kinematics of the stars surrounding the SMBH. This can be done in various bands, but most authors use visible line absorptions to measure the velocity and velocity dispersion (and other moments) of the stars in the regions surrounding the BH. If the kinematics cannot be explained without including a point mass in the middle, then you are done.

See this work, for a comparison of the two methods.

*(extragalactic means outside of our own galaxy)

## Giant black holes revealed in the nuclei of merging galaxies

Figure 1: Schematic diagrams of obscured AGNs. left: A doughnut-shaped dusty medium hides an active mass-accreting SMBH. Photons from the central AGN (= a mass accreting SMBH) can escape along the doughnut axis and ionize the gas there. Since the emission pattern of such AGN-ionized gas clouds differs from those in star-forming regions, we can use optical spectroscopy to easily infer the presence of an AGN hidden behind the dusty medium. right: Dust in all lines-of-sight obscures and buries the active mass-accreting SMBH, which is very difficult to detect with conventional optical spectroscopy. Credits: NASA for the images of mass accreting SMBH (lower), and mass accreting SMBH surrounded by doughnut-shaped dusty medium (upper left). NAOJ, Naomi Ishikawa, for the upper-right image.

Subaru Telescope research team led by Dr. Masatoshi Imanishi at the National Astronomical Observatory of Japan sampled many infrared bright, merging galaxies and determined the presence of active supermassive black holes (SMBH) deeply buried in their centers.

The scientists used the 8.2 m Subaru Telescope atop Mauna Kea (4200 m in elevation) as well as the Gemini South telescope at Cerro Pachon, Chile (2700 m in elevation) to perform high-spatial-resolution infrared imaging observations of nearby infrared luminous merging galaxies. Observations with both telescopes revealed that some samples show characteristics of rapid star-formation, while others display the signature of active galactic nuclei (AGN) that draw their energy from SMBH.

According to prevailing galaxy formation theories, small gas-rich galaxies with central SMBHs collide and merge, and then grow into the matured galaxies of the current universe. This is why the investigation of nearby infrared luminous merging galaxies helps to clarify the process of galaxy formation. The collision and compression of gas clouds from the galaxy merger causes the rapid formation of new stars, a heating-up of the surrounding dust, and the consequent production of strong infrared radiation. Also the supply of material increases the accretion to the SMBHs.

Although the merging galaxies enhance star formation as well as accretion to SMBHs, they also hinder these processes. A large amount of gas and dust are supplied to their nuclear regions, a process that can easily bury the compact SMBHs and make them difficult to find. By chance some objects have a ring-shaped distribution of the dust and gas, allowing observers to peek into the effect of the active SMBHs (Figure 1).

To detect emission behind dust and gas, the current research team made observations at 18 micrometers, using Subaru Telescope's COMICS (Cooled Mid-Infrared Camera and Spectrometer) as well as Gemini South's T-ReCS (Thermal-Region Camera Spectrograph). By utilizing the time exchange program, the team could use both telescopes to survey objects all over the sky. Subaru's observations captured images in the northern hemisphere and Gemini South, in the southern hemisphere.

How, then, could they confirm the presence of active SMBHs? It was neither an easy nor a trivial task to discover active SMBHs in merging galaxies. The researchers had used their methodology and choice of instruments to overcome a number of challenges. First they needed to identify an object had a bright infrared emission but was compact in size. Both AGN activity (a mass accreting SMBH) and compact star formation region are spatially confined. Measuring the luminosity in the infrared was the key for the finally categorizing their source. If the emission surface brightness at the nucleus of a merging galaxy is substantially higher than the maximum brightness expected from star-formation, then one can infer that the emission comes from a luminous buried AGN, because an accreting SMBH can emit radiation much more efficiently than a star. Observations at infrared 18 micrometers with both the Subaru and Gemini South telescopes demonstrated that some infrared luminous merging galaxies show a star formation type of emission (spatially extended with modest surface brightness) while others had an emission typical of AGNs (spatially compact with high surface brightness) (Figure 2). Ten of the current sample of eighteen objects showed the characteristics of the AGNs.

Figure 2: High-spatial-resolution infrared 18-micrometer images, obtained with the Subaru telescope (top) and the Gemini South telescope (bottom). The field of view (FOV) is 8 x 8 arcsec2. N and E indicate north and east directions, respectively. left: Image of a standard star. Three dots (top) or three dots with a ring pattern (bottom) demonstrate that the image has reached or is close to the limit for its highest possible resolution. middle: Image of an infrared luminous merging galaxy, with indication of a luminous AGN. The infrared emission is very compact, indistinguishable from the stellar image. The emission surface brightness is estimated to be significantly higher than the upper limit achieved by star-formation activity. right: Image of an infrared luminous merging galaxy, typical of a star-formation dominated source. The emission is spatially extended, and the emission surface brightness is within a range explained by star-formation activity.

The team's coherent, logical steps used to investigate the presence of supermassive black holes in merging galaxies yielded clear and important results, which were published in the Astronomical Journal: Imanishi et al. 2011 Astronomical Journal, 141, 156). Comparison of the results from high spatial resolution infrared observations with those from research using infrared spectroscopy to investigate deeply buried AGNs shows that both are reliable energy diagnostic tools and provide a consistent picture of the nature of hidden energy sources in merging galaxies.

## Thread: Excretion or accretion disc around the SMBH of Spiral galaxy?

It is vital to distinguish between accretion disc of Star to the accretion disc of SMBH in the core of Spiral galaxy. They have totally different characteristics. I wonder why our scientists put them in the same basket.
In order to understand that issue we need to look again on the evidences:

1. Accretion Mass – There are plenty of evidences for accretion Mass/gas around a star disc.
https://en.wikipedia.org/wiki/Accretion_disk
"An accretion disk is a structure (often a circumstellar disk) formed by diffused material in orbital motion around a massive central body. The central body is typically a star."

In the following image we see clearly that mass is drifting from outside to the center.
https://en.wikipedia.org/wiki/Accret. etion_disk.jpg
However, I personally couldn't find even one real evidence for mass/gas/star accretion around a SMBH disc in the center of spiral galaxy.
Actually, our astronomers are still trying to work out why the accretion disc in the Milky way didn't suck in a gas cloud as expected:
https://www.space.com/31524-black-ho. -goldmine.html
"Remember that mysterious cloud of gas that was supposed to be on a collision course with the supermassive black hole in the center of our galaxy? Well, astronomers are still trying to work out why it wasn’t sucked in, and why it didn’t spark the mother of all cosmic fireworks displays."

2. Excretion mass – There are evidences for gas emission from the accretion disc of SMBH:
A. The outer parts of an accretion disc surrounding a suppermassive black hole (SMBH) in NGC 7213 galaxy:
"Evolution of the accretion disc around the suppermassive black hole of NGC 7213"

"This is the first time that the double-peaked line profile of this nucleus – typical of gas emission from the outer parts of an accretion disc surrounding a suppermassive black hole (SMBH) – is reported to vary."
This evidence should be considered as the "smoking gun" for that understanding.
B. Evidences for a "wind" of hot matter blowing outward from the SMBH accretion disk of the Milky way:
http://www.dailygalaxy.com/my_weblog. that-glow.html

"The two beams, or jets, were revealed by NASA's Fermi space telescope. They extend from the galactic center to a distance of 27,000 light-years above and below the galactic plane."

"The jets were produced when plasma squirted out from the galactic center, following a corkscrew-like magnetic field that kept it tightly focused. The gamma-ray bubbles likely were created by a "wind" of hot matter blowing outward from the black hole's accretion disk. As a result, they are much broader than the narrow jets."

3. Mass structure in the accretion disc of SMBH:
If I understand it correctly, the plasma with the following characteristics is ONLY located at the accretion disc around a SMBH in the center of Spiral galaxy:
A. Plasma with different structure/feature at different location in the disc: At the most inwards of the disc – Ionized/broken Atoms/other elements (electrons, cations…?). At the most outwards – Unionized Atom, Hydrogen, Molecular.
Shaula - "There is a gradient from partly unionised to mostly ionised as you go in" Mostly hydrogen, some other elements."
chornedsnorkack - "Mostly electrons and cations, besides atoms. A few anions and molecules."
B. Different Temp based on location in the disc. At the most inwards side of the disc the Temp may reach to 10^9K. The highest temp records in the whole Universe. It is much more than the temp at the core of the Sun. However, at the most outwards of the disc it is much cooler.
Shaula – "Yes, but again only in the inner disk. The outer disk is much cooler (hence the comment about H-alpha before)."
C. Orbital velocity – Different orbital velocity based on location in the disc. At the most inwards side of the disc the orbital velocity is almost as high as the speed of light. It is the highest orbital velocity in the whole Universe. However, at the most outwards side, the orbital velocity is much more slowly.
Shaula – "Not throughout the disk though - the edges far from the centre rotate much more slowly."
D. Magnetic Energy – There is quite high magnetic energy in the disc.
Shaula - Yes, but the energy in magnetic fields is not the only thing about an accelerator. It is also very much dependent on the configuration. Accelerators work by confining a beam and accelerating it, the disk magnetic fields are much less ordered
However, I couldn't find the dispersion of the magnetic energy based on location in the disc. Based on the above structures of other elements, I would assume that at the most inwards side of the disc the magnetic energy is the highest while at most outwards it is fairly low.
E. Different pressures based on location in the disc. I assume that the Highest pressure must located at the most inwards side of the disc.

Conclusions – There are totally different characteristics of the mass in the accretion discs of star comparing to the accretion disc of SMBH. In accretion disc around a star there are no plasma with that Ultra high temp and Ultra high orbital velocity.
Actually, if you eat something there is no need to rotate it in your mouth at the speed of light and break its Atoms before you swallow it.
Therefore, it is quite enigma why the accretion disc around the SMBH should eat its mass from a nearby star or gas cloud, increase its orbital velocity as it gets dipper in his mouth, increase its temp to the highest temp in the universe, ionized the atoms just to blow it later on from the outer layer of the disc.
I would consider it as unrealistic activity. The accretion disc must work in one direction. The accretion disc around star does not spit his food while it accretes the mass inwards.
Therefore, what we really see around the SMBH is an activity of new mass creation.

## Astronomy Final Section Week 9 and 10 - SUMMARY

Spiral arms are stationary shock waves Stars and gas clouds orbit around the Galactic center and cross spiral arms Shocks initiate star formation.

O and B Associations trace out
3 spiral arms near the Sun

Black Hole at the Center of Our Galaxy

Using adaptive optics can distinguish numerous
stars orbits.

Many absorption lines also from heavier elements (metals)
Young stars contain more metals than older stars

Absorption lines almost exclusively from Hydrogen

Once ULIRGS use up all their gas => may become
ellipticals

Spirals are thought to not have gone through a
major merger (merger with galaxy of similar
size), but could have absorbed smaller galaxies
without affecting their structures

Both are "Standard-candle" methods:
Know absolute magnitude (luminosity) → compare to
apparent magnitude → find distance

Repeated
brightness
measurements
of a Cepheid
allow the
determination
of the period
and thus the
absolute
magnitude.
→ Distance
m - M = -5 + 5log(d)

Need brighter objects with known luminosity to
obtain distances of farther galaxies

Cepheid variables luminosity: L

400-40,000 Lsun
=> Can be used out to around

Both are "Standard-candle" methods:
Know absolute magnitude (luminosity) → compare to
apparent magnitude → find distance

Recall: Type Ia supernovae
(collapse of an accreting
white dwarf) have almost
uniform luminosity →
Absolute magnitude

evidence of SMBH- NGC 4261: Radio image reveals double-lobed jet structure close-up view by Hubble Space Telescope reveals a bright central source embedded in a dust torus
-Galaxies with extremely violent energy released from their nuclei (pl. of nucleus)
-Up to many thousand times more luminous than the entire Milky Way energy released within a region approx. the size of our solar system!
-Active galaxies are often associated with interacting galaxies, possibly the result of recent galaxy mergers.

Often gas outflowing at high velocities, up to 10,000 km/s, in opposite directions => jets

Most powerful type of AGN - Quasars - strong emission lines, variability, and redshift

Jets are powered by accretion of matter onto a supermassive black hole
-Many active galaxies show powerful radio jets
-Hot spots: Energy in the jets is released in interaction with surrounding material
-Typically consists of two radio lobes and a distorted galaxy between them
-Jet visible in both radio and X-rays show bright spots in similar locations

Twisted jets, probably because two galactic nuclei are orbiting each other

NGC 1265: Evidence for the galaxy moving through intergalactic material, jets trail behind

Cosmic Microwave Background: relic from earlier time in our universe.
-Formed during period of Recombination

The radiation from the very early phase of the universe should still be detectable today

Mostly observed from space since water molecules in atmosphere will absorb at λmax

After recombination, photons can travel freely through space.
Their wavelength is stretched (red shifted) by cosmic expansion.

The CMB photons are from when the universe became transparent, we cannot see further back in time. After recombination: The Dark Ages, The Universe filled with neutral hydrogen (and helium). No stars have formed yet

## Honey, it’s an Intermediate Mass Black Hole this time!

Facing trouble decrypting the title? Here’s a cue.

### Let’s talk Black Holes first…

Our universe is speculated to host a plethora of black holes, and they come in varied sizes: stellar mass (SBH), intermediate-mass (IMBH), and super-massive (SMBH). Of this assortment, SBHs, that have masses of a few to tens of times the mass of our Sun, as well as the SMBH variety that can be a million- to billion-fold heavier and found in the central engines of active galactic nuclei at large redshifts, have been studied and characterized for a long time. On the other hand, what remained elusive to this point, has been the existence and properties of a novel class of long-lived black holes (IMBHs), thought to represent the evolutionary bridge between SBHs and SMBHs.

In lack of a way of creating IMBHs, scientists have contemplated various formation mechanisms for them such as collapse of extremely massive primordial stars, core collapse of the centers of dense stellar systems or merger of smaller BHs, all of which favour a dense, primordial environment such as the center of a globular cluster. Considering this, faithful observational campaigns have targeted globular clusters for many decades seeking observational signatures of these objects, but the quest so far has been rife with uncertainty, unsatisfactory upper limits, and tantalizing but inconclusive evidence. For example, kinematic modeling of the globular cluster Mayall II in Andromeda galaxy indicated the presence of an IMBH, but alternative evolutionary dynamical modeling showed that the data could be explained in absence of such an object. In 2014, IMBH candidate M82 X-2 was revealed not to be a black hole at all but rather a neutron star, while plenty of other candidates lack sufficient flux ratio in radio vs. X-ray wavelengths to match the dynamical mass estimate of an accreting IMBH.

Figure 1. A false colour optical image of the 47 Tucanae (NGC 104) cluster. (Credits: ESO)

### The Real News…

Very recently, Kızıltan et al. published the evidence of an IMBH residing – in line with previous theoretical predictions – at the centre of 47 Tucanae (or NGC 104 see Figure 1.), one of the most massive globular clusters known. Kızıltan and colleagues made this discovery by examining the dynamical state of the cluster with the help of pulsars residing within it. (Pulsars are rapidly rotating neutron stars emitting a beam of electromagnetic radiation that appears as pulses of light sweeping in and out of our line of sight at regular and precisely measurable intervals, just like pulses from a rotating lighthouse lamp).

### How exactly did they do it?

Within a globular cluster, massive stars tend to sink towards the center. In the presence of a massive central mass (say, a black hole), as more stars sink inwards, the core heats up and stars start getting scattered off of the black hole, as well as each other. The propagation of the central black hole’s dynamical influence outwards (i.e., the inner stars scattering outer stars, and those scattering the ones even further from the center) ultimately affects the overall spatial arrangement and acceleration of stars in the cluster. Using the pulse-period measurements of pulsars to determine the distribution of their accelerations, the authors were able to infer a distinct imprint of the central massive object’s gravitational potential on the 3-D dynamical state of those pulsars. Moreover, using N-body numerical simulations of the cluster, the authors confirmed the mass of the object to be 2,200 times that of the Sun (Figure 2). This number is in vicinity of the previously established upper limit of 2,060 solar masses for the putative central object, and lies well within the mass regime of an IMBH.

Figure 2. The normalized probabilities of N-body models with different black hole masses, showing a conspicuous peak around 2200 solar masses. (Figure 4a. in the original paper)

### The Implications

1) One can study whether IMBHs can potentially host ultra-luminous X-ray sources (ULXs) that are too bright to be an accreting SBH and too distant from the centers of galaxies to be caused by SMBHs.

2) By the virtue of bridging the mass-gap between SBHs and SMBHs, a detailed analysis of IMBHs and their environments can shed light on accretionary growth models of SMBHs, and whether IMBHs can act as their initial seeds.

These questions in case of the IMBH in 47 Tuc, unfortunately, remain unanswered due to the absence of any detectable electromagnetic emission from it, suggesting that it is likely lurking in a gas-deficient environment. Whether this is typical for IMBHs or an exception, will become clear in time to come as scientists continue to expand our search for more objects through the cosmos.

## Multiple Black Holes may circle Sagittarius A (Sgr A*)

We now learn that multiple black holes may circle the super massive black hole at the center of our Milky Way Galaxy, known as Sagittarius A (Sgt A*).

Columbia Astrophysicist Chuck Hailey, co-director of the Columbia Astrophysics Lab says: "The Milky Way is really the only galaxy we have where we can study how supermassive black holes interact with little ones because we simply can't see their interactions in other galaxies. In a sense, this is the only laboratory we have to study this phenomenon."

"There are only about five dozen known black holes in the entire Milky Way Galaxy -- 100,000 light years wide -- and there are supposed to be (according to theory) 10,000 to 20,000 of these things in a region just six light years wide that no one has been able to find," Hailey said, adding that extensive fruitless searches have been made for black holes around Sgr A*, the closest SMBH to Earth and therefore the easiest to study. "There hasn't been much credible evidence."

Hailey explained that Sgr A* is surrounded by a halo of gas and dust that provides the perfect breeding ground for the birth of massive stars, which live, die and could turn into black holes there. Additionally, black holes from outside the halo are believed to fall under the influence of the SMBH as they lose their energy, causing them to be pulled into the vicinity of the SMBH, where they are held captive by its gravitational force.

While most of the trapped black holes remain isolated, some capture and bind to a passing star, forming a stellar binary. Researchers believe there is a heavy concentration of these isolated and mated black holes orbiting the Galactic Center, forming a density cusp, or ring, which gets more crowded as distance to the SMBH decreases.

"When black holes mate with a low mass star, the marriage emits X-ray bursts that are weaker, but consistent and detectable. If we could find black holes that are coupled with low mass stars and we know what fraction of black holes will mate with low mass stars, we could scientifically infer the population of isolated black holes out there," explained Hailey.

Hailey and his colleagues turned to archival data from the Chandra X-ray Observatory to test their technique. They searched for X-ray signatures of black hole-low mass binaries in their inactive state and were able to find 12 within three light years of Sgr A*. The researchers then analyzed the properties and spatial distribution of the identified binary systems and extrapolated from their observations that there must be anywhere from 300 to 500 black hole-low mass binaries and about 10,000 isolated black holes in the area surrounding Sgr A*.

This is an incredible identification, especially one based stored data from a past satellite's findings.

"This finding confirms a major theory and the implications are many," Hailey said. "It is going to significantly advance gravitational wave research because knowing the number of black holes in the center of a typical galaxy can help in better predicting how many gravitational wave events may be associated with them. All the information astrophysicists need is at the center of the galaxy."

Hailey's co-authors on the paper include: Kaya Mori, Michael E. Berkowitz, and Benjamin J. Hord, all of Columbia University Franz E. Bauer, of the Instituto de Astrofísica, Facultad de Física, Pontificia, Universidad Católica de Chile, Millennium Institute of Astrophysics, Vicuña Mackenna, and the Space Science Institute and Jaesub Hong, of Harvard-Smithsonian Center for Astrophysics.

When you visualize the center of our galaxy, the Milky Way, what the Romans called the Via Lactea, which stretches across our night sky in its beautiful multi-hued band, we now know that's its center is a place of roiling destruction, of deadly X-rays, of black holes devouring the stars they orbit as they, too, are in a ceaseless death orbit around a super massive black hole known as Sagittarius A.

## Compton-Thick or Thin? Classifying NGC 5347

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Title: A Hard Look at NGC 5347: Revealing a Nearby Compton-Thick AGN
Authors: E. S. Kammoun et al.
First Author’s Institution: University of Michigan
Status: Published in ApJ

Black holes are some of the most interesting and extreme objects in the universe. Fortunately, we think that almost every galaxy in the universe has a supermassive black hole (SMBH) at its center, giving us many opportunities to study their environments. As matter falls towards a black hole, it forms an accretion disk — a flattened disk of gas and other debris — outside of its event horizon. This accretion disk is hot and emits radiation, even though we can’t see any light from the black hole itself. When SMBHs at the centers (or nuclei) of galaxies are actively accreting mass and emitting a huge amount of energy, we call them active galactic nuclei (AGN). The different structural components of AGN, shown in Figure 1, emit radiation across a wide range of wavelengths, from low-energy radio to high-energy X-rays and gamma-rays.

Figure 1: The structural components of an AGN. Matter orbiting the black hole forms an accretion disk. There is also a torus, a donut-shaped cloud of neutral gas and dust, that could obscure the light emitted by the disk. [Aurore Simonnet, Sonoma State University]

#### Compton-Thick AGN

The X-ray emission in AGN comes from a hot atmosphere of gas called the corona, which surrounds the accretion disk. In the corona, ultraviolet (UV) photons get scattered by really fast electrons, gaining enough energy to become X-ray photons (this is the inverse of Compton scattering). As these X-ray photons leave the corona, they run the risk of being absorbed by the surrounding torus of neutral hydrogen and dust. If there is enough neutral hydrogen (at least 1.5 x 10 24 hydrogen atoms per cm 2 to be exact), most of the X-rays are absorbed and we call the gas “Compton-thick”.

In order to reproduce the observed cosmic X-ray background, we expect that 10–25% of AGN should be Compton-thick (CT AGN). However, actual observations of AGN so far have estimated the fraction of CT AGN to be less than 10%. Are there actually missing CT AGN, or are some AGN misclassified?

NGC 5347 is an AGN that has been classified as both Compton-thick and Compton-thin by different methods, with an estimated hydrogen content differing by a factor of 10 between measurements. Today’s paper re-opens this question by analyzing new high-sensitivity observations of NGC 5347, as well as incorporating more physical models. This investigation could solidify the classification of NGC 5347, as well as help explain the “missing” fraction of CT AGN.

#### You Have to Look Harder

X-rays can be divided into two classes depending on their energy lower energy X-rays are called soft, while higher energy X-rays are called hard. Hard X-rays are generally considered to have energies greater than 10 keV. In CT AGN, while most of the soft X-rays get absorbed, the hard X-rays can escape and reach telescopes!

The authors use observations from three different X-ray telescopes: previously available data from the Chandra X-ray Observatory and Suzaku, as well as new observations from NuSTAR. The new observations are particularly helpful, since NuSTAR is highly sensitive to the hard X-rays that actually make it out of the CT AGN.

Figure 2: X-ray spectra for NGC 5347. The y-axis shows the number of observed photons per time, per area, per energy. The x-axis corresponds to the energy of the X-ray photons. Data from Chandra and Suzaku are shown as red and black crosses, respectively. Data from NuSTAR, shown in pink and blue, make up the hard part of the spectrum. The best-fit model is shown in gray, and can be separated into the different components labeled in the legend. These different components represent different emission and absorption processes. [Kammoun et al. 2019]

#### Future Missions

One way for astronomers to classify sources as Compton-thick would be to measure the strength of iron emission lines such as Fe Kα, but current X-ray observatories are not able to resolve this emission line, at least not for NGC 5347. Simulating the spectrum of NGC 5347 as if it were observed by Athena a future X-ray observatory — suggests that a clear Fe Kα could be resolved (Figure 3), making the classification of NGC 5347 easier.

Figure 3: Simulated X-ray spectrum for NGC 5347. Counts in the y-axis represent the number of photons, and the x-axis corresponds to the energy of the X-ray photons. The smaller squares are zoomed in to the ∼ 6 keV region of the spectrum, to show the Fe Kα emission lines. [Adapted from Kammoun et al. 2019]

#### About the author, Gloria Fonseca Alvarez:

I’m a third year graduate student at the University of Connecticut. My current research focuses on the inner environments of supermassive black holes.

## Research Box Title

As if black holes weren't mysterious enough, astronomers using NASA's Hubble Space Telescope have found an unexpected thin disk of material furiously whirling around a supermassive black hole at the heart of the magnificent spiral galaxy NGC 3147, located 130 million light-years away.

The conundrum is that the disk shouldn't be there, based on current astronomical theories. However, the unexpected presence of a disk so close to a black hole offers a unique opportunity to test Albert Einstein's theories of relativity. General relativity describes gravity as the curvature of space and special relativity describes the relationship between time and space.

“We've never seen the effects of both general and special relativity in visible light with this much clarity,” said Marco Chiaberge of the European Space Agency, and the Space Telescope Science Institute and Johns Hopkins University, both in Baltimore, Maryland, a member of the team that conducted the Hubble study.

“This is an intriguing peek at a disk very close to a black hole, so close that the velocities and the intensity of the gravitational pull are affecting how the photons of light look,” added the study's first author, Stefano Bianchi of Università degli Studi Roma Tre, in Rome, Italy. “We cannot understand the data unless we include the theories of relativity.”

Black holes in certain types of galaxies like NGC 3147 are malnourished because there is not enough gravitationally captured material to feed them regularly. So, the thin haze of infalling material puffs up like a donut rather than flattening out in a pancake-shaped disk. Therefore, it is very puzzling why there is a thin disk encircling a starving black hole in NGC 3147 that mimics much more powerful disks found in extremely active galaxies with engorged, monster black holes.

“We thought this was the best candidate to confirm that below certain luminosities, the accretion disk doesn't exist anymore,” explained Ari Laor of the Technion-Israel Institute of Technology located in Haifa, Israel. “What we saw was something completely unexpected. We found gas in motion producing features we can explain only as being produced by material rotating in a thin disk very close to the black hole.”

The astronomers initially selected this galaxy to validate accepted models about lower-luminosity active galaxies—those with black holes that are on a meager diet of material. Models predict that an accretion disk forms when ample amounts of gas are trapped by a black hole’s strong gravitational pull. This infalling matter emits lots of light, producing a brilliant beacon called a quasar, in the case of the most well-fed black holes. Once less material is pulled into the disk, it begins to break down, becomes fainter, and changes structure.

"The type of disk we see is a scaled-down quasar that we did not expect to exist," Bianchi said. "It's the same type of disk we see in objects that are 1,000 or even 100,000 times more luminous. The predictions of current models for gas dynamics in very faint active galaxies clearly failed."

The disk is so deeply embedded in the black hole's intense gravitational field that the light from the gas disk is modified, according to Einstein’s theories of relativity, giving astronomers a unique look at the dynamic processes close to a black hole.

Hubble clocked material whirling around the black hole as moving at more than 10% of the speed of light. At those extreme velocities, the gas appears to brighten as it travels toward Earth on one side, and dims as it speeds away from our planet on the other side (an effect called relativistic beaming). Hubble's observations also show that the gas is so entrenched in the gravitational well the light is struggling to climb out, and therefore appears stretched to redder wavelengths. The black hole's mass is around 250 million Suns.

The researchers used Hubble's Space Telescope Imaging Spectrograph (STIS) to observe matter swirling deep inside the disk. A spectrograph is a diagnostic tool that divides light from an object into its many individual wavelengths to determine its speed, temperature, and other characteristics at a very high precision. The astronomers needed STIS's sharp resolution to isolate the faint light from the black-hole region and block out contaminating starlight.

"Without Hubble, we wouldn't have been able to see this because the black-hole region has a low luminosity," Chiaberge said. "The luminosities of the stars in the galaxy outshine anything in the nucleus. So if you observe it from the ground, you're dominated by the brightness of the stars, which drowns the feeble emission from the nucleus."

The team hopes to use Hubble to hunt for other very compact disks around low-wattage black holes in similar active galaxies.

The team's paper will appear online today in the Monthly Notices of the Royal Astronomical Society.

The international team of astronomers in this study consists of Stefano Bianchi (Università degli Studi Roma Tre, Rome, Italy) Robert Antonucci (University of California, Santa Barbara, California) Alessandro Capetti (INAF - Osservatorio Astrofisico di Torino, Pino Torinese, Italy) Marco Chiaberge (Space Telescope Science Institute and Johns Hopkins University, Baltimore, Maryland) Ari Laor (Israel Institute of Technology, Haifa, Israel) Loredana Bassani (INAF/IASF Bologna, Italy) Francisco Carrera (CSIC-Universidad de Cantabria, Santander, Spain) Fabio La Franca, Andrea Marinucci, Giorgio Matt, and Riccardo Middei (Università degli Studi Roma Tre, Roma, Italy) and Francesca Panessa (INAF Istituto di Astrofisica e Planetologia Spaziali, Rome, Italy).

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, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Credits:NASA, ESA, S. Bianchi (Università degli Studi Roma Tre, Italy) and M. Chiaberge (ESA, STScI, and JHU)

## Supplemental and Background Information

One of the most significant astronomical discoveries in the last decade is the realization that almost every massive galaxy harbors a supermassive black hole (SMBH) at its center. SMBHs gain millions to billions times the mass of the Sun by accreting material from their immediate surroundings. The matter is forced into a rotating accretion disk around the black hole. As the matter spirals down onto the black hole, it reaches temperatures of millions of degrees, generating large amounts of X-rays in the process. The intensity of these X-rays tells us how active a SMBH currently is. Part of the X-ray radiation is absorbed by dust in the immediate vicinity of the black hole. Consequently, the dust heats up and re-emits the energy absorbed at thermal mid-infrared wavelengths, from which astronomers can also infer the SMBH's current activity level. The most active of these galaxies are known as quasars.

The remainder of the X-ray radiation escaping from the galactic centers ionizes the interstellar gas in the host galaxies. This can be seen in the galaxies' spectra by extended emission of double ionized oxygen, [OIII]. Usually, these so-called narrow-line regions (NLRs) have an extent of 1000-10,000 light-years, i.e. 1-10 percent of the diameter of a typical galaxy. Their luminosity correlates well with the X-ray luminosity of the SMBHs. Today, the centers of most galaxies are largely quiescent, but the presence of a SMBH shows that in their past they must have undergone a very active phase.

In this new class of &ldquogreen-bean galaxies,&rdquo [OIII] luminosities rival those of the brightest quasars known but astronomers haven&rsquot observed a quasar at the centers of these galaxies. Therefore, either the quasars radiation is absorbed by large amounts of dust, or they have been shutting down in the very recent past &ndash&ndash so recent, in fact, that the lower X-ray flux hasn't yet reached the farther regions of the NLR. In the latter scenario one would observe a light echo, in which the ionized gas further from the nucleus still reflects the earlier, more active quasar state.

Since mid-infrared photons aren&rsquot affected by dust obscuration it reveals that the [OIII] emission in these galaxies is indeed 5-50 times higher than expected, confirming the light echo. While active galaxies are known to change their luminosity on hours to decades timescales, and by 10 percent up to a factor of 10, respectively, time scales longer than about two decades have not been studied previously. Accretion models predict that the luminosity of the central SMBH engine can drop by factors of 10,000 over some 100,000 years.

## Future Missions

One way for astronomers to classify sources as Compton-thick would be to measure the strength of iron emission lines such as Fe Kα, but current X-ray observatories are not able to resolve this emission line, at least not for NGC 5347 . S imulating the spectrum of NGC 5347 as if it were observed by Athena — a future X-ray observatory — suggests that a clear Fe Kα could be resolved (Figure 3), making the classification of NGC 5347 easier.

Figure 3: Simulated X-ray spectrum for NGC 5347. Counts in the y-axis represent the number of photons, and the x-axis corresponds to the energy of the X-ray photons . The smaller squares are zoomed in to the ∼ 6 keV region of the spectrum, to show the Fe Kα emission lines. (Excerpt from Figure 4 in the paper.)

The previous classification of NGC 5347 as Compton-thin could mean that other CT AGN have not been correctly classified due to a lack of high quality observations. NuStar and future X-ray missions like XRISM and Athena could provide the higher quality X-ray spectra necessary to identify CT AGN.