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erl

An important discovery in the past quarter century or so of extragalactic astronomy is the existence of scaling relations which very simply relate different properties of those galaxies.

One of the most intriguing relations connects the total stellar mass of a galaxy to the mass of its black hole. Virtually every evolved galaxy in the sky has a supermassive black hole in its center, and the mass of each of those black holes is about 0.2% the mass of the stars in its galaxy.

At first glance, this relation is pretty simple -- big galaxies have big black holes. But more profoundly this relation suggests that supermassive black holes co-evolve with the galaxies they live in.

This winds up begin a really useful tool: If you know the mass of all the stars in a galaxy, you can reasonably estimate the mass of its black hole! So if we want to build a census of black holes, we can start out with a census of galaxy stellar masses!

2/8

23 July at 3:39 | Open on chaosfem.tw
8 comments
erl

Okay, so how do you measure the stellar mass of a galaxy? The complicated and difficult approach (ya know, the way that's a great PhD project) is to make careful measurements of the motions of the stars then carefully model those motions and infer the mass distribution. This is extremely expensive, taking several months of grad-student-time to produce a single reliable and well-tested measurement.

The simpler approach is to notice that brighter galaxies are probably heavier! But it's not trivial to relate the brightness and the mass. More massive galaxies tend to have older stars in them, and those older stars tend to be fainter than other younger stars with the same masses.

Can we approximate this? Yep! there are now about a dozen high-precision measurements of the stellar mass for galaxies above about 300 billion solar masses within 100 megaparsecs of us. In the figure, we do a very simple fit to relate the brightness of these galaxies to their stellar masses.

3/8

Okay, so how do you measure the stellar mass of a galaxy? The complicated and difficult approach (ya know, the way that's a great PhD project) is to make careful measurements of the motions of the stars then carefully model those motions and infer the mass distribution. This is extremely expensive, taking several months of grad-student-time to produce a single reliable and well-tested measurement.
23 July at 3:40 | Open on chaosfem.tw
erl

So we've now linked black hole masses to galaxy stellar masses and galaxy stellar masses to galaxy luminosities. Thankfully, one of our group members made a series of extremely deep observations of about 90 nearby massive elliptical galaxies giving us a starting point for our census. Even more, a member of our collaboration performed deep spectroscopic observations of 41 nearby massive ellipticals to provide an alternate measurement of the relation between mass and light for those objects using stellar population models. When we use measurements based on the dynamical models I described in toot 3 we find **very** similar masses as when we use these stellar population models.

In the figure we show the distribution of galaxy stellar masses for galaxies within 100 Mpc that we find using these two methods. They agree pretty well!

4/8

So we've now linked black hole masses to galaxy stellar masses and galaxy stellar masses to galaxy luminosities. Thankfully, one of our group members made a series of extremely deep observations of about 90 nearby massive elliptical galaxies giving us a starting point for our census. Even more, a member of our collaboration performed deep spectroscopic observations of 41 nearby massive ellipticals to provide an alternate measurement of the relation between mass and light for those objects using stellar...
23 July at 3:41 | Open on chaosfem.tw
erl

We combined these observations with those for much less massive galaxies to build a model for the distribution of galaxy stellar masses. In the figure we compare our model against those which folks have previously constructed. Overall we find that there are about **twice** as many very massive galaxies (>300 billion times the mass of the sun) than folks have previously counted.

This is probably the result of a couple major improvements: more careful measurements of the faint outer region of these massive galaxies and the use of dynamical masses to calibrate the mass-luminosity relation.

This is one of the first important results of this paper -- previous papers have found that the high-mass end of this function is basically unchanged over the past 8 or so billion years. This is strange! We expect galaxies to still be growing and merging so this relation should be evolving a lot over time! Our new model **might** give evidence of that evolution

5/8

We combined these observations with those for much less massive galaxies to build a model for the distribution of galaxy stellar masses. In the figure we compare our model against those which folks have previously constructed. Overall we find that there are about **twice** as many very massive galaxies (>300 billion times the mass of the sun) than folks have previously counted.
23 July at 3:43 | Open on chaosfem.tw
erl

Okay, so what does that mean about the black hole mass distribution? Well, let's apply that scaling relation from toot #2. In the figure we find that there are about **twice** as many very massive black holes than suggested from previous models (which we might expect given that there are more big galaxies than prior models!). Our model (and its uncertainty) is shown with the violet band.

The number of black holes above 10 billion times the mass of our sun predicted by our model actually pretty closely matches what folks have found (the grey histogram)!

6/8

Okay, so what does that mean about the black hole mass distribution? Well, let's apply that scaling relation from toot #2. In the figure we find that there are about **twice** as many very massive black holes than suggested from previous models (which we might expect given that there are more big galaxies than prior models!). Our model (and its uncertainty) is shown with the violet band.
23 July at 3:43 | Open on chaosfem.tw
erl

Now things get spicy. Over the past couple years, folks have measured the **cosmic gravitational wave background**, a low background hum in spacetime itself coming from all directions. This hum is thought to be due to the mergers of supermassive black holes across the universe, which each ring out with their own characteristic amplitude and frequency. Those billions of mergers add incoherently and produce something that sounds like static on Earth.

The amplitude of that static is a **huge** open question -- last year there were a number of groups which measured something that looked a lot like that static, with a characteristic strain amplitude of 2.4 parts in 1000000000000000.

If this signal is due to supermassive black holes, what does our new model say about it? Well, with some reasonable assumptions you can compute it! And we find their number!

In the figure we see our measured strain along with several other prior measurements.

7/8

Now things get spicy. Over the past couple years, folks have measured the **cosmic gravitational wave background**, a low background hum in spacetime itself coming from all directions. This hum is thought to be due to the mergers of supermassive black holes across the universe, which each ring out with their own characteristic amplitude and frequency. Those billions of mergers add incoherently and produce something that sounds like static on Earth.
23 July at 3:44 | Open on chaosfem.tw
erl

This is really important -- if using the previous models for the numbers of black holes we find a much smaller strain than folks actually measure. This has led folks to suggest that either the gravitational wave background is coming from something else or that there are missing black holes.

We can also count up the total mass density in black holes. When we do this we get **much** larger values than folks have found in the past. These prior measurements use the brightness of active galactic nuclei to infer the accretion history of present-day black holes. The fact that we find a much larger value could suggest a couple things! This likely suggests that the link between observed AGN luminosity and the mass accretion rate is a bit fuzzier than folks have assumed, and perhaps the overall corrections folks make for the presence of dust or the efficiency of accretion is a bit off.

Anyway, feel free to check out the paper: arxiv.org/abs/2407.14595 It's a pretty accessible and fun read!

8/8

This is really important -- if using the previous models for the numbers of black holes we find a much smaller strain than folks actually measure. This has led folks to suggest that either the gravitational wave background is coming from something else or that there are missing black holes.

We can also count up the total mass density in black holes. When we do this we get **much** larger values than folks have found in the past. These prior measurements use the brightness of active galactic nuclei...

23 July at 3:45 | Open on chaosfem.tw
Spatula

@erl I find it so fascinating that we're all being compressed and rarefacted infinitesimally by the spacetime we inhabit all the time, without ever realizing it.

23 July at 4:46 | Open on tech.lgbt
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