Date: January 18, 2011
Title: What’s New With Supermassive Black Holes
Podcaster: Rob Knop
Organization: Quest University Canada
Links: My home page : http://www.questu.ca/academics/faculty/rob_knop.php
MICA : http://www.mica-vw.org
MICA Public Events : http://www.mica-vw.org/wiki/index.php/MICA_Events
Description: Of all astronomical objects, there are few that inspire the imagination more than black holes. I’ll tell you about a couple of results that have come out recently having to do with supermassive black holes.
Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. He now teaches physics the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Physics Department at Eastern Illinois University: “Caring faculty guiding students through teaching and research” at www.eiu.edu/~physics/.
Transcript:
What’s New With Supermassive Black Holes
Hello! This is Dr. Rob Knop, Professor of Physical Science at Quest Unviersity Canada. Thank you for listening to 365 Days of Astronomy.
Of all astronomical objects, there are few that inspire the imagination more than black holes. Compact objects so dense that light cannot escape from them. The event horizon, this boundary in space— once you cross it, you can never come back. The effects of clocks slowing down, and light being redshifted, as something moves closer and closer to this fearsome event horizon. As if that weren’t enough, what is it that produces black holes? Supernova. The deaths of massive stars, as their cores collapse even past the unimaginable densities of neutron stars, in a tremendous explosion that can be seen billions of light years away. Gamma ray bursts, signaling a particularly spectacular death of a star and the formation of a black hole, lighthouses that can be seen clear across the observable universe.
Now that the sales pitch is done, what I want to do is tell you about a couple of results that have come out recently having to do with supermassive black holes. But, before that, some background. It is often said that there are two types of astronomers: those that classify all objects into two types, and those that don’t. There is some justification for this stereotype; frequently, especially as we are first learning about objects, we tend to classify them as “Type I” and “Type II”. While this may sound intellectually lazy, in fact it’s a very rational way to proceed. The first step in identifying patterns in a group of objects is recognizing that some are different from others in a consistent way. It’s not just that they’re all sort of the same, or that each one is an individual, but that, hey, it seems that this group has a certain characteristic that this other group does not. Invariably, as we learn more about the objects, we figure out that our simple two-set classification is too simple, and that there’s more subtlety to it. But, it’s a great way to get started.
So, you will probably not be surprised when I tell you that we know of two broad categories of black holes. First, there are the “stellar mass” black holes. These are black holes that range from a few times the mass of the Sun up to something like twenty times the mass of the Sun. They are produced in supernovae, left behind as the collapsed core of a massive star after it explodes. We know of several of these stellar mass black holes in our own galaxy. “How can we see them if light can’t escape from them?”, you may ask. A good question. In fact, we only see them because of their interactions with the material around them. The stellar mass black holes we know in our galaxy are in what we call X-ray binaries. The black hole has a companion star, and is pulling gas off of that companion star. The gas eventually disappears down into the black hole, but before that it builds up in a disk rapidly swirling around the black hole. This “accretion disk” gets so hot that it glows in the X-ray band of the electromagnetic spectrum, allowing us to observe it.
The other class of black hole we know about is composed of what we call supermassive black holes. These are black holes that are between a million and tens of billions times the mass of the Sun. You can see why they’ve acquired the name supermassive! Supermassive black holes are found at the cores of galaxies, and indeed today we believe that nearly every big galaxy has a supermassive black hole at its core.
(As an aside, what about black holes with masses too small to be a supermassive black hole, but too big to be a stellar mass black hole? We call those intermediate mass black holes. At the moment, there are no known examples of these, but of course that doesn’t stop astronomers from looking for them.)
The best known example of a supermassive black hole is the one at the center of our own Galaxy. You are probably familiar with the idea that our galaxy is a spiral galaxy, a broad disk of stars showing a spiral pattern when observed in optical light. In addition to the disk, there are also a couple of structures at the center of the Galaxy, a bar and a bulge. The bar is an elongated structure composed of stars, and the bulge is just an elliptical blob of stars around the center of the bar. At the core of the bulge is a black hole that is four million times the mass of the Sun. It’s a monster! Again, we can’t observe this black hole directly. Rather, we have observed it because of its effects on the stars at the center of the Milky Way’s bulge. Astronomers have watched those stars over the course of the last twenty years, and have seen them orbiting around a much more massive unseen object. From analyzing those orbits, they are able to determine the location and mass of the Galactic center black hole.
One word of caution. We are used to thinking of black holes as objects with immense gravity. The reason that’s true is because they are so compact, because you can get so close to them. At a greater distance away from the black hole, the gravity is no different than it is from any other object. For instance, were our Sun to be suddenly replaced with a black hole of exactly the same mass, the gravity in the rest of the Solar System would not change one bit. Earth would stay in its orbit. As regards this galactic center black hole, it may be 4 million times the mass of the Sun– but that’s less than one percent the mass of the bulge of the Galaxy, and it’s utterly insignificant compared to the mass of the Galaxy as a whole. So, don’t fall into the trap of thinking that the whole Galaxy is orbiting around this black hole, or that the spiral arms represent stars going “down the drain” or any such. The black hole is very important to the orbits of stars right near it, but is insignificant compared to the mass of the Galaxy as a whole.
Looking at other galaxies, astronomers have discovered a strong correlation between the size of a galaxy’s bulge and the mass of the black hole at its core. The largest supermassive black holes are found in giant elliptical galaxies– galaxies that are “all bulge”. What’s more, whereas the black hole at the core of our galaxy is quiescent, in a few galaxies these black holes are the engine powering an active galactic nucleus, or a quasar. This is the phenomenon that results when you feed a supermassive black hole. Just as the accretion disks around black holes in our Galaxy emit X-rays, the accretion disk around a supermassive black hole will also emit radiation, only on a much larger scale.
For the quasar phenomenon to happen, you need to get enough gas down to the center of a galaxy, so that it can gather in a glowing accretion disk around the black hole. For a long time, many astronomers, including myself, have suspected and believed that interactions between galaxies would be a great way to do this. We know that galaxies have a lot of gas. Our galaxy has about 50 billion times the mass of the Sun in stars, and about one tenth that much mass in gas. There’s plenty of gas, but it’s spread out throughout the disk of the galaxy. We’ve observed that when galaxies run into each other, often a fair amount of gas gathers in the inner few hundred or thousand light-years of the resulting merged galaxies. In many of these mergers, we see the result of this in a vigorous burst of star formation near the center of the galaxy. As such, we’ve long suspected that the gas processes associated with a merger would be a great way to funnel gas down to the center of the galaxy to feed a supermassive black hole.
That brings me to the first new result about supermassive black holes. A group of astronomers led by Mauricio Cisternas selected 140 active galactic nuclei from an X-ray survey. They then went and obtained high-resolution images of the host galaxies using the Hubble Space Telescope, and looked for the sorts of disturbed morphologies you see when galaxies interact strongly. The result was that most of the active galaxies did not show the signs of a recent strong interaction! What’s more, when comparing these galaxies to a control sample, active galaxies and quiet galaxies were just as likely to show signs of recent strong interactions. This leaves us with the conclusion that major mergers aren’t responsible for triggering most of the active galaxies we see, at least in the last 8 billion years or so. This was a result that was surprising to me; I had fully expected that we would have come to the opposite conclusion. That’s why you go and do these observations— just because we have good reasons to suspect that something might be true doesn’t mean that we know it’s true until we’ve actually run the test.
The other new result about supermassive black holes was announced at the meeting of the American Astronomical Society in Seattle at the beginning of this year. A group of astronomers at the University of Virginia and at the National Astronomical Radio Observatory, led by graduate student Amy Reines, discovered a supermassive black hole at the core of a nearby dwarf galaxy. This black hole has a mass a million times the mass of the Sun— and the galaxy is far too small, and of the wrong type, to be harboring it! The dwarf galaxy has been described as a “fluffy” galaxy, and is similar in some ways to the Large Magellanic Cloud, a satellite of our own galaxy. It does not have a big stellar bulge that you’d suspect of harboring a million solar mass black hole. And, yet, there it is— there’s a black hole, and it’s accreting enough gas that it shows up in X-rays. What does this mean? This doesn’t invalidate the earlier observations that the size of galaxy bulges correlate with the mass of the supermassive black holes at their centers, but it doesn’t fit with it either.
Note that this new black hole is towards the low end of the mass range we’ve traditionally considered to be supermassive black holes. While we have some good ideas about how the growth of black holes will relate to the growth of stellar bulges, thus giving the observed mass correlation, we don’t really know how the “seed” black holes that started the process were formed. This new black hole suggests that you can form black holes and grow them to at least a million solar masses even in dwarf galaxies— and, it may be that at least sometimes, it was dwarf galaxies in the early Universe that were host to the seed black holes that have since grown to the monsters we see in elliptical galaxies today.
Astronomy is always exciting; we’re always learning new things. Black holes are wacky enough by themselves, but the fact that they then give us phenomena like active galactic nuclei only makes them all the more interesting. And, as we struggle to understand how it is that all of these things fit together, new observations are always forcing us to modify and refine our ideas. Stay tuned, for the last word on supermassive black holes has not been written yet!
End of podcast:
365 Days of Astronomy
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