Daniel Kennefick
Updated: 2008 October 09

The Arkansas Galaxy Evolution Survey


The Collaboration

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Project Overview
The Evolution of Supermassive Black Holes in the Universe

The AGES Collaboration consists of four astronomers based in Arkansas working on issues relating to the mass function of supermassive black holes in the Universe. AGES stands for Arkansas Galaxy Evolution Survey. The four members are (in alphabetical order) Daniel Kennefick, Julia Kennefick and Claud Lacy, at the University of Arkansas, Fayetteville, and Marc Seigar at the University of Arkansas, Little Rock. The group is working on several issues relating to supermassive black holes, and the research behind the collaborations first publication is discussed here.

One of the dramatic discoveries of astronomy in the late 20th century has been that apparently nearly all galaxies have, at their center, a supermassive black hole, with a mass of between 10,000 and 1 billion times the mass of the Sun. Such black holes, when they are swallowing material, are the source of the enormous outpouring of energy characteristic of active galactic nuclei such as quasars. But in most galaxies there is a void around the central black hole, where all the matter has already disappeared into the hole and so the black hole, starved of matter to devour, is quiescent. This makes the job of detecting these black holes very difficult, so that we know very little about the typical size of the black holes in normal galaxies.

This has given rise to the problem of cosmic downsizing. Most the supermassive black holes we can measure the mass of are in quasars and other active galactic nuclei and most of the closer ones are smaller than the ones which are farther away. This is odd because farther away means that the objects are seen when the Universe was younger. The nearer objects are, in general, older. But we expect that black holes should never grow smaller, only bigger, as they swallow more matter without letting anything escape. Shouldn't the nearer ones, which are older, be generally bigger? As an aside for those who know about such things, it is true that Hawking Radiation does provide one mechanism by which black holes can lose mass, but we expect that supermassive black holes will be much too "cool," in terms of black hole thermodynamics, to lose any appreciable mass to Hawking Radiation. The explanation for cosmic downsizing is that the bigger black holes are more efficient at hoovering up matter in their neighborhoods, and so they become quiescent more quickly. Thus in our own epoch only the smaller supermassive black holes are still active. To prove this it would be nice to be able to find some way of estimating the masses of the central black holes for large numbers of galaxies in the more recent Universe. We should expect to able to locate those large black holes now sitting at the centers of their host galaxies doing nothing.

The AGES collaboration of astronomers in Arkansas believes we have found a useful tool for doing just this. Looking at the disk galaxies for which supermassive black hole mass has been measured, we notice that there appears to be a relatively strong correlation between the mass of the supermassive black hole and the pitch angle of the spiral arms. This relation could be an extremely useful tool, because it would allow us to use the rich store of images of near and distant galaxies built up in recent decades by the recent generation of advanced telescopes, foremost amongst then the Hubble Space Telescope. Spiral Arm Pitch angle can be measured just from images of galaxies and does not require the sort of spectroscopic work which is more expensive of telescope time and which is normally required to come up with an estimate of supermassive black hole mass.

Question: What is spiral arm pitch angle?

Our correlation shows that galaxies which have more tightly wound spiral arms (the shape of whose arms are more nearly like a circle) have larger supermassive black holes. Galaxies with loosely wound arms have smaller central black holes. See the graph below.


At first glance it seems extraordinary that something as localized as the central black hole (whose radius, even for a very large one, is generally no greater than the width of the Earth's orbit around the Sun) should correlate so well to a structure which spans the width of a galaxy, a matter of a couple of hundred thousand light years across. One possible explanation is that both features are tied strongly to the dark matter halo of the galaxy. Dark matter is a mysterious feature of galaxies which astronomers have come to believe in because of their study of the way disk galaxies rotate. (link to Wikipedia).

If dark matter exists, it seems that it does not interact with forms of electromagnetic energy such as light, but it does interact gravitationally with other matter. Since it is dark and transparent, we cannot see it using telescopes, but its effect may be seen through its gravitational influence on objects we can see. So for instance, astronomers have come to believe that dark matter exists because of its apparent effect on the way stars in the spiral arms of disk galaxies orbit the center of the galaxies, the so-called galactic rotation curve.

Now dark matter ought in principle to have a bearing on both of the quantities in our correlation. Since dark matter can fall into black holes just like any other matter, it ought to contribute to the growth of the central black hole, and this contribution should depend on the density of the halo of dark matter which threads through the galaxy. The denser the halo the more dark matter is available to swell the mass of the black hole and the bigger it will grow.

But what of the spiral arm pitch angle? Most people assume the spiral arms of galaxies are a fixed feature which rotates around with the slow rotation of the galaxy, but actually this is not so. Since not all stars in the galaxy rotate around the center at the same rate, the spiral arms would be smeared out over time if they were simply a fixed feature. In fact it seems they are a kind of standing wave in the distribution of stars and galactic gas and dust. As all of the these masses interact with other, they resonate with each other if their orbits are in certain ratios. A typical orbital resonace (Wikipedia link) occurs when one star takes exactly twice as long as another star to orbit around the galactic center. This means that every couple of hundred million years these stars pass close to each other, i.e. they do so regularly! That is to say, they come close to each other every time (or every second time) they go around, rather than haphazardly, randomly. They thus pull on each other more strongly than other stars not in resonance, and thus move closer together, tending to create gaps and denser areas in the pattern in the galactic disk. (In the same way the gaps in Saturn's rings are created, Wikipedia link).

The resonances create denser areas of gas and dust, and these increases in density set off star formation, causing the gas and dust to collapse locally to form a solar system. Thus certain parts of the distribution of matter in the disk, at any one moment in time, will tend to have a lot of young stars. Because the brightest stars are the shortest lived (because they burn their fuel so quickly), regions with more young stars contain many more bright stars, and stand out to the eye. These are the arms of the spiral. In between there are stars also, but they are dimmer and older. This pattern is to be thought of as a sort of standing wave in the material of the disk, set up by gravitational interactions within the disk. Although it has not been possible to develop an exact model of how this pattern is formed, it certainly seems plausible that the denisty of the dark matter in the galaxy could impact how it is formed, and it would be very interesting to find some evidence for this. Thus it is possible that work on supermassive black holes and their role in galactic structure might eventually shed some light on the mystery of dark matter.

See Article in the Astrophysical Journal Letters on astroph