Galaxies with growing Black Holes: What makes them special?

Active Galaxies in the CANDELS survey with bright blue nuclear emission
from a growing Supermassive Black Hole (courtesy: David Rosario)

Supermassive Black Holes (we like to call them SMBHs for short) lie lurking in the heart of every large galaxy in the Universe, and possibly in many little galaxies too. A popular misconception is that black holes like these are continuously devouring stuff: stars, planets, the thin plasma that fills up most of space. In actual fact, one of the big mysteries in galaxy physics is why such monstrous black holes - some as massive as a billion suns - remain, for most of their existence, on a severe diet, not growing at all. In the area of space within a few hundred million light years of our own Milky Way galaxy, a region known as the Local Universe, there are hundreds of thousands of large galaxies like our own. Remarkably, only a few percent of them show the tell-tale signs of SMBH growth, manifested as Active Galactic Nuclei (or AGNs - see the blog post by Dale Kocevski for an introduction to these rare and remarkable objects). Nineteen out of any twenty SMBHs, while entirely capable of huge outbursts of energy as they feed on in-falling gas and stars, seem instead to be quietly biding their time in the nucleus of their 'host' galaxy. These black holes are being starved - not enough fuel comes their way to keep them active. Why exactly these black holes remain this way is not understood, but may have to do with the way that gas settles down into the centers of galaxies, and how black holes themselves influence these flows of gas.

One interesting clue to this conundrum is the observation that in the distant cosmos, far beyond the Local Universe, AGNs are both brighter and more common. In fact, much of the mass in Local SMBHs fell in many billions of years ago, during a period in the history of the Universe that we can only probe by looking deep into space (and, by extension, far back into time). Something about galaxies in the early Universe allowed black holes in their centers to grow faster than they do today. By comparing these distant galaxies to the ones we see today, we can figure out what affects black hole growth and shed some light on the question of why nearby SMBHs stay anorexic.

This is where CANDELS comes in. With the superb new capabilities of our survey, we can actually get a good look at the very galaxies that contain growing SMBHs across a vast swath of cosmic history and compare them to galaxies that don't host AGNs, to see if these galaxies are special in some way. Since CANDELS provides pictures of galaxies across a wide range of wavelengths (or colors), we can separate out the light that comes from the powerful shining active nucleus and home in on the galaxy itself.

In a recent paper from the CANDELS collaboration, we carefully looked at galaxies that shine powerfully in the X-rays, a sure sign of gas heated to millions of degrees as it falls into a growing black hole. We found that such 'active galaxies' only really distinguish themselves in one way - they contain a lot of stars. This is not too surprising; we know that such massive galaxies harbor massive black holes, and massive black holes are exactly what is needed to produce AGNs. After accounting for this difference between AGNs and other galaxies, AGN hosts have the same basic range in shape and color as normal galaxies of similar mass. Another thing we can do with the superb CANDELS images is place good constraints on the past history of star-formation in these distant galaxies. We found that active galaxies form stars in much the same way as normal galaxies, both at the time we observe them and in their past history.

What does this mean? From our study, we can tell that galaxies that host growing black holes are not remarkable in any way. In other words, the mechanisms that carry gas into the black hole mostly work on small scales, probably within a few tens of light years of the nucleus itself, and don't immediately affect the rest of the host galaxy. As the project progresses, we will be able to build on this early work by looking at more and brighter AGNs. In time, CANDELS will be a powerful tool as we uncover the complete story of how black holes and galaxies evolve in the early Universe. Stay tuned to this blog as the picture develops.