by Sarah Scoles
Just like some people are active and some are not, some galaxies have active nuclei, and some do not. But while for people inactivity means parking in the space closest to the grocery store's automatic doors, inactivity for a galaxy's nucleus means that it is not actively accreting, or eating, matter. Active galactic nuclei are eating things all the time, and all that feasting presumably affects the way the galaxy grows up. But how? That's the question researchers like Lisa Winter, a Hubble Fellow at the University of Colorado, Boulder, are asking.
What is a galactic nucleus?
Like the nucleus of an atom, the term nucleus, in this context, means the part at the center that is considerably massive. For an atom, this is a combo of protons and neutrons. For a galaxy, this is a supermassive black hole. An active galactic nuclei, or AGN, has matter around it, and that matter is falling into the black hole.
But even though light doesn't come from the black hole, we can see light from the matter that hasn't fallen in--the material that is swirling around the black hole, and the material that, while swirling, gets directed outward in jets.
For more information on seeing evidence of black holes, see this post.
Are there different types of AGN, or are all active galaxies active in the same way?
While all people who are active do not throw javelins on the weekends, all AGN might be active in the same way. While we observe AGN with lots of different properties, unified models of AGN state that different subclasses could be united by considering the angle at which we see the galaxy. This viewing angle could cause us both to see or to miss, say, soft X-ray emission.
The universe doesn't care at all whether we look an AGN head-on or see it from the side, so we see AGN from all different sides, and, depending on the side, we can see different properties.
Does this mean all AGN are the same? No, they are all special snowflakes, and they do have differing characteristics. But mechanisms and structures are likely similar across time and space.
What don't we know about AGN?
1. Why are some galaxies active and some not?
2. Why were there more AGN in the early universe than there are now?
3. Were galaxies that are now couch-potatoes once marathon runners?
4. If so, what processes, exactly, led them from one way of being to the other?
5. If an AGN is eating all this gas, does that mean it will have a lower star formation rate, since it will have less star-birthing material?
6. Can we build a wormhole to one that comes out right at the event horizon and just see what is going on? And then die? For science?
Astronomers are also still figuring out how, exactly, to find AGN. If a galaxy is active, that's because there's a bunch of gas around its black hole. This gas blocks and absorbs light. Light is all our telescopes get. If the light isn't reaching us, we can't learn anything about its source.
However, different kinds of light waves are blocked and absorbed more than others. Optical light and soft (lower energy) X-rays have a hard time getting from the AGN to us. Infrared light looks the same as light from newly forming stars, so we can't know for sure whether it's coming from a black hole or from baby stars.
Higher energy X-rays to the rescue!
These "hard" X-rays are unambiguous indicators of nucleic activity.
There's a telescope for that.
It's called SWIFT. It has an instrument called the BAT that can detect these kinds of X-rays. And the results from the BAT suggest that by looking at infrared, optical, and wussy soft X-rays, we have missed a significant population of AGN. According to the Lisa Winter's, and her collaborators', results, 24% of the galaxies BAT detected don't show up in anything but hard X-rays. There's a whole demographic of "hidden" AGN that no one knew and few cared about.
Hidden AGN are important to general AGN study: if you're missing a whole subset of an object whose physics you're trying to untangle, you can't really untangle the physics very well.
For a NASA article on this topic, click here.
Why do AGN matter to me?
Well, maybe our galaxy used to be active (it current eats some stuff, but not enough for anyone to use the acronym). Maybe that affected the way the galaxy evolved, which means that it was a key factor in determining how the galaxy (and you) look now. Or maybe it wasn't active. Maybe some galaxies are never active. But, then, that would have affected the way the galaxy evolved, too. If having an active nucleus is a factor in how galaxies grow up, then our galaxy became the stand-up fellow it is today by either having an active nucleus, or not.
Now here's a different question to consider before you go to bed tonight: How did galaxies get supermassive black holes at their centers? How did they form?
Or, alternatively, as a student I was helping with a science fair project said, "What are galaxies for?"
Lisa M. Winter (2011). Uncovering Local Absorbed Active Galactic Nuclei with Swift and Suzaku 2011 arXiv: 1112.0545v1