Date: May 17, 2011
Title: Quasars in Galaxy Clusters
Podcasters: Rob Sparks and Dr. Dara Norman
Organization: National Optical Astronomy Observatory (NOAO) – http://www.noao.edu
Links: http://twitter.com/#!/NOAONorth
Description: Quasars are some of the most luminous objects in the universe. Quasars are ancient galaxies that harbor massive black holes at their centers. The black holes emit huge amounts of energy across the spectrum as they consume matter. In this podcast, Dara Norman discusses her research on how quasars interact with their environment. Many quasars occur in galaxy clusters which can play a role in turning on quasars as well as their evolution.
Bio: Dr. Dara Norman is a research astronomer at the NOAO. Her research interests are in the area of Active Galactic Nuclei, including Quasars, and their cluster environments, in particular the triggering of AGN, and their influence on galactic evolution. She is also interested in how Quasars can be used to understand large-scale structure in the universe.
In addition to her research Dr. Norman holds the title of Diversity Co-Advocate at NOAO. She previously held an NSF Astronomy and Astrophysics Postdoctoral Fellowship. She is an active member of the American Astronomical Society’s Committee on the Status of Minorities and Chair of the Astronomy section of the National Society of Black Physicists.
Dr. Norman holds an M.S. and Ph.D. in Astronomy from the University of Washington and a B.S. in Earth, Atmospheric and Planetary Science from the Mass. Inst. of Technology.
Rob Sparks is a science education specialist in the EPO group at NOAO and works on the Galileoscope project (www.galileoscope.org), providing design, dissemination and professional development. He also blogs at halfastro.wordpress.com.
Sponsors:
INTRO – 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/
OUTRO – This episode has also been sponsored by the National Optical Astronomy Observatory. NOAO is a US national research and development center for ground-based nighttime astronomy. We provide astronomers access to world-class observing facilities on a peer-reviewed basis. Our mission is to engage in programs to develop the next generation of telescopes, instruments, and software tools necessary to enable exploration and investigation through the observable Universe. For information on observing proposals or our public programs, please visit www.noao.edu for more information.
Transcript:
Rob: Hi, this is Rob Sparks from the National Optical Astronomy Observatory and I would like to welcome you to this episode of the 365 Days of Astronomy podcast. I am sitting here today with Dara Norman from NOAO. Good morning, Dara.
Dara: Good morning, how are you?
Rob: First, could you tell me a little bit about yourself and your background and what you do here at NOAO.
Dara: Okay, well, I am originally from Chicago, Illinois and I grew up on the south side of Chicago. I went to MIT as an undergraduate and then I went to work actually before graduate school at NASA Goddard in Greenbelt, Maryland. I worked with a team that had an instrument on the Hubble Space Telescope, the Goddard High Resolution Spectrograph team, and when Hubble had its problem with the mirror I decided, well, maybe this is a good time to go back to graduate school. So I went to the University of Washington where I worked with both Craig Hogan, my advisor there, and Chris Impey who is actually here at the University of Arizona.
Rob: That sounds like a great way to take some time off, working at Goddard!
Dara: Yeah, Right!
Rob: Today we are going to talk about quasars in Galaxy clusters. First, could you briefly describe what a quasar is for our listeners.
Dara: So a quasar stands for Quasi Stellar Object. And a quasi stellar object just means that it looks like a star but it isn’t a star. People originally got these spectra of what they thought were bright stars and these objects turned out to be much brighter, give off much more energy than an entire galaxy of billions of stars. So they clearly were not regular stars. It turns out these objects are the bright centers of active galaxies. And by active I just mean that the center of the galaxy is giving off a lot of energy, is very luminous. And the reason that these objects look like stars is that they were so bright that they were swamping all the rest of the light in the galaxy and so you could only see the very bright center that looked like a point source. It turns out, we think, that all galaxies have black holes at their center but only some of these galaxies have active black holes and what I mean by active is that cool gas is actively falling into the center of the black hole producing a lot of energy and making gas shine near the center of the black hole.
Rob: For those of you listening to this podcast, last month we interviewed Todd Lauer about black holes so if you want more information about black holes, listen to the April 17th podcast and you will hear all kinds of great stuff from him.
Galaxies occur in clusters so it’s not surprising that some of these galaxies have quasars in them. What have we learned about studying these quasars in galaxy clusters?
Dara: So what’s really interesting, what we want to understand, is why some of these quasars are actually bright and some black holes are not bright at all. In order to try and do this, we are looking to understand what is it about the environment of an object that actually has a quasar at the center. What is it about the environment of that galaxy that has made it shine very brightly, that has made cool gas fall into the center of the galaxy. And so one thing I am very interested in trying to understand is looking to see if there seem to be more quasars shining in galaxy clusters where you have a lot of galaxies that are all interacting with each other. Does that in some way inspire or trigger gas to fall into the center of the galaxy making a bright quasar.
Rob: So its possible that galaxy clustering plays a role in turning these quasars on and off.
Dara: That’s the idea. The idea is that we want to understand if because of gravitational interaction between these galaxies in the cluster. Does that in some way trigger gas to fall into the center of some of the galaxies. So in order to look at this we are looking at not just the numbers of quasars that are found in clusters but also the distribution. Are there more quasars that occur at the center of clusters are there more that occur in the outskirts of clusters. If there are more galaxies in a very large cluster does it tend to have more quasars or does it tend to have fewer quasars, things like that. What’s interesting too though is that quasars are identified in very different ways. A lot of times they are identified by their optical spectra but that’s very costly as far as telescope time. An easier way to identify quasars is how bright they are in x-rays and that’s because very few things are very bright in X-rays and so if you get an object that has a luminosity, a very large luminosity you can know that it’s probably a quasar. You need to get spectra to find out where it actually is. Is it in the cluster or is it in front of the cluster or behind the cluster. But you can find objects in the x-rays very easily that are quasar like.
Rob: Yeah, it’s a little tougher to build up that 3D map isn’t it?
Dara: That’s right. The 3d map is the hard part, which is a very good point. What’s interesting is that a lot of work has been done saying there are a lot of quasars near this cluster, but to understand if its actually in the cluster, is actually being gravitationally bound to the cluster, you need that 3d information and that’s the really hard part.
Rob: Or it could just be a line of sight thing.
Dara: Exactly.
Rob: Galaxy clusters evolve over time, so is it possible that the quasars also evolve over time in these clusters?
Dara: Absolutely. In fact, as I was telling you, there are many different ways that we identify quasars and in fact, X-ray and optical spectra as just two of the ways. We can also identify them through their infrared luminosities and people are working on doing that with the Spitzer Space Telescope. What we think might be happening is that at any given stage in the evolution of a quasar, you might be able to identify it at different wavelengths. It might give off more x-rays, it might be more luminous in x-rays later in its life after its gotten rid of a bunch of gas and dust from the center that might be blocking x-rays from getting through. This is another interesting aspect of trying to understand the triggering of what makes a quasar but also trying to understand the evolution of once it gets triggered, how do we see it. Do we see it as an optical quasar? Do we see it as an X-ray quasar? How do we identify these things and how do we piece together what the timeline is or what’s an older quasar or when it has been triggered or retriggered at different stages of its evolution.
Rob: So you really need to use a variety of different telescopes to look at these because it might be bright in the infrared at one part of its life and then bright in the visible and bright in the x-rays at a different part of its life.
Dara: Yeah, that’s the idea. And you know this is work that’s all really being done now so we don’t really know is an x-ray quasar older than an optical quasar. It’s not really quite clear yet.
Rob: So what are the next big questions you would like to answer?
Dara: So what I am really interested in trying to work is how do you really get a quasar to ignite in the center of a galaxy. Do you really need a merger event, do you really need two galaxies running into each other to really push the gas into the black holes of one or both of them or are there other ways to get gas, get enough gas in a galaxy into the center of the black hole. This is a hard thing to do because of angular momentum arguments. Trying to get stuff that’s rotating in a disc down into the center of a point source like a black hole, do you really need something as violent as a merger or can you really just get this to fall in because a galaxy is falling through a cluster. The galaxy is falling into a cluster, is that enough of a push to get the gas to fall into the center of the black hole.
Rob: How do you shed that energy and angular momentum to get it in there.
Dara:Exactly.
Rob: Thank you very much for joining me today Dara.
Dara: Thank you.
Rob: I look forward to hearing more about your research in the future and keeping track of it. It sounds very interesting. Thanks for joining us for this episode of the 365 Days of Astronomy podcast. This is Rob Sparks with Dara Norman.
End of podcast:
365 Days of Astronomy
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