Date: November 2, 2011
Title: Black Hole Formation in the Early Universe: Q & A
Podcaster: Marta Volonteri by Chris Lindsay
Link: For more information about Dr. Marta Volonteri, please see http://www.astro.lsa.umich.edu/~martav/
For more information about the Ann Arbor Science & Skeptics, please see http://www.annarborscienceskeptic.com.
For more information about the Critical Wit Podcast, please see http://criticalwitpodcast.com
Description: This episode consists of an interview with Marta Volonteri about her recent research in the formation of black holes in the early universe.
Bio: Marta Volonteri is a professor of astronomy at the University of Michigan. Her research is in black hole formation, and she was co-author on a recent paper in Nature about black hole formation in the early universe. http://www.nature.com/nature/journal/v474/n7351/full/nature10103.html
Chris Lindsay is the organizer of the Ann Arbor Science & Skeptics, and host of the Critical Wit podcast, a weekly, twenty-minute podcast about science, literature, and the arts.
Sponsor:This episode of “365 Days of Astronomy” has been sponsored by the Ann Arbor Science & Skeptics group in Ann Arbor, Michigan. You can find them at www.annarborscienceskeptic.com.
Hello. Thank you for listening to 365 Days of Astronomy. I’m Chris Lindsay, the organizer of the Ann Arbor Science and Skeptics and host of Critical Wit, a short weekly podcast about science, literature, and the arts.
On June 16, in a paper published in Nature, Dr. Marta Volonteri and several other co-authors concluded that the formation of black holes in the early universe, while difficult to observe, helped set the development and evolution of the first galaxies. And some super massive black holes have been observed, through the Chandra space satellite , in the X-ray spectrum of light. I spoke with Marta about her paper and how some of the observations compare to theoretical models.
Chris Lindsay: Marta, you recently authored a paper about the development of black holes in the early universe. Can you provide a summary of that paper?
Marta Volonteri: Uh, sure. It was an interesting collaboration between observers, those of us who look at observational data and actually use telescopes, and some theorists, me and another collaborator who tend to just write equations and codes, and develop theoretical models. And so what I really like about the paper was the collaboration that ensued between observers and theorists, which is what we hope happens as often as possible. So in that particular paper, we were looking at observations – the deepest observations that we have of the universe in x-rays and optical. In optical, what we detect is star light, so galaxies – all the way from the nearby galaxies to the most far away galaxies – and with the far away galaxies because light travels at the speed of light, it means it took a long time for this light to get to us. So far away galaxies are young galaxies – that existed at the beginning of the universe. These observations are in optical, so these observations of the youngest galaxies that we have, were coupled with x-ray observations. X-ray observations are high energy, and in particular, they select light coming from accreting black holes. Now for something to be luminous enough to be detected today, but existed for just a few billion years, like one billion years after the Big Bang, means it’s very luminous, it’s very big. These are massive black holes. These black holes weigh about a few million to up to several hundred million solar masses, so they are really big. Compare to stellar-mass black holes seen around our neighborhood. However, these black holes that we observed in the first galaxies were not super-duper massive. Up to now, the only super-massive black holes were detected in faraway young galaxies, were really enormous. The reason why we detect the most luminous, big black holes is because they must be extremely luminous for the light to get to us. So we use a specific technique which is called “stacking analysis.” So we summed the light that was coming from all of these systems, trying to find any sort of detection. And so we found the detection that some of these black holes indeed existed, because we could find the x-ray signature coming from them. So this brought us to compare these findings with the theoretical models that I had developed the last few years, and we compared them. And the agreement was very good. This means that my models, and the models developed by the other groups, explain that the first black holes were probably born in the very first galaxies and grew up with their own galaxies in a sort of tandem evolution, giving a sensible picture of the first billion years of the universe. So the most interesting part was the result, in my opinion, the first time using this particular type of analysis, we don’t just see the largest, brightest, most enormous sources, but more normal black holes were developing, but there much harder to find because they are not as luminous.
Chris: Can you talk about the Chandra Deep Field South Observatory, such as what it is exactly, and how you were able to use it for the observations that you made?
Marta: Yes. So Chandra is a telescope, a satellite launched by NASA. It’s a satellite that observes the universe in x-rays. So, in principle, x-rays detect very energetic photons that come from something which is very hot, or has a very deep potential will, a particular example is a black hole. Now, Chandra was used to observe this deep field – a field just means a patch of the sky that has been observed for a very long time – and in particular, in this field, we stared at the same galaxy, the same black holes, for many hours. This allowed us to have the deepest fields to date light for a very long time. And so that’s how we used this particular field, looking for the most faraway galaxies and black holes. It was observed for actually, four million seconds, which is a very long time for any standards. This is several thousand hours.
Chris: Now, are there any new questions that you have, or your group has, that have emerged from these findings, such as questions that may call for new observations or new models to be made?
Marta: Oh yes. If you are a scientist, you’re never happy. You always want more knowledge. So one thing that we noticed was… so there are currently two main theories for how the first black holes formed and what they were like. So one theory predicts that the first black holes were kind of smallish, let’s say about a hundred times the mass of the sun. But these black holes were very common. There was one in every, let’s say, about ten hundred galaxies. The second theory predicts, instead, the first black holes were more massive. Let’s say about, between ten thousand and a million times massive than the sun. Just to give you a reference, the mass of the super-massive black hole at the center of the Milky Way [galaxy] today is four million times the mass of the sun. So the first black holes were already quite substantial, but they were rare.
There was one in every thousand galaxies. So what our original plan was, with these observations, to try and distinguish between these two models. But unfortunately, the data is not good enough. So we need deeper and larger observations, to have more statistics, to see which of the two models are correct. So these are a very immediate need that came out of working on the paper. That’s something else we want to do is have more information on the properties of the galaxies that hosts these black holes, and see, for instance, how many galaxies do host a black hole? Today, we see that black holes are really common in galaxies. Whenever a black hole was looked for in a substantial galaxy, it was found, except in a couple of cases. But we don’t know anything about the red-shift, about the early universe. So this is definitely something that we want to get to, using new data.
Chris: And my last question is, I don’t suppose that your findings here have any relevance or impact on existing theories about the formation of the early universe? Is that right?
Marta: That’s right. These black holes form quite late. So the theories of the early universe describe what happened in the universe in the first few seconds, or let’s say a few minutes of its existence. These black holes were forming and evolving a few hundred million years after the Big Bang. So, that by that time, all theories had to converge on to what we actually observe.
Chris: Well, great. Thank you very much, Marta Volonteri, for taking the time to talk with me on 365 Days Of Astronomy.
End of podcast:
365 Days of Astronomy
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