Oct 27 - Cosmic Mergers & Acquisitions: Mergers and Black Holes and the Growth of Galaxies

Our speaker this week was Dr. Jenny Greene, a Professor of Astrophysics at Princeton University.

Much of Jenny’s research concerns some of the most mind-boggling objects in the Universe: the supermassive black holes that exit at the centers of most galaxies. Millions to billions of times more massive than the Sun, these huge black holes are thought to play a crucial role in the development of galaxies.

Jenny began by reviewing what a black hole is: an incredibly dense object. She used the visual of compressing the Sun to the size of Jupiter, then the Earth, then Manhattan; as it gets smaller, the speed you need to escape its gravity gets larger and larger until it exceeds the speed of light. Since nothing moves faster than that, nothing can escape from a black hole.

Even though we can’t see black holes directly, Jenny showed us evidence of the supermassive black hole in the center of our Galaxy. Careful observations of the Galactic center reveal stars moving at phenomenal speeds. Measuring their positions over decades, astronomers can reconstruct their motions and show that they must be orbiting an incredibly dense but invisible object: a black hole.

Astronomers can’t directly see stars moving in other galaxies but their collective motions can be measured very precisely and it is clear that almost all galaxies require a supermassive black hole in their centers to explain these observations. Interestingly, it seems like the black holes’ sizes ‘know about’ the galaxy that they live in: the mass of the black hole can be predicted if the mass of the galaxy is known. How is this possible? Galaxies grow mostly by merging with other galaxies. This process brings in lots of new stars and gas. The gas both forms new stars and feeds the black hole, but as the black hole eats it forms an ‘accretion disk’ of very hot gas that heats up the remaining gas, eventually causing both star formation and its own growth to stop. In this way the size of the galaxy and the size of the black hole can become related.

If this picture is correct we should also see evidence of pairs of black holes, since each galaxy should have brought one along with it. Jenny described one way she and her students have been looking for these binary black holes. When they get close together, they will be orbiting each other at very high speeds — probably thousands of kilometers per second. The light from their accretion disks will be shifted to redder and bluer colors periodically due to the Doppler shift; if we can observe these color-changing black holes it would be good evidence in favor of our cosmological picture. So far the observations haven’t found anything but astronomers are still looking!





-- David Hendel (graduate student)

Oct 24: Stripping Stars

This week's Astronomy public lecture was given by post-doc fellow, Nick Stone, from Columbia Astronomy.  His talk "Stripping Stars: The Exciting Lives and Untimely Demise of Stars Near Supermassive Black Holes" led the audience to discover the dynamic processes associated with black holes.  Starting with an introduction to basic dynamics predicted by Newton's laws of motion and then general relativity, Nick familiarized the audience with the concepts of four dimension time-space and interactions between objects via gravity. Then he explained in detail the physics of black hole accretion, and broadened the case to dual-black hole interactions with the help of simulations. Finally, Nick showed what phenomena when we can expect to see as observers on Earth and how we can observe these energetic events in the Universe. 

About 150 people attended the lecture, after which they headed up to the Rutherfurd Observatory for to observe the Pleides open cluster and the spiral galaxy M31 (aka our nearest neighbor, Andromeda). Those who didn't go to the roof were treated to a slideshow on 'Gas Beyond the Milky Way Disk', by graduate student Yong Zheng, which introduced the very diffuse hot gas with more than 10 thousand degrees in the Milky Way potential well.

-- Yong Zheng (graduate student) 

November 30: Galactic Neighborhoods

On November 30th, about 100 people joined us to hear Columbia Astronomy Professor Jacqueline van Gorkom talk about how galaxies are influenced by their surroundings. Professor van Gorkom is a radio astronomer and does extensive research on galaxies. She showed us how strikingly different galaxies look in dense clusters of galaxies compared with fields. She explained that when a galaxy enters a galaxy cluster filled with hot intra-cluster gas, the cold gas inside the galaxy gets stripped out of it and thus star formation is quenched, which changes the color of the galaxy from blue to red. The audience raised a lot of interesting questions about galaxies and gravity.

After the talk, some people went to see the 3D movies on the 13th floor shown by Columbia undergraduate student Gladys Velez-Caicedo. Unfortunately, the sky was not clear on Friday, but Columbia Astronomy graduate students Adrian Price-Whelan and Jenna Lemonias gave a telescope tour to a group of people on the roof of Pupin. In the lecture hall, graduate student Yuan Li presented a slide show on spiral galaxies and Dr. Jana Grcevich talked about the Big Bang and the expansion of the universe.

Join us on December 14th for the last event of this semester: a panel discussion on Life, the Universe, and Everything!

--Yuan

February 11, 2010: Galactic Sleuthing Lecture and Public Observing

This past Friday, Dr. Allyson Sheffield, a current post-doc of the Columbia Astronomy department, gave a talk entitled, ''Galactic Sleuthing: Unraveling the Milky Way’s Past.'' In her talk, Allyson described two prevailing models for galactic formation. First, 'monolithic collapse' posits that a galactic-sized gas reservoir (of Hydrogen and Helium) collapses all at once, creating an old population halo of stars first and then flattens out into a disk composed of younger stars. Second, she presented the theory of 'hierarchical merging' that posits that galaxies are built up from the accretion of smaller galaxies and the merging of bigger ones.

Allyson then talked about how we can determine which scenario better describes the evolutionary path of our own galaxy by looking at both the group motions of stars and their shared chemical abundances. By showing us data and simulations by Rodrigo Ibata and his collaborators and by our very own Prof. Kathryn Johnston, respectively, she showed us how the merging Sagittarius dwarf galaxy was discovered by surveying stellar velocities in the galaxy along with models that describe how the dwarf galaxy would currently look which were later confirmed by additional chemical abundance observations.

Finally, she showed us how one can merge the two indicators of common ancestry to look at moving groups closer to us and determine if evidence for other accreted dwarf galaxy remnants remain in the galaxy by looking at the spectra of stars with similar peculiar velocities. She found that she could likely link certain groups to larger ones seen in the galactic halo stellar streams by models that would predict the location of these peculiar stars. Given her research and the other research she presented, we learned that merging plays a significant role in galaxy evolution and that one can recount this evolution based on remnants of past accretion events.

We had a great turn out and our attendees made full use of our facilities by watching astronomy visualizations on our 3-D wall, ran by graduate student Jana Grcevich, and by going to the roof to stargaze. It's cool to think that many of the stars we observed, including our own, exhibit group origins or movements that were probably even more defined in the past.

Thanks to the 8 volunteers and the 130 people who attended our lecture and observing night!