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)

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

March 30: Space Rocks in the Neighborhood

During our last public outreach night, Columbia Astronomy Professor, Greg Bryan, took a break from his normal schedule of simulating the processes of cosmic galactic evolution to inform us about objects a little closer to home. In his talk entitled, ''Space Rocks in the Neighborhood: The Dangers of Near Earth Objects,'' Prof. Bryan discusses the forces governing the orbits of near-earth objects (NEOs) and how many have impacted the Earth in the past. He explained that there are different types of NEOs depending on whether they cross the Earth's orbit or circle just inside or outside of it. What makes these objects dangerous is that for many asteroids and a few comets larger than a 1 kilometer (~.6 miles) across, real catastrophic damage can be done, with many global causalties.

Prof. Bryan also described the ranking system for potential threats by NEOs which is based on the size of the approaching object and the "projected" distance from impacting Earth. He pointed out that the key to adverting disaster by impact is to know the orbits of all concerning NEOs well in advance of a potential impact. If the orbits are known, then various efforts can be utilized to nudge the object out of an impacting trajectory towards Earth. While new telescopes and surveys have greatly expanded our knowledge of these potential "doomsday" rocks, only a complete survey of these objects, with multiple snapshots (to determine orbits), can help us prepare to deflect them out of harm's way.

Finally, Prof. Bryan pointed out different methods that can be used to deflect the objects in orbit ranging from painting the rock in a highly reflective coating such that photon pressure from solar rays can slowly push the rock away to detonating a nuclear warhead next to it to give it a more pronounced and immediate impulse away from a harmful trajectory towards Earth. You can read about NASA's Near Earth Object Program at their website and explore the near Earth environment for yourself, virtually of course, at the Minor Planet Center.

It seems that many people are concerned about our ability to track and advert a catastrophic impact, considering we had 260+ people in attendance. Even though the weather wasn't cooperating that evening, many people remained to enjoy our post lecture activities by either staying in the lecture hall and discussing the science in Astronomy Picture of the Day images, led by undergrad Jose Montelongo, or watching 3-D astronomy shows, run by graduating grad student, Jana Grcevich.

Our next public outreach lecture is on Friday, April 13th and is entitled "Where the Ocean Meets the Sky."

--Duane

March 2: To the Moon on a Budget

On Friday, March 2nd Columbia Professor Arlin Crotts discussed the history and status of the exploration of the moon. Starting with robotic and manned missions in the 1960's, Prof. Crotts explained how America's space program was a symbol of national pride and a major "battlefield" in the Cold War with the Soviet Union. In the 1970's and 1980's, after the Apollo program was shutdown, there was a long lull in lunar exploration, leading to major scientific questions, which are only beginning to be answered now due to a resurgence over the last decade in the number of robotic missions to the Moon. Prof. Crotts proceeded to give the current status of NASA's investment into lunar science, as well as the status of some private ventures to send robotic missions to reach the Moon. The talk concluded with some of the ideas from Prof. Crotts's upcoming book on lunar exploration, about how future lunar missions may not only be scientifically beneficial but also profitable.

The lecture was followed by some excellent questions from the 85 attendees, especially by several keen younger audience members. Unfortunately, the weather did not permit observing with our telescopes, but we gave tours of our Rutherford Observatory, showed 3D movies with our 3D projector, and showed Cosmic Collisions, an astronomical movie put together by the American Museum of Natural History.

--Jeff