Astronomy and Astrophysics

Waiting for Fireworks at the Center of our Galaxy
Tuan Do, University of Toronto

Observations of the center of the Milky Way has taught us a great deal about what lies at the heart of galaxies. We now know there exists a supermassive black hole at the Galactic center with a mass greater than four million times that of our sun. This is also a region that is packed with stars, with more than a million stars in the same volume as between the sun and its nearest star. The sheer density of objects in this region in combination with the strong gravitational field of the black hole makes it one of the most interesting dynamical regions in the universe to study, and one which is a source of many surprising discoveries. The most recent of these discoveries is that of an object called G2. Earlier this year, G2 was the fastest moving object we know of in our Galaxy when it approached the supermassive black hole at over 1% the speed of light. While there are some fast stars that can achieve this velocity in this region, its other properties make it unusual compared to stars: it is very red at infrared wavelengths, and has strong hydrogen line emission features. These properties led to the hypothesis that G2 is a gas cloud. If G2 is a gravitationally unbound gas cloud, then it will be tidally stripped by the black hole as it goes through closest approach, providing us with an unprecedented view of the feeding of a supermassive black hole. However, a gravitationally unbound gas cloud should not be able to survive in this region for very long. Alternatively, G2 may be of stellar nature -- such as a proto-planetary disk, embedded protostar, a merger of two stars, etc. While these objects should be rarer than typical stars found in this region, they would have a much longer lifetime. The nature of the source is important because there will be significantly less material feeding the black hole if G2 is stellar in nature. I will present an observer’s point of view on this puzzling object. I will show imaging and spectroscopic observations of G2 over the past eight years from the Keck Telescopes, and how these observations compare to theoretical predictions for the nature of this object. Unlike most other astronomical sources, these hypotheses can be tested in almost real time.

Background Review Article:

Why galactic black hole fireworks were a flop. Nature. Ron Cowen. 21 July 2014 Corrected: 23 July 2014.

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Astronomy and Astrophysics

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