4 Feb. 2011

 

A disrupted galaxy right next to Earth

 

At a distance of between 16,000 and 32,000 light years from the Earth there is a galaxy; or, to be more precise, a stellar stream, whose dynamics show that it represents the remnants of a dwarf galaxy that merged with the Milky Way.
This new stellar stream (called the "Aquarius Stream" because most of its component stars lie in that constellation) was discovered by an international team of astronomers led by Mary Williams (Astrophysical Institute Potsdam), using data collected by the Radial Velocity Experiment (RAVE).
This ambitious spectroscopic survey aims to determine, with unprecedented precision, the radial velocity, luminosity, temperature and metallicity of a million stars in the solar neighbourhood. The survey uses the historic UK Schmidt Telescope of Siding Spring Observatory (Australia).
Although the Aquarius Stream is the closest known stream, its proximity to the densely populated Galactic disk had kept it hidden until now. Of the 25,000 radial velocity measurements made to date, 15 stars were identified as having anomalous velocities, something which can be reproduced in models that include the merger of a satellite galaxy.
In the case of this new stream, models show that a dwarf galaxy was consumed by our own about 700 million years ago, leaving the Aquarius Stream as the only trace of the event. This is not the first such stream to be identified, but is certainly the most recent and closest found so far.
Thanks to the RAVE survey, that will continue until 2012, we will have a very complete picture of the dynamical history of our galaxy. By reconstructing its recent dynamical evolution, we will be able to estimate the frequency and importance of these merger events, which will be an important constraint for models of galaxy evolution in general. It is incredible to think that such nearby structures can tell us about how galaxies evolve on cosmological timescales.

 

by Michele Ferrara & Marcel Clemens

credit: Arman Khalatyan, AIP