22 Jul 2011

 

A dive into the stars of M31

 

It is becoming common nowadays to discover something new and interesting while searching through archival data from the world's great telescopes. The 4 images above were taken by the Hubble Space Telescope between 2004 and 2007, and show some unusual views of the nearby spiral galaxy, M31 in Andromeda.
We've all seen thousands of images of M31, more or less well done, but also (necessarily) more or less the same, but Hubble shows us M31 as if we were inside it, in fact almost all of the stars visible in the above images A-D belong to M31.
In particular, image A shows a small region of the outskirts of the disk, that is, the part of the galaxy containing the spiral arms. Here the stellar density is relatively high, as can be seen by compering with images C and D in particular, that are of regions of the halo. The stellar density in the halo is very low, but the dynamical properties of the galaxy suggest that this nonetheless contains a great deal of dark matter.
Image B, instead, is an image of part of the so-called "Giant Stellar Stream". Because of the increased stellar density in this region compared to the surroundings and its dynamical properties, this is thought to be the remains of a dwarf galaxy that was destroyed, and then consumed by, M31.
The great depth of the images, which are actually the sum of many individual exposures, was necessary for the study of individual stars within M31, that researchers wished to categorise on the basis of their type, age and chemical composition, so as to build up a picture of the evolutionary history of the whole galaxy.
The long exposures also show an enormous quantity of background galaxies, all very much more distant than the 2.5 million light years estimated for M31.

 

by Michele Ferrara & Marcel Clemens

credit: NASA, ESA and T.M. Brown (STScI), Digitized Sky Survey 2