The outer halo regions of galaxies are important for studies of galactic evolution as much of the stellar populations (stars, globular clusters) formed before most stars on the galaxy proper (for example, the disks in spiral galaxies), and thus provide us insight as to galactic formation and early galactic evolution. However, typical galaxy halos comprise a small fraction of all of the galactic stars, and as a result are very difficult to study.
The halo of the Milky Way (comprising about 1% of the visible mass of the Galaxy) is difficult to study due to our location in the Galaxy -- visible halo stars are literally swamped by the much larger number of closer disk stars (although very important studies on the Milky Way halo are being conducted as you read this!)
An ideal opportunity to study a galactic halo in detail lies with the nearby Andromeda galaxy M31. Its brightest halo stars (red giants) are readily visible with 4m class ground-based telescopes (eg. CFHT/KPNO), and have been actively studied for over a decade (eg. Mould & Kristian 1986, Davidge 1993). Based on a CFHT study of a halo field 10' from the nucleus (Durrell et al. 1994) , we have noted a rather high metallicity ([Fe/H] = -0.6) for the halo stars, suggesting a higher level of metal enrichment than present in the (for example) Milky Way halo. These results have been confirmed by HST studies of fields at other locations in the M31 halo (Rich et al. 1996, Holland et al. 1996). Although there are several hundred globular clusters in the M31 halo, there are not enough to fully explore the outermost regions of the halo. There are plenty of halo stars, but they are just so faint...An excellent recent review on the chemical evolution of the M31 halo has been done by Rich (2004).
M31 image, courtesy of Raja Guhathakurta
More recently, there have been a number of very exciting results by a number of groups have found many coherent streams of stars in the M31 halo, which are likely the disrupted remains of satellite galaxies that were once part of the M31 system. An ongoing question is how much of the M31 halo is formed by the disruption of such satellites?
As part of a large scale survey of the M31 halo, we (C. Pritchet, W. Harris and myself) have taken large (32') CCD mosaic images (with the CFHT UH8K camera of outer regions of the halo in order to (a) trace the extent of the halo (to levels far below what is possible from standard surface photometry), (b) the shape of the halo by studying fields at different locations of the halo, and (c) the metallicity of the stellar populations, looking for (perhaps) a metallicity gradient if one exists. This project has recently finished, with two papers on the studiesi of 3 M31 halo fields now published or in press. I have since been collaborating with Pat Cote (Rutgers) on another photometric and spectroscopic study of the M31 halo.
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