Discovery of an extended halo of metal-poor stars in the Andromeda spiral galaxy

Details Discovery of an extended halo of metal-poor stars in the Andromeda spiral galaxy Discovery of an extended halo of metal-poor stars in the Andromeda spiral galaxy

2005-02-18

arxiv.org/abs/astro-ph/0502366v5

Astronomy and Astrophysics

Astrophysics

48 pages, 7 figs (main manuscript: 16 pages, 3 figs; supplementary information section: 32 pages, 4 figs) - revised through tw

Scientific paper

Understanding galaxy formation involves look-back and fossil-record studies of distant and nearby galaxies, respectively. Debris trails in our Galaxy's spheroidal halo of old stars provide evidence of "bottom-up" formation via tidal disruption/merging of dwarf satellite galaxies, but it is difficult to study our Galaxy's large-scale structure from within. Studies of our neighbouring Andromeda galaxy have concluded that its spheroid contains chemically enriched ("metal-rich") stars out to a radius of 30 kiloparsecs with an exponential r^1/4 fall-off in density thereby resembling a galactic "bulge". Were Andromeda's true halo to be found, our detailed yet global view of its stellar dynamics, substructure, chemical abundance, and age distribution would directly constrain hierarchical halo formation models. We report here on the discovery of a hitherto elusive halo of metal-poor Andromeda stars, distinct from its bulge, with a power-law brightness profile extending beyond r = 160 kiloparsecs. This is 3 - 5 times larger than any previously mapped Andromeda spheroidal/disk component. Together, the Galactic and Andromeda halos span >1/3 of the distance between them, suggesting that stars occupy a substantial volume fraction of our Local Group, and possibly most galaxy groups.

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