Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-05-17
Astrophys.J. 592 (2003) L17-L20
Astronomy and Astrophysics
Astrophysics
4 pages, Latex. 1 color figure. Uses corrected version of emulateapj.sty and apjfonts.sty (included). Accepted for publication
Scientific paper
10.1086/376935
We present a color-magnitude diagram (CMD) for a minor-axis field in the halo of the Andromeda galaxy (M31), 51 arcmin (11 kpc) from the nucleus. These observations, taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, are the deepest optical images yet obtained, attaining 50% completeness at V = 30.7 mag. The CMD, constructed from approximately 300,000 stars, reaches more than 1.5 mag fainter than the old main-sequence turnoff. Our analysis is based on direct comparisons to ACS observations of four globular clusters through the same filters, as well as chi-squared fitting to a finely-spaced grid of calibrated stellar-population models. We find that the M31 halo contains a major (approximately 30% by mass) intermediate-age (6-8 Gyr) metal-rich ([Fe/H] > -0.5) population, as well as a significant globular-cluster age (11-13.5 Gyr) metal-poor population. These findings support the idea that galaxy mergers played an important role in the formation of the M31 halo.
Brown Thomas M.
Ferguson Henry C.
Kimble Randy A.
Renzini Alvio
Rich Robert Michael
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