2003/05/17 by Thomas M. Brown, T. M. Brown, Henry C. Ferguson +9 · 9 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Galaxies: Formation, Evolution, Phenomena #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/376935
published as Astrophys.J. 592 (2003) L17-L20 · 4 pages, Latex. 1 color figure. Uses corrected version of emulateapj.sty and apjfonts.sty (included). Accepted for publication in ApJ Letters. Images available at http://hubblesite.org/newscenter/archive/2003/15/
arxiv created 2003/05/17 · openalex publication_date 2003/06/24 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
We present a color-magnitude diagram (CMD) for a minor-axis field in the halo of the Andromeda galaxy (M31), 51' (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 m V = 30.7 mag. The CMD, constructed from ~3 × 10 5 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 χ 2 fitting to a finely spaced grid of calibrated stellar-population models. We find that the M31 halo contains a major (~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.