Astrophysics, abstract
astro-ph/0607637

From: Thomas M. Brown [view email]

Date: Fri, 28 Jul 2006 04:04:15 GMT (880kb)

The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy

Authors: Thomas M. Brown, Ed Smith, Henry C. Ferguson (STScI), R. Michael Rich (UCLA), Puragra Guhathakurta (UCO/Lick), Alvio Renzini (INAF), Allen V. Sweigart, Randy A. Kimble (NASA/GSFC)

Comments: Accepted for publication in The Astrophysical Journal. 29 pages, 23 figures (including 9 in color), latex. Images and CMDs are significantly smoothed and degraded in this version; a version with high-quality figures is available at this http URL

Report-no: STScI Eprint #1730

[Abridged] Using the HST/ACS, we have obtained deep optical images reaching stars well below the oldest main sequence turnoff in the spheroid, tidal stream, and outer disk of M31. These data allow a complete reconstruction of the star formation history in these fields. We analyze the CMDs of these populations by comparison to a grid of isochrones calibrated to Galactic globular clusters observed in the same ACS bands. All 3 fields exhibit an extended star formation history, with many stars younger than 10 Gyr but few younger than 4 Gyr. Considered together, the star count maps, kinematics, and population characteristics of the spheroid argue against some explanations for its intermediate-age, metal-rich population, such as a significant contribution from stars residing in the disk or a chance intersection with the orbit of the stream. Instead, it is likely that these intermediate-age, metal-rich stars are intrinsic to the inner spheroid, whose highly-disturbed structure is clearly distinct from the pressure-supported metal-poor halo that dominates at large distances from the galaxy's center. The stream and spheroid populations are similar, but not identical, with the stream's mean age being ~1 Gyr younger; this similarity suggests that the inner spheroid is largely polluted by material stripped from the stream's progenitor or by objects like it. The disk population is considerably younger and more metal-rich than the stream and spheroid populations, but not as young as the thin disk population of the solar neighborhood; instead, the M31 outer disk is dominated by intermediate-age stars of age 4-8 Gyr, resembling the Milky Way's thick disk population. The disk data are inconsistent with a population dominated by ages older than 10 Gyr, and in fact do not require any stars older than 10 Gyr.

http://arxiv.org/abs/astro-ph/0607637