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

By T. Brown, E. Smith, H. Ferguson, R. Rich, P. Guhathakurta, A. Renzini, A. Sweigart, R. Kimble. 2006

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Using 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 Andromeda. We have reconstructed the star formation history in these fields by comparing their color-magnitude diagrams to a grid of isochrones calibrated to Galactic globular clusters observed in the same bands. Each field exhibits an extended star formation history, with many stars younger than 10 Gyr but few younger than 4 Gyr. Considered together, the star counts, 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 stream's orbit. Instead, it is likely that this population is intrinsic to the inner spheroid, whose highly disturbed structure is clearly distinct from the pressure-supported metal-poor halo that dominates farther 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 either the stream's progenitor or similar objects. 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 outer disk of Andromeda is dominated by stars of age 4-8 Gyr, resembling the Milky Way's thick disk. 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/e67d9945e45db9573ffc867a0995dd08a4568821

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THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY (2012, background): “…on extended disk stars (e.g., Cuillandre et al. 2001; Durrell et al. 2001; Ferguson & Johnson 2001; Sarajedini & Van Duyne 2001; Rich et al. 2004; Brown et al. 2006; Kalirai et al. 2006a,b; Brown et al. 2007, 2008; Richardson et al. 2008; Brown et al. 2009b; Bernard et al. 2011, and many…”
THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. X. ULTRAVIOLET TO INFRARED PHOTOMETRY OF 117 MILLION EQUIDISTANT STARS (2014, methodology): “…nature (Rich et al. 1996), and very deep observations have provided the detailed metallicity and age distribution of a handful of halo pointings (Brown et al. 2003, 2006, 2007, 2008, 2009) There have been several isolated HST fields in the disk analyzed as well (Sarajedini & Van Duyne 2001;…”
A Search for Extended Ultraviolet Disk (XUV-Disk) Galaxies in the Local Universe (2007, background): “The stellar population of a small section within the outer M 31 disk was found to have an intermediate age of 4-8 Gyr by Brown et al. (2006).”
THE GHOSTS SURVEY. I. HUBBLE SPACE TELESCOPE ADVANCED CAMERA FOR SURVEYS DATA (2011, result): “…has been targeted with deep Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) pointings, revealing a complex composition of stellar populations, which, in contrast to our Galaxy, is indicative of substantial mixing in the M31 system (Brown et al. 2006; Richardson et al. 2008).”
Cosmological simulations of the formation of the stellar haloes around disc galaxies (2011, background): “…stellar populations, larger and more numerous tidal streams and surviving satellite galaxies (Guhathakurta et al. 2005; Irwin et al. 2005; Brown et al 2006a,b; Gilbert et al. 2006; Kalirai et al. 2006; McConnachie & Irwin 2006; Gilbert et al. 2007; Ibata et al. 2007; Koch et al. 2008;…”
The Haunted Halos of Andromeda and Triangulum: A Panorama of Galaxy Formation in Action (2007, background): “This is consistent also with the deep photometric survey in a Giant Stream field undertaken by Brown et al. (2006b) with the Advanced Camera for Surveys (ACS) on board the HST.”
THE LARGE-SCALE STRUCTURE OF THE HALO OF THE ANDROMEDA GALAXY. I. GLOBAL STELLAR DENSITY, MORPHOLOGY AND METALLICITY PROPERTIES (2013, background): “…current ability to probe the stellar populations in M31 both to extreme photometric depths (Richardson et al. 2009), including down to the main sequence turnoff in a handful of HST fields (Brown et al. 2006, 2007; Bernard et al. 2012), and over an extremely wide area, such as in the present study.”