Abstract / Description of output
We present deep Hubble Space Telescope ACS observations of the G1 clump,
a distinct stellar overdensity lying at ~30 kpc along the southwestern
major axis of M31, close to the G1 globular cluster (from the work of
Ferguson and coworkers). Our well-populated color-magnitude diagram
reaches ~7 mag below the red giant branch tip with 90% completeness, and
allows the detection of various morphological features that can be used
to derive detailed constraints on the age and metallicity of the
constituent stellar population. We find that the color-magnitude diagram
is best described by a population with a large age range (gtrsim10 Gyr)
and a relatively high mean metallicity [M/H] = -0.4. The spread in
metallicity is constrained to be lesssim0.5 dex. The star formation rate
in this region has declined over time, with the bulk of the stellar mass
having formed >6 Gyr ago. Nonetheless, a nonnegligible mass fraction
(approx10%) of the population has formed in the last 2 Gyr. We discuss
the nature of the G1 clump in light of these new stellar population
constraints and argue that the combination of stellar content and
physical size make it unlikely that the structure is the remnant of an
accreted dwarf galaxy. Instead, the strong similarity between the
stellar content of the G1 clump and that of the M31 outer disk suggests
that the substructure is a fragment of the outer disk, perhaps torn off
from the main body during a past accretion/merger event; this
interpretation is consistent with extant kinematical data. If this
interpretation is correct, our analysis of the stellar content provides
further evidence that the outskirts of large disk galaxies have been in
place for a significant time.
Based on observations made with the NASA/ESA Hubble Space Telescope,
obtained at the Space Telescope Science Institute, which is operated by
the Association of Universities for Research in Astronomy, Inc., under
NASA contract NAS5-26555. These observations are associated with program
GO9458.
Original language | English |
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Pages (from-to) | 1275-1286 |
Journal | Astronomical Journal |
Volume | 133 |
Issue number | 4 |
DOIs | |
Publication status | Published - 1 Apr 2007 |
Keywords / Materials (for Non-textual outputs)
- galaxies: evolution
- galaxies: formation
- galaxies: halos
- galaxies: individual: M31
- galaxies: stellar content
- galaxies: structure