The Formation History of Andromeda's Extended Metal-Poor Halo

Details The Formation History of Andromeda's Extended Metal-Poor Halo The Formation History of Andromeda's Extended Metal-Poor Halo

Jul 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006hst..prop10816b&link_type=abstract

HST Proposal ID #10816. Cycle 15

Computer Science

Scientific paper

We propose deep ACS imaging in the outer spheroid of the Andromeda galaxy, in order to measure the star formation history of its true halo. For the past 20 years, nearly all studies of the Andromeda "halo" were focused on the spheroid within 30 kpc of the galaxy's center, a region now known to host significant substructure and populations with high metallicity and intermediate ages. However, two groups have recently discovered an extended metal-poor halo beyond 30 kpc; this population is distinct in its surface-brightness profile, abundance distribution, and kinematics. In earlier cycles, we obtained deep images of the inner spheroid 11 kpc on the minor axis, outer disk 25 kpc on the major axis, and giant tidal stream, yielding the complete star formation history in each field. We now propose deep ACS imaging of 4 fields bracketing this 30 kpc transition point in the spheroid, so that the inner spheroid and the extended halo populations can be disentangled, enabling a reconstruction of the star formation history in the halo. A wide age distribution in the halo, as found in the inner spheroid, would imply the halo was assembled through ongoing accretion of satellite galaxies, while a uniformly old population would be a strong indication that the halo was formed during the early rapid collapse of the Andromeda proto-galaxy.

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