Puragra Guhathakurta

Lick Obs

                   Studies of M31's Stellar Halo

A combination of imaging and spectroscopic data is used to study the
structure, metallicity, and dynamics of M31's stellar halo.  Deep Kitt Peak
4-meter UBRI CCD images of a 19 kpc minor axis field are analyzed.  A new
photometric technique is developed for isolating faint stars from the
contaminating dense background of field galaxies.  Coupled with standard
morphological star-galaxy separation (based on high resolution Keck I-band
images) and statistical subtraction relative to a well-matched comparison
field, this enables us to identify a plausible sample of candidate red
giant branch stars in M31's outer halo.  These data on the outer halo, taken
together with HST / WFPC2 images at comparable or smaller distances out in
the halo, indicate that M31's stellar halo is an order of magnitude denser
and/or larger than that of the Milky Way galaxy.  The broadband colors of
these M31 halo RGB candidates are consistent with HST measurements and
suggest that they are significantly more metal rich than their Galactic
counterparts: [Fe/H] >~ -1, indicative of substantial pre-enrichment of the
M31 halo.  Recent spectroscopy with the Keck 10-meter / Low Resolution
Imaging Spectrograph of the Ca II near infrared triplet is used to make
secure identification of M31 halo RGB stars from amongst ~ 75 photometrically
isolated candidates by discriminating them from M31 disk stars, foreground
Galactic dwarf stars, and background field galaxies.  Spectroscopic estimates
of the mean metallicity and metallicity spread are compared to color-based
estimates.  The kinematics of the outer halo RGB stars also serve as a tracer
of the M31 halo mass profile.

Time permitting, I will also briefly describe ongoing studies of M31's
recently discovered dSph satellites and tidal distortion of M32 and NGC205.