The Stellar Halos of Massive Elliptical Galaxies
  We use the Mitchell Spectrograph (formerly VIRUS-P) on the McDonald
Observatory 2.7m Harlan J. Smith Telescope to search for the chemical
signatures of massive elliptical galaxy assembly. The Mitchell Spectrograph is
an integral-field spectrograph with a uniquely wide field of view (107x107 sq
arcsec), allowing us to achieve remarkably high signal-to-noise ratios of
~20-70 per pixel in radial bins of 2-2.5 times the effective radii of the eight
galaxies in our sample. Focusing on a sample of massive elliptical galaxies
with stellar velocity dispersions sigma* > 150 km/s, we study the radial
dependence in the equivalent widths (EWs) of key metal absorption lines. By
twice the effective radius, the Mgb EWs have dropped by ~50%, and only a weak
correlation between sigma* and Mgb EW remains. The Mgb EWs at large radii are
comparable to those seen in the centers of elliptical galaxies that are
approximately an order of magnitude less massive. We find that the well-known
metallicity gradients often observed within an effective radius continue
smoothly to 2.5R_e, while the abundance ratio gradients remain flat. Much like
the halo of the Milky Way, the stellar halos of our galaxies have low
metallicities and high alpha-abundance ratios, as expected for very old stars
formed in small stellar systems. Our observations support a picture in which
the outer parts of massive elliptical galaxies are built by the accretion of
much smaller systems whose star formation history was truncated at early times.
