Spitzer Observations of the Prototypical Extremely Red Objects HR10 and   LBDS53W091: Separating Dusty Starbursts from Old Elliptical Galaxies
  We present Spitzer Space Telescope observations of the well-studied extremely red objects (EROs) HR10 and LBDS53W091 from 3.6 to 160 microns. These galaxies are the prototypes of the two primary classes of EROs: dusty starbursts and old, evolved galaxies, respectively. Both galaxies, as well as LBDS53W069, another example of an old, quiescent galaxy, are well-detected out to 8 microns. However, only the dusty starburst HR10 is detected in the far-infrared. All three EROs have stellar masses of a few times 10^11 M(sun). Using evolutionary model fits to their multiband photometry, we predict the infrared colors of similar EROs at 1<z<2. We find that blueward of observed 10 microns, the two ERO classes are virtually indistinguishable photometrically. Deep spectroscopy and 24 micron data allow the classes to be separated. 
Daniel Stern (1), Ranga-Ram Chary (2), Peter Eisenhardt (1), Leonidas   Moustakas (1) ((1) JPL/Caltech, (2) SSC/Caltech)