SpitzerObservations of the Prototypical Extremely Red Objects HR 10 and LBDS 53W091: Separating Dusty Starbursts from Old Elliptical Galaxies

We present Spitzer Space Telescope observations of the well-studied extremely red objects (EROs) HR 10 and LBDS 53W091 from 3.6 to 160 μm. These galaxies are the prototypes of the two primary classes of EROs: dusty starbursts and old, evolved galaxies, respectively. Both galaxies, as well as LBDS 53W069, another example of an old, quiescent galaxy, are well detected out to 8 μm. However, only the dusty starburst HR 10 is detected in the far-infrared. All three EROs have stellar masses of a few × 10 11 M ⊙ . 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 μm, the two ERO classes are virtually indistinguishable photometrically. Deep spectroscopy and 24 μm data allow the classes to be separated.