Spitzer observations 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 mu m. 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 mu m. However, only the dusty starburst HR 10 is detected in the far-infrared. All three EROs have stellar masses of a few x 10(11) M-circle dot. 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 mu m, the two ERO classes are virtually indistinguishable photometrically. Deep spectroscopy and 24 mu m data allow the classes to be separated.