Spitzer 24micron Observations of Optical/Near-IR Selected Extremely Red Galaxies: Evidence for Assembly of Massive Galaxies at z approximately 1 - 2?
We carried out the direct measurement of the fraction of dusty sources in a sample of extremely red galaxies with (R-K(sub s)) greater than or equal to 5.3 mag and K(sub s) less than 20.2 mag, using from the Spitzer Space Telescope. Combining deep 24 micrometers, K(sub s)- and R-band data over an area of approximately 64 sq.arcmin in the ELAIS N1 field of the Spitzer First Look Survey (FLS), we find that 50 +/- 60% of our ERO sample have measurable 24 micrometer flux above the 3(sigma) flux limit of 40 microns Jy. This flux limit corresponds to a SFR of 12 solar mass/yr at z approximately 1, much mo previous long wavelength measurement. The 24fJ,m-detected EROs have 24-to2.2 and 24-to-0.7micrometr flux ratios consistent with infrared luminous, dusty sources at z approx. 1, and an order of magnitude too red to be explained by an infrared quiescent spiral or a pure old stellar population at any redshift. Some of these 24 micrometer-detected EROs could be AGN, however, the fraction among the whole ERO sample is probably small, 10-20%, as suggested by deep X-ray observations as well as optical spectroscopy. Keck optical spectroscopy of a sample of similarly selected EROs in the FLS field suggests that most of the EROs in ELAIS Nl are probably at z approx. 1.