Spitzer 24 micron observations of optical/near-infrared-selected extremely red galaxies:: Evidence for assembly of massive galaxies at z ∼ 1-2?

We carried out direct measurement of the fraction of dusty sources in a sample of extremely red galaxies with (R - K-s) greater than or equal to 5.3 mag and K-s < 20.2 mag, using 24 μm data from the Spitzer Space Telescope. Combining deep 24 μm K-s- and R-band data over an area of ∼ 64 arcmin(2) in ELAIS N1 of the Spitzer First Look Survey (FLS), we find that 50% +/- 6% of our extremely red object (ERO) sample have measurable 24 μm flux above the 3 σ flux limit of 40 μJy. This flux limit corresponds to a star formation rate (SFR) of 12 M-. yr(-1) at z ∼ 1, much more sensitive than any previous long-wavelength measurement. The 24 μm-detected EROs have 24 μm/2.2 μm and 24 μm/0.7 μm flux ratios consistent with infrared luminous, dusty sources at z ≥ 1, and are 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 μm-detected EROs could be active galactic nuclei; 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 N1 are probably at z ∼ 1. The mean 24 μm flux (167 μJy) of the 24 μm-detected ERO sample roughly corresponds to the rest-frame 12 μm luminosity, νL-ν(12 μm), of 3 x 10(10) L-. at z ∼ 1. Using the correlation between IRAS νLν(12 μm) and infrared luminosity L-IR(8-1000 μm), we infer that the [L-IR] of the 24 μm - detected EROs is 3 x 10(11) and 1 x 10(12) L-. at z = 1.0 and 1.5, respectively, similar to that of local luminous infrared galaxies (LIRGs) and ultraluminous infrared galaxies (ULIRGs). The corresponding SFR would be roughly 50 - 170 M-. yr(-1). If the timescale of this starbursting phase is on the order of 10(8) yr as inferred for the local LIRGs and ULIRGs, the lower limit on the masses of these 24 μm-detected EROs is 5 x 10(9) to 2 x 10(10) M-.. It is plausible that some of the starburst EROs are in the midst of a violent transformation to become massive early type galaxies at the epoch of z ∼ 1-2.