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) ≥ 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 arcmin² 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 × 10¹⁰ L_⊙ at z ∼ 1. Using the correlation between IRAS νL_ν(12 μm) and infrared luminosity L_IR(8-1000 μm), we infer that the of the 24 μm-detected EROs is 3 × 10¹¹ and 1 × 10¹² 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 10⁸ yr as inferred for the local LIRGs and ULIRGs, the lower limit on the masses of these 24 μm-detected EROs is 5 × 10⁹ to 2 × 10¹⁰ 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.