Spitzer 24 Micron Observations of Optical/Near-Infrared-Selected Extremely Red Galaxies: Evidence for Assembly of Massive Galaxies at z~1-2?

Details Spitzer 24 Micron Observations of Optical/Near-Infrared-Selected Extremely Red Galaxies: Evidence for Assembly of Massive Galaxies at z~1-2? Spitzer 24 Micron Observations of Optical/Near-Infrared-Selected Extremely Red Galaxies: Evidence for Assembly of Massive Galaxies at z~1-2?

Sep 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004apjs..154...75y&link_type=abstract

The Astrophysical Journal Supplement Series, Volume 154, Issue 1, pp. 75-79.

Astronomy and Astrophysics

Astronomy

35

Galaxies: Bulges, Galaxies: Evolution, Galaxies: High-Redshift, Galaxies: Starburst, Infrared: Galaxies

Scientific paper

We carried out direct measurement of the fraction of dusty sources in a sample of extremely red galaxies with (R-Ks)>=5.3 mag and Ks<20.2 mag, using 24 μm data from the Spitzer Space Telescope. Combining deep 24 μm Ks- 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 Msolar 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, νLν(12μm), of 3×1010Lsolar at z~1. Using the correlation between IRAS νLν(12μm) and infrared luminosity LIR(8-1000μm), we infer that the IR> of the 24 μm-detected EROs is 3×1011 and 1×1012Lsolar at z=1.0and1.5, respectively, similar to that of local luminous infrared galaxies (LIRGs) and ultraluminous infrared galaxies (ULIRGs). The corresponding SFR would be roughly 50-170 Msolar 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 5×109to2×1010Msolar. 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.

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