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Title: Spitzer 24 micron observations of optical/near-infrared–selected extremely red galaxies : evidence for assembly of massive galaxies at Z ~1–2?
Authors: Yan, Lin
Choi, Philip I.
Fadda, D.
Marleau, F. R.
Soifer, B. T.
Im, M.
Armus, L.
Frayer, D. T.
Storrie-Lombardi, L. J.
Thompson, D. J.
Teplitz, H. I.
Helou, G.
Appleton, P. N.
Chapman, S.
Fan, F.
Heinrichsen, I.
Lacy, M.
Shupe, D. L.
Squires, G. K.
Surace, J.
Wilson, G.
Keywords: galaxies
high redshift
Issue Date: Sep-2004
Publisher: American Astronomical Society
Citation: The Astrophysical Journal Supplement Series, 154:75–79, 2004 September
Abstract: 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 M[suns] 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(12 µm), of 3x 10¹°L[suns] at z ~1. Using the correlation between IRAS νLν(12 µm) and infrared luminosity LIR(8-1000 µm), we infer that the (LIR) of the 24 µm– detected EROs is 3 x 10¹¹ and 1 x 10¹² L[suns] 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[suns] 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 x 109 to 2 x 10¹º M[suns]. 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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