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Published January 20, 2006 | Published + Submitted
Journal Article Open

Star Formation Rates and Extinction Properties of IR-luminous Galaxies in the Spitzer First Look Survey


We investigate the instantaneous star formation rates (SFRs) and extinction properties for a large (N = 274), near-infrared (NIR: 2.2 μm) + mid-infrared (MIR: 24 μm)-selected sample of normal to ultraluminous infrared galaxies (ULIRGs; 10^9 < L_(IR)/L_☉ < 10^(12.5)) with〈 z〉~ 0.8 in the Spitzer Extragalactic First Look Survey (FLS). We combine 24 μm observations with high-resolution Keck DEIMOS spectroscopy to derive optical emission-line (Hα, Hβ, and [O II]) and infrared star formation rates (SFR_(opt) and SFR_(IR), respectively). Comparison of SFR diagnostics reveals a wide extinction range (1.0 < A_V < 4.0 mag) for this sample, even after removing spectroscopic and IRAC color-selected AGN candidates (≈12% of the sample). Objects with SFRs of a few M_☉ yr^(-1) have extinction values consistent with normal spirals (A_V ≈ 1.0 mag). By contrast, LIRGs at z ≳ 1, which comprise a fraction of our sample, have SFR ≈ 100 M_☉ yr^(-1) and a mean A_V ≈ 2.5 mag. This translates to a 97% mean [O II] λλ3727 attentuation and in extreme cases is as high as 99.7%. We derive an IR-luminosity-dependent A^(IR)_V function [A^(IR)_V = 0.75 log(L_(IR)/L_☉) - 6.35 mag] that we use to extinction correct our line luminosities. The resulting correlation between SFR_(IR) and SFR_(opt) has a dispersion of ~0.2 dex (semi-interquartile range). Comparison of the A_V dependence on redshift and LIR reveals that for a fixed L_(IR), there is no significant AV evolution. Comparison to previous studies reveals a mean attenuation that is intermediate between that of local optical/UV- and radio-selected samples with a marginally stronger L_(IR) dependence.

Additional Information

© 2006. The American Astronomical Society. Received 2005 June 19; accepted 2005 September 27. We thank the anonymous referee for insightful comments that have significantly improved this paper. We thank Dave Thomson for his contribution to the observation and reduction of the WIRC data. We would also like to thank Carol Lonsdale, Robert Kennicutt, and Rolf Jansen for early discussions that helped shape this project. This work is based (in part) on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL, Caltech. M. I. was supported by grant R01-2005-000-10610-0 from the Basic Research Program of the Korea Science and Engineering Foundation. We are indebted to Grant Hill, Greg Wirth, and the rest of the Keck observatory staff for their phenomenal observation support. The analysis pipeline used to reduce the DEIMOS data was developed by the DEEP2 group at UC Berkeley with support from NSF grant AST 00-71048. Finally, we wish to recognize and acknowledge the significant cultural role and reverence that the summit of Mauna Kea has within the indigenous Hawaiian community. We are grateful for the opportunity to conduct observations from this mountain.

Attached Files

Published - Choi_2006_ApJ_637_227.pdf

Submitted - 0509894.pdf


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