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Published October 2011 | Published
Journal Article Open

Dust-obscured star formation and the contribution of galaxies escaping UV/optical color selections at z ~ 2


Context. A substantial amount of the stellar mass growth across cosmic time occurred within dust-enshrouded environments. So far, identification of complete samples of distant star-forming galaxies from the short wavelength range has been strongly biased by the effect of dust extinction. Nevertheless, the exact amount of star-forming activity that took place in high-redshift dusty galaxies but that has currently been missed by optical surveys has barely been explored. Aims. Our goal is to determine the number of luminous star-forming galaxies at 1.5 ≲ z ≲ 3 that are potentially missed by the traditional color selection techniques because of dust extinction. We also aim at quantifying the contribution of these sources to the IR luminosity and cosmic star formation density at high redshift. Methods. We based our work on a sample of 24 μm sources brighter than 80 μJy and taken from the Spitzer survey of the COSMOS field. Almost all of these sources have accurate photometric redshifts. We applied to this mid-IR selected sample the BzK and BM/BX criteria, as well as the selections of the IRAC peakers and the Optically-Faint IR-bright (OFIR) galaxies. We analyzed the fraction of sources identified with these techniques. We also computed 8 μm rest-frame luminosity from the 24 μm fluxes of our sources, and considering the relationships between L_(8μm) and L_(Pa_α) and between L_(8μm) and L_IR, we derived ρIR and then ρ_SFR for our MIPS sources. Results. The BzK criterion offers an almost complete (~90%) identification of the 24 μm sources at 1.4 < z < 2.5. In contrast, the BM/BX criterion misses 50% of the MIPS sources. We attribute this bias to the effect of extinction, which reddens the typical colors of galaxies. The contribution of these two selections to the IR luminosity density produced by all the sources brighter than 80 μJy are on the same order. Moreover the criterion based on the presence of a stellar bump in their spectra (IRAC peakers) misses up to 40% of the IR luminosity density, while only 25% of the IR luminosity density at z ~ 2 is produced by OFIR galaxies characterized by extreme mid-IR to optical flux ratios. Conclusions. Color selections of distant star-forming galaxies must be used with care given the substantial bias they can suffer. In particular, the effect of dust extinction strongly affects the completeness of identifications at the bright end of the bolometric luminosity function, which implies large and uncertain extrapolations to account for the contribution of dusty galaxies missed by these selections. In the context of forthcoming facilities that will operate at long wavelengths (e.g., JWST, ALMA, SAFARI, EVLA, SKA), this emphasizes the importance of minimizing the extinction biases when probing the activity of star formation in the early Universe.

Additional Information

© 2011 ESO. Article published by EDP Sciences. Received 24 March 2011; Accepted 31 May 2011. Published online 06 October 2011. It is a pleasure to acknowledge the contribution from all our colleagues in the COSMOS collaboration. More information on the COSMOS survey is available at http://www.astro.caltech.edu/cosmos. This work is based on observations made with the Spitzer Space Telescope, a facility operated by NASA/JPL. Financial supports were provided by NASA through contracts Nos. 1289085, 1310136, 1282612, and 1298231 issued by the Jet Propulsion Laboratory. We also want to warmly thank our referee for his/her critical review of the manuscript, as well as Tanio Díaz-Santos for useful discussions related to our work. We are finally grateful to Marc Sauvage for his help and for his useful and numerous comments.

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Published - Riguccini2011p16336Astron_Astrophys.pdf


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