A MULTIWAVELENGTH STUDY OF A SAMPLE OF 70 μm SELECTED GALAXIES IN THE COSMOS FIELD. I. SPECTRAL ENERGY DISTRIBUTIONS AND LUMINOSITIES

Abstract

We present a large robust sample of 1503 reliable and unconfused 70 µm selected sources from the multiwavelength data set of the Cosmic Evolution Survey (COSMOS). Using the Spitzer IRAC and MIPS photometry, we estimate the total infrared luminosity, L IR (8-1000 µm), by finding the best fit template from several different template libraries. The long wavelength 70 and 160 µm data allow us to obtain a reliable estimate of L IR , accurate to within 0.2 and 0.05 dex, respectively. The 70 µm data point enables a significant improvement over the luminosity estimates possible with only a 24 µm detection. The full sample spans a wide range in L IR , L IR ≈ 10 8 − 10 14 L , with a median luminosity of 10 11.4 L . We identify a total of 687 luminous, 303 ultraluminous, and 31 hyperluminous infrared galaxies (LIRGs, ULIRGs, and HyLIRGs) over the redshift range 0.01 < z < 3.5 with a median redshift of 0.5. Presented here are the full spectral energy distributions for each of the sources compiled from the extensive multiwavelength data set from the ultraviolet (UV) to the far-infrared (FIR). A catalog of the general properties of the sample (including the photometry, redshifts, and L IR ) are included with this paper. We find that the overall shape of the spectral energy distribution (SED) and trends with L IR (e.g., IR color temperatures and optical-IR ratios) are similar to what has been seen in studies of local objects, however, our large sample allows us to see the extreme spread in UV to nearinfrared (NIR) colors spanning nearly three orders of magnitude. In addition, using SED fits we find possible evidence for a subset of cooler ultraluminous objects than observed locally. However, until direct observations at longer wavelengths are obtained, the peak of emission and the dust temperature cannot be well constrained. We use these SEDs, along with the deep radio and X-ray coverage of the field, to identify a large sample of candidate active galactic nuclei (AGN). We find that the fraction of AGN increases strongly with L IR , as it does in the local universe, and that nearly 70% of ULIRGs and all HyLIRGs likely host a powerful AGN.

Hidden starbursts and active galactic nuclei at 0 < z < 4 from theHerschel-VVDS-CFHTLS-D1 field: Inferences on coevolution and feedback

“…Still, the redshift evolution of these fractions was investigated in J13 by comparing their observed 70 μm-selected sample to an equivalently selected sample drawn from Sloan Digital Sky Survey (SDSS), finding essentially no evolution in the AGN fraction of IR-selected sources as a function of L TIR between the redshifts of 0.05 < z < 1. Here, we broadly reaffirm the results found by Kartaltepe et al (2010a) and J13 using a sample limited to the host galaxies of more powerful AGN. Considering only a SPIRE volumelimited sample between 0 < z < 1.5, a redshift range in which A90, page 32 of 39 a volume-limited sample does not restrict us only to ULIRGs, the fraction of galaxies hosting any type of AGN at the highest SFRs (equal to log(L TIR ) ∼ 12.5) is ∼60%, roughly equivalent to that found at the same L TIR in J13 and identical within the errors to the fraction computed for low-redshift IR-selected sources.…”

“…This is not the first time this trend has been observed. In two different studies covering different fields, Kartaltepe et al (2010a) and Juneau et al (2013;hereafter J13) observed that the fraction of galaxies which host AGN, AGN selected by a wide variety of methods, was a strong function of L TIR (see also the review of this trend for galaxies hosting X-ray AGN in Lapi et al 2014). These samples were, however, selected at shorter wavelengths, meaning that the contamination to L TIR from the AGN was more problematic than it is in a SPIRE-selected sample.…”

Encoding of the infrared excess in theNUVrKcolor diagram for star-forming galaxies

“…Indeed, the conversion of the 24 µm luminosity to total IR luminiosity becomes uncertain for ULIRGs (i.e., for L IR ≥ 10 12 L , see Bavouzet et al 2008;Goto et al 2011). Also, the merger nature of ULIRGs at low z (Kartaltepe et al 2010) makes predictions inaccurate in absence of a complete set of far-IR observations.…”

Disentangling star formation and AGN activity in powerful infrared luminous radio galaxies at 1 <z< 4

“…The broad wavelength coverage from UV to FIR allows us to probe multiple regimes of energy and therefore to better isolate Kartaltepe et al (2010), making use of Spitzer data. The blue squares denote the GOODS samples from Elbaz et al (2011) and the dark green dots represent the selection from Symeonidis et al (2013), both making use of Herschel data.…”