The properties of 70 μm-selected high-redshift galaxies in the Extended Groth Strip

Abstract

We examine the infrared properties of 43 high‐redshift (0.1 < z < 1.2), infrared‐luminous galaxies in the Extended Groth Strip (EGS), selected by a deep 70 μm survey with the Multiband Imaging Photometer on Spitzer (MIPS). In addition and with reference to starburst‐type spectral energy distributions (SEDs), we derive a set of equations for estimating the total infrared luminosity (LIR) in the range 8–1000 μm using photometry from at least one MIPS band. 42 out of 43 of our sources' optical/infrared SEDs (λobserved < 160 μm) are starburst type, with only one object displaying a prominent power‐law near‐infrared continuum. For a quantitative analysis, models of radiation transfer in dusty media are fit on to the infrared photometry, revealing that the majority of galaxies are represented by high extinction, Av > 35, and for a large fraction (∼50 per cent) the SED turns over into the Rayleigh–Jeans regime at wavelengths longward of 90 μm. For comparison, we also fit semi‐empirical templates based on local galaxy data; however, these underestimate the far‐infrared SED shape by a factor of at least 2 and in extreme cases up to 10 for the majority (∼70 per cent) of the sources. Further investigation of SED characteristics reveals that the mid‐infrared (70/24 μm) continuum slope is decoupled from various galaxy properties such as the total infrared luminosity and far‐infrared peak, quantified by the L160/L70 ratio. In view of these results, we propose that these high‐redshift galaxies have different properties to their local counterparts, in the sense that large amounts of dust cause heavy obscuration and are responsible for an additional cold emissive component, appearing as a far‐infrared excess in their SEDs.

UV-TO-FIR ANALYSIS OF SPITZER /IRAC SOURCES IN THE EXTENDED GROTH STRIP. II. PHOTOMETRIC REDSHIFTS, STELLAR MASSES, AND STAR FORMATION RATES

“…The blue line joins the median values of SFR TIR (24)/SFR TIR (8,24,70) per luminosity bin (similarly to the colored stars in the top-left panel of Figure 19). The black stars depict the ratio of SFRs for galaxies in common with the sample of Symeonidis et al (2008, filled stars at z < 1.2) and Huang et al (2009, open stars at z ∼ 2). The IR-based SFR for these sources is one of the most accurate available at the moment, and it is in overall good agreement with our estimates with SFR TIR .…”

Gas fraction and star formation efficiency atz< 1.0

“…We chose to compute their F FIR fluxes similarly, with a linear combination of these two fluxes, since all sources are ultraluminous objects; i.e., their spectral energy distributions (SEDs) fall into the same category, and this gives comparable results to those of SED fitting (e.g. Symeonidis et al 2008). The FIR-toradio ratio q = log([F FIR /(3.75 × 10 12 Hz)]/[ f ν (1.4 GHz)]) has been computed for sources where radio data are available; the radio fluxes are listed in Table 2.…”

Fueling the central engine of radio galaxies

“…The 70 μm luminosity can be used to establish lower and upper limits on the SFR (Seymour et al 2011;Symeonidis et al 2008;Kennicutt 1998a) The upper limit is obtained assuming that all the 70 μm emission is due to star formation. Using the 70 μm flux of 3C 293 measured by Dicken et al (2010) with Spitzer (F 70 μm = 1.29 × 10 −11 erg s −1 cm −2 ), we obtained 5.8 < SFR < 6.0 M yr −1 .…”