Star Formation Rates in Lyman Break Galaxies: Radio Stacking of LBGs in the COSMOS Field and the Sub‐μJy Radio Source Population

Abstract

We present an analysis of the radio properties of large samples of Lyman Break Galaxies (LBGs) at z ∼ 3, 4, and 5 from the COSMOS field. The median stacking analysis yields a statistical detection of the z ∼ 3 LBGs (U-band dropouts), with a 1.4 GHz flux density of 0.90 ± 0.21µJy. The stacked emission is unresolved, with a size < 1", or a physical size < 8kpc. The total star formation rate implied by this radio luminosity is 31 ± 7 M ⊙ year −1 , based on the radio-FIR correlation in low redshift star forming galaxies. The star formation rate derived from a similar analysis of the UV luminosities is 17 M ⊙ year −1 , without any correction for UV dust attenuation. The simplest conclusion is that the dust attenuation factor is 1.8 at UV wavelengths. However, this factor is considerably ⋆ Based on observations in the COSMOS Legacy Survey including those taken on the HST, Keck, NRAO-VLA, Subaru, KPNO 4m, CTIO 4m, and CFHT 3.6m.-2smaller than the standard attenuation factor ∼ 5, normally assumed for LBGs. We discuss potential reasons for this discrepancy, including the possibility that the dust attenuation factor at z ≥ 3 is smaller than at lower redshifts. Conversely, the radio luminosity for a given star formation rate may be systematically lower at very high redshift. Two possible causes for a suppressed radio luminosity are: (i) increased inverse Compton cooling of the relativistic electron population due to scattering off the increasing CMB at high redshift, or (ii) cosmic ray diffusion from systematically smaller galaxies. The radio detections of individual sources are consistent with a radio-loud AGN fraction of 0.3%. One source is identified as a very dusty, extreme starburst galaxy (a 'submm galaxy').

UV-CONTINUUM SLOPES ATz∼ 4-7 FROM THE HUDF09+ERS+CANDELS OBSERVATIONS: DISCOVERY OF A WELL-DEFINED UV COLOR-MAGNITUDE RELATIONSHIP FORz⩾ 4 STAR-FORMING GALAXIES

“…Chary & Pope (2011) argue for low dust extinction in high-redshift galaxies based upon extragalactic background light stacking results. It also been argued that the Carilli et al (2008) stacking results of the radio emission in z ∼ 3 galaxies from COSMOS also suggest lower values for the dust extinction, but Reddy et al (2012a) The above arguments aside, there is circumstantial evidence that dust obscuration in high-redshift galaxies is likely at least as large as implied by the Meurer et al (1999) relationship (see also discussion in Section 6.3). Perhaps the strongest piece of evidence for substantial dust extinction is provided by the large number of high mass (∼1-3×10 10 M ) galaxies found at z ∼ 5-6 in the GOODS fields (Eyles et al 2005;Yan et al 2005Yan et al , 2006.…”

The Astrophysics of Star Formation Across Cosmic Time at >10 GHz with the Square Kilometre Array

“…Additionally, global investigations of starburst radio spectra (M 82, NGC 253, and NGC 4945) similarly suggest 80% thermal fractions at 33 GHz (Peel et al 2011). As we push the detection of star-forming galaxies out to significantly higher redshifts, we anticipate even larger thermal fractions due to increased inverse-Compton losses to cosmic-ray electrons off of the CMB with redshift, whose energy density scales as U CMB ∼ (1+z) 4 ; non-thermal emission from galaxies should become severely suppressed with increasing redshift, making this frequency range ideal for accurate estimates of star formation activity at high z, unbiased by dust (Carilli et al 2008;Murphy 2009).…”

Cosmic Rays: A View Into Galactic Interactions and the New Physics

“…One of the well-established tracers of the star-formation rate (SFR) is the Hα emission of a galaxy [21], since the Hα emission traces HII regions around young O and B type stars. Other tracers used to determine SFR are the infrared emission of the heated dust and radio observations at 1.4 GHz [22,23,24]. Radio continuum emission from star forming and starburst galaxies originates from…”