Abstract
Abstract Ultra-deep radio surveys are an invaluable probe of dust-obscured star formation, but require a clear understanding of the relative contribution from radio active galactic nuclei (AGNs) to be used to their fullest potential. We study the composition of the μJy radio population detected in the Karl G. Jansky Very Large Array COSMOS-XS survey based on a sample of 1540 sources detected at 3 GHz over an area of ∼350 arcmin2. This ultra-deep survey consists of a single pointing in the well-studied COSMOS field at both 3 and 10 GHz and reaches rms sensitivities of 0.53 and 0.41 μJy beam−1, respectively. We find multiwavelength counterparts for 97% of radio sources, based on a combination of near-UV/optical to sub-millimeter data, and through a stacking analysis at optical/near-IR wavelengths we further show that the sources lacking such counterparts are likely to be high-redshift in nature (typical z ∼ 4−5). Utilizing the multiwavelength data over COSMOS, we identify AGNs through a variety of diagnostics and find these to make up 23.2 ± 1.3% of our sample, with the remainder constituting uncontaminated star-forming galaxies. However, more than half of the AGNs exhibit radio emission consistent with originating from star formation, with only 8.8 ± 0.8% of radio sources showing a clear excess in radio luminosity. At flux densities of ∼30 μJy at 3 GHz, the fraction of star formation-powered sources reaches ∼90%, and this fraction is consistent with unity at even lower flux densities. Overall, our findings imply that ultra-deep radio surveys such as COSMOS-XS constitute a highly effective means of obtaining clean samples of star formation-powered radio sources.
Highlights
One of the key quests in extragalactic astronomy is to understand how the build-up and subsequent evolution of galaxies proceeds across cosmic time
In order to disentangle true extended sources from blended ones, we examined whether the Gaussian components making up an island can be individually crossmatched to separate sources in the recent Super-deblended catalog over COSMOS (Jin et al 2018, see Section 2.2), which contains MIR to radio photometry based on positional priors from a combination of Ks, Spitzer/MIPS 24 μm, and Very Large Array (VLA) 1.4 and 3 GHz observations
The Ultra-faint Radio Population In Figure 11, we show both the fractional and cumulative contribution of the different radio populations as a function of approximately z ∼ 1, illustrating the well-known result that the redshift distribution of radio sources is nearly independent of flux density (Condon 1989)
Summary
One of the key quests in extragalactic astronomy is to understand how the build-up and subsequent evolution of galaxies proceeds across cosmic time. Two populations of AGNs can be distinguished in radio surveys: sources that can be identified through an excess in radio emission compared to what is expected from the FIRC ( referred to as radio-excess AGNs) and sources that have radio emission compatible with originating from star formation, but can be identified as AGNs through any of several multiwavelength diagnostics (Padovani et al 2011; Bonzini et al 2013; Heckman & Best 2014; Delvecchio et al 2017; Smolčić et al 2017a; Calistro Rivera et al 2017).8 The latter class generally exhibit AGN-related emission throughout the bulk of their non-radio spectral energy distribution (SED), in the form of, e.g., strong X-ray emission or mid-IR (MIR) dust emission from a warm torus surrounding the AGN (e.g., Evans et al 2006; Hardcastle et al 2013). The radio spectral index, α, is defined through Sν represents the flux density at frequency ν
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