The star formation history of the Universe as revealed by deep radio observations

Abstract

Discerning the exact nature of the sub-mJy radio population has been historically difficult due to the low luminosity of these sources at most wavelengths. Using deep ground based optical follow-up and observations from the Spitzer Space Telescope we are able to disentangle the radio-selected active galactic nuclei (AGN) and star-forming galaxy (SFG) populations for the first time in a deep multifrequency VLA/MERLIN Survey of the 13 H XMM-Newton/Chandra Deep Field. The discrimination diagnostics include radio morphology, radio spectral index, radio/near-infrared (near-IR) and mid-IR/radio flux density ratios. We are now able to calculate the extragalactic Euclidean normalized source counts separately for AGN and SFGs. We find that while SFGs dominate at the faintest flux densities and account for the majority of the upturn in the counts, AGN still make up around one quarter of the counts at ∼50 μJy (1.4 GHz). Using radio luminosity as an unobscured star formation rate (SFR) measure we are then able to examine the comoving SFR density of the Universe up to z = 3 which agrees well with measures at other wavelengths. We find a rough correlation of SFR with stellar mass for both the sample presented here and a sample of local radio-selected SFGs from the 6df-NVSS survey. This work also confirms the existence of, and provides alternative evidence for, the evolution of distribution of star formation by galaxy mass: 'downsizing'. As both these samples are SFR-selected, this result suggests that there is a maximum SFR for a given galaxy that depends linearly on its stellar mass. The low 'characteristic times' (inverse specific SFR) of the SFGs in our sample are similar to those of the 6dF-NVSS sample, implying that most of these sources are in a current phase of enhanced star formation.

The ATLAS 5.5 GHz survey of the extendedChandraDeep Field South: the second data release

“…Hopkins et al 1998). However, a growing number of studies are finding that lower luminosity AGN, both radio-loud and weakly radio emitting sources (radio-loud and radio-quiet AGN respectively), make a significant contribution to the sub-mJy population (Jarvis & Rawlings 2004;Huynh et al 2008;Seymour et al 2008;Smolčić et al 2008;Padovani et al 2009Padovani et al , 2011Bonzini et al 2013).…”

The eMERGE Survey – I: Very Large Array 5.5 GHz observations of the GOODS-North Field

“…Gruppioni et al 1999;Muxlow et al 2005;Simpson et al 2006;Bondi et al 2007;Seymour et al 2008;Smolčić et al 2009;Bonzini et al 2013;Smolčić et al 2017a). The unexpected detection in deep radio surveys of large numbers of RE-AGN (e.g Heckman & Best 2014), emitting over a wide range of the electromagnetic spectrum, from mid-IR (MIR) to X-rays, but typically radio-quiet, has opened the exciting prospect of studying the whole AGN population through deep radio surveys.…”

Radio Continuum Surveys with Square Kilometre Array Pathfinders

“…Recent results (e.g. Seymour et al 2008;Smolčić et al 2008;Padovani et al 2009), which are summarised in Figure 5, show that both SF galaxies and AGNs contribute significantly to the source counts. About half of these AGNs are radio-quiet, characterised by relatively low radio-to-optical flux density ratios and radio powers, as compared with radio quasars (see also Prandoni 2010).…”

“…Converting this radio luminosity to a SFR depends on a conversion factor which is principally based on the infrared/radio correlation (Yun et al 2001;Bell 2003;Seymour et al 2008). However, this correlation has not yet been well determined at the high end of the luminosity function, since the number of ultraluminous IR galaxies (ULIRGs) in these papers is very small, or at high redshifts, although evidence so far suggests the correlation remains constant .…”