The Vla Survey of Chandra Deep Field South. V. Evolution and Luminosity Functions of Sub-Millijansky Radio Sources and the Issue of Radio Emission in Radio-Quiet Active Galactic Nuclei

Abstract

We present the evolutionary properties and luminosity functions of the radio sources belonging to the Chandra Deep Field South VLA survey, which reaches a flux density limit at 1.4 GHz of 43 µJy at the field center and redshift ∼ 5, and which includes the first radio-selected complete sample of radio-quiet active galactic nuclei (AGN). We use a new, comprehensive classification scheme based on radio, farand near-IR, optical, and X-ray data to disentangle star-forming galaxies from AGN and radio-quiet from radio-loud AGN. We confirm our previous result that star-forming galaxies become dominant only below 0.1 mJy. The sub-millijansky radio sky turns out to be a complex mix of star-forming galaxies and radio-quiet AGN evolving at a similar, strong rate; non-evolving low-luminosity radio galaxies; and declining radio powerful (P 3 × 10 24 W Hz −1 ) AGN. Our results suggest that radio emission from radio-quiet AGN is closely related to star formation. The detection of compact, high brightness temperature cores in several nearby radio-quiet AGN can be explained by the co-existence of two components, one non-evolving and AGN-related and one evolving and star-formation-related. Radio-quiet AGN are an important class of sub-millijansky sources, accounting for ∼ 30% of the sample and ∼ 60% of all AGN, and outnumbering radio-loud AGN at 0.1 mJy. This implies that future, large area sub-millijansky surveys, given the appropriate ancillary multi-wavelength data, have the potential of being able to assemble vast samples of radio-quiet AGN by-passing the problems of obscuration, which plague the optical and soft X-ray bands.

Observational evidence that positive and negative AGN feedback depends on galaxy mass and jet power

“…Helou et al 1985) and also typical radio-quiet AGN (e.g. Padovani et al 2011;Sargent et al 2010) and the mean q = −0.38 for a sample of radioloud AGN from Evans et al (2005). For the separation between 'radio-normal' and radio-excess sources we have picked the mean q = 1.2 value that perfectly separates the RLQs and RQQs in our sample.…”

Star formation properties of sub-mJy radio sources

“…The faint radio sky is a complex mixture of star forming galaxies (SFG) and active galactic nuclei (AGN) (e.g., Mauch & Sadler 2007;Smolčić et al 2008;Padovani et al 2009Padovani et al , 2011. Indeed, radio emission can either be due to the relativistic jets powered by the AGN or to synchrotron emission from electrons accelerated by supernova explosions.…”

An overview of jet-mode AGN feedback and the prospects for studying its cosmic evolution with LOFAR

“…As the simulations from Wilman et al (2008) show (see Figure 3), above a flux of 1 mJy at 120 MHz LOFAR can detect hundreds of systems per square degree at z ∼ 1 and tens at z ∼ 5. The challenge will be to separate the RL AGN (LERGs and HERGs) from SFGs and RQ AGN (Padovani et al 2011;Bonzini et al 2013), both of which might be related with SF processes (Padovani et al 2011) and not important for AGN feedback studies. Additionally, although radio-loud HERGs are often located in cluster environments at high redshift (Crawford & Fabian 1996;Worrall et al 2001;Siemiginowska et al 2010;Russell et al 2012), their importance to jet-mode AGN feedback is not clear, since the radiative mode is expected to be the dominate feedback mechanism in these sources.…”

“…From the radio perspective, AGN can be separated in radio-loud (RL) AGN and radio-quiet (RQ) AGN., but the physical separation between the two categories is still unclear (e.g., star forming processes, BH spin, BH mass, etc. ; Dunlop & McLure 2003;Tchekhovskoy et al 2010;Padovani et al 2011). The RL AGN of the radiative mode are also known as high-excitation radio galaxies (HERGs) and have strong QSO and Seyfert-like lines (Hine & Longair 1979;Best & Heckman 2012) and powerful jets, usualy with a FRII morphology.…”