The faint radio sky: radio astronomy becomes mainstream

Abstract

Radio astronomy has changed. For years it studied relatively rare sources, which emit mostly non-thermal radiation across the entire electromagnetic spectrum, i.e. radio quasars and radio galaxies. Now it is reaching such faint flux densities that it detects mainly star-forming galaxies and the more common radio-quiet active galactic nuclei. These sources make up the bulk of the extragalactic sky, which has been studied for decades in the infrared, optical, and Xray bands. I follow the transformation of radio astronomy by reviewing the main components of the radio sky at the bright and faint ends, the issue of their proper classification, their number counts, luminosity functions, and evolution. The overall "big picture" astrophysical implications of these results, and their relevance for a number of hot topics in extragalactic astronomy, are also discussed. The future prospects of the faint radio sky are very bright, as we will soon be flooded with survey data. This review should be useful to all extragalactic astronomers, irrespective of their favourite electromagnetic band(s), and even stellar astronomers might find it somewhat gratifying.

Radio Selection of the Most Distant Galaxy Clusters

“…This is quite promising news for future large area radio surveys, which will reach very faint flux densities (Padovani 2016). For example, even in phase1, SKA will likely carry out a full-sky survey to few μJy sensitivity and with~ 2 resolution at 1-2GHz, while covering ∼1000 deg 2 to ∼1 μJy depth and subarcsec resolution (Prandoni & Seymour 2015), surpassing the depth and sensitivity of our 3 GHz VLA data, allowing us in principle to find W16-like clusters over the whole sky, and perhaps much further away and in great numbers.…”

Active Galactic Nuclei at All Wavelengths and from All Angles

“…Namely, while the bright radio sky (f r 1 mJy around 1 GHz, where 1 Jy is 10 −23 erg cm −2 s −1 Hz −1 ) is populated mostly by radio galaxies (RGs) and radio quasars, that is largely non-thermal sources, at faint radio flux densities we are now detecting mainly star-forming galaxies (SFGs) and the more common non-jetted AGN (see Padovani, 2016, for a review). This change is also apparent by looking at the Euclidean normalized 1.4 GHz source counts, which show an upturn around ≈ 0.1 mJy (see Figure 1).…”

“…The Faint Radio Sky: f r 1 mJy Sources include both non-jetted AGN and a quickly decreasing fraction of jetted AGN (see Figure 7 of Padovani, 2016); the former are the dominant type. Selection requires data in various bands to single out the AGN, especially the non-jetted ones, from the SFGs, as the optical counterparts are very faint (as detailed in section 5.1 of Padovani, 2016).…”

“…The Faint Radio Sky: f r 1 mJy Sources include both non-jetted AGN and a quickly decreasing fraction of jetted AGN (see Figure 7 of Padovani, 2016); the former are the dominant type. Selection requires data in various bands to single out the AGN, especially the non-jetted ones, from the SFGs, as the optical counterparts are very faint (as detailed in section 5.1 of Padovani, 2016). Apart from probing the jet in jetted AGN, radio emission is mostly due to star formation (through synchrotron emission from relativistic plasma accelerated in supernova remnants) in non-jetted ones (at least for radio-selected sources: see discussion in section 6.2 of Padovani, 2016).…”

“…Selection requires data in various bands to single out the AGN, especially the non-jetted ones, from the SFGs, as the optical counterparts are very faint (as detailed in section 5.1 of Padovani, 2016). Apart from probing the jet in jetted AGN, radio emission is mostly due to star formation (through synchrotron emission from relativistic plasma accelerated in supernova remnants) in non-jetted ones (at least for radio-selected sources: see discussion in section 6.2 of Padovani, 2016). There is some chance of contamination from SFGs (especially if no X-ray detection is available).…”