The Faint Radio Sky: Radio Astronomy Becomes Mainstream
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Astronomy Astrophysics Review manuscript No. (will be inserted by the editor) The faint radio sky: radio astronomy becomes mainstream Paolo Padovani Received: June 7, 2016 / Accepted: August 9, 2016 Abstract Radio astronomy has changed. For years it stud- 2.4 Star-forming galaxies . 6 ied relatively rare sources, which emit mostly non-thermal 2.5 FR0radiogalaxies ................... 7 radiation across the entire electromagnetic spectrum, i.e. ra- 3 Radionumbercounts ..................... 7 dio quasars and radio galaxies. Now it is reaching such faint 3.1 Number count basics . 7 3.2 Observed radio number counts and their complications 8 flux densities that it detects mainly star-forming galaxies and 4 The bright radio sky population . 10 the more common radio-quiet active galactic nuclei. These 5 The faint radio sky population . 11 sources make up the bulk of the extragalactic sky, which 5.1 Source classification . 11 has been studied for decades in the infrared, optical, and X- 5.2 The importance of selection effects........... 14 ray bands. I follow the transformation of radio astronomy 5.3 Radio number counts by population . 15 by reviewing the main components of the radio sky at the 5.4 Luminosity functions . 16 5.4.1 SFGs ...................... 17 bright and faint ends, the issue of their proper classifica- 5.4.2 RLAGN .................... 17 tion, their number counts, luminosity functions, and evolu- 5.4.3 RQAGN .................... 18 tion. The overall “big picture” astrophysical implications of 5.4.4 SFGsandAGN................. 18 these results, and their relevance for a number of hot top- 5.5 Evolution ........................ 18 ics in extragalactic astronomy, are also discussed. The fu- 5.5.1 SFGs ...................... 19 ture prospects of the faint radio sky are very bright, as we 5.5.2 RLAGN .................... 19 5.5.3 RQAGN .................... 20 will soon be flooded with survey data. This review should 6 The big picture: what does it all mean? . 20 be useful to all extragalactic astronomers, irrespective of 6.1 Astrophysics of faint radio sources . 20 their favourite electromagnetic band(s), and even stellar as- 6.1.1 SFGs and cosmic star formation history . 20 tronomers might find it somewhat gratifying. 6.1.2 RL AGN and quiescent galaxies . 21 arXiv:1609.00499v1 [astro-ph.GA] 2 Sep 2016 6.2 The origin of radio emission in (sub-mJy) RQ AGN . 21 Keywords Radio continuum: galaxies · Galaxies: active · 6.3 Radio emitting AGN in the larger context . 23 Galaxies: starburst · Quasars: general · Galaxies: statistics · 7 Thefuture........................... 23 surveys 7.1 Newradiofacilities . 23 7.2 How many sources, and of what type, will the new ra- diofacilitiesdetect?. 24 Contents 7.2.1 Predictions ................... 24 7.2.2 Source classification in the era of the SKA and its precursors . 25 1 Introduction.......................... 2 7.3 The astrophysical impact of future radio observations . 26 2 The main components of the radio sky . 3 2.1 Radiogalaxies...................... 3 7.3.1 SFGs and cosmic star formation history . 26 2.2 Radio quasars and blazars . 4 7.3.2 Galaxyevolution . 26 2.3 Radio-quietAGN .................... 5 7.3.3 WhydoRLAGNexist? . 27 7.3.4 Andmore.................... 28 P. Padovani 8 Conclusions .......................... 29 European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 8.1 Messages to all astronomers . 29 Garching bei M¨unchen, Germany 8.2 Messages to radio astronomers . 29 Tel.: +49-89-32006478 Abbreviations ........................... 29 E-mail: [email protected] References............................. 30 2 Paolo Padovani Fig. 1 The radio sky at 4.85 GHz above an optical photograph of the NRAO site in Green Bank, West Virginia (USA). The former 300-foot (a) 3C 31 telescope made this image, which is about 45◦ across. Increasing radio brightness is indicated by lighter shades to indicate how the sky would appear to someone with a ”radio eye” 300 feet (∼ 91 metres) in diame- ter. The flux density limit is ∼ 25 mJy (Gregory et al., 1996). Copyright NRAO. 1 Introduction Theradiosky is verydifferentfrom the optical sky.When we look at the sky with the naked eye, we practically see only stars. These approximate blackbody radiators and therefore their emission covers a relatively narrow range of frequen- cies, centred at values ranging from the ultraviolet (UV) to (b) Fornax A the infrared (IR), depending on the star temperature. There- Fig. 2 (a): large scale radio map at 1.4 GHz of 3C fore, most bright stars are extremely faint at radio frequen- 31, showing filamentary plumes extending over 400 kpc cies. Figure 1 displays the radio sky as it would appear to from the galaxy. Copyright NRAO 1996. Image from http://www.cv.nrao.edu/∼abridle/images.htm. (b): Radio emission someone with a “radio eye” with a diameter equal to that of (orange) associated with the giant elliptical galaxy NGC1316 (centre the former National Radio Astronomy Observatory (NRAO) of the image), consisting of two large radio lobes, each extending over Green Bank 300-foot radio telescope. Most of the “dots” ∼ 180 kpc. Image courtesy of NRAO/AUI and J. M. Uson. in the figure, which are unresolved radio sources, are actu- ally distant (z ≈ 0.8; Condon, 1989) luminous radio galax- AB ∼ 29 for point sources, contains at least 10,000 objects, ies (RGs) and quasars. The very few extended sources are almost all of them galaxies (Beckwith et al., 2006). Never- mostly Galactic supernova remnants. Radio emission from theless, these galaxies are very different from those seen in RGs and quasars is due to ultra-relativistic (E ≫ m c2) elec- e the radio “bright”1 sky. Radio quasars and RGs, in fact, are trons moving in a magnetic field and thereby emitting syn- somewhat rare, atypical, mostly non-thermal sources across chrotron radiation, which, unlike blackbody emission, can the entire electromagnetic spectrum, in which a large frac- cover a very large range in frequency,reaching ∼ 10 decades tion of the total emission comes from relativistic jets, that in some sources. is streams of plasma with speeds getting close to the speed As one goes fainter by using telescopes, galaxies take of light, and associated lobes (Fig. 2). Most of the galax- over even in the optical sky: for AB & 20 − 22 mag, de- pending on the filter, galaxies outnumber stars by a large 1 The standard flux density unit in radio astronomy is the Jansky −23 −2 −1 −1 margin (Windhorst et al., 2011). The Hubble Ultra Deep (Jy), which is equivalent to 10 erg cm s Hz . By today’s stan- dards, strong radio sources have S & 1 Jy, intermediate ones have 1 Field (HUDF), which covers 11 arcmin2 in four filters (B r mJy . S r . 1 Jy, while weak radio sources are below the mJy (soon to z) down to approximately uniform limiting magnitudes µJy) level. The faint radio sky: radio astronomy becomes mainstream 3 ies detected in the HUDF, on the other hand, are undergoing 2 The main components of the radio sky episodes of star formation (SF) and thereforeare strong ther- mal emitters. I describe here the main properties of the sources, which 2 populatethe radio sky, namely galaxies (radio and star-form- This review aims to discuss very recent developments ing), radio quasars and blazars, and radio-quiet AGN. These in our understanding of the faint radio sky and how our ra- properties are also extremely important for a proper classifi- dio view of the Universe has changed and got much similar cation of faint radio sources (Sect. 5.1). A recent review of to the optical one. By going radio faint one is in fact detect- 3 24 −1 z < 0.7 AGN with radio powers P1.4GHz > 10 W Hz is ing the bulk of the active galactic nuclei (AGN) population, given by Tadhunter (2016). and not only the small minority of radio quasars and RGs, and also plenty of star-forming galaxies (SFGs). These de- velopments are having (or should have) a strong effect on 2.1 Radio galaxies radio astronomy, but should also change the perception that astronomers working in other bands have of it. One of the Normal galaxies, with GHz radio powers . 4 × 1020 W main messages of this review, in fact, is that radio astron- Hz−1, are thought to have their radio emission dominated omy is not a “niche” activity but is extremely relevant also by synchrotron radiation from interstellar relativistic elec- to more classical aspects of extragalactic astronomy, such as trons (Phillips et al., 1996; Sadler et al., 1989). RGs are in- star formation and galaxy evolution. stead associated with relativistic jets extending well beyond I wrote this review primarily with non radio-astronomers the host galaxy (see Fig. 2), which is typically a giant ellip- in mind but I believe that some radio astronomers might still tical. They are characterised by GHz radio powers & 1022 be not fully aware of their changing landscape and espe- W Hz−1 (e.g., Sadler et al., 1989; Ledlow & Owen, 1996), cially of what is in store for them. Therefore, this review which represents the faint end of the RG LF (e.g., Urry & should be useful to all extragalactic astronomers irrespec- Padovani, 1995; van Velzen et al., 2012; Capetti & Rai- tive of their preferred electromagnetic band(s). Stellar as- teri, 2015) and, therefore, is a natural threshold for “radio- tronomers might still find some satisfaction in learning that loudness” in galaxies (some papers have suggested the pres- the faint radio sky is dominated by SF related processes ence of non-thermal, jet emission in early type galaxies as (which is another “take home” message)! faint as 1020 WHz−1: e.g., Balmaverde & Capetti, 2006). Fa- naroff & Riley (1974) recognized that RGs separate into two The structure of this review is as follows: Sect.