Spectroscopy of faint radio sources: the nature of the sub-mJy radio-source population

We present very long baseline interferometry (VLBI) observations of 179 radio sources in the COSMOS field with extremely high sensitivity using the Green Bank Telescope (GBT) together with the Very Long Baseline Array (VLBA) (VLBA+GBT) at 1.4 GHz, to explore the faint radio population in the flux density regime of tens of μJy. Here, the identification of active galactic nuclei (AGN) is based on the VLBI detection of the source, meaning that it is independent of X-ray or infrared properties. The milli-arcsecond resolution provided by the VLBI technique implies that the detected sources must be compact and have large brightness temperatures, and therefore they are most likely AGN (when the host galaxy is located at z ≥ 0.1). On the other hand, this technique only allows us to positively identify when a radio-active AGN is present, in other words, we cannot affirm that there is no AGN when the source is not detected. For this reason, the number of identified AGN using VLBI should be always treated as a lower limit. We present a catalogue containing the 35 radio sources detected with the VLBA+GBT, ten of which were not previously detected using only the VLBA. We have constructed the radio source counts at 1.4 GHz using the samples of the VLBA and VLBA+GBT detected sources of the COSMOS field to determine a lower limit for the AGN contribution to the faint radio source population. We found an AGN contribution of >40−75% at flux density levels between 150 μJy and 1 mJy. This flux density range is characterised by the upturn of the Euclidean-normalised radio source counts, which implies a contribution of a new population. This result supports the idea that the sub-mJy radio population is composed of a significant fraction of radio-emitting AGN, rather than solely by star-forming galaxies, in agreement with previous studies.

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Unveiling the high-frequency radio source population with the AT20G survey

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314003780 ◽

2013 ◽

Vol 9 (S304) ◽

pp. 205-208

Author(s):

Elizabeth K. Mahony

Keyword(s):

Radio Source ◽

Flux Density ◽

High Frequency ◽

Gamma Ray ◽

Low Frequency ◽

Radio Sources ◽

Source Population ◽

Frequency Source ◽

Galaxy Population ◽

5 Ghz

AbstractUntil recently, the radio sky above 5 GHz was relatively unexplored. This has changed with the completion of the Australia Telescope 20 GHz survey (AT20G; Murphy et al., 2010); a blind survey of the southern sky down to a limiting flux density of 40 mJy. The AT20G survey provides by far the largest and most complete sample of high-frequency radio sources yet obtained, offering new insights into the nature of the high-frequency active galaxy population. Whilst the radio data provides a unique sample of objects, these data alone are insufficient to completely constrain models of radio source properties and the evolution of radio galaxies. Complementary multiwavelength data is vital in understanding the physical properties of the central black hole.In this talk I will provide a brief overview of the AT20G survey, followed by a discussion of the multiwavelength properties of the high-frequency source population. In particular, I will focus on the optical properties of AT20G sources, which are very different to those of a low-frequency selected sample, along with the gamma-ray properties where we find a correlation between high-frequency radio flux density and gamma-ray flux density. By studying the multiwavelength properties of a large sample of high-frequency radio sources we gain a unique perspective on the inner dynamics of some of the most active AGN.

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Cosmological Studies from Radio Source Samples

Symposium - International Astronomical Union ◽

10.1017/s0074180900168512 ◽

2002 ◽

Vol 199 ◽

pp. 34-49 ◽

Cited By ~ 1

Author(s):

S. Rawlings

Keyword(s):

Star Formation ◽

Radio Source ◽

Accretion Rate ◽

Simple Calculation ◽

Physical Models ◽

Radio Sources ◽

Source Population ◽

Low Frequencies ◽

Close Link ◽

Redshift Surveys

I review some recent cosmological studies based on redshift surveys of radio sources selected at low frequencies. The accretion rate onto the central black hole is identified as the basis of a crude physical division of the low-frequency population into two sub-populations: the first comprises ‘Eddington-tuned’ (high accretion rate) quasars and their torus-hidden counterparts; the second comprises ‘starved quasars’ like M87. There exist remarkable similarities between the shapes and evolutionary behaviours of the luminosity functions of radio sources and radio-quiet quasars; all luminous AGN are ∼ 300—times rarer at epochs corresponding to z ∼ 0 than at z ∼ 2.5. I argue that any evidence that quasars were intrinsically rarer at z ∼ 5 than at z ∼ 2.5 is as yet both tentative and indirect. A simple calculation suggests that the radio source population has been over-looked as a potentially important contributor to the entropy budget of the Universe. A recent sub-mm survey of radio sources is used to demonstrate a connection between the events which trigger jets and intense bursts of star formation, and a close link between the histories of star formation and AGN activity is proposed. I discuss the aims and methods of future large redshift surveys of radio sources, emphasising the importance of dovetailing these with the development of robust physical models for radio sources and their epoch-dependent environments.

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Cosmological Size Evolution of Extragalactic Radio Sources

Symposium - International Astronomical Union ◽

10.1017/s0074180900081869 ◽

1996 ◽

Vol 175 ◽

pp. 563-566

Author(s):

Ashok K. Singal

Keyword(s):

Radio Source ◽

Size Distribution ◽

Flux Density ◽

Angular Size ◽

Radio Sources ◽

Source Population ◽

Extragalactic Radio Source ◽

Quarter Century ◽

Extragalactic Radio Sources ◽

Size Evolution

The study of cosmological evolution of the sizes of extragalactic radio sources started about a quarter century back. From the very first angular size-redshift (θ-z) plots (Miley 1968, 1971; Legg 1970) and angular size-flux density (θ-S) plots (Swarup 1975; Kapahi 1975) it became evident that some sort of cosmic epoch-dependent evolution in the size distribution for the population of extragalactic radio source needs to be proposed; the sources at earlier epochs appeared on the average to have smaller physical sizes. However, a suitable luminosity-linear size (P-l) correlation among the radio source population could also explain the observations, without invoking a size evolution with redshift. The only reliable way to disentangle these two separate effects is to investigate the size distribution in the luminosity-redshift plane, where one could examine not only the l-z relation for a given luminosity class, but could also check for a P-l correlation in a given redshift bin.

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The Physics of Radio Sources and Cosmology

Symposium - International Astronomical Union ◽

10.1017/s0074180900016314 ◽

1977 ◽

Vol 74 ◽

pp. 343-352

Author(s):

P. A. G. Scheuer

Keyword(s):

Radio Source ◽

Radio Sources ◽

Source Population ◽

General Survey ◽

Present Epoch ◽

Hubble Expansion ◽

Definite Answer ◽

Compact Sources

There are two important questions in which the physics of radio sources impinges upon cosmology. The first is whether the large apparent expansion velocities of certain compact sources can be explained satisfactorily within the hypothesis that their red-shifts are due to the Hubble expansion. The second is the whole broad question of the evolution of the radio source population with epoch. I do not have a new and convincing answer to the first, and the second is pretty nebulous, since we do not even understand radio sources at the present epoch very well. So I shall not present a general survey: instead, I shall use my allotted time to discuss a smaller question to which one can now give a fairly definite answer.

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High Frequency Peakers

Publications of the Astronomical Society of Australia ◽

10.1071/as03005 ◽

2003 ◽

Vol 20 (1) ◽

pp. 79-84 ◽

Cited By ~ 19

Author(s):

Daniele Dallacasa

Keyword(s):

Radio Source ◽

High Frequency ◽

Peak Frequency ◽

Source Size ◽

Host Galaxy ◽

Radio Sources ◽

Inner Region

AbstractThere is quite a clear anticorrelation between the intrinsic peak frequency and the overall radio source size in compact steep spectrum (CSS) and gigahertz peaked spectrum (GPS) radio sources. This feature is interpreted in terms of synchrotron self-absorption (although free–free absorption may play a role as well) of the radiation emitted by a small radio source which is growing within the inner region of the host galaxy. This leads to the hypothesis that these objects are young and that the radio source is still developing/expanding within the host galaxy itself.Very young radio sources must have the peak in their radio spectra occurring above a few tens of gigahertz, and for this reason they are termed high frequency peakers (HFPs). These newly born radio sources must be very rare given that they spend very little time in this stage. Ho = 100 km s−1 Mpc−1 and qo = 0.5 are used throughout this paper.

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Nowhere to Hide: Radio-faint AGN in GOODS-N field

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833399 ◽

2018 ◽

Vol 619 ◽

pp. A48 ◽

Cited By ~ 5

Author(s):

J. F. Radcliffe ◽

M. A. Garrett ◽

T. W. B. Muxlow ◽

R. J. Beswick ◽

P. D. Barthel ◽

...

Keyword(s):

Galaxy Evolution ◽

Cosmic Time ◽

Primary Beam ◽

Radio Sources ◽

Source Population ◽

Wide Field ◽

Beam Model ◽

Phase Errors ◽

Long Baseline

Context. The occurrence of active galactic nuclei (AGN) is critical to our understanding of galaxy evolution and formation. Radio observations provide a crucial, dust-independent tool to study the role of AGN. However, conventional radio surveys of deep fields ordinarily have arc-second scale resolutions often insufficient to reliably separate radio emission in distant galaxies originating from star-formation and AGN-related activity. Very long baseline interferometry (VLBI) can offer a solution by identifying only the most compact radio emitting regions in galaxies at cosmological distances where the high brightness temperatures (in excess of 105 K) can only be reliably attributed to AGN activity. Aims. We present the first in a series of papers exploring the faint compact radio population using a new wide-field VLBI survey of the GOODS-N field. This will expand upon previous surveys, permitting the characterisation of the faint, compact radio source population in the GOODS-N field. The unparalleled sensitivity of the European VLBI Network (EVN) will probe a luminosity range rarely seen in deep wide-field VLBI observations, thus providing insights into the role of AGN to radio luminosities of the order 1022 WHz−1 across cosmic time. Methods. The newest VLBI techniques are used to completely cover an entire 7′̣5 radius area to milliarcsecond resolutions, while bright radio sources (S > 0.1 mJy) are targeted up to 25′ from the pointing centre. Multi-source self-calibration, and a primary beam model for the EVN array are used to correct for residual phase errors and primary beam attenuation respectively. Results. This paper presents the largest catalogue of VLBI detected sources in GOODS-N comprising of 31 compact radio sources across a redshift range of 0.11–3.44, almost three times more than previous VLBI surveys in this field. We provide a machine-readable catalogue and introduce the radio properties of the detected sources using complementary data from the e-MERLIN Galaxy Evolution survey (eMERGE).

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Extended Structure in High-Redshift Radio Sources

Symposium - International Astronomical Union ◽

10.1017/s0074180900027327 ◽

1982 ◽

Vol 97 ◽

pp. 59-60

Author(s):

P. J. Duffett-Smith ◽

A. Purvis

Keyword(s):

Radio Source ◽

High Redshift ◽

Radio Sources ◽

List Type ◽

Type Number ◽

Extended Structure ◽

Structure Changes ◽

Lower Luminosity ◽

Source Structure

We have compared measurements of several hundred 3C and 4C radio sources at large redshifts to investigate how radio-source structure changes over a factor of 5–10 in luminosity. Our results show that for z ≳ 0.6: (i)most sources (both 3C and 4C) have hotspots about 3.5 kpc in size (Ho = 50 km s−1 Mpc−1, Ω = 1);(ii)lower-luminosity sources (bottom of 4C) have less-extended outer lobes.

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The Milliarcsecond Core of 3C147 at 6 cm

Symposium - International Astronomical Union ◽

10.1017/s0074180900077573 ◽

1984 ◽

Vol 110 ◽

pp. 29-30 ◽

Cited By ~ 2

Author(s):

E. Preuss ◽

W. Alef ◽

N. Whyborn ◽

P.N. Wilkinson ◽

K.I. Kellermann

Keyword(s):

Radio Source ◽

Radio Sources ◽

Radio Structure ◽

Extragalactic Radio Sources ◽

North East ◽

The North ◽

Vlbi Observations ◽

Spectrum Radio ◽

The One

3C147 is a compact (≲1″), steep spectrum radio source identified with a quasar at z = 0.545 (0″.001 = 7.4 pc; c/Ho = 6000 Mpc and qo = 0.5). The radio structure shown by VLBI observations at 18 cm (Readhead & Wilkinson, 1980; Simon et al., this volume), at 50 cm (Wilkinson et al., 1977), and at 90 cm (Simon et al., 1980 and 1983) shows a bright ‘core’ (60 pc at one end of a ‘jet’ ~0″.2 (1.5 kpc) in length oriented in p.a. ~ −130°. In this sense 3C147 is typical of the one-sided ‘core-jet’ structures commonly found in the centres of other extragalactic radio sources. However, MERLIN observations at 6 cm (Wilkinson, this vol.) and VLA observations at 2 cm (Crane & Kellermann, unpubl.; Readhead et al., 1980) show a larger elongated feature extending ~0″.5 (3.7 kpc) to the North East of the bright core in p.a. ~25° or on the opposite side to the 0″.2 jet.

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