VLBA+GBT observations of the COSMOS field and radio source counts at 1.4 GHz

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.

Download Full-text

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.

Download Full-text

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.

Download Full-text

CHILES VERDES: Radio Variability at an Unprecedented Depth and Cadence in the COSMOS Field

The Astrophysical Journal ◽

10.3847/1538-4357/ac2239 ◽

2021 ◽

Vol 923 (1) ◽

pp. 31

Author(s):

Sumit K. Sarbadhicary ◽

Evangelia Tremou ◽

Adam J. Stewart ◽

Laura Chomiuk ◽

Charee Peters ◽

...

Keyword(s):

Active Galactic Nuclei ◽

Flux Density ◽

Galactic Nuclei ◽

Density Variation ◽

Time Variable ◽

Host Galaxy ◽

Radio Sources ◽

Star Forming ◽

Lower Variability ◽

Cross Matching

Abstract Although it is well established that some extragalactic radio sources are time-variable, the properties of this radio variability, and its connection with host galaxy properties, remain to be explored—particularly for faint sources. Here we present an analysis of radio variable sources from the CHILES Variable and Explosive Radio Dynamic Evolution Survey (CHILES VERDES)—a partner project of the 1.4 GHz COSMOS H i Large Extragalactic Survey. CHILES VERDES provides an unprecedented combination of survey depth, duration, and cadence, with 960 hr of 1–2 GHz continuum VLA data obtained over 209 epochs between 2013 and 2019 in a 0.44 deg2 section of the well-studied extragalactic deep field, COSMOS. We identified 18 moderate-variability sources (showing 10%–30% flux density variation) and 40 lower-variability sources (2%–10% flux density variation). They are mainly active galactic nuclei (AGNs) with radio luminosities in the range of 1022–1027 W Hz−1, based on cross-matching with COSMOS multiwavelength catalogs. The moderate-variability sources span redshifts z = 0.22–1.56, have mostly flat radio spectra (α > −0.5), and vary on timescales ranging from days to years. The lower-variability sources have similar properties, but generally have higher radio luminosities than the moderate-variability sources, extending to z = 2.8, and have steeper radio spectra (α < −0.5). No star-forming galaxy showed statistically significant variability in our analysis. The observed variability likely originates from scintillation on short (∼week) timescales, and Doppler-boosted intrinsic AGN variability on long (month–year) timescales.

Download Full-text

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.

Download Full-text

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).

Download Full-text

The Over-estimation of the Flux Density of Weak Radio Sources

Publications of the Astronomical Society of Australia ◽

10.1017/s132335800001170x ◽

1969 ◽

Vol 1 (5) ◽

pp. 233-234

Author(s):

H. S. Murdoch

Keyword(s):

Monte Carlo ◽

Flux Density ◽

Lower Limit ◽

Error Distribution ◽

Radio Sources ◽

Systematic Bias ◽

Digital Recording ◽

Analysis Techniques ◽

Monte Carlo Techniques ◽

The Past

The over-estimation of the flux density of radio sources near the lower limit of a survey has often been considered in the past. The use of digital recording and analysis techniques now enables a quantitative approach to the problem. Monte Carlo techniques may be used to determine the error distribution, including any systematic bias.

Download Full-text

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

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/263.1.98 ◽

1993 ◽

Vol 263 (1) ◽

pp. 98-122 ◽

Cited By ~ 78

Author(s):

C. R. Benn ◽

M. Rowan-Robinson ◽

R. G. McMahon ◽

T. J. Broadhurst ◽

A. Lawrence

Keyword(s):

Radio Source ◽

Radio Sources ◽

Source Population

Download Full-text

A closer look at the deep radio sky: Multi-component radio sources at 3 GHz VLA-COSMOS

Astronomy and Astrophysics ◽

10.1051/0004-6361/201832982 ◽

2019 ◽

Vol 627 ◽

pp. A142

Author(s):

E. Vardoulaki ◽

E. F. Jiménez Andrade ◽

A. Karim ◽

M. Novak ◽

S. K. Leslie ◽

...

Keyword(s):

Radio Emission ◽

Radio Continuum ◽

Continuum Emission ◽

High Angular Resolution ◽

Radio Sources ◽

Source Population ◽

Data Set ◽

Star Forming ◽

Large Project ◽

Automatic Code

Context. Given the unprecedented depth achieved in current large radio surveys, we are starting to probe populations of radio sources that have not been studied in the past. However, identifying and categorising these objects, differing in size, shape and physical properties, is becoming a more difficult task. Aims. In this data paper we present and characterise the multi-component radio sources identified in the VLA-COSMOS Large Project at 3 GHz (0.75 arcsec resolution, 2.3 μJy beam−1 rms), i.e. the radio sources which are composed of two or more radio blobs. Methods. The classification of objects into multi-components was done by visual inspection of 351 of the brightest and most extended blobs from a sample of 10,899 blobs identified by the automatic code BLOBCAT. For that purpose we used multi-wavelength information of the field, such as the 1.4 GHz VLA-COSMOS data and the Ultra Deep Survey with the VISTA telescope (UltraVISTA) stacked mosaic available for COSMOS. Results. We have identified 67 multi-component radio sources at 3 GHz: 58 sources with active galactic nucleus (AGN) powered radio emission and nine star-forming galaxies. We report eight new detections that were not observed by the VLA-COSMOS Large Project at 1.4 GHz, due to the slightly larger area coverage at 3 GHz. The increased spatial resolution of 0.75 arcsec has allowed us to resolve (and isolate) multiple emission peaks of 28 extended radio sources not identified in the 1.4 GHz VLA-COSMOS map. We report the multi-frequency flux densities (324 MHz, 325 MHz, 1.4 GHz & 3 GHz), star formation rates, and stellar masses of these objects. We find that multi-component objects at 3 GHz VLA-COSMOS inhabit mainly massive galaxies (> 1010.5 M⊙). The majority of the multi-component AGN lie below the main sequence of star-forming galaxies (SFGs), in the green valley and the quiescent region. Furthermore, we provide detailed descriptions of the objects and find that amongst the AGN there are two head-tail, ten core-lobe, nine wide-angle-tail (WAT), eight double-double or Z-/X-shaped, three bent-tail radio sources, and 26 symmetric sources, while amongst the SFGs we find the only star-forming ring seen in radio emission in COSMOS. Additionally, we report a large number (32 out of 58) of disturbed/bent multi-component AGN, 18 of which do not lie within X-ray groups in COSMOS (redshift range 0.08 ≤ z < 1.53). Conclusion. The high angular resolution and sensitivity of the 3 GHz VLA-COSMOS data set give us the opportunity to identify peculiar radio structures and sub-structures of multi-component objects, and relate them to physical phenomena such as AGN or star-forming galaxies. This study illustrates the complexity of the μJy radio-source population; at the sensitivity and resolution of 3 GHz VLA-COSMOS, the radio structures of AGN and SFG both emitting radio continuum emission, become comparable in the absence of clear, symmetrical jets. Thus, disentangling the AGN and SFG contributions using solely radio observations can be misleading in a number of cases. This has implications for future surveys, such as those done by square kilometre array (SKA) and precursors, which will identify hundreds of thousands of multi-component objects.

Download Full-text

An Evolutionary Model for Quasars

Australian Journal of Physics ◽

10.1071/ph760097 ◽

1976 ◽

Vol 29 (2) ◽

pp. 97

Author(s):

Natalia V Zotov ◽

W Davidson

Keyword(s):

Population Dynamics ◽

Radio Source ◽

Flux Density ◽

Evolutionary Model ◽

Source Population ◽

Evolutionary Trend ◽

Wide Range ◽

Unique Model ◽

Luminosity Evolution ◽

Good Agreement

A simple, though not necessarily unique, model for the cosmic population dynamics of quasars is presented. This model incorporates a double evolutionary trend: (i) a strong luminosity evolution, the presence of which we have discussed previously (Zotov and Davidson 1970, 1973) and (ii) an evolution of total coordinate density. This double trend is the same as that already found to be applicable to the total radio source population, if account is taken only of radio characteristics (Davidson et at. 1971). The predictions of the scheme are compared with five sets of data, covering a wide range of frequencies and flux density limits, and they are found to give good agreement in all cases.

Download Full-text