The GLEAM 4-Jy Sample: The brightest radio-sources in the southern sky

2019 ◽  
Vol 15 (S356) ◽  
pp. 375-375
Author(s):  
Sarah White
Keyword(s):  
Radio Emission ◽  
Active Galactic Nuclei ◽  
Visual Inspection ◽  
Low Frequency ◽  
Galactic Nuclei ◽  
Radio Sources ◽  
Relativistic Jets ◽  
The Galaxy ◽  
Murchison Widefield Array ◽  
Past Activity

AbstractLow-frequency radio emission allows powerful active galactic nuclei (AGN) to be selected in a way that is unaffected by dust obscuration and orientation of the jet axis. It also reveals past activity (e.g. radio lobes) that may not be evident at higher frequencies. Currently, there are too few “radio-loud” galaxies for robust studies in terms of redshift-evolution and/or environment. Hence our use of new observations from the Murchison Widefield Array (the SKA-Low precursor), over the southern sky, to construct the GLEAM 4-Jy Sample (1,860 sources at S151MHz > 4 Jy). This sample is dominated by AGN and is 10 times larger than the heavily relied-upon 3CRR sample (173 sources at S178MHz > 10 Jy) of the northern hemisphere. In order to understand how AGN influence their surroundings and the way galaxies evolve, we first need to correctly identify the galaxy hosting the radio emission. This has now been completed for the GLEAM 4-Jy Sample – through repeated visual inspection and extensive checks against the literature – forming a valuable, legacy dataset for investigating relativistic jets and their interplay with the environment.

scholarly journals Polarization Distributions in Compact Radio Sources

1986 ◽  
Vol 119 ◽  
pp. 141-147
Author(s):  
David H. Roberts ◽  
John F. C. Wardle
Keyword(s):  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Radio Sources ◽  
Bl Lacertae ◽  
The Galaxy ◽  
5 Ghz

We present milliarcsecond-resolution 5 GHz polarization maps of several active galactic nuclei: one epoch each for the quasar 3C345, the galaxy 3C120, and the BL Lacertae object 0735+178, and two epochs for the BL Lacertae object OJ287.

scholarly journals Radio spectral properties of cores and extended regions in blazars in the MHz regime

2019 ◽  
Vol 490 (4) ◽  
pp. 5798-5806 ◽  
Author(s):  
D d’Antonio ◽  
M Giroletti ◽  
G Giovannini ◽  
A Maini
Keyword(s):  
Radio Frequency ◽  
Active Galactic Nuclei ◽  
High Frequency ◽  
Spectral Properties ◽  
Gamma Ray ◽  
Angular Resolution ◽  
Low Frequency ◽  
Galactic Nuclei ◽  
High Angular Resolution ◽  
Murchison Widefield Array

ABSTRACT Low-frequency radio surveys allow in-depth studies and new analyses of classes of sources that were previously known and characterized only in other bands. In recent years, low radio frequency observations of blazars have become available as a result of new surveys, such as the GaLactic and Extragalactic All-sky Murchison Widefield Array (MWA) survey (GLEAM). We search for gamma-ray blazars in a low-frequency (ν < 240 MHz) survey, to characterize the spectral properties of the spatial components. We cross-correlate GLEAM with the fourth catalogue of active galactic nuclei (4LAC) detected by the Fermi satellite. This improves on previous works by using a low-frequency catalogue that is wider and deeper, with a better spectral coverage and the latest and most sensitive gamma-ray source list. Compared with a previous study based on the commissioning survey, the detection rate increased from 35 to 70 per cent. We include data from the Australia Telescope 20-GHz (AT20G) survey in order to extract high-frequency high-angular resolution information about the radio cores of blazars. We find low radio frequency counterparts for 1274 out of 1827 blazars in the range of 72–231 MHz. Blazars have flat spectra at the ∼100-MHz regime, with a mean spectral index α = −0.44 ± 0.01 (assuming Sν ∝ να). Low synchrotron peaked objects have a flatter spectrum than high synchrotron peaked objects. Low frequency radio and gamma-ray emissions show a significant but scattered correlation. The ratio between lobe and core radio emission in gamma-ray blazars is smaller than previously estimated.

scholarly journals The ATLAS 9.0 GHz survey of the extended Chandra Deep Field South: the faint 9.0 GHz radio population

2019 ◽  
Vol 491 (3) ◽  
pp. 3395-3410 ◽  
Author(s):  
M T Huynh ◽  
N Seymour ◽  
R P Norris ◽  
T Galvin
Keyword(s):  
Radio Emission ◽  
Active Galactic Nuclei ◽  
Radio Source ◽  
Galactic Nuclei ◽  
Radio Galaxies ◽  
Significant Fraction ◽  
Radio Sources ◽  
Star Forming ◽  
Source Populations ◽  
Compact Array

ABSTRACT We present a new image of the 9.0 GHz radio emission from the extended Chandra Deep Field South. A total of 181 h of integration with the Australia Telescope Compact Array has resulted in a 0.276 deg2 image with a median sensitivity of ∼20 µJy beam−1 rms, for a synthesized beam of 4.0 × 1.3 arcsec. We present a catalogue of the 9.0 GHz radio sources, identifying 70 source components and 55 individual radio galaxies. Source counts derived from this sample are consistent with those reported in the literature. The observed source counts are also generally consistent with the source counts from simulations of the faint radio population. Using the wealth of multiwavelength data available for this region, we classify the faint 9 GHz population and find that 91 per cent are radio-loud active galactic nuclei (AGNs), 7 per cent are radio-quiet AGNs, and 2 per cent are star-forming galaxies. The 9.0 GHz radio sources were matched to 5.5 and 1.4 GHz sources in the literature and we find a significant fraction of flat or inverted spectrum sources, with 36 per cent of the 9 GHz sources having $\alpha _{5.5\,\mathrm{ GHz}}^{9.0\,\mathrm{ GHz}}$ > −0.3 (for S ∝ να). This flat or inverted population is not well reproduced by current simulations of radio source populations.

scholarly journals Deep LOFAR 150 MHz imaging of the Boötes field: Unveiling the faint low-frequency sky

2018 ◽  
Vol 620 ◽  
pp. A74 ◽  
Author(s):  
E. Retana-Montenegro ◽  
H. J. A. Röttgering ◽  
T. W. Shimwell ◽  
R. J. van Weeren ◽  
I. Prandoni ◽  
...  
Keyword(s):  
Active Galactic Nuclei ◽  
Angular Resolution ◽  
Low Frequency ◽  
Galactic Nuclei ◽  
Radio Sources ◽  
Low Frequencies ◽  
Star Forming ◽  
Radio Imaging ◽  
Deep Survey ◽  
Systematic Effects

We have conducted a deep survey (with a central rms of 55 μJy) with the LOw Frequency ARray (LOFAR) at 120–168 MHz of the Boötes field, with an angular resolution of 3.98″ × 6.45″, and obtained a sample of 10 091 radio sources (5σ limit) over an area of 20 deg2. The astrometry and flux scale accuracy of our source catalog is investigated. The resolution bias, incompleteness and other systematic effects that could affect our source counts are discussed and accounted for. The derived 150 MHz source counts present a flattening below sub-mJy flux densities, that is in agreement with previous results from high- and low- frequency surveys. This flattening has been argued to be due to an increasing contribution of star-forming galaxies and faint active galactic nuclei. Additionally, we use our observations to evaluate the contribution of cosmic variance to the scatter in source counts measurements. The latter is achieved by dividing our Boötes mosaic into 10 non-overlapping circular sectors, each one with an approximate area of 2 deg2. The counts in each sector are computed in the same way as done for the entire mosaic. By comparing the induced scatter with that of counts obtained from depth observations scaled to 150 MHz, we find that the 1σ scatter due to cosmic variance is larger than the Poissonian errors of the source counts, and it may explain the dispersion from previously reported depth source counts at flux densities S <  1 mJy. This work demonstrates the feasibility of achieving deep radio imaging at low-frequencies with LOFAR.

scholarly journals Physical Conditions in Active Galactic Nuclei

1989 ◽  
Vol 134 ◽  
pp. 535-536
Author(s):  
V. S. Artyukh
Keyword(s):  
Magnetic Field ◽  
Synchrotron Radiation ◽  
Active Galactic Nuclei ◽  
Relativistic Electron ◽  
Angular Resolution ◽  
Low Frequency ◽  
Galactic Nuclei ◽  
Radio Sources ◽  
Electron Number ◽  
Physical Conditions

Self-absorption of synchrotron radiation in compact radio sources gives rise to a low-frequency cutoff in their spectra. To study this effect it is necessary to observe the radio sources at meter wavelengths with angular resolution < 1″. Such observations of compact radio sources located in active galactic nuclei lead to estimates of magnetic field strengths and relativistic electron number densities in the AGNs.

scholarly journals Absorbed relativistic jets in radio-quiet narrow-line Seyfert 1 galaxies

2020 ◽  
Vol 636 ◽  
pp. A64
Author(s):  
M. Berton ◽  
E. Järvelä ◽  
L. Crepaldi ◽  
A. Lähteenmäki ◽  
M. Tornikoski ◽  
...  
Keyword(s):  
Radio Emission ◽  
Active Galactic Nuclei ◽  
High Frequency ◽  
Galactic Nuclei ◽  
Large Array ◽  
Narrow Line ◽  
Relativistic Jets ◽  
Ionized Gas ◽  
Very Large Array ◽  
Relativistic Jet

Narrow-line Seyfert 1 (NLS1) galaxies are peculiar active galactic nuclei. Most of them do not show strong radio emission, but seven radio-quiet (or radio-silent) NLS1s have recently been detected flaring multiple times at 37 GHz by the Metsähovi Radio Telescope, indicating relativistic jets in these peculiar sources. We observed them with the Karl G. Jansky Very Large Array (JVLA) in A configuration at 1.6, 5.2, and 9.0 GHz. Our results show that these sources are either extremely faint or not detected in the JVLA bands. At these frequencies, the radio emission from their relativistic jet must be absorbed, either through synchrotron self-absorption as it occurs in gigahertz-peaked sources, or more likely, through free-free absorption by a screen of ionized gas associated with starburst activity or shocks. Our findings cast new shadows on the radio-loudness criterion, which seems to be increasingly frequently a misleading parameter. New high-frequency and high-resolution radio observations are essential to test our hypotheses.

scholarly journals Polarimetric Properties of Blazars Caught by the WEBT

Galaxies ◽  
2021 ◽  
Vol 9 (2) ◽  
pp. 42
Author(s):  
Claudia M. Raiteri ◽  
Massimo Villata
Keyword(s):  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Plasma Jets ◽  
Line Of Sight ◽  
Optical Behaviour ◽  
Relativistic Jets ◽  
Comprehensive Picture ◽  
Multi Wavelength ◽  
Extreme Variability ◽  
Continuous Range

Active galactic nuclei come in many varieties. A minority of them are radio-loud, and exhibit two opposite prominent plasma jets extending from the proximity of the supermassive black hole up to megaparsec distances. When one of the relativistic jets is oriented closely to the line of sight, its emission is Doppler beamed and these objects show extreme variability properties at all wavelengths. These are called “blazars”. The unpredictable blazar variability, occurring on a continuous range of time-scales, from minutes to years, is most effectively investigated in a multi-wavelength context. Ground-based and space observations together contribute to give us a comprehensive picture of the blazar emission properties from the radio to the γ-ray band. Moreover, in recent years, a lot of effort has been devoted to the observation and analysis of the blazar polarimetric radio and optical behaviour, showing strong variability of both the polarisation degree and angle. The Whole Earth Blazar Telescope (WEBT) Collaboration, involving many tens of astronomers all around the globe, has been monitoring several blazars since 1997. The results of the corresponding data analysis have contributed to the understanding of the blazar phenomenon, particularly stressing the viability of a geometrical interpretation of the blazar variability. We review here the most significant polarimetric results achieved in the WEBT studies.

scholarly journals Understanding the origin of radio outflows in Seyfert galaxies using radio polarimetry

2019 ◽  
Vol 15 (S356) ◽  
pp. 247-251
Author(s):  
Biny Sebastian ◽  
Preeti Kharb ◽  
Christopher P. O’ Dea ◽  
Jack F. Gallimore ◽  
Stefi A. Baum ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Radio Emission ◽  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Seyfert Galaxy ◽  
Seyfert Galaxies ◽  
Starburst Galaxies ◽  
Galactic Magnetic Fields ◽  
5 Ghz

AbstractThe role of starburst winds versus active galactic nuclei (AGN) jets/winds in the formation of the kiloparsec scale radio emission seen in Seyferts is not yet well understood. In order to be able to disentangle the role of various components, we have observed a sample of Seyfert galaxies exhibiting kpc-scale radio emission suggesting outflows, along with a comparison sample of starburst galaxies, with the EVLA B-array in polarimetric mode at 1.4 GHz and 5 GHz. The Seyfert galaxy NGC 2639, shows highly polarized secondary radio lobes, not observed before, which are aligned perpendicular to the known pair of radio lobes. The additional pair of lobes represent an older epoch of emission. A multi-epoch multi-frequency study of the starburst-Seyfert composite galaxy NGC 3079, reveals that the jet together with the starburst superwind and the galactic magnetic fields might be responsible for the well-known 8-shaped radio lobes observed in this galaxy. We find that many of the Seyfert galaxies in our sample show bubble-shaped lobes, which are absent in the starburst galaxies that do not host an AGN.

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