The 6C Survey

A new survey of radio sources at 151 MHz, which has not been described previously, is in progress at Cambridge. There are several of us working on it including Warner, Kenderdine, Waggett, Masson and Mayer. The results of the first observations are at present in a preliminary state but we hope that in time they will form the first part of the 6C survey. The purpose of the survey is not to reach the faintest sources detected so far in aperture synthesis observations but to study moderately faint sources at a low observing frequency and to cover a large part of the northern sky rapidly. The deepest survey made so far at a low frequency is that of Ryle and Neville (1962) at 178 MHz over a region of 50 square degrees near the north celestial pole. The faintest sources detected had flux densities of 0.25 Jy, corresponding to a source density of 104 sr−1. It is already 15 years since that survey, which was the first trial of aperture synthesis using the earth's rotation, and much more is now technically possible. One of the most interesting features of a low frequency survey is its ability to detect preferentially sources with steep radio spectra and to be sensitive to sources of very low surface brightness. We know that in many cases these two properties go together and are associated with old radio sources, or at least with those parts of sources which are old. Many of the weak radio galaxies in nearby clusters are obvious examples of this type of source while the final, and so far unidentified, stages of the development of the most powerful double sources may be exciting candidates for discovery.

Download Full-text

Spatially resolved studies of extragalactic jets in high redshift radio galaxies

Proceedings of the International Astronomical Union ◽

10.1017/s1743921315002239 ◽

2014 ◽

Vol 10 (S313) ◽

pp. 231-235

Author(s):

Leah K. Morabito ◽

Adam Deller ◽

J. B. R. Oonk ◽

Huub Röttgering ◽

George Miley

Keyword(s):

High Redshift ◽

Low Frequency ◽

Radio Galaxies ◽

Radio Sources ◽

Spatially Resolved ◽

Long Baseline ◽

Early Results ◽

Spectrum Radio ◽

Extragalactic Jets ◽

Frequency Survey

AbstractThe correlation between radio spectral steepness and redshift has been successfully used to find high redshift (z ⩾ 2) radio galaxies, but the origin of this relation is unknown. The ultra-steep spectra of high-z radio sources make them ideally suited for studies with the Low Band Antenna of the new Low Frequency Array, which covers 10–80 MHz and has baselines up to about 1300 km. As part of an ongoing survey, we use the longest baselines to map the low-frequency (< 70 MHz) spatial distributions along the jets of 5 bright extended steep spectrum high-z radio sources. From this, we will determine whether the spectra change over these spatially resolved sources, thereby constraining particle acceleration processes. We present early results from our low-frequency survey of ultra-steep spectrum radio galaxies. The first low frequency long baseline images of these objects are presented.

Download Full-text

Remnant radio galaxies discovered in a multi-frequency survey

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2020.49 ◽

2021 ◽

Vol 38 ◽

Author(s):

Benjamin Quici ◽

Natasha Hurley-Walker ◽

Nicholas Seymour ◽

Ross J. Turner ◽

Stanislav S. Shabala ◽

...

Keyword(s):

Surface Brightness ◽

Radio Galaxy ◽

Radio Galaxies ◽

Low Surface Brightness ◽

Show Evidence ◽

Frequency Survey ◽

Square Kilometer Array ◽

Murchison Widefield Array ◽

Limiting Case ◽

Compact Array

Abstract The remnant phase of a radio galaxy begins when the jets launched from an active galactic nucleus are switched off. To study the fraction of radio galaxies in a remnant phase, we take advantage of a $8.31$ deg $^2$ subregion of the GAMA 23 field which comprises of surveys covering the frequency range 0.1–9 GHz. We present a sample of 104 radio galaxies compiled from observations conducted by the Murchison Widefield Array (216 MHz), the Australia Square Kilometer Array Pathfinder (887 MHz), and the Australia Telescope Compact Array (5.5 GHz). We adopt an ‘absent radio core’ criterion to identify 10 radio galaxies showing no evidence for an active nucleus. We classify these as new candidate remnant radio galaxies. Seven of these objects still display compact emitting regions within the lobes at 5.5 GHz; at this frequency the emission is short-lived, implying a recent jet switch off. On the other hand, only three show evidence of aged lobe plasma by the presence of an ultra-steep-spectrum ( $\alpha<-1.2$ ) and a diffuse, low surface brightness radio morphology. The predominant fraction of young remnants is consistent with a rapid fading during the remnant phase. Within our sample of radio galaxies, our observations constrain the remnant fraction to $4\%\lesssim f_{\mathrm{rem}} \lesssim 10\%$ ; the lower limit comes from the limiting case in which all remnant candidates with hotspots are simply active radio galaxies with faint, undetected radio cores. Finally, we model the synchrotron spectrum arising from a hotspot to show they can persist for 5–10 Myr at 5.5 GHz after the jets switch of—radio emission arising from such hotspots can therefore be expected in an appreciable fraction of genuine remnants.

Download Full-text

FR II radio galaxies at low frequencies – II. Spectral ageing and source dynamics

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stx820 ◽

2017 ◽

Vol 469 (1) ◽

pp. 639-655 ◽

Cited By ~ 17

Author(s):

Jeremy J. Harwood ◽

Martin J. Hardcastle ◽

Raffaella Morganti ◽

Judith H. Croston ◽

Marcus Brüggen ◽

...

Keyword(s):

Surface Brightness ◽

Spatial Scales ◽

Low Frequency ◽

Radio Galaxies ◽

Speed Of Light ◽

Low Frequencies ◽

Very Large Array ◽

Low Surface Brightness ◽

Initial Electron Energy

AbstractIn this paper, the second in a series investigating Fanaroff–Riley type II (FR II) radio galaxies at low frequencies, we use LOw Frequency ARray (LOFAR) and Very Large Array (VLA) observations between 117 and 456 MHz, in addition to archival data, to determine the dynamics and energetics of two radio galaxies, 3C 452 and 3C 223, by fitting spectral ageing models on small spatial scales. We provide improved measurements for the physical extent of the two sources, including a previously unknown low surface brightness extension to the northern lobe of 3C 223, and revised energetics based on these values. We find spectral ages of $77.05^{+9.22}_{-8.74}$ and $84.96^{+15.02}_{-13.83}$ Myr for 3C 452 and 3C 223, respectively, suggesting a characteristic advance speed for the lobes of around 1 per cent of the speed of light. For 3C 452, we show that, even for a magnetic field strength not assumed to be in equipartition, a disparity of a factor of approximately 2 exists between the spectral age and that determined from a dynamical standpoint. We confirm that the injection index of both sources (as derived from the lobe emission) remains steeper than classically assumed values, even when considered on well-resolved scales at low frequencies. However, we find an unexpected sharp discontinuity between the spectrum of the hotspots and the surrounding lobe emission. We suggest that this discrepancy is a result of the absorption of hotspot emission and/or non-homogeneous and additional acceleration mechanisms; as such, hotspots should not be used in the determination of the underlying initial electron energy distribution.

Download Full-text

HI Searches in the Zone of Avoidance: Past and Present (and Future)

Publications of the Astronomical Society of Australia ◽

10.1071/as97021 ◽

1997 ◽

Vol 14 (1) ◽

pp. 21-24 ◽

Cited By ~ 3

Author(s):

P. A. Henning

Keyword(s):

Feasibility Study ◽

Surface Brightness ◽

The South ◽

The North ◽

Low Surface Brightness ◽

Complete Survey ◽

Zone Of Avoidance ◽

Low Surface Brightness Galaxies

AbstractIn the regions of highest optical obscuration and infrared confusion, only 21-cm emission can be used to find galaxies in the Zone of Avoidance. A feasibility study conducted with the 300-ft telescope successfully uncovered galaxies which seem to be consistent with populations of optically-selected low surface brightness galaxies. A complete survey is currently being conducted in the north with the Dwingeloo telescope. The big breakthrough should come in the south, however, with the advent of the Parkes telescope multibeam system.

Download Full-text

Giant low surface brightness haloes in distant radio galaxies: USS0828+193

Monthly Notices of the Royal Astronomical Society ◽

10.1046/j.1365-8711.2002.05751.x ◽

2002 ◽

Vol 336 (2) ◽

pp. 436-444 ◽

Cited By ~ 31

Author(s):

M. Villar-Martín ◽

J. Vernet ◽

S. Di Serego Alighieri ◽

R. Fosbury ◽

L. Pentericci ◽

...

Keyword(s):

Surface Brightness ◽

Radio Galaxies ◽

Low Surface Brightness

Download Full-text

LOFAR view of NGC 3998, a sputtering AGN

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936796 ◽

2020 ◽

Vol 634 ◽

pp. A108 ◽

Cited By ~ 2

Author(s):

Sarrvesh S. Sridhar ◽

Raffaella Morganti ◽

Kristina Nyland ◽

Bradley S. Frank ◽

Jeremy Harwood ◽

...

Keyword(s):

Low Power ◽

Galaxy Evolution ◽

Spectral Index ◽

Surface Brightness ◽

Large Scale ◽

Radio Sources ◽

The Core ◽

Low Surface Brightness ◽

High Turbulence ◽

The Rich

Low-power radio sources dominate the radio sky. They tend to be small in size and dominated by their cores, but the origin of their properties and the evolution of their radio plasma are not well constrained. Interestingly, there is mounting evidence that low-power radio sources can significantly affect their surrounding gaseous medium and may therefore be more relevant for galaxy evolution than previously thought. In this paper, we present low radio frequency observations obtained with LOFAR at 147 MHz of the radio source hosted by NGC 3998. This is a rare example of a low-power source that is extremely dominated by its core, but that has two large-scale lobes of low surface brightness. We combine the new 147 MHz image with available 1400 MHz data to derive the spectral index over the source. Despite the low surface brightness, reminiscent of remnant structures, the lobes show an optically thin synchrotron spectral index (∼0.6). We interpret this as being due to rapid decollimation of the jets close to the core, to high turbulence of the plasma flow, and to entrainment of thermal gas. This could be the result of intermittent activity of the central active galactic nucleus, or, more likely, temporary disruption of the jet due to the interaction of the jet with the rich circumnuclear interstellar matter. Both would result in sputtering energy injection from the core, which would keep the lobes fed, albeit at a low rate. We discuss these results in connection with the properties of low-power radio sources in general. Our findings show that amorphous low surface brightness lobes should not be interpreted by default as remnant structures. Large deep surveys (in particular the LOFAR 150 MHz LoTSS and the recently started 1400 MHz Apertif survey) will identify a growing number of objects similar to NGC 3998 where these ideas can be further tested.

Download Full-text

Diffuse Cluster Radio Sources

Symposium - International Astronomical Union ◽

10.1017/s0074180900081006 ◽

1996 ◽

Vol 175 ◽

pp. 333-338 ◽

Cited By ~ 21

Author(s):

L. Feretti ◽

G. Giovannini

Keyword(s):

Surface Brightness ◽

Radio Galaxy ◽

Radio Sources ◽

Clusters Of Galaxies ◽

Rare Phenomenon ◽

Low Surface Brightness ◽

Extended Sources ◽

Formation And Evolution

Diffuse radio sources in clusters remain a poorly understood phenomenon. They are very extended sources (0.4-0.6 Mpc), of low surface brightness and steep spectrum, which cannot be identified with any active radio galaxy. They are a rare phenomenon, as they have been found so far in few clusters of galaxies. This paper reviews the current findings about this kind of sources, and the suggestions about their formation and evolution.

Download Full-text

LOFAR observations of the XMM-LSS field

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833906 ◽

2019 ◽

Vol 622 ◽

pp. A4 ◽

Cited By ~ 8

Author(s):

C. L. Hale ◽

W. Williams ◽

M. J. Jarvis ◽

M. J. Hardcastle ◽

L. K. Morabito ◽

...

Keyword(s):

Large Scale ◽

Angular Resolution ◽

Low Frequency ◽

Primary Beam ◽

Electromagnetic Spectrum ◽

Radio Sources ◽

Spectral Indices ◽

The North ◽

True Source ◽

Scale Calibration

We present observations of the XMM Large-Scale Structure (XMM-LSS) field observed with the LOw Frequency ARray (LOFAR) at 120–168 MHz. Centred at a J2000 declination of −4.5°, this is a challenging field to observe with LOFAR because of its low elevation with respect to the array. The low elevation of this field reduces the effective collecting area of the telescope, thereby reducing sensitivity. This low elevation also causes the primary beam to be elongated in the north-south direction, which can introduce side lobes in the synthesised beam in this direction. However the XMM-LSS field is a key field to study because of the wealth of ancillary information, encompassing most of the electromagnetic spectrum. The field was observed for a total of 12 h from three four-hour LOFAR tracks using the Dutch array. The final image presented encompasses ∼27 deg2, which is the region of the observations with a >50% primary beam response. Once combined, the observations reach a central rms of 280μJy beam−1at 144 MHz and have an angular resolution of 7.5 × 8.5″. We present our catalogue of detected sources and investigate how our observations compare to previous radio observations. This includes investigating the flux scale calibration of these observations compared to previous measurements, the implied spectral indices of the sources, the observed source counts and corrections to obtain the true source counts, and finally the clustering of the observed radio sources.

Download Full-text

Spectral Index - Redshift Relation for Radio Galaxies and Quasars

Symposium - International Astronomical Union ◽

10.1017/s0074180900081882 ◽

1996 ◽

Vol 175 ◽

pp. 569-570

Author(s):

R.D. Dagkesamanskii

Keyword(s):

Flux Density ◽

Spectral Index ◽

Low Frequency ◽

Radio Galaxies ◽

Cosmological Evolution ◽

Radio Sources ◽

Spectral Indices ◽

Extragalactic Radio Sources

Cosmological evolution of synchrotron spectra of the powerful extragalactic radio sources was studied by many authors. Some indications of such an evolution had been found firstly by analysis of ‘spectral index - flux density’ (α – S) relation for the sample of relatively strong radio sources. Later Gopal-Krishna and Steppe extended the analysis to weaker sources and found that the slope of αmed(S) curve changes dramatically at intermediate flux densities. Gopal-Krishna and Steppe pointed out that the maxima of the αmed(S) curve and of differential source counts are at almost the same flux density ranges (see, Fig. 2). It has to be noticed that the all mentioned results were obtained using the low-frequency spectral indices and on the basis of low frequency samples.

Download Full-text