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

2017 ◽  
Vol 469 (1) ◽  
pp. 639-655 ◽  
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.

scholarly journals The 6C Survey

1977 ◽  
Vol 74 ◽  
pp. 3-7
Author(s):  
J. E. Baldwin
Keyword(s):  
Surface Brightness ◽  
Low Frequency ◽  
Radio Galaxies ◽  
Aperture Synthesis ◽  
Radio Sources ◽  
Source Density ◽  
The North ◽  
Low Surface Brightness ◽  
Celestial Pole ◽  
Frequency 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.

scholarly journals The UTR-2 Very Low Frequency Sky Survey and Its Main Results

1998 ◽  
Vol 179 ◽  
pp. 100-102
Author(s):  
K.P. Sokolov
Keyword(s):  
High Frequency ◽  
Surface Brightness ◽  
Low Frequency ◽  
Source Population ◽  
Low Frequencies ◽  
Physical Conditions ◽  
The Past ◽  
Sky Survey ◽  
Low Surface Brightness ◽  
Compact Sources

During the past decade there has been a dramatic increase in the amount of high-frequency (v > 1000 MHz) data currently underlying the studies of bright compact sources (Tb ∼ 1011–12K, l ≤ 1 kpc) with flat spectra. But in order to determine physical conditions inside extragalactic radio sources at different stages of their evolution the studies of old extended (l ∼ 100 kpc) sources with low surface brightness and steep spectra which constitute the dominant radio source population at very low frequencies (v ≪ 100 MHz) are also needed. These sources are known to represent the final stage in the evolution of extragalactic objects.

scholarly journals Investigation of the cosmic ray population and magnetic field strength in the halo of NGC 891

2018 ◽  
Vol 615 ◽  
pp. A98 ◽  
Author(s):  
D. D. Mulcahy ◽  
A. Horneffer ◽  
R. Beck ◽  
M. Krause ◽  
P. Schmidt ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Low Frequency ◽  
Cosmic Ray ◽  
Radio Continuum ◽  
Continuum Emission ◽  
Low Frequencies ◽  
Very Large Array ◽  
Field Strengths

Context. Cosmic rays and magnetic fields play an important role for the formation and dynamics of gaseous halos of galaxies. Aims. Low-frequency radio continuum observations of edge-on galaxies are ideal to study cosmic-ray electrons (CREs) in halos via radio synchrotron emission and to measure magnetic field strengths. Spectral information can be used to test models of CRE propagation. Free–free absorption by ionized gas at low frequencies allows us to investigate the properties of the warm ionized medium in the disk. Methods. We obtained new observations of the edge-on spiral galaxy NGC 891 at 129–163 MHz with the LOw Frequency ARray (LOFAR) and at 13–18 GHz with the Arcminute Microkelvin Imager (AMI) and combine them with recent high-resolution Very Large Array (VLA) observations at 1–2 GHz, enabling us to study the radio continuum emission over two orders of magnitude in frequency. Results. The spectrum of the integrated nonthermal flux density can be fitted by a power law with a spectral steepening towards higher frequencies or by a curved polynomial. Spectral flattening at low frequencies due to free–free absorption is detected in star-forming regions of the disk. The mean magnetic field strength in the halo is 7 ± 2 μG. The scale heights of the nonthermal halo emission at 146 MHz are larger than those at 1.5 GHz everywhere, with a mean ratio of 1.7 ± 0.3, indicating that spectral ageing of CREs is important and that diffusive propagation dominates. The halo scale heights at 146 MHz decrease with increasing magnetic field strengths which is a signature of dominating synchrotron losses of CREs. On the other hand, the spectral index between 146 MHz and 1.5 GHz linearly steepens from the disk to the halo, indicating that advection rather than diffusion is the dominating CRE transport process. This issue calls for refined modelling of CRE propagation. Conclusions. Free–free absorption is probably important at and below about 150 MHz in the disks of edge-on galaxies. To reliably separate the thermal and nonthermal emission components, to investigate spectral steepening due to CRE energy losses, and to measure magnetic field strengths in the disk and halo, wide frequency coverage and high spatial resolution are indispensable.

scholarly journals Superluminal Motion in CTA 102

1989 ◽  
Vol 134 ◽  
pp. 529-530
Author(s):  
Ann E. Wehrle
Keyword(s):  
Flux Density ◽  
High Speed ◽  
Low Frequency ◽  
Precise Determination ◽  
Flux Variation ◽  
Low Frequencies ◽  
Superluminal Motion ◽  
Low Frequency Variability ◽  
Expansion Speed

Sholomitskii (1965) discovered that the flux density of the quasar CTA 102 varies at low frequencies on a timescale of a few months. Low-frequency variability can be explained by “superluminal flux variation” (Romney et al. 1984): If the intrinsic brightness of a component moving in a relativistically beamed source varies by only a few percent, the observer sees its flux density change by a much larger factor δ3-α when the optically thin blob moves almost directly toward the observer. Such a relativistically beamed source is likely to exhibit superluminal motion if studied with sufficient resolution and sensitivity. Superluminal motion in CTA 102 was discovered by Bååth (1987) who concluded on the basis of maps made at three epochs at a frequency of 932 MHz that two components were separating at a rate of 0.65 milliarcseconds (mas) per year. Using a redshift z = 1.037 and H0 = 100 km s−1 Mpc−1, q0 = 0.5, this expansion speed corresponds to (18 ± 4)h−1c. The extraordinarily high speed led us to make VLBI images of the source at a higher frequency in order to increase the resolution and make a more precise determination of the speed.

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

2002 ◽  
Vol 336 (2) ◽  
pp. 436-444 ◽  
Author(s):  
M. Villar-Martín ◽  
J. Vernet ◽  
S. Di Serego Alighieri ◽  
R. Fosbury ◽  
L. Pentericci ◽  
...  
Keyword(s):  
Surface Brightness ◽  
Radio Galaxies ◽  
Low Surface Brightness

Generation of Low-Frequency Spiciness Variability in the Thermocline*

2011 ◽  
Vol 41 (2) ◽  
pp. 365-377 ◽  
Author(s):  
Thomas Kilpatrick ◽  
Niklas Schneider ◽  
Emanuele Di Lorenzo
Keyword(s):  
Markov Model ◽  
Large Scale ◽  
Spatial Scales ◽  
Low Frequency ◽  
Passive Tracer ◽  
Wind Stress Curl ◽  
Frequency Spectra ◽  
Low Frequencies ◽  
Geostrophic Advection

Abstract The generation of variance by anomalous advection of a passive tracer in the thermocline is investigated using the example of density-compensated temperature and salinity anomalies, or spiciness. A coupled Markov model is developed in which wind stress curl forces the large-scale baroclinic ocean pressure that in turn controls the anomalous geostrophic advection of spiciness. The “double integration” of white noise atmospheric forcing by this Markov model results in a frequency (ω) spectrum of large-scale spiciness proportional to ω−4, so that spiciness variability is concentrated at low frequencies. An eddy-permitting regional model hindcast of the northeast Pacific (1950–2007) confirms that time series of large-scale spiciness variability are exceptionally smooth, with frequency spectra ∝ ω−4 for frequencies greater than 0.2 cpy. At shorter spatial scales (wavelengths less than ∼500 km), the spiciness frequency spectrum is whitened by mesoscale eddies, but this eddy-forced variability can be filtered out by spatially averaging. Large-scale and long-term measurements are needed to observe the variance of spiciness or any other passive tracer subject to anomalous advection in the thermocline.

scholarly journals Stellar Over-Densities in the Outer Halo of the Milky Way

2010 ◽  
Vol 27 (1) ◽  
pp. 45-55 ◽  
Author(s):  
Stefan C. Keller
Keyword(s):  
Surface Brightness ◽  
Dwarf Galaxy ◽  
Local Density ◽  
Spatial Scales ◽  
Sloan Digital Sky Survey ◽  
Tidal Disruption ◽  
Giant Stars ◽  
Sky Survey ◽  
Low Surface Brightness ◽  
Model Predictions

AbstractThis study presents a tomographic survey of a subset of the outer halo (10–40 kpc) drawn from the Sloan Digital Sky Survey Data Release 6. Halo substructure on spatial scales of >3 degrees is revealed as an excess in the local density of sub-giant stars. With an appropriate assumption of a model stellar isochrone it is possible for us to then derive distances to the sub-giant population. We describe three new candidate halo substructures; the 160- and 180-degree over-densities (at distances of 17 and 19 kpc respectively and radii of 1.3 and 1.5 kpc respectively) and an extended feature at 28 kpc that covers at least 162 deg2, the Virgo Equatorial Stream. In addition, we recover the Sagittarius dwarf galaxy (Sgr) leading-arm material and the Virgo Over-Density.The derived distances, together with the number of sub-giant stars associated with each substructure, enables us to derive the integrated luminosity for the features. The tenuous, low surface brightness of the features strongly suggests an origin from the tidal disruption of an accreted galaxy or galaxies. Given the dominance of the tidal debris of Sgr in this region of the sky we investigate if our observations can be accommodated by tidal disruption models for Sgr. The clear discordance between observations and model predictions for known Sgr features means it is difficult to tell unambiguously if the new substructures are related to Sgr or not. Radial velocities in the stellar over-densities will be critical in establishing their origins.

scholarly journals Bioinspired Low-Frequency Material Characterisation

2012 ◽  
Vol 2012 ◽  
pp. 1-12 ◽  
Author(s):  
C. Hopper ◽  
S. Assous ◽  
P. B. Wilkinson ◽  
D. A. Gunn ◽  
P. D. Jackson ◽  
...  
Keyword(s):  
Sound Velocity ◽  
Medical Physics ◽  
Low Frequency ◽  
High Sensitivity ◽  
Prediction Errors ◽  
Low Frequencies ◽  
Longitudinal Sound Velocity ◽  
Velocity Prediction ◽  
Thickness Measurements

New-coded signals, transmitted by high-sensitivity broadband transducers in the 40–200 kHz range, allow subwavelength material discrimination and thickness determination of polypropylene, polyvinylchloride, and brass samples. Frequency domain spectra enable simultaneous measurement of material properties including longitudinal sound velocity and the attenuation constant as well as thickness measurements. Laboratory test measurements agree well with model results, with sound velocity prediction errors of less than 1%, and thickness discrimination of at least wavelength/15. The resolution of these measurements has only been matched in the past through methods that utilise higher frequencies. The ability to obtain the same resolution using low frequencies has many advantages, particularly when dealing with highly attenuating materials. This approach differs significantly from past biomimetic approaches where actual or simulated animal signals have been used and consequently has the potential for application in a range of fields where both improved penetration and high resolution are required, such as nondestructive testing and evaluation, geophysics, and medical physics.

scholarly journals Unveiling the nature of the unidentified gamma-ray sources: blazar counterparts at low radio frequencies

2013 ◽  
Vol 9 (S304) ◽  
pp. 95-95
Author(s):  
Francesco Massaro ◽  
R. D'Abrusco ◽  
M. Giroletti ◽  
A. Paggi ◽  
N. Masetti ◽  
...  
Keyword(s):  
Radio Emission ◽  
Gamma Ray ◽  
Low Frequency ◽  
Radio Galaxies ◽  
Large Array ◽  
New Approach ◽  
Very Large Array ◽  
Very Low Frequency ◽  
Sky Survey ◽  
Order Of Magnitude

AbstractAbout one third of the gamma-ray sources detected by Fermi have still no firmly established counterpart at lower energies. Here we propose a new approach to find candidate counterparts for the unidentified gamma-ray sources (UGSs) based on the 325 MHz radio survey performed with Westerbork Synthesis Radio Telescope (WSRT) in the northern hemisphere. First we investigate the low-frequency radio properties of blazars, the largest known population of gamma-ray sources; then we search for sources with similar radio properties combining the information derived from the Westerbork Northern Sky Survey (WENSS) with those of the NRAO VLA Sky survey (NVSS). We present a list of candidate counterparts for 32 UGSs with at least one counterpart in the WENSS. We also performed an extensive research in literature to look for infrared and optical counterparts of the gamma-ray blazar candidates selected with the low-frequency radio observations to confirm their nature. On the basis of our multifrequency research we identify 23 new gamma-ray blazar candidates out of 32 UGSs investigated. I will also present the first analysis of very low frequency radio emission of blazars based on the recent Very Large Array Low-Frequency Sky Survey (VLSS) at 74 MHz. I show that blazars present radio flat spectra when evaluated at 74 MHz, about an order of magnitude in frequency lower than previous analyses. The implications of these findings in the contest of the blazars – radio galaxies connection will be discussed.

scholarly journals Radio Observations of AGN in Low Surface Brightness Galaxies

2007 ◽  
Vol 3 (S244) ◽  
pp. 352-353
Author(s):  
M. Das ◽  
S. S. McGaugh ◽  
N. Kantharia ◽  
S. N. Vogel
Keyword(s):  
Star Formation ◽  
Surface Brightness ◽  
Low Frequency ◽  
Radio Continuum ◽  
Continuum Emission ◽  
Preliminary Results ◽  
Low Surface Brightness ◽  
Low Surface Brightness Galaxies ◽  
Lsb Galaxies ◽  
Extended Emission

AbstractWe present preliminary results of a study of the low frequency radio continuum emission from the nuclei of Giant Low Surface Brightness (LSB) galaxies. We have mapped the emission and searched for extended features such as radio lobes/jets associated with AGN activity. LSB galaxies are poor in star formation and generally less evolved compared to nearby bright spirals. This paper presents low frequency observations of 3 galaxies; PGC 045080 at 1.4 GHz, 610 MHz, 325MHz, UGC 1922 at 610 MHz and UGC 6614 at 610 MHz. The observations were done with the GMRT. Radio cores as well as extended structures were detected and mapped in all three galaxies; the extended emission may be assocated with jets/lobes associated with AGN activity. Our results indicate that although these galaxies are optically dim, their nuclei can host AGN that are bright in the radio domain.

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