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901. LOFAR view of NGC 3998
ID:
ivo://CDS.VizieR/J/A+A/634/A108
Title:
LOFAR view of NGC 3998
Short Name:
J/A+A/634/A108
Date:
21 Oct 2021
Publisher:
CDS
Description:
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 aect 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 147MHz 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 147MHz image with available 1400MHz 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 150MHz LoTSS and the recently started 1400MHz Apertif survey) will identify a growing number of objects similar to NGC 3998 where these ideas can be further tested.
902. Log of CGPS exposures
ID:
ivo://CDS.VizieR/VI/128
Title:
Log of CGPS exposures
Short Name:
VI/128
Date:
21 Oct 2021
Publisher:
CDS
Description:
The Dominion Radio Astrophysical Observatory at Penticton (HIA/NRC), in collaboration with an international Consortium of astronomers, has begun a high-resolution survey of the atomic hydrogen and radio continuum emission from our Milky Way galaxy. By constructing a mosaic of 190 synthesis fields, the survey will cover Galactic longitudes from L=74.2 to 147.3 degrees and latitudes from B=-3.6 to +5.6 degrees, resolving features as small as 1 arcminute. For each mosaic (1024x1024 pixels), atomic hydrogen observations are presented as a data "cube" with 272 spectral channels having a velocity resolution of 1.3km/s. Stokes I continuum images at 1420MHz and 408MHz are produced, as well as selected Stokes Q, U and V images at 1420MHz. Complementary images in the four IRAS infrared bands and data cubes of CO (J=1-0) emission have also been created as part of the Survey. In addition to the 5x5{deg} mosaics, a series of nine 15x15{deg} mosaics of 408MHz emission, and possibly other lower radio frequencies, will be created. Mosaics are archived as they are processed and released to the Consortium. Archived data are in FITS format, except for the occasional text file. Imaging of the molecular phase of the ISM, traced by the CO molecule, has been realized at the Five College Radio Astronomy Observatory. High-resolution IRAS images of the dust component, seen through its infrared emission, have been completed at far-infrared wavelengths at the California Institute of Technology, and in the mid-infrared at the Canadian Institute for Theoretical Astrophysics at the University of Toronto. Complementary radio continuum images at frequencies below 408MHz are being contributed by the Mullard Radio Astronomy Observatory of the University of Cambridge and at the Beijing Astronomical Observatory.
903. LoLSS-Deep Bootes 54MHz catalog
ID:
ivo://CDS.VizieR/J/A+A/655/A40
Title:
LoLSS-Deep Bootes 54MHz catalog
Short Name:
J/A+A/655/A40
Date:
22 Feb 2022
Publisher:
CDS
Description:
We present the first sub-mJy (~0.7mJy/beam) survey to be completed below 100 MHz, which is over an order of magnitude deeper than previously achieved for widefield imaging of any field at these low frequencies. The high-resolution (15x15arcsec) image of the Bootes field at 34-75MHz is made from 56 hours of observation with the LOw Frequency ARray (LOFAR) Low Band Antenna (LBA) system. We present a radio source catalogue containing 1,948 sources detected over an area of 23.6 sq. deg, with a peak flux density threshold of 5{sigma}. Using the available deep 144-MHz data, we identified 144-MHz counterparts to all the 54-MHz sources, and produced a matched catalogue within the deep optical coverage area containing 829 sources.
904. Long-term radio variations of QSO J1819+3845
ID:
ivo://CDS.VizieR/J/A+A/574/A125
Title:
Long-term radio variations of QSO J1819+3845
Short Name:
J/A+A/574/A125
Date:
21 Oct 2021
Publisher:
CDS
Description:
We examine the long-term evolution of the intra-hour variable quasar, J1819+3845, whose variations have been attributed to interstellar scintillation by extremely local turbulent plasma, located only 1-3pc from Earth. The variations in this source ceased some time between June 2006 and February 2007. The evolution of the source spectrum and the long-term lightcurve, and the persistent compactness of the source VLBI structure indicates that the cessation of rapid variability was associated with the passage of the scattering material out of the line of sight to the quasar. We present an analysis of the linear polarization variations and their relation to total intensity variations. The proper motion of polarized features in the quasar jet is found to be subluminal. Systematic time delays between Stokes I, Q and U, in combination with the structure of the source obtained from 8.4GHz VLBI data, confirm the estimate of the screen distance: 1-2pc, making the screen one of the nearest objects to the Solar System. We determine the physical properties of this scattering material. We examine the rotation measures of sources and the diffuse polarized emission in the surrounding region. We place a limit of 10rad/m^2^ on the RM change. The variability of sources near J1819+3845 is used to deduce that the screen must therefore be either very small (~100AU) or patchy.
905. LOTSS HETDEX Faraday depth cube
ID:
ivo://CDS.VizieR/J/A+A/623/A71
Title:
LOTSS HETDEX Faraday depth cube
Short Name:
J/A+A/623/A71
Date:
21 Oct 2021
Publisher:
CDS
Description:
Faraday tomography allows us to map diffuse polarized synchrotron emission from our Galaxy and use it to interpret the magnetic field in the interstellar medium (ISM). We have applied Faraday tomography to 60 observations from the LOFAR Two-Meter Sky Survey (LOTSS) and produced a Faraday depth cube mosaic covering 568 square degrees at high Galactic latitudes, at 4.3 angular resolution and 1rad/m^2^ Faraday depth resolution, with a typical noise level of 50-100uJy per point spread function (PSF) per rotation measure spread function (RMSF) (40-80mK/RMSF). While parts of the images are strongly affected by instrumental polarization, we observe diffuse polarized emission throughout most of the field, with typical brightness between 1 and 6K/RMSF, and Faraday depths between -7 and +25rad/m^2^. We observe many new polarization features, some up to 15deg in length. These include two regions with very uniformly structured, linear gradients in the Faraday depth; we measured the steepness of these gradients as 2.6 and 13rad/m^2^/deg. We also observe a relationship between one of the gradients and an HI filament in the local ISM. Other ISM tracers were also checked for correlations with our polarization data and none were found, but very little signal was seen in most tracers in this region. We conclude that the LOTSS data are very well suited for Faraday tomography, and that a full-scale survey with all the LOTSS data has the potential to reveal many new Galactic polarization features and map out diffuse Faraday depth structure across the entire northern hemisphere.
906. LoTSS/HETDEX. Optical quasars. I.
ID:
ivo://CDS.VizieR/J/A+A/622/A11
Title:
LoTSS/HETDEX. Optical quasars. I.
Short Name:
J/A+A/622/A11
Date:
21 Oct 2021
Publisher:
CDS
Description:
The radio-loud/radio-quiet (RL/RQ) dichotomy in quasars is still an open question. Although it is thought that accretion onto supermassive black holes in the centre the host galaxies of quasars is responsible for some radio continuum emission, there is still a debate as to whether star formation or active galactic nuclei (AGN) activity dominate the radio continuum luminosity. To date, radio emission in quasars has been investigated almost exclusively using high-frequency observations in which the Doppler boosting might have an important effect on the measured radio luminosity, whereas extended structures, best observed at low radio frequencies, are not affected by the Doppler enhancement. We used a sample of quasars selected by their optical spectra in conjunction with sensitive and high-resolution low-frequency radio data provided by the LOw Frequency ARray (LOFAR) as part of the LOFAR Two-Metre Sky Survey (LoTSS) to investigate their radio properties using the radio loudness parameter (R=L_144MHz_/L_iband_). The examination of the Li band radio continuum emission and RL/RQ dichotomy in quasars exhibits that quasars show a wide continuum of radio properties (i.e. no clear bimodality in the distribution of R). Radio continuum emission at low frequencies in low-luminosity quasars is consistent with being dominated by star formation. We see a significant albeit weak dependency of R on the source nuclear parameters. For the first time, we are able to resolve radio morphologies of a considerable number of quasars. All these crucial results highlight the impact of the deep and high-resolution low-frequency radio surveys that foreshadow the compelling science cases for the Square Kilometre Array (SKA).
907. Low-frequency flat spectrum sources (LORCAT)
ID:
ivo://CDS.VizieR/J/ApJS/213/3
Title:
Low-frequency flat spectrum sources (LORCAT)
Short Name:
J/ApJS/213/3
Date:
21 Oct 2021
Publisher:
CDS
Description:
A well known property of the {gamma}-ray sources detected by Cos-B in the 1970s, by the Compton Gamma-Ray Observatory in the 1990s, and recently by the Fermi observations is the presence of radio counterparts, particularly for those associated with extragalactic objects. This observational evidence is the basis of the radio-{gamma}-ray connection established for the class of active galactic nuclei known as blazars. In particular, the main spectral property of the radio counterparts associated with {gamma}-ray blazars is that they show a flat spectrum in the GHz frequency range. Our recent analysis dedicated to search blazar-like candidates as potential counterparts for the unidentified {gamma}-ray sources allowed us to extend the radio-{gamma}-ray connection in the MHz regime. We also showed that blazars below 1GHz maintain flat radio spectra. Thus, on the basis of these new results, we assembled a low-frequency radio catalog of flat-spectrum sources built by combining the radio observations of the Westerbork Northern Sky Survey and of the Westerbork in the southern hemisphere catalog with those of the NRAO Very Large Array Sky survey (NVSS). This could be used in the future to search for new, unknown blazar-like counterparts of {gamma}-ray sources. First, we found NVSS counterparts of Westerbork Synthesis Radio Telescope radio sources, and then we selected flat-spectrum radio sources according to a new spectral criterion, specifically defined for radio observations performed below 1GHz. We also described the main properties of the catalog listing 28358 radio sources and their logN-logS distributions. Finally, a comparison with the Green Bank 6cm radio source catalog was performed to investigate the spectral shape of the low-frequency flat-spectrum radio sources at higher frequencies.
908. Low-frequency (115-175MHz) image of M51
ID:
ivo://CDS.VizieR/J/A+A/568/A74
Title:
Low-frequency (115-175MHz) image of M51
Short Name:
J/A+A/568/A74
Date:
21 Oct 2021
Publisher:
CDS
Description:
The grand-design spiral galaxy M51 was observed with the LOFAR High Frequency Antennas (HBA) and imaged in total intensity and polarization. This observation covered the frequencies between 115MHz and 175MHz. We produced an image of total emission of M51 at the mean frequency of 151MHz with 20-arcsec resolution and 0.3mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300MHz so far. The integrated spectrum of total radio emission is well described by a power law, while flat spectral indices in the central region indicates thermal absorption. We observe the disk to extend out to 16kpc and a break in the radial profile near the optical radius of the disk. The radial scale lengths in the inner and outer disks are larger at 151MHz, and the break is smoother at 151MHz than those observed at 1.4GHz. The arm--interarm contrast is lower at 151MHz than at 1400MHz, indicating propagation of cosmic ray electrons (CRE) from spiral arms into interarm regions.
909. Low-mass black holes in Sy1 galaxies. II. SDSS-DR7
ID:
ivo://CDS.VizieR/J/ApJS/235/40
Title:
Low-mass black holes in Sy1 galaxies. II. SDSS-DR7
Short Name:
J/ApJS/235/40
Date:
21 Oct 2021
Publisher:
CDS
Description:
A new sample of 204 low-mass black holes (LMBHs) in active galactic nuclei (AGNs) is presented with black hole masses in the range of (1-20)x10^5^M_{sun}_. The AGNs are selected through a systematic search among galaxies in the Seventh Data Release (DR7) of the Sloan Digital Sky Survey (SDSS), and careful analyses of their optical spectra and precise measurement of spectral parameters. Combining them with our previous sample selected from SDSS DR4 makes it the largest LMBH sample so far, totaling over 500 objects. Some of the statistical properties of the combined LMBH AGN sample are briefly discussed in the context of exploring the low-mass end of the AGN population. Their X-ray luminosities follow the extension of the previously known correlation with the [OIII] luminosity. The effective optical-to-X-ray spectral indices {alpha}_OX_, albeit with a large scatter, are broadly consistent with the extension of the relation with the near-UV luminosity L_2500{AA}_. Interestingly, a correlation of {alpha}_OX_ with black hole mass is also found, with {alpha}OX being statistically flatter (stronger X-ray relative to optical) for lower black hole masses. Only 26 objects, mostly radio loud, were detected in radio at 20cm in the FIRST survey, giving a radio-loud fraction of 4%. The host galaxies of LMBHs have stellar masses in the range of 10^8.8^-10^12.4^M_{sun}_ and optical colors typical of Sbc spirals. They are dominated by young stellar populations that seem to have undergone continuous star formation history.
910. Low-z BQS Quasars
ID:
ivo://CDS.VizieR/J/MNRAS/263/425
Title:
Low-z BQS Quasars
Short Name:
J/MNRAS/263/425
Date:
21 Oct 2021
Publisher:
CDS
Description:
(no description available)