- ID:
- ivo://CDS.VizieR/J/A+A/648/A3
- Title:
- LOFAR Two-metre Sky Survey Deep Fields DR1
- Short Name:
- J/A+A/648/A3
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the source associations, cross-identifications, and multi-wavelength properties of the faint radio source population detected in the deep tier of the LOFAR Two Metre Sky Survey (LoTSS): the LoTSS Deep Fields. The first LoTSS Deep Fields data release consists of deep radio imaging at 150MHz of the ELAIS-N1, Lockman Hole, and Bootes fields, down to RMS sensitives of around 20, 22, and 32uJy/beam, respectively. These fields are some of the best studied extra-galactic fields in the northern sky, with existing deep, wide-area panchromatic photometry from X-ray to infrared wavelengths, covering a total of ~26 square degrees. We first generated improved multi-wavelength catalogues in ELAIS-N1 and Lockman Hole; combined with the existing catalogue for Bootes, we present forced, matched aperture photometry for over 7.2 million sources across the three fields. We identified multi-wavelength counterparts to the radio detected sources, using a combination of the Likelihood Ratio method and visual classification, which greatly enhances the scientific potential of radio surveys and allows for the characterisation of the photometric redshifts and the physical properties of the host galaxies. The final radio-optical cross-matched catalogue consists of 81951 radio-detected sources, with counterparts identified and multi-wavelength properties presented for 79820 (>97%) sources. We also examine the properties of the host galaxies, and through stacking analysis find that the radio population with no identified counterpart is likely dominated by AGN at z~3-4. This dataset contains one of the largest samples of radio-selected star-forming galaxies and active galactic nuclei (AGN) at these depths, making it ideal for studying the history of star-formation, and the evolution of galaxies and AGN across cosmic time.
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- ID:
- ivo://CDS.VizieR/J/A+A/622/A1
- Title:
- LOFAR Two-metre Sky Survey DR1 source catalog
- Short Name:
- J/A+A/622/A1
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45{deg}00'00'' to 57{deg}00'00'') were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325694 sources are detected with a signal of at least five times the noise, and the source density is a factor of ~10 higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S_144MHz_=71{mu}Jy/beam and the point-source completeness is 90% at an integrated flux density of 0.45mJy. The resolution of the images is 6'' and the positional accuracy is within 0.2''. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further ~20 articles that highlight the scientific potential of LoTSS.
- ID:
- ivo://CDS.VizieR/J/A+A/659/A1
- Title:
- LOFAR Two-metre Sky Survey (LoTSS) DR2
- Short Name:
- J/A+A/659/A1
- Date:
- 25 Feb 2022 07:37:08
- Publisher:
- CDS
- Description:
- In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) we present 120-168MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44{deg}30' and 1h00m +28{deg}00' and spanning 4178 and 1457 square degrees respectively. The images were derived from 3,451 hrs (7.6 PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4396228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6"resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144MHz have: a median rms sensitivity of 83mJy/beam; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2"; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8mJy/beam. By creating three 16MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of >+/-0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20" resolution 120-168 MHz continuum images have a median rms sensitivity of 95uJy/beam, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480x97.6kHz wide planes and have a median rms sensitivity per plane of 10.8mJy/beam at 4' and 2.2mJy/beam at 20"; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data to facilitate the thorough scientific exploitation of this unique dataset.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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).
- 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.