- ID:
- ivo://CDS.VizieR/J/A+A/606/A142
- Title:
- L1544 1.2 and 2mm emission maps
- Short Name:
- J/A+A/606/A142
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In dense and cold molecular clouds dust grains are surrounded by thick icy mantles. It is however not clear if dust growth and coagulation take place before the switch-on of a protostar. This is an important issue, as the presence of large grains may affect the chemical structure of dense cloud cores, including the dynamically important ionization fraction, and the future evolution of solids in protoplanetary disks. To study this further, we focus on L1544, one of the most centrally concentrated pre-stellar cores on the verge of star formation, and with a well-known physical structure. We observed L1544 at 1.2 and 2mm using NIKA, a new receiver at the IRAM 30 m telescope, and we used data from the Herschel Space Observatory archive. We find no evidence of grain growth towards the center of L1544 at the available angular resolution. Therefore, we conclude that single dish observations do not allow us to investigate grain growth towards the pre-stellar core L1544 and high sensitivity interferometer observations are needed. We predict that dust grains can grow to 200um in size toward the central ~300au of L1544. This will imply a dust opacity change by a factor of ~2.5 at 1.2mm, which can be detected using the Atacama Large Millimeter and submillimeter Array (ALMA) at different wavelengths and with an angular resolution of 2".
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Search Results
- ID:
- ivo://CDS.VizieR/J/MNRAS/429/2080
- Title:
- Lockman Hole 10C sources radio spectral indices
- Short Name:
- J/MNRAS/429/2080
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have studied a sample of 296 faint (>0.5mJy) radio sources selected from an area of the Tenth Cambridge (10C) survey at 15.7GHz in the Lockman Hole. By matching this catalogue to several lower frequency surveys (e.g. including a deep GMRT survey at 610MHz, a WSRT survey at 1.4GHz, NVSS, FIRST and WENSS) we have investigated the radio spectral properties of the sources in this sample; all but 30 of the 10C sources are matched to one or more of these surveys. We have found a significant increase in the proportion of flat spectrum sources at flux densities below 1mJy - the median spectral index between 15.7GHz and 610MHz changes from 0.75 for flux densities greater than 1.5mJy to 0.08 for flux densities less than 0.8mJy. This suggests that a population of faint, flat spectrum sources is emerging at flux densities around 1mJy. The spectral index distribution of this sample of sources selected at 15.7GHz is compared to those of two samples selected at 1.4GHz from FIRST and NVSS. We find that there is a significant flat spectrum population present in the 10C sample which is missing from the samples selected at 1.4GHz. The 10C sample is compared to a sample of sources selected from the SKADS Simulated Sky by Wilman et al. (2008MNRAS.388.1335W) and we find that this simulation fails to reproduce the observed spectral index distribution and significantly underpredicts the number of sources in the faintest flux density bin. It is likely that the observed faint, flat spectrum sources are a result of the cores of FRI sources becoming dominant at high frequencies. These results highlight the importance of studying this faint, high frequency population.
- ID:
- ivo://CDS.VizieR/J/MNRAS/463/2997
- Title:
- Lockman Hole low-frequency radio sources
- Short Name:
- J/MNRAS/463/2997
- Date:
- 02 Nov 2021 11:21:03
- Publisher:
- CDS
- Description:
- The Lockman Hole is a well-studied extragalactic field with extensive multi-band ancillary data covering a wide range in frequency, essential for characterizing the physical and evolutionary properties of the various source populations detected in deep radio fields (mainly star-forming galaxies and AGNs). In this paper, we present new 150-MHz observations carried out with the LOw-Frequency ARray (LOFAR), allowing us to explore a new spectral window for the faint radio source population. This 150-MHz image covers an area of 34.7 square degrees with a resolution of 18.6x14.7-arcsec and reaches an rms of 160{mu}Jy/beam at the centre of the field. As expected for a low-frequency selected sample, the vast majority of sources exhibit steep spectra, with a median spectral index of {alpha}_150_^1400^=-0.78+/-0.015. The median spectral index becomes slightly flatter (increasing from {alpha}_150_^1400^=-0.84 to {alpha}_150_^1400^=-0.75) with decreasing flux density down to S_150_ ~10mJy before flattening out and remaining constant below this flux level. For a bright subset of the 150-MHz selected sample, we can trace the spectral properties down to lower frequencies using 60-MHz LOFAR observations, finding tentative evidence for sources to become flatter in spectrum between 60 and 150MHz. Using the deep, multi-frequency data available in the Lockman Hole, we identify a sample of 100 ultra-steep-spectrum sources and 13 peaked-spectrum sources. We estimate that up to 21 per cent of these could have z>4 and are candidate high-z radio galaxies, but further follow-up observations are required to confirm the physical nature of these objects.
- ID:
- ivo://CDS.VizieR/J/A+A/653/A155
- Title:
- Lockman Hole Polarised Sources at 1.4GHz
- Short Name:
- J/A+A/653/A155
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In the context of structure formation and galaxy evolution, the contribution of magnetic fields is not well understood. Feedback processes originating from active galactic nucleus (AGN) activity and star formation can be actively influenced by magnetic fields, depending on their strength and morphology. One of the best tracers of magnetic fields is polarised radio emission. Tracing this emission over a broad redshift range therefore allows an investigation of these fields and their evolution. We aim to study the nature of the faint, polarised radio source population whose source composition and redshift dependence contain information about the strength, morphology, and evolution of magnetic fields over cosmic timescales. We use a 15-pointing radio continuum L-band mosaic of the Lockman Hole, observed in full polarisation, generated from archival data of the Westerbork Synthesis Radio Telescope (WSRT). The data were analysed using the rotation measure synthesis technique. We achieved a noise of 7uJy/beam in polarised intensity, with a resolution of 15". Using infrared and optical images and source catalogues, we were able to cross-identify and determine redshifts for one-third of our detected polarised sources. Results. We detected 150 polarised sources, most of which are weakly polarised with a mean fractional polarisation of 5.4%. No source was found with a fractional polarisation higher than 21%. With a total area of 6.5deg^2^ and a detection threshold of 6.25{sigma}, we find 23 polarised sources per deg^2^. Based on our multi-wavelength analysis, we find that our sample consists of AGN only. We find a discrepancy between archival number counts and those present in our data, which we attribute to sample variance (i.e. large-scale structures). Considering the absolute radio luminosity, we find a general trend of increased probability of detecting weak sources at low redshift and strong sources at high redshift. We attribute this trend to a selection bias. Further, we find an anti-correlation between fractional polarisation and redshift for our strong-source sample at z>=0.6. A decrease in the fractional polarisation of strong sources with increasing redshift cannot be explained by a constant magnetic field and electron density over cosmic scales; however, the changing properties of cluster environments over cosmic time may play an important role. Disentangling these two effects requires deeper and wider polarisation observations as well as better models of the morphology and strength of cosmic magnetic fields.
- ID:
- ivo://CDS.VizieR/J/A+A/658/A4
- Title:
- LOFAR imaging of Arp299 at 150MHz
- Short Name:
- J/A+A/658/A4
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- We present the spatially resolved emission of Arp 299 revealed by 150 MHz international baseline Low-Frequency Array (LOFAR) and 1.4, 5.0, and 8.4GHz Very Large Array (VLA) observations. Nowadays, LOFAR allows to compare its resolution with the VLA ones. These will allow us to obtain pixel-by-pixel spectral imdex maps and local radio SEDs. With this information we recover the nature of the ISM around the main structures (either clumpy or continuous), as well as magnetic fields. The 150MHz image also shows structures never seen before, pointing out that these radio-frequencies observations are a perfect tool to detect them.
- ID:
- ivo://CDS.VizieR/J/A+A/648/A104
- Title:
- LOFAR LBA Sky Survey. I.
- Short Name:
- J/A+A/648/A104
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- LOFAR is the only radio telescope that is presently capable of high-sensitivity, high-resolution (i.e. <1mJy/b and <15") observations at ultra-low frequencies (<100MHz). To utilise these capabilities, the LOFAR Surveys Key Science Project is undertaking a large survey to cover the entire northern sky with Low Band Antenna (LBA) observations. The LOFAR LBA Sky Survey (LoLSS) aims to cover the entire northern sky with 3170 pointings in the frequency range 42-66MHz, at a resolution of 15-arcsec and at a sensitivity of 1mJy/beam (1{sigma}. Here we outline the survey strategy, the observational status, the current calibration techniques, and briefly describe several scientific motivations. We also describe the preliminary public data release. The preliminary images were produced using a fully automated pipeline that aims to correct all direction-independent effects in the data. Whilst the direction-dependent effects, such as those from the ionosphere, are not yet corrected, the images presented in this work are still 10 times more sensitive than previous surveys available at these low frequencies. The preliminary data release covers 740deg^2^ around the HETDEX spring field region at a resolution of 47" with a median noise level of 5mJy/beam. The images and the catalogue with 25247 sources are publicly released. We demonstrate that the system is capable of reaching an rms noise of 1mJy/beam and the resolution of 15" once direction-dependent effects are corrected for. LoLSS will provide the ultra-low-frequency information for hundreds of thousands of radio sources, providing critical spectral information and producing a unique dataset that can be used for a wide range of science topics such as: the search for high redshift galaxies and quasars, the study of the magnetosphere of exoplanets, and the detection of the oldest populations of cosmic-rays in galaxies, clusters of galaxies, and from AGN activity.
- ID:
- ivo://CDS.VizieR/J/A+A/658/A2
- Title:
- LOFAR Long-Baseline Calibrator Survey. II.
- Short Name:
- J/A+A/658/A2
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to provide a set of calibrators for the LOFAR array. Here we present the complete survey, for which a preliminary analysis was published in 2016 using 20% of the survey data. The final catalogue consists of 30007 observations of 24713 sources in the northern sky, selected for a combination of bright low-frequency radio flux density and flat spectral index using existing surveys (WENSS, VLSS and MSSS). Approximately 1 calibrator per square degree is identified by the detection of compact flux, for declinations north of 30{deg} and a considerably lower density south of this point. The catalogue contains indicators of degree of correlated flux on baselines between the Dutch core and each of the international stations for all of the observations. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact fluxes, on scales between 300mas and 2-arcseconds, for sources observed in the survey.
- ID:
- ivo://CDS.VizieR/J/A+A/574/A73
- Title:
- LOFAR long baselines at 140MHz Calibrators
- Short Name:
- J/A+A/574/A73
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- An efficient means of locating calibrator sources for international LOw Frequency ARray (LOFAR) is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140MHz. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full international LOFAR array. Sources were preselected on the basis of 325MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators.
- ID:
- ivo://CDS.VizieR/J/A+A/598/A104
- Title:
- LOFAR Two-metre Sky Survey
- Short Name:
- J/A+A/598/A104
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The LOFAR Two-metre Sky Survey (LoTSS) is a deep 120-168MHz imaging survey that will eventually cover the entire Northern sky. Each of the 3170 pointings will be observed for 8 hrs, which, at most declinations, is sufficient to produce ~5-arcsec resolution images with a sensitivity of ~0.1mJy/beam and accomplish the main scientific aims of the survey which are to explore the formation and evolution of massive black holes, galaxies, clusters of galaxies and large-scale structure. Due to the compact core and long baselines of LOFAR, the images provide excellent sensitivity to both highly extended and compact emission. For legacy value, the data are archived at high spectral and time resolution to facilitate subarcsecond imaging and spectral line studies. In this paper we provide an overview of the LoTSS. We outline the survey strategy, the observational status, the current calibration techniques, a preliminary data release, and the anticipated scientific impact. The preliminary images that we have released were created using a fully-automated but direction-independent calibration strategy and are significantly more sensitive than those produced by any existing large-area low-frequency survey. In excess of 44,000 sources are detected in the images that have a resolution of 25-arcsec, typical noise levels of less than 0.5mJy/beam, and cover an area of over 350 square degrees in the region of the HETDEX Spring Field (right ascension 10:45:00 to 15:30:00 and declination 45:00:00 to 57:00:00).
- 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.
