Search

Start Over Subject radio astronomy Content Level Research
881. Local Group dSph radio survey with ATCA
ID:
ivo://CDS.VizieR/J/MNRAS/448/3731
Title:
Local Group dSph radio survey with ATCA
Short Name:
J/MNRAS/448/3731
Date:
21 Oct 2021
Publisher:
CDS
Description:
Dwarf spheroidal (dSph) galaxies are key objects in near-field cosmology, especially in connection to the study of galaxy formation and evolution at small scales. In addition, dSphs are optimal targets to investigate the nature of dark matter. However, while we begin to have deep optical photometric observations of the stellar population in these objects, little is known so far about their diffuse emission at any observing frequency, and hence on thermal and non-thermal plasma possibly residing within dSphs. In this paper, we present deep radio observations of six local dSphs performed with the Australia Telescope Compact Array (ATCA) at 16cm wavelength. We mosaicked a region of radius of about 1{deg} around three 'classical' dSphs, Carina, Fornax, and Sculptor, and of about half of degree around three 'ultrafaint' dSphs, BootesII, Segue2, and Hercules. The rms noise level is below 0.05 mJy for all the maps. The restoring beams full width at half-maximum ranged from 4.2"x2.5" to 30.0"x2.1" in the most elongated case. A catalogue including the 1392 sources detected in the six dSph fields is reported. The main properties of the background sources are discussed, with positions and fluxes of brightest objects compared with the FIRST, NVSS, and SUMSS observations of the same fields. The observed population of radio emitters in these fields is dominated by synchrotron sources. We compute the associated source number counts at 2GHz down to fluxes of 0.25mJy, which prove to be in agreement with AGN count models.
882. Lockman Hole Apertif map at 1.4GHz
ID:
ivo://CDS.VizieR/J/A+A/648/A9
Title:
Lockman Hole Apertif map at 1.4GHz
Short Name:
J/A+A/648/A9
Date:
21 Oct 2021
Publisher:
CDS
Description:
Supermassive black holes at the centres of galaxies can cycle through periods of activity and quiescence. Characterising the duty cycle of active galactic nuclei (AGN) is crucial for understanding the impact of the energy they release on the host galaxy. For radio AGN, this can be done by identifying dying (remnant) and restarted radio galaxies from their radio spectral properties. Using the combination of the images at 1400MHz produced by Apertif, the new phased-array feed receiver installed on the Westerbork Synthesis Radio Telescope, and images at 150MHz provided by LOFAR, we have derived resolved spectral index images (at a resolution of 15 arcsec) for all the sources within an approximately 6 deg^2^ area of the Lockman Hole region. In this way, we were able to select 15 extended radio sources with emission (partly or entirely) characterised by extremely steep spectral indices (steeper than 1.2). These objects represent cases of radio sources in the remnant or the restarted phases of their life cycle. Our findings confirm that these objects are not as rare as previously thought, suggesting a relatively fast cycle. They also show a variety of properties that can be relevant for modelling the evolution of radio galaxies. For example, the restarted activity can occur while the remnant structure from a previous phase of activity is still visible. This provides constraints on the duration of the 'off' (dying) phase. In extended remnants with ultra-steep spectra at low frequencies, the activity likely stopped a few hundred megayears ago, and they correspond to the older tail of the age distribution of radio galaxies, in agreement with the results of simulations of radio source evolution. We find remnant radio sources with a variety of structures (from double-lobed to amorphous), possibly suggesting different types of progenitors. The present work sets the stage for exploiting the powerful tool of low-frequency spectral index studies of extended sources by taking advantage of the large areas common to the LOFAR and the Apertif surveys.
883. Lockman Hole 10C sources radio spectral indices
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.
884. Lockman Hole low-frequency radio sources
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.
885. Lockman Hole North 3GHz catalogue
ID:
ivo://CDS.VizieR/J/MNRAS/462/2934
Title:
Lockman Hole North 3GHz catalogue
Short Name:
J/MNRAS/462/2934
Date:
21 Oct 2021
Publisher:
CDS
Description:
This is the second of two papers describing the observations and source catalogues derived from sensitive $3$-GHz images of the Lockman Hole North using the Karl G. Jansky Very Large Array. We describe the reduction and cataloguing process, which yielded an image with 8-arcsec resolution and instrumental noise of {sigma}_n=1.01uJy/beam rms (before primary beam corrections) and a catalogue of 558 sources detected above 5{sigma}_n. We include details of how we estimate source spectral indices across the 2GHz VLA bandwidth, finding a median index of -0.76+/-0.04. Stacking of source spectra reveals a flattening of spectral index with decreasing flux density. We present a source count derived from the catalogue. We show a traditional count estimate compared with a completely independent estimate made via a P(D) confusion analysis, and find very good agreement. Cross-matches of the catalogue with X-ray, optical, infrared, radio, and redshift catalogues are also presented. The X-ray, optical and infrared data, as well as AGN selection criteria allow us to classify 10 per cent as radio-loud AGN, 28 percent as radio-quiet AGN, and 58 per cent as star-forming galaxies, with only 4 per cent unclassified.
886. Lockman Hole Polarised Sources at 1.4GHz
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.
887. Lockman Hole region 325MHz source catalogue
ID:
ivo://CDS.VizieR/J/MNRAS/495/4071
Title:
Lockman Hole region 325MHz source catalogue
Short Name:
J/MNRAS/495/4071
Date:
21 Oct 2021
Publisher:
CDS
Description:
One of the key science goals for the most sensitive telescopes, both current and upcoming, is the detection of the redshifted 21-cm signal from the Cosmic Dawn and Epoch of Reionization. The success of detection relies on accurate foreground modelling for their removal from data sets. This paper presents the characterization of astrophysical sources in the Lockman Hole region. Using 325-MHz data obtained from the Giant Metrewave Radio Telescope, a 6{deg}x6{deg} mosaiced map is produced with an rms reaching 50uJy per beam. A source catalogue containing 6186 sources is created, and the Euclidean normalized differential source counts have been derived from it, consistent with previous observations as well as simulations. A detailed comparison of the source catalogue is also made with previous findings - at both lower and higher frequencies. The angular power spectrum (APS) of the diffuse Galactic synchrotron emission is determined for three different Galactic latitudes using the tapered gridded estimator. The values of the APS lie between ~1 and ~100mK^2^. Fitting a power law of the form Al^-{beta}^ gives values of A and {beta} varying across the latitudes considered. This paper demonstrates, for the first time, the variation of the power-law index for diffuse emission at very high Galactic locations. It follows the same trend that is seen at locations near the Galactic plane, thus emphasizing the need for low-frequency observations for developing better models of the diffuse emission.
888. Lockman Hole/XMM VLBI observations
ID:
ivo://CDS.VizieR/J/A+A/551/A97
Title:
Lockman Hole/XMM VLBI observations
Short Name:
J/A+A/551/A97
Date:
21 Oct 2021
Publisher:
CDS
Description:
Active galactic nuclei (AGN) play a decisive role in galaxy evolution, particularly so when they launch powerful jets, which reshape their surroundings. However, identifying them is difficult, since radio observations typically have a resolution between 1arcsec and 10arcsec, which is equally sensitive to radio emission from star-forming activity and from AGN. Very Long Baseline Interferometry (VLBI) observations allow only the most compact non-thermal emission to be to be filtered from radio survey data. The observational and computational demands to do this in large surveys have, until recently, been too high to make this practical. Only the recent advent of wide-field observing techniques have enabled such observations, and we here present the results of a survey of 217 radio sources in the Lockman Hole/XMM field. We detected 65 out of the 217 radio sources and were able to construct, for the first time, the source counts of VLBI-detected AGN.
889. LOFAR Bootes and 3C295 field sources
ID:
ivo://CDS.VizieR/J/ApJ/793/82
Title:
LOFAR Bootes and 3C295 field sources
Short Name:
J/ApJ/793/82
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present Low Frequency Array (LOFAR) Low Band observations of the Bootes and 3C 295 fields. Our images made at 34, 46, and 62 MHz reach noise levels of 12, 8, and 5 mJy/beam, making them the deepest images ever obtained in this frequency range. In total, we detect between 300 and 400 sources in each of these images, covering an area of 17-52 deg^2^. From the observations, we derive Euclidean-normalized differential source counts. The 62 MHz source counts agree with previous GMRT 153 MHz and Very Large Array 74 MHz differential source counts, scaling with a spectral index of -0.7. We find that a spectral index scaling of -0.5 is required to match up the LOFAR 34 MHz source counts. This result is also in agreement with source counts from the 38 MHz 8C survey, indicating that the average spectral index of radio sources flattens toward lower frequencies. We also find evidence for spectral flattening using the individual flux measurements of sources between 34 and 1400 MHz and by calculating the spectral index averaged over the source population. To select ultra-steep spectrum ({alpha}<-1.1) radio sources that could be associated with massive high-redshift radio galaxies, we compute spectral indices between 62 MHz, 153 MHz, and 1.4 GHz for sources in the Bootes field. We cross-correlate these radio sources with optical and infrared catalogs and fit the spectral energy distribution to obtain photometric redshifts. We find that most of these ultra-steep spectrum sources are located in the 0.7<~z<~2.5 range.
890. LOFAR census of pulsars at low frequencies
ID:
ivo://CDS.VizieR/J/A+A/635/A76
Title:
LOFAR census of pulsars at low frequencies
Short Name:
J/A+A/635/A76
Date:
21 Oct 2021
Publisher:
CDS
Description:
To date, only 69 pulsars have been identified with a detected pulsed radio emission below 100MHz. A LOFAR-core LBA census and a dedicated campaign with the Nancay LOFAR station in stand-alone mode were carried out in the years 2014-2017 in order to extend the known population in this frequency range. In this paper, we aim to extend the sample of known radio pulsars at low frequencies and to produce a catalogue in the frequency range of 25-80 MHz. This will allow future studies to probe the local Galactic pulsar population, in addition to helping explain their emission mechanism, better characterising the low-frequency turnover in their spectra, and obtaining new information about the interstellar medium through the study of dispersion, scattering, and scintillation. We observed 102 pulsars that are known to emit radio pulses below 200MHz and with declination above -30{deg}. We used the the Low Band Antennas (LBA) of the LOw Frequency ARray (LOFAR) international station FR606 at the Nancay Radio Observatory in stand-alone mode, recording data between 25-80MHz. Out of our sample of 102 pulsars, we detected 64. We confirmed the existence of ten pulsars detected below 100 MHz by the LOFAR LBA census for the first time (Bilous et al., 2019, submitted) and we added two more pulsars that had never before been detected in this frequency range. We provided average pulse profiles, DM values, and mean flux densities (or upper limits in the case of non-detections). The comparison with previously published results allows us to identify a hitherto unknown spectral turnover for five pulsars, confirming the expectation that spectral turnovers are a widespread phenomenon.