Diffuse cluster radio sources, in the form of radio halos and relics, reveal the presence of cosmic rays and magnetic fields in the intracluster medium (ICM). These cosmic rays are thought to be ...(re)accelerated through the ICM turbulence and shock waves generated by cluster merger events. Here we characterize the presence of diffuse radio emission in known galaxy clusters in the HETDEX Spring Field, covering 424 deg
2
. For this, we developed a method to extract individual targets from LOFAR observations processed with the LoTSS DDF-pipeline software. This procedure enables improved calibration as well as the joint imaging and deconvolution of multiple pointings of selected targets. The calibration strategy can also be used for LOFAR low-band antenna and international-baseline observations. The fraction of
Planck
PSZ2 catalog clusters with any diffuse radio emission apparently associated with the ICM is 73 ± 17%. We detect a total of ten radio halos and twelve candidate halos in the HETDEX Spring Field. Of these ten radio halos, four are new discoveries, two of which are located in PSZ2 clusters. Five clusters host radio relics, two of which are new discoveries. The fraction of radio halos in
Planck
PSZ2 clusters is 31 ± 11%, or 62 ± 15% when including the candidate radio halos. Based on these numbers, we expect that there will be at least 183 ± 65 radio halos found in the LoTSS survey in PSZ2 clusters, in agreement with past predictions. The integrated flux densities for the radio halos were computed by fitting exponential models to the radio images. From these flux densities, we determine the cluster mass (
M
500
) and Compton
Y
parameter (
Y
500
) 150 MHz radio power (
P
150 MHz
) scaling relations for
Planck
PSZ2-detected radio halos. Using bivariate correlated errors and intrinsic scatter orthogonal regression, we find slopes of 6.13 ± 1.11 and 3.32 ± 0.65 for the
M
500
–
P
150 MHz
and
M
500
–
P
150 MHz
relations, respectively. These values are consistent with the results of previous works.
ABSTRACT
We examine the distribution of radio emission from ∼42 000 quasars from the Sloan Digital Sky Survey, as measured in the LOFAR Two-metre Sky Survey (LoTSS). We present a model of the radio ...luminosity distribution of the quasars that assumes that every quasar displays a superposition of two sources of radio emission: active galactic nuclei (jets) and star formation. Our two-component model provides an excellent match to the observed radio flux density distributions across a wide range of redshifts and quasar optical luminosities; this suggests that the jet-launching mechanism operates in all quasars but with different powering efficiency. The wide distribution of jet powers allows for a smooth transition between the ‘radio-quiet’ and ‘radio-loud’ quasar regimes, without need for any explicit bimodality. The best-fitting model parameters indicate that the star formation rate of quasar host galaxies correlates strongly with quasar luminosity and also increases with redshift at least out to z ∼ 2. For a model where star formation rate scales as $L_{\rm bol}^{\alpha } (1+z)^{\beta }$, we find α = 0.47 ± 0.01 and β = 1.61 ± 0.05, in agreement with far-infrared studies. Quasars contribute ≈0.15 per cent of the cosmic star formation rate density at z = 0.5, rising to 0.4 per cent by z ∼ 2. The typical radio jet power is seen to increase with both increasing optical luminosity and black hole mass independently, but does not vary with redshift, suggesting intrinsic properties govern the production of the radio jets. We discuss the implications of these results for the triggering of quasar activity and the launching of jets.
Context. The phase of radio galaxy evolution after the jets have switched off, often referred to as the remnant phase, is poorly understood and very few sources in this phase are known. Aims. In this ...work we present an extensive search for remnant radio galaxies in the Lockman Hole, a well-studied extragalactic field. We create mock catalogues of low-power radio galaxies based on Monte Carlo simulations to derive first-order predictions of the fraction of remnants in radio flux limited samples for comparison with our Lockman-Hole sample. Methods. Our search for remnant radio galaxies is based on LOFAR observations at 150 MHz combined with public survey data at higher frequencies. To enhance the selection process, and obtain a more complete picture of the remnant population, we use spectral criteria such as ultra-steep spectral index and high spectral curvature, and morphologre biased toward tinuum: galaxie ical criteria such as low radio core prominence and relaxed shapes to identify candidate remnant radio galaxies. Mock catalogues of radio galaxies are created based on existing spectral and dynamical evolution models combined with observed source properties. Results. We have identified 23 candidate remnant radio galaxies which cover a variety of morphologies and spectral characteristics. We suggest that these different properties are related to different stages of the remnant evolution. We find that ultra-steep spectrum remnants represent only a fraction of our remnant sample suggesting a very rapid luminosity evolution of the radio plasma. Results from mock catalogues demonstrate the importance of dynamical evolution in the remnant phase of low-power radio galaxies to obtain fractions of remnant sources consistent with our observations. Moreover, these results confirm that ultra-steep spectrum remnants represent only a small subset of the entire population (~50%) when frequencies higher than 1400 MHz are not included in the selection process, and that they are biased towards old ages.
Aims.
We provide the most accurate estimate yet of the bright end of the infrared (IR) luminosity functions (LFs) and the abundance of hyperluminous IR galaxies (HLIRGs) with IR luminosities >10
13
L
...⊙
, thanks to the combination of the high sensitivity, angular resolution, and large area of the LOFAR Deep Fields, which probes an unprecedented dynamic range of luminosity and volume.
Methods.
We cross-match
Herschel
sources and LOFAR sources in Boötes (8.63 deg
2
), Lockman Hole (10.28 deg
2
), and ELAIS-N1 (6.74 deg
2
) with rms sensitivities of ~32, 22, and 20
μ
Jy beam
−1
, respectively. We divide the matched samples into “unique” and “multiple” categories. For the multiple matches, we de-blend the
Herschel
fluxes using the LOFAR positions and the 150-MHz flux densities as priors. We perform spectral energy distribution fitting, combined with multi-wavelength counterpart identifications and photometric redshift estimates, to derive IR luminosities.
Results.
The depth of the LOFAR data allows us to identify highly complete (~92% completeness) samples of bright
Herschel
sources with a simple selection based on the 250
μ
m flux density (45, 40, and 35 mJy in Boötes, Lockman Hole, and ELAIS-N1, respectively). Most of the bright
Herschel
sources fall into the unique category (i.e. a single LOFAR counterpart). For the multiple matches, there is excellent correspondence between the radio emission and the far-IR emission. We find a good agreement in the IR LFs with a previous study out to
z
~ 6 which used de-blended
Herschel
data. Our sample gives the strongest and cleanest indication to date that the population of HLIRGs has surface densities of ~5 to ~18/deg
2
(with variations due to a combination of the applied flux limit and cosmic variance) and an uncertainty of a factor of ≲2. In comparison, the GALFORM semi-analytic model significantly under-predicts the abundance of HLIRGs.
Abstract
This paper presents a study of the redshift evolution of radio-loud active galactic nuclei (AGN) as a function of the properties of their galaxy hosts in the Boötes field. To achieve this we ...match low-frequency radio sources from deep 150-MHz LOFAR (LOw Frequency ARray) observations to an I-band-selected catalogue of galaxies, for which we have derived photometric redshifts, stellar masses, and rest-frame colours. We present spectral energy distribution (SED) fitting to determine the mid-infrared AGN contribution for the radio sources and use this information to classify them as high- versus low-excitation radio galaxies (HERGs and LERGs) or star-forming galaxies. Based on these classifications, we construct luminosity functions for the separate redshift ranges going out to z = 2. From the matched radio-optical catalogues, we select a sub-sample of 624 high power (P150 MHz > 1025 W Hz−1) radio sources between 0.5 ≤ z < 2. For this sample, we study the fraction of galaxies hosting HERGs and LERGs as a function of stellar mass and host galaxy colour. The fraction of HERGs increases with redshift, as does the fraction of sources in galaxies with lower stellar masses. We find that the fraction of galaxies that host LERGs is a strong function of stellar mass as it is in the local Universe. This, combined with the strong negative evolution of the LERG luminosity functions over this redshift range, is consistent with LERGs being fuelled by hot gas in quiescent galaxies.
Coherent radio emission from stars can be used to constrain fundamental coronal plasma parameters, such as plasma density and magnetic field strength. It is also a probe of chromospheric and ...magnetospheric acceleration mechanisms. Close stellar binaries, such as RS Canum Venaticorum (RS CVn) systems, are particularly interesting as their heightened level of chromospheric activity and possible direct magnetic interaction make them a unique laboratory to study coronal and magnetospheric acceleration mechanisms. RS CVn binaries are known to be radio-bright but coherent radio emission has only conclusively been detected previously in one system. Here, we present a population of 14 coherent radio emitting RS CVn systems. We identified the population in the ongoing LOFAR Two Metre Sky Survey as circularly polarised sources at 144 MHz that are astrometrically associated with known RS CVn binaries. We show that the observed emission is powered by the electron cyclotron maser instability. We use numerical calculations of the maser’s beaming geometry to argue that the commonly invoked ‘loss-cone’ maser cannot generate the necessary brightness temperature in some of our detections and that a more efficient instability, such as the shell or horseshoe maser, must be invoked. Such maser configurations are known to operate in planetary magnetospheres. We also outline two acceleration mechanisms that could produce coherent radio emission, one where the acceleration occurs in the chromosphere and one where the acceleration is due to an electrodynamic interaction between the stars. We propose radio and optical monitoring observations that can differentiate between these two mechanisms.
Context.
Radio halos are megaparsec-scale diffuse radio sources mostly located at the centres of merging galaxy clusters. The common mechanism invoked to explain their origin is the re-acceleration ...of relativistic particles caused by large-scale turbulence.
Aims.
Current re-acceleration models predict that a significant number of halos at high redshift should be characterised by very steep spectra (
α
< −1.5) because of increasing inverse Compton energy losses. In this paper, we investigate the spectral index properties of a sample of nine clusters selected from the second
Planck
Sunyaev-Zel’dovich catalogue showing diffuse radio emission with the Low Frequency Array (LOFAR) in the 120–168 MHz band. This is the first time that radio halos discovered at low frequencies are followed up at higher frequencies.
Methods.
We analysed upgraded Giant Metrewave Radio Telescope (uGMRT) observations in Bands 3 and 4, that is, 250–500 and 550–900 MHz respectively. These observations were combined with existing LOFAR data to obtain information on the spectral properties of the diffuse radio emission.
Results.
We find diffuse radio emission in the uGMRT observations for five of the nine high-
z
radio halos previously discovered with LOFAR. For those, we measure spectral indices in the range of −1 to −1.4. For the uGMRT non-detections, we estimated that the halos should have a spectral index steeper than −1.5. We also confirm the presence of one candidate relic.
Conclusions.
Despite the small number of clusters, we find evidence that about half of the massive and merging clusters at high redshift host radio halos with a very steep spectrum. This is in line with theoretical predictions, although larger statistical samples are necessary to test models.
Galaxy clusters grow by mergers with other clusters and galaxy groups. These mergers create shock waves within the intracluster medium (ICM) that can accelerate particles to extreme energies. In the ...presence of magnetic fields, relativistic electrons form large regions emitting synchrotron radiation, the so-called radio relics. Behind the shock front, synchrotron and inverse Compton losses cause the radio spectral index to steepen away from the shock front. In a binary cluster merger, two shock waves are generated which move diametrically outwards along the merger axis. Two radio relics can then form on both sides of the cluster centre. An example of such a cluster is CIZA J2242.8+5301, where very clear spectral steepening in the downstream region is observed. The main relic has a total extent of 1700 kpc, while its width is only 55 kpc. Together with the high observed polarization fraction, this implies that the relic is seen very close to edge-on which makes it easier to constrain the merger geometry. Here we present hydrodynamical simulations of idealized binary cluster mergers with the aim of constraining the merger scenario for this cluster. From our simulations, we find that CIZA J2242.8+5301 is probably undergoing a merger in the plane of the sky (less than 10° from edge-on) with a mass ratio (M
1:M
2) of about 2:1, and an impact parameter ≲400 kpc. We find that the core passage of the clusters happened about 1 Gyr ago. We conclude that double relics can set constraints on the mass ratios, impact parameters, time-scales and viewing geometry of binary cluster mergers, which is particularly useful when detailed X-ray observations are not available. In addition, the presence of large radio relics can be used to constrain the degree of clumping in the outskirts of the ICM, which is important to constrain the baryon fraction, density and entropy profiles, around the virial radius and beyond. We find that the amplitude of density fluctuations, with sizes of ≲200 kpc, in the relic in CIZA J2242.8+5301 is not larger than 30 per cent.
ABSTRACT
We present LOFAR observations at 150 MHz of the borderline FRI/FRII giant radio galaxy NGC 6251. This paper presents the most sensitive and highest resolution images of NGC 6251 at these ...frequencies to date, revealing for the first time a low-surface-brightness extension to the northern lobe, and a possible backflow associated with the southern lobe. The integrated spectra of components of NGC 6251 are consistent with previous measurements at higher frequencies, similar to results from other LOFAR studies of nearby radio galaxies. We find the outer structures of NGC 6251 to be either at equipartition or slightly electron dominated, similar to those of FRII sources rather than FRIs, but this conclusion remains tentative because of uncertainties associated with the geometry and the extrapolation of X-ray measurements to determine the external pressure distribution on the scale of the outer lobes. We place lower limits on the ages of the extension of the northern lobe and the backflow of the southern lobe of t ≳ 250 Myr and t ≳ 210 Myr, respectively. We present the first detection of polarization at 150 MHz in NGC 6251. Taking advantage of the high Faraday resolution of LOFAR, we place an upper limit on the magnetic field in the group of $B \lt 0.2 \, (\Lambda _B / 10\, {\rm kpc})^{-0.5}\, \mu$G for a coherence scale of $\Lambda _B \lt 60\, {\rm kpc}$ and $B \lt 13\, \mu \textrm{G~for} \Lambda _B = 240$ kpc.
The LOFAR Two-metre Sky Survey (LoTSS) will cover the full northern sky and, additionally, aims to observe the LoTSS deep fields to a noise level of ≲10
μ
Jy beam
−1
over several tens of square ...degrees in areas that have the most extensive ancillary data. This paper presents the ELAIS-N1 deep field, the deepest of the LoTSS deep fields to date. With an effective observing time of 163.7 h, it reaches a root mean square noise level of ≲20
μ
Jy beam
−1
in the central region (and below 30
μ
Jy beam
−1
over 10 square degrees). The resolution is ~6 arcsecs and 84 862 radio sources were detected in the full area (68 square degrees) with 74 127 sources in the highest quality area at less than 3 degrees from the pointing centre. The observation reaches a sky density of more than 5000 sources per square degree in the central region (~5 square degrees). We present the calibration procedure, which addresses the special configuration of some observations and the extended bandwidth covered (115–177 MHz; central frequency 146.2 MHz) compared to standard LoTSS. We also describe the methods used to calibrate the flux density scale using cross-matching with sources detected by other radio surveys in the literature. We find the flux density uncertainty related to the flux density scale to be ~6.5 per cent. By studying the variations of the flux density measurements between different epochs, we show that relative flux density calibration is reliable out to about a 3 degree radius, but that additional flux density uncertainty is present for all sources at about the 3 per cent level; this is likely to be associated with residual calibration errors, and is shown to be more significant in datasets with poorer ionosphere conditions. We also provide intra-band spectral indices, which can be useful to detect sources with unusual spectral properties. The final uncertainty in the flux densities is estimated to be ~10 per cent for ELAIS-N1.
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