Search

We investigate the nature of low-luminosity radio-loud active galactic nuclei (RLAGN) selected from the LOFAR Two-metre Sky Survey (LoTSS) first data release (DR1). Using optical, mid-infrared, and radio data, we have conservatively selected 55 radiative AGN candidates from DR1 within the redshift range $0.03$$1-3$ kpc, 42 are compact at the limiting resolution of 0.35 arcsec (taking an upper limit on the projected physical size, this corresponds to less than 1 kpc), and three are undetected. The extended objects display a wide range of radio morphologies: two-jet (5), one-jet (4), and double-lobed (1). We present the radio spectra of all detected radio sources which range from steep to flat/inverted and span the range seen for other compact radio sources such as compact symmetric objects (CSOs), compact steep spectrum (CSS) sources, and gigahertz peaked-spectrum (GPS) sources. Assuming synchrotron self-absorption (SSA) for flat/inverted radio spectrum sources, we predict small physical sizes for compact objects to range between $2-53$ pc. Alternatively, using free-free absorption (FFA) models, we have estimated the free electron column depth for all compact objects, assuming a homogeneous absorber. We find that these objects do not occupy a special position on the power/linear size ($P-D$) diagram but some share a region with radio-quiet quasars (RQQs) and so-called `FR0' sources in terms of radio luminosity and linear size., Comment: Accepted for publication in the Monthly Notices of the Royal Astronomy Society (MNRAS) journal

Radio jets are present in a diverse sample of AGN. However, the mechanisms of jet powering are not fully understood, and it is yet unclear to what extent they obey mass-invariant scaling relations, similar to those found for the triggering and fuelling of X-ray selected AGN. We study the incidence of eROSITA/eFEDS X-ray and LOFAR radio AGN as a function of several stellar mass normalised AGN power indicators. A new sample of radio AGN from the LOFAR-eFEDS survey is defined and we publicly release this catalogue, including host galaxy counterparts from the Legacy Survey DR9, LOFAR radio morphologies and host galaxy properties from the complete, spectroscopic (z<0.4) GAMA09 survey. The fraction of GAMA09 galaxies hosting radio, X-ray and both radio and X-ray AGN are calculated as a function of the specific black hole kinetic ($\lambda_{\rm Jet}$) and radiative ($\lambda_{\rm Edd}$) power. The incidence of eFEDS X-ray AGN as a function of $\lambda_{\rm Edd}$ shows the same mass-invariance as found in past studies. Meanwhile, radio AGN, regardless of their morphology, are more likely to be hosted in more massive galaxies, at all $\lambda_{\rm Jet}$. Across the stellar mass range, the compact radio AGN incidence follows the same power-law distribution, showing that it is not only high mass galaxies that host high power radio AGN and vice versa. On the other hand, the incidence of compact and complex radio AGN is boosted at the highest jet powers, diverging from a simple power-law. Interestingly, this increased incidence cannot be explained by more powerful radio AGN lying in more dense environments which could naturally boost their radio luminosity. Overall, we show that statistical incidence studies are a powerful method to probe disk-jet coupling for different AGN accretion modes, although future work on a more reliable determination of jet power for diverse samples of radio AGN is needed., Comment: 30 pages, 32 figures, accepted for publication in A&A

The second data release of the LOFAR Two-Metre Sky Survey (LoTSS) covers 27% of the northern sky, with a total area of $\sim 5,700$ deg$^2$. The high angular resolution of LOFAR with Dutch baselines (6 arcsec) allows us to carry out optical identifications of a large fraction of the detected radio sources without further radio followup; however, the process is made more challenging by the many extended radio sources found in LOFAR images as a result of its excellent sensitivity to extended structure. In this paper we present source associations and identifications for sources in the second data release based on optical and near-infrared data, using a combination of a likelihood-ratio cross-match method developed for our first data release, our citizen science project Radio Galaxy Zoo: LOFAR, and new approaches to algorithmic optical identification, together with extensive visual inspection by astronomers. We also present spectroscopic or photometric redshifts for a large fraction of the optical identifications. In total 4,116,934 radio sources lie in the area with good optical data, of which 85% have an optical or infrared identification and 58% have a good redshift estimate. We demonstrate the quality of the dataset by comparing it with earlier optically identified radio surveys. This is by far the largest ever optically identified radio catalogue, and will permit robust statistical studies of star-forming and radio-loud active galaxies., Comment: 29 pages. Accepted by A&A; data products available at https://lofar-surveys.org/dr2_release.html

A Faraday rotation measure (RM) catalogue, or RM Grid, is a valuable resource for the study of cosmic magnetism. Using the second data release (DR2) from the LOFAR Two-metre Sky Survey (LoTSS), we have produced a catalogue of 2461 extragalactic high-precision RM values across 5720 deg$^{2}$ of sky (corresponding to a polarized source areal number density of $\sim$0.43 deg$^{-2}$). The linear polarization and RM properties were derived using RM synthesis from the Stokes $Q$ and $U$ channel images at an angular resolution of 20'' across a frequency range of 120 to 168 MHz with a channel bandwidth of 97.6 kHz. The fraction of total intensity sources ($>1$ mJy beam$^{-1}$) found to be polarized was $\sim$0.2%. The median detection threshold was 0.6 mJy beam$^{-1}$ ($8\sigma_{QU}$), with a median RM uncertainty of 0.06 rad m$^{-2}$ (although a systematic uncertainty of up to 0.3 rad m$^{-2}$ is possible, after the ionosphere RM correction). The median degree of polarization of the detected sources is 1.8%, with a range of 0.05% to 31%. Comparisons with cm-wavelength RMs indicate minimal amounts of Faraday complexity in the LoTSS detections, making them ideal sources for RM Grid studies. Host galaxy identifications were obtained for 88% of the sources, along with redshifts for 79% (both photometric and spectroscopic), with the median redshift being 0.6. The focus of the current catalogue was on reliability rather than completeness, and we expect future versions of the LoTSS RM Grid to have a higher areal number density. In addition, 25 pulsars were identified, mainly through their high degrees of linear polarization., Comment: 20 pages, 23 figures, 3 tables. Accepted for publication in MNRAS. Go to https://lofar-mksp.org/data/ to download the catalogue and associated data products

In this data release from the 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$^\circ$30' and 1h00m +28$^\circ$00' and spanning 4178 and 1457 square degrees respectively. The images were derived from 3,451hrs (7.6PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4,396,228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6" resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144MHz have: a median rms sensitivity of 83$\mu$Jy/beam; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2"; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8mJy/beam. By creating three 16MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of +/-0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20" resolution 120-168MHz continuum images have a median rms sensitivity of 95$\mu$Jy/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 480 x 97.6 kHz 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., Comment: 23 figures, 1 table and 29 pages. The catalogues, images and uv-data associated with this data release are publicly available via https://lofar-surveys.org/

Radio-loud active galaxies have two accretion modes [radiatively inefficient (RI) and radiatively efficient (RE)], with distinct optical and infrared signatures, and two jet dynamical behaviours, which in arcsec- to arcmin-resolution radio surveys manifest primarily as centre- or edge-brightened structures [Fanaroff-Riley (FR) class I and II]. The nature of the relationship between accretion mode and radio morphology (FR class) has been the subject of long debate. We present a comprehensive investigation of this relationship for a sample of 286 well-resolved radio galaxies in the LOFAR Two-metre Sky Survey Deep Fields (LoTSS-Deep) first data release, for which robust morphological and accretion mode classifications have been made. We find that two-thirds of luminous FRII radio galaxies are RI, and identify no significant differences in the visual appearance or source dynamic range (peak/mean surface brightness) of the RI and RE FRIIs, demonstrating that both RI and RE systems can produce FRII structures. We also find a significant population of low-luminosity FRIIs (predominantly RI), supporting our earlier conclusion that FRII radio structures can be produced at all radio luminosities. We demonstrate that in the luminosity range where both morphologies are present, the probability of producing FRI or FRII radio morphology is directly linked to stellar mass, while across all morphologies and luminosities, RE accretion occurs in systems with high specific star formation rate, presumably because this traces fuel availability. In summary, the relationship between accretion mode and radio morphology is very indirect, with host-galaxy environment controlling these two key parameters in different ways., Comment: 22 pages, 17 figures, 7 tables. Accepted for publication in MNRAS. Update with minor changes to match published version

Extended radio sources are an important minority population in modern deep radio surveys, because they enable detailed investigation of the physics governing radio-emitting regions such as active galaxies and their environments. Cross-identification of radio sources with optical host galaxies is challenging for this extended population, due to their morphological complexity and multiple potential counterparts. In the first data release of the Low-frequency array (LOFAR) Two-metre Sky Survey (LoTSS DR1) the automated likelihood ratio for compact sources was supplemented by a citizen science visual identification process for extended sources. In this paper we present a novel method for automating the host identification of extended sources by using ridgelines, which trace the assumed direction of fluid-flow through the points of highest flux density. Applying a new code, RL-Xid, to LoTSS DR1, we demonstrate that ridgelines are versatile; by providing information about spatial structure and brightness distributions, they can be used both for optical host identification and morphological studies in radio surveys. RL-Xid draws ridgelines for 85 per cent of sources brighter than 10 mJy and larger than 15 arcsec, with an improved performance of 96 per cent for the subset >30 mJy and >60 arcsec. Using a sample of sources with known hosts from LoTSS DR1, we demonstrate that RL-Xid successfully identifies the host for 98 per cent of the sources with successfully drawn ridgelines, and performs at a comparable level to visual identification via citizen science. We also demonstrate that ridgeline brightness profiles provide a promising automated technique for morphological classification., Comment: 16 pages, 7 figures, accepted for publication in MNRAS

Previous studies have shown that physically small, low-luminosity radio galaxies, which we refer to as galaxy scale jets (GSJ), could potentially have a significant effect upon the host galaxy's evolution. Using 6 arcsec resolution images taken from the first release of the LOFAR Two Metre Sky Survey (LoTSS DR1), we identified a representative sample of nine potential GSJ for which we obtained high-resolution, 2-4 GHz data using the Karl G. Jansky Very Large Array (VLA). Using these data we aim to verify the GSJ nature of these sources as well as investigating the potential role of feedback. Our VLA images reveal a diversity of structures, confirm the hosts for four of the sources and find that a fifth is the first known example of a galaxy-scale remnant showing that some radio galaxies never grow beyond the GSJ stage. We also derive spectral ages and the first estimates of the lobe expansion speeds of GSJ. We find our GSJ have maximum spectral ages of 60 Myr with most between about 5 and 20 Myr, consistent with being located along an evolutionary path joining compact sources and larger radio galaxies. We find lobe advance speeds a few times the local sound speed, with most GSJ predicted to be driving strong shocks into their environment and having a significant impact upon the host's evolution. Our discovery of a remnant GSJ, which will eventually transfer all of its energy directly into the local environment, represents an important and previously hidden aspect of AGN life cycles., Comment: 19 pages, 12 figures, accepted by MNRAS

[abridged] The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2,000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz, although this is technically and logistically challenging. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LOFAR Two-metre Sky Survey (LoTSS) pointing. We perform in-field delay calibration, solution referencing to other calibrators, self-calibration, and imaging of example directions of interest in the field. For this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5 degrees away, while phase solution transferral works well over 1 degree. We demonstrate a check of the astrometry and flux density scale. Imaging in 17 directions, the restoring beam is typically 0.3" x 0.2" although this varies slightly over the entire 5 square degree field of view. We achieve ~80 to 300 $\mu$Jy/bm image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 $\mu$Jy/bm for the 8 hour observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image ~900 sources per LoTSS pointing. This equates to ~3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution (LoTSS-HR) makes this estimate a lower limit., Comment: Accepted to a special issue of A&A on sub-arcsecond imaging with LOFAR. 24 pages, 16 figures

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 150~MHz of the ELAIS-N1, Lockman Hole, and Bo\"{o}tes fields, down to RMS sensitives of around 20, 22, and 32$~\mu$Jy\,beam$^{-1}$, 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 $\approx$~26~\mbox{deg$^{2}$}. We first generated improved multi-wavelength catalogues in ELAIS-N1 and Lockman Hole; combined with the existing catalogue for Bo\"{o}tes, 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 81\,951 radio-detected sources, with counterparts identified and multi-wavelength properties presented for 79\,820 ($>$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 $z\sim3-4$. 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., Comment: 34 pages, 14 figures and 14 tables. Accepted for publication in A&A special issue. This paper is part of the 1st data release of the LoTSS Deep Fields. Electronic data catalogues will be made available on journal publication

The effects of feedback from high luminosity radio-loud AGN have been extensively discussed in the literature, but feedback from low-luminosity radio-loud AGN is less well understood. The advent of high sensitivity, high angular resolution, large field of view telescopes such as LOFAR is now allowing wide-area studies of such faint sources for the first time. Using the first data release of the LOFAR Two Metre Sky Survey (LoTSS) we report on our discovery of a population of 195 radio galaxies with 150 MHz luminosities between $3\times10^{22}$ and $1.5\times10^{25}\text{ W Hz}^{-1}$ and total radio emission no larger than 80 kpc. These objects, which we term galaxy-scale jets (GSJ), are small enough to be directly influencing the evolution of the host on galaxy scales. We report upon the typical host properties of our sample, finding that 9 per cent are hosted by spirals with the remainder being hosted by elliptical galaxies. Two of the spiral-hosted GSJ are highly unusual with low radio luminosities and FRII-like morphology. The host properties of our GSJ show that they are ordinary AGN observed at a stage in their life shortly after the radio emission has expanded beyond the central regions of the host. Based on our estimates, we find that about half of our GSJ have internal radio lobe energy within an order of magnitude of the ISM energy so that, even ignoring any possible shocks, GSJ are energetically capable of affecting the evolution of the host. The current sample of GSJ will grow in size with future releases of LoTSS and can also form the basis for further studies of feedback from low-luminosity radio sources., Comment: 23 pages, accepted by MNRAS

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 $t \gtrsim 250$ Myr and $t \gtrsim 210$ Myr respectively. We present the first detection of polarisation 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 $B < 0.2 (\Lambda_B / 10 {\rm kpc})^{-0.5} \mu$G for a coherence scale of $\Lambda_B < 60 {\rm kpc}$ and $B < 13 \mu$G for $\Lambda_B = 240$ kpc., Comment: 19 pages, 17 figures; accepted by MNRAS

We review current understanding of the population of radio galaxies and radio-loud quasars from an observational perspective, focusing on their large-scale structures and dynamics. We discuss the physical conditions in radio galaxies, their fuelling and accretion modes, host galaxies and large-scale environments, and the role(s) they play as engines of feedback in the process of galaxy evolution. Finally we briefly summarise other astrophysical uses of radio galaxy populations, including the study of cosmic magnetism and cosmological applications, and discuss future prospects for advancing our understanding of the physics and feedback behaviour of radio galaxies., Comment: To appear in New Astronomy Reviews special volume '100 Years of Astrophysical Jets' (eds. Rob Fender and Ralph Wijers). 43 pages

The relative positions of the high and low surface brightness regions of radio-loud active galaxies in the 3CR sample were found by Fanaroff and Riley to be correlated with their luminosity. We revisit this canonical relationship with a sample of 5805 extended radio-loud AGN from the LOFAR Two-Metre Sky Survey (LoTSS), compiling the most complete dataset of radio-galaxy morphological information obtained to date. We demonstrate that, for this sample, radio luminosity does *not* reliably predict whether a source is edge-brightened (FRII) or centre-brightened (FRI). We highlight a large population of low-luminosity FRIIs, extending three orders of magnitude below the traditional FR break, and demonstrate that their host galaxies are on average systematically fainter than those of high-luminosity FRIIs and of FRIs matched in luminosity. This result supports the jet power/environment paradigm for the FR break: low-power jets may remain undisrupted and form hotspots in lower mass hosts. We also find substantial populations that appear physically distinct from the traditional FR classes, including candidate restarting sources and ``hybrids''. We identify 459 bent-tailed sources, which we find to have a significantly higher SDSS cluster association fraction (at $z<0.4$) than the general radio-galaxy population, similar to the results of previous work. The complexity of the LoTSS faint, extended radio sources demonstrates the need for caution in the automated classification and interpretation of extended sources in modern radio surveys, but also reveals the wealth of morphological information such surveys will provide and its value for advancing our physical understanding of radio-loud AGN., Comment: 21 pages, 4 tables, 16 Figures. Accepted for publication on Monthly Notices of the Royal Astronomical Society (MNRAS). Our LoTSS morphological catalogue containing classifications for 5805 extended radio-loud AGN is available from www.lofar-surveys.org/releases.html

We present new 144-MHz LOFAR observations of the prototypical `X-shaped' radio galaxy NGC 326, which show that the formerly known wings of the radio lobes extend smoothly into a large-scale, complex radio structure. We argue that this structure is most likely the result of hydrodynamical effects in an ongoing group or cluster merger, for which pre-existing X-ray and optical data provide independent evidence. The large-scale radio structure is hard to explain purely in terms of jet reorientation due to the merger of binary black holes, a previously proposed explanation for the inner structure of NGC 326. For this reason, we suggest that the simplest model is one in which the merger-related hydrodynamical processes account for all the source structure, though we do not rule out the possibility that a black hole merger has occurred. Inference of the black hole-black hole merger rate from observations of X-shaped sources should be carried out with caution in the absence of deep, sensitive low-frequency observations. Some X-shaped sources may be signposts of cluster merger activity, and it would be useful to investigate the environments of these objects more generally., Comment: 7 pages, 2 colour figures. Accepted by MNRAS

Next-generation radio surveys will yield an unprecedented amount of data, warranting analysis by use of machine learning techniques. Convolutional neural networks are the deep learning technique that has proven to be the most successful in classifying image data. Capsule networks are a more recently developed technique that use capsules comprised of groups of neurons, that describe properties of an image including the relative spatial locations of features. The current work explores the performance of different capsule network architectures against simpler convolutional neural network architectures, in reproducing the classifications into the classes of unresolved, FRI and FRII morphologies. We utilise images from a LOFAR survey which is the deepest, wide-area radio survey to date, revealing more complex radio-source structures compared to previous surveys, presenting further challenges for machine learning algorithms. The 4- and 8-layer convolutional networks attain an average precision of 93.3% and 94.3% respectively, compared to 89.7% obtained with the capsule network, when training on original and augmented images. Implementing transfer learning achieves a precision of 94.4%, that is within the confidence interval of the 8-layer convolutional network. The convolutional networks always outperform any variation of the capsule network, as they prove to be more robust to the presence of noise in images. The use of pooling appears to allow more freedom for the intra-class variability of radio galaxy morphologies, as well as reducing the impact of noise., Comment: 17 pages, 11 figures, Accepted for publication in MNRAS

Double-double radio galaxies (DDRGs) represent a short but unique phase in the life-cycle of some of the most powerful radio-loud active galactic nuclei (RLAGN). These galaxies display large-scale remnant radio plasma in the intergalactic medium left behind by a past episode of active galactic nuclei (AGN) activity, and meanwhile, the radio jets have restarted in a new episode. The knowledge of what causes the jets to switch off and restart is crucial to our understanding of galaxy evolution, while it is important to know if DDRGs form a host galaxy dichotomy relative to RLAGN. We utilised the LOFAR Two-Metre Sky Survey DR1, using a visual identification method to compile a sample of morphologically selected candidate DDRGs, showing two pairs of radio lobes. To confirm the restarted nature in each of the candidate sources, we obtained follow-up observations with the VLA at higher resolution to observe the inner lobes or restarted jets, the confirmation of which created a robust sample of 33 DDRGs. We created a comparison sample of 777 RLAGN from the DR1 catalogue, and compared the optical and infrared magnitudes and colours of their host galaxies. We find that there is no statistically significant difference in the brightness of the host galaxies between double-doubles and single-cycle RLAGN. The DDRG and RLAGN samples also have similar distributions in WISE mid-infrared colours, indicating similar ages of stellar populations and dust levels in the hosts of DDRGs. We conclude that DDRGs and 'normal' RLAGN are hosted by galaxies of the same type, and that DDRG activity is simply a normal part of the life cycle of RLAGN. Restarted jets, particularly for the class of low-excitation radio galaxies, rather than being a product of a particular event in the life of a host galaxy, must instead be caused by smaller scale changes, such as in the accretion system surrounding the black hole., Comment: 20 pages, 2 tables, 54 figures. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available on lofar-surveys.org upon publication of the journal

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$^\circ$00$'$00$''$ to 57$^\circ$00$'$00$''$) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325,694 sources are detected with a signal of at least five times the noise, and the source density is a factor of $\sim 10$ higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S$_{\rm 144 MHz} = 71\,\mu$Jy beam$^{-1}$ 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 $\sim$20 articles that highlight the scientific potential of LoTSS., Comment: 16 figures, 1 table and 22 pages. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available on lofar-surveys.org upon publication of the journal

The LOFAR Two-metre Sky Survey (LoTSS) is a sensitive, high-resolution 120-168 MHz survey of the Northern sky. The LoTSS First Data Release (DR1) presents 424 square degrees of radio continuum observations over the HETDEX Spring Field (10h45m00s $<$ right ascension $<$ 15h30m00s and 45$^\circ$00$'$00$'$ $<$ declination $<$ 57$^\circ$00$'$00$''$) with a median sensitivity of 71$\mu$Jy/beam and a resolution of 6$''$. In this paper we present photometric redshifts (photo-$z$) for 94.4% of optical sources over this region that are detected in the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) 3$\pi$ steradian survey. Combining the Pan-STARRS optical data with mid-infrared photometry from the Wide-field Infrared Survey Explorer, we estimate photo-$z$s using a novel hybrid photometric redshift methodology optimised to produce the best possible performance for the diverse sample of radio continuum selected sources. For the radio-continuum detected population, we find an overall scatter in the photo-$z$ of 3.9% and an outlier fraction ($\left | z_{\rm{phot}} - z_{\rm{spec}} \right | / (1+z_{\rm{spec}}) > 0.15$) of 7.9%. We also find that, at a given redshift, there is no strong trend in photo-$z$ quality as a function of radio luminosity. However there are strong trends as a function of redshift for a given radio luminosity, a result of selection effects in the spectroscopic sample and/or intrinsic evolution within the radio source population. Additionally, for the sample of sources in the LoTSS First Data Release with optical counterparts, we present rest-frame optical and mid-infrared magnitudes based on template fits to the consensus photometric (or spectroscopic when available) redshift., Comment: 16 figures, 1 table and 18 pages. This paper is part of the LOFAR surveys Data Release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, Volume 622. The catalogues and images from the data release will be publicly available at http://www.lofar-surveys.org upon publication of the volume

We constructed a sample of 23,344 radio-loud active galactic nuclei (RLAGN) from the catalogue derived from the LOFAR Two-Metre Sky Survey (LoTSS) survey of the HETDEX Spring field. Although separating AGN from star-forming galaxies remains challenging, the combination of spectroscopic and photometric techniques we used gives us one of the largest available samples of candidate RLAGN. We used the sample, combined with recently developed analytical models, to investigate the lifetime distribution of RLAGN. We show that large or giant powerful RLAGN are probably the old tail of the general RLAGN population, but that the low-luminosity RLAGN candidates in our sample, many of which have sizes $<100$ kpc, either require a very different lifetime distribution or have different jet physics from the more powerful objects. We then used analytical models to develop a method of estimating jet kinetic powers for our candidate objects and constructed a jet kinetic luminosity function based on these estimates. These values can be compared to observational quantities, such as the integrated radiative luminosity of groups and clusters, and to the predictions from models of RLAGN feedback in galaxy formation and evolution. In particular, we show that RLAGN in the local Universe are able to supply all the energy required per comoving unit volume to counterbalance X-ray radiative losses from groups and clusters and thus prevent the hot gas from cooling. Our computation of the kinetic luminosity density of local RLAGN is in good agreement with other recent observational estimates and with models of galaxy formation., Comment: 21 pages, 11 colour figures. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available at https://lofar-surveys.org/ upon publication of the journal

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168 MHz survey of the Northern sky with diverse and ambitious science goals. Many of the scientific objectives of LoTSS rely upon, or are enhanced by, the association or separation of the sometimes incorrectly catalogued radio components into distinct radio sources, and the identification and characterisation of the optical counterparts to these sources. Here we present the source associations and optical and/or IR identifications for sources in the first data release, which are made using a combination of statistical techniques and visual association and identification. We document in detail the colour- and magnitude-dependent likelihood ratio method used for statistical identification as well as the Zooniverse project, called LOFAR Galaxy Zoo, used for the visual classification. We describe the process used to select which of these two different methods is most appropriate for each LoTSS source. The final LoTSS-DR1-IDs value-added catalogue presented contains 318,520 radio sources, of which 231,716 (73%) have optical and/or IR identifications in Pan-STARRS and WISE. The value-added catalogue is available online at https://lofar-surveys.org/, as part of this data release., Comment: 8 figures, 5 tables and 23 pages. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available at https://lofar-surveys.org/ upon publication in the journal

An understanding of the relationship between radio-loud active galaxies and their large-scale environments is essential for realistic modelling of radio-galaxy evolution and environmental impact, for understanding AGN triggering and life cycles, and for calibrating galaxy feedback in cosmological models. We use the LOFAR Two-Metre Sky Survey (LoTSS) Data Release 1 catalogues to investigate this relationship. We cross-matched 8,745 radio-loud AGN with 0.08

This paper presents a study of the local radio source population, by cross-comparing the data from the first data release (DR1) of the LOFAR Two-Metre Sky Survey (LoTSS) with the Sloan Digital Sky Survey (SDSS) DR7 main galaxy spectroscopic sample. The LoTSS DR1 provides deep data (median rms noise of 71 $\mathrm{\mu}$Jy at 150 MHz) over 424 square degrees of sky, which is sufficient to detect 10615 (32 per cent) of the SDSS galaxies over this sky area. An improved method to separate active galactic nuclei (AGN) accurately from sources with radio emission powered by star formation (SF) is developed and applied, leading to a sample of 2121 local ($z < 0.3$) radio AGN. The local 150 MHz luminosity function is derived for radio AGN and SF galaxies separately, and the good agreement with previous studies at 1.4 GHz suggests that the separation method presented is robust. The prevalence of radio AGN activity is confirmed to show a strong dependence on both stellar and black hole masses, remarkably reaching a fraction of 100 per cent of the most massive galaxies ($> 10^{11} \mathrm{M_{\odot}}$) displaying radio-AGN activity with $L_{\rm 150 MHz} \geq 10^{21}$W Hz$^{-1}$; thus, the most massive galaxies are always switched on at some level. The results allow the full Eddington-scaled accretion rate distribution (a proxy for the duty cycle) to be probed for massive galaxies. More than 50 per cent of the energy is released during the $\le 2$ per cent of the time spent at the highest accretion rates, $L_{\mathrm{mech}}/L_{\mathrm{Edd}} > 10^{-2.5}$. Stellar mass is shown to be a more important driver of radio-AGN activity than black hole mass, suggesting a possible connection between the fuelling gas and the surrounding halo. This result is in line with models in which these radio AGN are essential for maintaining the quenched state of galaxies at the centres of hot gas haloes., Comment: 14 pages, 8 figures, 3 tables. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available on lofar-surveys.org upon publication of the journal

We use new and archival Chandra observations of Cygnus A, totalling $\sim$1.9 Ms, to investigate the distribution and temperature structure of gas lying within the projected extent of the cocoon shock and exhibiting a rib-like structure. We confirm that the X-rays are dominated by thermal emission with an average temperature of around 4 keV, and have discovered an asymmetry in the temperature gradient, with the southwestern part of the gas cooler than the rest by up to 2 keV. Pressure estimates suggest that the gas is a coherent structure of single origin located inside the cocoon, with a mass of roughly $2\times10^{10} M_{\odot}$. We conclude that the gas is debris resulting from disintegration of the cool core of the Cygnus A cluster after the passage of the jet during the early stages of the current epoch of activity. The 4 keV gas now lies on the central inside surface of the hotter cocoon rim. The temperature gradient could result from an offset between the centre of the cluster core and the Cygnus A host galaxy at the switch-on of current radio activity., Comment: 15 pages, 12 figures, accepted by MNRAS

Ongoing and future radio surveys aim to trace the evolution of black hole growth and feedback from active galactic nuclei (AGN) throughout cosmic time; however, there remain major uncertainties in translating radio luminosity functions into a reliable assessment of the energy input as a function of galaxy and/or dark matter halo mass. A crucial and long-standing problem is the composition of the radio-lobe plasma that traces AGN jet activity. In this paper, we carry out a systematic comparison of the plasma conditions in Fanaroff & Riley class I and II radio galaxies to demonstrate conclusively that their internal composition is systematically different. This difference is best explained by the presence of an energetically dominant proton population in the FRI, but not the FRII radio galaxies. We show that, as expected from this systematic difference in particle content, radio morphology also affects the jet-power/radio-luminosity relationship, with FRII radio galaxies having a significantly lower ratio of jet power to radio luminosity than the FRI cluster radio sources used to derive jet-power scaling relations via X-ray cavity measurements. Finally we also demonstrate conclusively that lobe composition is unconnected to accretion mode (optical excitation class): the internal conditions of low- and high-excitation FRII radio lobes are indistinguishable. We conclude that inferences of population-wide AGN impact require careful assessment of the contribution of different jet sub-classes, particularly given the increased diversity of jet evolutionary states expected to be present in deep, low-frequency radio surveys such as the LOFAR Two-Metre Sky Survey., Comment: 10 pages, 6 figures. Accepted for publication in MNRAS

Only a small fraction of observed Active Galactic Nuclei display large-scale radio emission associated with jets, yet these radio-loud AGN have become increasingly important in models of galaxy evolution. In determining the dynamics and energetics of the radio sources over cosmic time, a key question concerns what happens when their jets switch off. The resulting `remnant' radio-loud AGN have been surprisingly evasive in past radio surveys, and therefore statistical information on the population of radio-loud AGN in their dying phase is limited. In this paper, with the recent developments of LOFAR and the VLA, we are able to provide a systematically selected sample of remnant radio-loud AGN in the Herschel-ATLAS field. Using a simple core-detection method, we constrain the upper limit on the fraction of remnants in our radio-loud AGN sample to 9 per cent, implying that the extended lobe emission fades rapidly once the core/jets turn off. We also find that our remnant sample has a wide range of spectral indices ($-1.5\leqslant \alpha^{1400}_{150}\leqslant -0.5$), confirming that the lobes of some remnants may possess flat spectra at low frequencies just as active sources do. We suggest that, even with the unprecedented sensitivity of LOFAR, our sample may still only contain the youngest of the remnant population., Comment: 23 pages, 5 tables, 5 Figures

We report L-band VLA observations of 1RXS J0603.3+4214, a cluster that hosts a bright radio relic, known as the Toothbrush, and an elongated giant radio halo. These new observations allow us to study the surface brightness distribution down to one arcsec resolution with very high sensitivity. Our images provide an unprecedented detailed view of the Toothbrush, revealing enigmatic filamentary structures. To study the spectral index distribution, we complement our analysis with published LOFAR and GMRT observations. The bright `brush' of the Toothbrush shows a prominent narrow ridge to its north with a sharp outer edge. The spectral index at the ridge is in the range $-0.70\leq\alpha\leq-0.80$. We suggest that the ridge is caused by projection along the line of sight. With a simple toy model for the smallest region of the ridge, we conclude that the magnetic field is below $5\,\rm\mu G$ and varies significantly across the shock front. Our model indicates that the actual Mach number is higher than that obtained from the injection index and agrees well with the one derived from the overall spectrum, namely ${\cal M}=3.78^{+0.3}_{-0.2}$. The radio halo shows an average spectral index of $\alpha=-1.16\pm0.05$ and a slight gradient from north to south. The southernmost part of the halo is steeper and possibly related to a shock front. Excluding the southernmost part, the halo morphology agrees very well with the X-ray morphology. A power-law correlation is found between the radio and X-ray surface brightness, Comment: 23 pages, 21 figures, accepted for publication in ApJ

We present a deep, low-frequency radio continuum study of the nearby Fanaroff--Riley class I (FR I) radio galaxy 3C 31 using a combination of LOw Frequency ARray (LOFAR; 30--85 and 115--178 MHz), Very Large Array (VLA; 290--420 MHz), Westerbork Synthesis Radio Telescope (WSRT; 609 MHz) and Giant Metre Radio Telescope (GMRT; 615 MHz) observations. Our new LOFAR 145-MHz map shows that 3C 31 has a largest physical size of $1.1$ Mpc in projection, which means 3C 31 now falls in the class of giant radio galaxies. We model the radio continuum intensities with advective cosmic-ray transport, evolving the cosmic-ray electron population and magnetic field strength in the tails as functions of distance to the nucleus. We find that if there is no in-situ particle acceleration in the tails, then decelerating flows are required that depend on radius $r$ as $v\propto r^{\beta}$ ($\beta\approx -1$). This then compensates for the strong adiabatic losses due to the lateral expansion of the tails. We are able to find self-consistent solutions in agreement with the entrainment model of Croston & Hardcastle, where the magnetic field provides $\approx$$1/3$ of the pressure needed for equilibrium with the surrounding intra-cluster medium (ICM). We obtain an advective time-scale of $\approx$$190$ Myr, which, if equated to the source age, would require an average expansion Mach number ${\cal M} \approx 5$ over the source lifetime. Dynamical arguments suggest that instead, either the outer tail material does not represent the oldest jet plasma or else the particle ages are underestimated due to the effects of particle acceleration on large scales., Comment: MNRAS accepted, 17 pages, 9 figures, 2 tables + 3 pages appendix with 3 figures and 2 tables

We present a new analysis of the widely used relation between cavity power and radio luminosity in clusters of galaxies with evidence for strong AGN feedback. We study the correlation at low radio frequencies using two new surveys - the First Alternative Data Release of the TIFR GMRT Sky Survey (TGSS ADR1) at 148 MHz and LOFAR's first all-sky survey, the Multifrequency Snapshot Sky Survey (MSSS) at 140 MHz. We find a scaling relation $P_{\rm cav} \propto L_{148}^{\beta}$, with a logarithmic slope of $\beta = 0.51 \pm 0.14$, which is in good agreement with previous results based on data at 327 MHz. The large scatter present in this correlation confirms the conclusion reached at higher frequencies that the total radio luminosity at a single frequency is a poor predictor of the total jet power. We show that including measurements at 148 MHz alone is insufficient to reliably compute the bolometric radio luminosity and reduce the scatter in the correlation. For a subset of four well-resolved sources, we examine the detected extended structures at low frequencies and compare with the morphology known from higher frequency images and Chandra X-ray maps. In Perseus we discuss details in the structures of the radio mini-halo, while in the 2A 0335+096 cluster we observe new diffuse emission associated with multiple X-ray cavities and likely originating from past activity. For A2199 and MS 0735.6+7421, we confirm that the observed low-frequency radio lobes are confined to the extents known from higher frequencies. This new low-frequency analysis highlights the fact that existing cavity power to radio luminosity relations are based on a relatively narrow range of AGN outburst ages. We discuss how the correlation could be extended using low frequency data from the LOFAR Two-metre Sky Survey (LoTSS) in combination with future, complementary deeper X-ray observations.

The 2Jy sample is a survey of radio galaxies with flux densities above 2 Jy at 2.7 GHz. As part of our ongoing work on the southern subset of 2Jy sources, in paper I of this series we analysed the X-ray cores of the complete 2Jy sample with redshifts 0.05

We describe a new method for identifying and characterizing the thermodynamic state of large samples of evolved galaxy groups at high redshifts using high-resolution, low-frequency radio surveys, such as those that will be carried out with LOFAR and the Square Kilometre Array (SKA). We identify a sub-population of morphologically regular powerful (FRII) radio galaxies and demonstrate that, for this sub-population, the internal pressure of the radio lobes is a reliable tracer of the external intragroup/intracluster medium (ICM) pressure, and that the assumption of a universal pressure profile for relaxed groups enables the total mass and X-ray luminosity to be estimated. Using a sample of well-studied FRII radio galaxies, we demonstrate that our method enables the estimation of group/cluster X-ray luminosities over three orders of magnitude in luminosity to within a factor of ~2 from low-frequency radio properties alone. Our method could provide a powerful new tool for building samples of thousands of evolved galaxy groups at z>1 and characterizing their ICM., Comment: 7 pages, 3 figures. Accepted for publication in MNRAS

We report on the discovery in the LOFAR Multifrequency Snapshot Sky Survey (MSSS) of a giant radio galaxy (GRG) with a projected size of $2.56 \pm 0.07$ Mpc projected on the sky. It is associated with the galaxy triplet UGC 9555, within which one is identified as a broad-line galaxy in the Sloan Digital Sky Survey (SDSS) at a redshift of $0.05453 \pm 1 \times 10^{-5} $, and with a velocity dispersion of $215.86 \pm 6.34$ km/s. From archival radio observations we see that this galaxy hosts a compact flat-spectrum radio source, and we conclude that it is the active galactic nucleus (AGN) responsible for generating the radio lobes. The radio luminosity distribution of the jets, and the broad-line classification of the host AGN, indicate this GRG is orientated well out of the plane of the sky, making its physical size one of the largest known for any GRG. Analysis of the infrared data suggests that the host is a lenticular type galaxy with a large stellar mass ($\log~\mathrm{M}/\mathrm{M}_\odot = 11.56 \pm 0.12$), and a moderate star formation rate ($1.2 \pm 0.3~\mathrm{M}_\odot/\mathrm{year}$). Spatially smoothing the SDSS images shows the system around UGC 9555 to be significantly disturbed, with a prominent extension to the south-east. Overall, the evidence suggests this host galaxy has undergone one or more recent moderate merger events and is also experiencing tidal interactions with surrounding galaxies, which have caused the star formation and provided the supply of gas to trigger and fuel the Mpc-scale radio lobes., Comment: Compressed for arXiv, see journal for full resolution images

We have observed the steep spectrum radio source B2 0924+30 using the LOw Frequency ARray (LOFAR) telescope. Hosted by a z=0.026 elliptical galaxy, it has a relatively large angular size of 12' (corresponding to 360 kpc projected linear size) and a morphology reminiscent of a remnant Fanaroff-Riley type II (FRII) radio galaxy. We combine LOFAR and archival images to study the spectral properties at a spatial resolution of 1'. We derive low frequency spectral index maps and use synchrotron ageing models to infer ages for different regions of the source. Using radiative ageing model fitting we show that the AGN activity ceased around 50 Myr ago. We note that the outer regions of the lobes are younger than the inner regions which is interpreted as a sign that those regions are remnant hotspots. We demonstrate the usefulness of maps of AGN radio remnants taken at low frequencies and suggest caution over the interpretation of spectral ages derived from integrated flux density measurements versus age mapping. The spectral index properties as well as the derived ages of B2 0924+30 are consistent with it being an FRII AGN radio remnant. LOFAR data are proving to be instrumental in extending our studies to the lowest radio frequencies and enabling analysis of the oldest source regions., Comment: 11 pages, 8 figures, online content

LOFAR, the Low-Frequency Array, is a powerful new radio telescope operating between 10 and 240 MHz. LOFAR allows detailed sensitive high-resolution studies of the low-frequency radio sky. At the same time LOFAR also provides excellent short baseline coverage to map diffuse extended emission. However, producing high-quality deep images is challenging due to the presence of direction dependent calibration errors, caused by imperfect knowledge of the station beam shapes and the ionosphere. Furthermore, the large data volume and presence of station clock errors present additional difficulties. In this paper we present a new calibration scheme, which we name facet calibration, to obtain deep high-resolution LOFAR High Band Antenna images using the Dutch part of the array. This scheme solves and corrects the direction dependent errors in a number of facets that cover the observed field of view. Facet calibration provides close to thermal noise limited images for a typical 8 hr observing run at $\sim$ 5arcsec resolution, meeting the specifications of the LOFAR Tier-1 northern survey., Comment: 17 pages, 12 figures, accepted for publication in ApJS

We present deep LOFAR observations between 120-181 MHz of the "Toothbrush" (RX J0603.3+4214), a cluster that contains one of the brightest radio relic sources known. Our LOFAR observations exploit a new and novel calibration scheme to probe 10 times deeper than any previous study in this relatively unexplored part of the spectrum. The LOFAR observations, when combined with VLA, GMRT, and Chandra X-ray data, provide new information about the nature of cluster merger shocks and their role in re-accelerating relativistic particles. We derive a spectral index of $\alpha = -0.8 \pm 0.1$ at the northern edge of the main radio relic, steepening towards the south to $\alpha \approx - 2$. The spectral index of the radio halo is remarkably uniform ($\alpha = -1.16$, with an intrinsic scatter of $\leq 0.04$). The observed radio relic spectral index gives a Mach number of $\mathcal{M} = 2.8^{+0.5}_{-0.3}$, assuming diffusive shock acceleration (DSA). However, the gas density jump at the northern edge of the large radio relic implies a much weaker shock ($\mathcal{M} \approx 1.2$, with an upper limit of $\mathcal{M} \approx 1.5$). The discrepancy between the Mach numbers calculated from the radio and X-rays can be explained if either (i) the relic traces a complex shock surface along the line of sight, or (ii) if the radio relic emission is produced by a re-accelerated population of fossil particles from a radio galaxy. Our results highlight the need for additional theoretical work and numerical simulations of particle acceleration and re-acceleration at cluster merger shocks., Comment: 20 pages, 18 figures, accepted for publication in ApJ

We report on deep Chandra observations of the nearby broad-line radio galaxy Pictor A, which we combine with new Australia Telescope Compact Array (ATCA) observations. The new X-ray data have a factor 4 more exposure than observations previously presented and span a 15-year time baseline, allowing a detailed study of the spatial, temporal and spectral properties of the AGN, jet, hotspot and lobes. We present evidence for further time variation of the jet, though the flare that we reported in previous work remains the most significantly detected time-varying feature. We also confirm previous tentative evidence for a faint counterjet. Based on the radio through X-ray spectrum of the jet and its detailed spatial structure, and on the properties of the counterjet, we argue that inverse-Compton models can be conclusively rejected, and propose that the X-ray emission from the jet is synchrotron emission from particles accelerated in the boundary layer of a relativistic jet. For the first time, we find evidence that the bright western hotspot is also time-varying in X-rays, and we connect this to the small-scale structure in the hotspot seen in high-resolution radio observations. The new data allow us to confirm that the spectrum of the lobes is in good agreement with the predictions of an inverse-Compton model and we show that the data favour models in which the filaments seen in the radio images are predominantly the result of spatial variation of magnetic fields in the presence of a relatively uniform electron distribution., Comment: 21 pages, 11 colour figures. Accepted by MNRAS

Determining the energy spectrum of an electron population can give key insights into the underlying physics of a radio source; however, the lack of high resolution, broad-bandwidth observations has left many ambiguities in our understanding of radio galaxies. The improved capabilities of telescopes such as the JVLA and LOFAR mean that within the bandwidth of any given observation, a detailed spectral shape can now be produced. We present recent investigations of powerful FR-II radio galaxies at GHz and MHz frequencies and show for the first time their small-scale spectral structure. We highlight problems in traditional methods of analysis and demonstrate how these issues can now be addressed. We present the latest results from low frequency studies which suggest a potential increase in the total energy content of radio galaxy lobes with possible implications for the energetics of the population as a whole., Comment: 4 Pages, 1 Figures, to appear in the proceedings of the conference `The Metre Wavelength Sky: Celebrating 50 years of Radio Astronomy at TIFR'

CIZA J2242.8+5301, a merging galaxy cluster at z=0.19, hosts a double-relic system and a faint radio halo. Radio observations at frequencies ranging from a few MHz to several GHz have shown that the radio spectral index at the outer edge of the N relic corresponds to a shock of Mach number 4.6+/-1.1, under the assumptions of diffusive shock acceleration of thermal particles in the test particle regime. Here, we present results from new Chandra observations of the cluster. The Chandra surface brightness profile across the N relic only hints to a surface brightness discontinuity (<2-sigma detection). Nevertheless, our reanalysis of archival Suzaku data indicates a temperature discontinuity across the relic that is consistent with a Mach number of 2.5+/-0.5, in agreement with previously published results. This confirms that the Mach number at the shock traced by the N relic is much weaker than predicted from the radio. Puzzlingly, in the Chandra data we also identify additional inner small density discontinuities both on and off the merger axis. Temperature measurements on both sides of the discontinuities do not allow us to undoubtedly determine their nature, although a shock front interpretation seems more likely. We speculate that if the inner density discontinuities are indeed shock fronts, then they are the consequence of violent relaxation of the dark matter cores of the clusters involved in the merger., Comment: 11 pages, 11 figures. Accepted for publication in MNRAS

We present the results of high resolution VLBI observations at 1.6 and 4.9 GHz of the radio-loud Seyfert galaxy, Mrk 6. These observations are able to detect a compact radio core in this galaxy for the first time. The core has an inverted spectral index ($\alpha^{1.6}_{4.9}$=+1.0$\pm$0.2) and a brightness temperature of $1\times10^8$ K. Three distinct radio components which resemble jet elements and/or hot spots, are also detected. The position angles of these elongated jet elements point, not only to a curved jet in Mrk 6, but also towards a connection between the AGN and the kpc-scale radio lobes/bubbles in this galaxy. Firmer constraints on the star formation rate provided by new Herschel observations (SFR $<0.8$ M$_\sun$ yr$^{-1}$) make the starburst-wind powered bubble scenario implausible. From plasma speeds obtained via prior Chandra X-ray observations, and ram pressure balance arguments for the ISM and radio bubbles, the north-south bubbles are expected to take $7.5\times10^6$ yr to form, and the east-west bubbles $1.4\times10^6$ yr. We suggest that the jet axis has changed at least once in Mrk 6 within the last $\approx10^7$ yr. A comparison of the nuclear radio-loudness of Mrk 6 and a small sample of Seyfert galaxies with a subset of low-luminosity FRI radio galaxies reveals a continuum in radio properties., Comment: 17 pages, 8 figures, MNRAS, accepted for publication

We carry out a systematic study of the X-ray emission from the active nuclei of the 0.02

The synchrotron-radiating particles and magnetic fields in low-power radio galaxies (including most nearby cluster-centre sources), if at equipartition, can provide only a small fraction of the total internal energy density of the radio lobes or plumes, which is now well constrained via X-ray observations of their external environments. We consider the constraints on models for the dominant energy contribution in low-power radio-galaxy lobes obtained from a detailed comparison of how the internal equipartition pressure and external pressure measured from X-ray observations evolve with distance for two radio galaxies, 3C 31 and Hydra A. We rule out relativistic-lepton dominance of the radio lobes, and conclude that models in which magnetic field or relativistic protons/ions carried up the jet dominate lobe energetics are unlikely. Finally, we argue that entrainment of material from the jet surroundings can provide the necessary pressure, and construct a simple self-consistent model of the evolution of the entrainment rate required for pressure balance along the 100-kpc scale plumes of 3C 31. Such a model requires that the entrained material is heated to temperatures substantially above that of the surrounding intra-group or cluster medium, and that the temperature of the thermal component of the jet increases with distance, though remaining sub-relativistic., Comment: 12 pages, 7 figures. Accepted for publication in MNRAS

Relativistic and magnetised plasma ejected by radio loud AGNs through jets form the diffuse lobes of radio galaxies. The radiating particles (electron/electron-positron) in lobes emit in radio via the synchrotron process and X-ray via inverse-Compton scattering of cosmic microwave background photons. The thermal environment around radio galaxies emits X-rays via the thermal bremsstrahlung process. By combining information from these processes we can measure physical conditions in and around the radio lobes and thus study the dynamics of radio galaxies, including double-double radio galaxies., Comment: 11 pages, 4 figures, Diffuse Radio Plasma Conference proceedings (held in Raman Research Institute, Bangalore, India)

We present multifrequency radio observations of two well known episodic FR\,II radio galaxies (J0116$-$4722 and J1158+2621) and a detailed investigation of the life-cycle of episodic radio galaxies from their spectral and radiative properties. Combining our previous work with the present results, we either constrain or place very good limits on the active and quiescent phases of a small sample of episodic FR\,II radio galaxies. The duration of the quiescent phase can be as small as the hotspot fading time of the previous episode, and as high as a few tens of Myr; however, for none of the sources in our sample is it close to the duration of the active phase of the previous episode. We also find that for many episodic radio galaxies, the nucleus is variable at radio wavelengths. For our small sample of 7 episodic radio galaxies, we find 4 to have strongly variable cores, a much larger proportion than is generally found in samples of normal FR\,II radio galaxies. Studies with larger samples will be required to establish a statistical association between core variability and episodic radio activity., Comment: 17 pages, 6 figures, 10 tables, published in MNRAS

We present radio observations of two well-known double-double radio galaxies, J0041+3224 and J1835+6204, at frequencies ranging from 150 to 8460 MHz, using both the Giant Metrewave Radio Telescope and the Very Large Array. These observations, over a large radio frequency range, enable us to determine the spectra of the inner and outer lobes. Our detailed spectral ageing analysis of their inner and outer lobes demonstrates that the outer doubles of double-double radio galaxies are created by the previous cycle of activity, while the inner doubles are due to the present cycle of activity. The (core subtracted) spectra of the inner doubles of both sources are power laws over a large frequency range. We found that the duration of the quiescent phase of J0041+3224 is between 4 and 28 per cent of the active phase of the previous activity. The outer north-western lobe of J1835+6204 has a compact hotspot and the regions of both the outer hotspots have close to power-law (rather than curved) spectra, which indicates that the outer lobes are still fed by jet material ejected in the previous episode just before the central engine stopped powering the jet. We estimate that the duration of the quiescent phase of J1835+6204 is $\lapp$5 per cent of the duration of the active phase of the previous activity. Therefore, we conclude that the duration of the quiescent phase can be as short as a few per cent of the active phase in radio galaxies of this type., Comment: 18 pages, 7 figures, 9 tables, published in MNRAS. (This paper constrain duration of quiescent phase of episodic jet forming activity in AGNs)

Mechanical feedback via Active Galactic Nuclei (AGN) jets in the centres of galaxy groups and clusters is a crucial ingredient in current models of galaxy formation and cluster evolution. Jet feedback is believed to regulate gas cooling and thus star formation in the most massive galaxies, but a robust physical understanding of this feedback mode is currently lacking. The large collecting area, excellent spectral resolution and high spatial resolution of Athena+ will provide the breakthrough diagnostic ability necessary to develop this understanding, via: (1) the first kinematic measurements on relevant spatial scales of the hot gas in galaxy, group and cluster haloes as it absorbs the impact of AGN jets, and (2) vastly improved ability to map thermodynamic conditions on scales well-matched to the jets, lobes and gas disturbances produced by them. Athena+ will therefore determine for the first time how jet energy is dissipated and distributed in group and cluster gas, and how a feedback loop operates in group/cluster cores to regulate gas cooling and AGN fuelling. Athena+ will also establish firmly the cumulative impact of powerful radio galaxies on the evolution of baryons from the epoch of group/cluster formation to the present day., Comment: Supporting paper for the science theme The Hot and Energetic Universe to be implemented by the Athena+ X-ray observatory (http://www.the-athena-x-ray-observatory.eu). 9 pages, 7 figures

As the nodes of the cosmic web, clusters of galaxies trace the large-scale distribution of matter in the Universe. They are thus privileged sites in which to investigate the complex physics of structure formation. However, the complete story of how these structures grow, and how they dissipate the gravitational and non-thermal components of their energy budget over cosmic time, is still beyond our grasp. Fundamental questions such as How do hot diffuse baryons accrete and dynamically evolve in dark matter potentials? How and when was the energy that we observe in the ICM generated and distributed? Where and when are heavy elements produced and how are they circulated? are still unanswered. Most of the cluster baryons exists in the form of a diffuse, hot, metal-enriched plasma that radiates primarily in the X-ray band (the intracluster medium, ICM), allowing the X-ray observations of the evolving cluster population to provide a unique opportunity to address these topics. Athena+ with its large collecting area and unprecedented combination of high spectral and angular resolution offers the only way to make major advances in answering these questions. Athena+ will show how the baryonic gas evolves in the dark matter potential wells by studying the motions and turbulence in the ICM. Athena+ will be able to resolve the accreting region both spatially and spectroscopically, probing the true nature and physical state of the X-ray emitting plasma. Athena+ has the capabilities to permit a definitive understanding of the formation and evolution of large-scale cosmic structure through the study of the cluster population., Comment: Supporting paper for the science theme The Hot and Energetic Universe to be implemented by the Athena+ X-ray observatory (http://www.the-athena-x-ray-observatory.eu). 10 pages, 5 figures

Major astrophysical questions related to the formation and evolution of structures, and more specifically of galaxy groups and clusters, will still be open in the coming decade and beyond: what is the interplay of galaxy, supermassive black hole, and intergalactic gas evolution in the most massive objects in the Universe - galaxy groups and clusters? What are the processes driving the evolution of chemical enrichment of the hot diffuse gas in large-scale structures? How and when did the first galaxy groups in the Universe, massive enough to bind more than 10^7 K gas, form? Focussing on the period when groups and clusters assembled (0.5

We present here the first results from the Chandra ERA (Environments of Radio-loud AGN) Large Project, characterizing the cluster environments of a sample of 26 radio-loud AGN at z ~ 0.5 that covers three decades of radio luminosity. This is the first systematic X-ray environmental study at a single epoch, and has allowed us to examine the relationship between radio luminosity and cluster environment without the problems of Malmquist bias. We have found a weak correlation between radio luminosity and host cluster X-ray luminosity, as well as tentative evidence that this correlation is driven by the subpopulation of low-excitation radio galaxies, with high-excitation radio galaxies showing no significant correlation. The considerable scatter in the environments may be indicative of complex relationships not currently included in feedback models., Comment: Accepted for publication in ApJ, 16 pages, 12 figures, 7 tables

The cluster 1RXS J0603.3+4214 is a merging galaxy cluster that hosts three radio relics and a giant radio halo. The northern relic, the Toothbrush, is 1.9-Mpc long and has an unusual linear morphology. According to simple diffusive shock acceleration theory, its radio spectral index indicates a Mach number of 3.3-4.6. Here, we present results from a deep XMM-Newton observation of the cluster. We observe two distinct cluster cores that have survived the merger. The presence of three shocks at or near the locations of the radio relics is confirmed by density and temperature discontinuities. However, the observation poses several puzzles that challenge our understanding of radio relics: (i) at the Toothbrush, the shock Mach number is not larger than 2, in apparent conflict with the shock strength predicted from the radio spectrum; (ii) at the Toothbrush, the shock front is, in part, spatially offset from the radio emission; (iii) at the eastern relic, we detect a temperature jump corresponding to a Mach number of approximately 2.5, but there is no associated surface brightness discontinuity. We discuss possible explanations for these findings., Comment: 14 pages, 15 figures; submitted to MNRAS