We explore the low-frequency radio properties of the sources in the Fanaroff-Riley class 0 catalog (FR0CAT) as seen by the LOw Frequency ARray (LOFAR) observations at 150 MHz. This sample includes 104 compact radio active galactic nuclei (AGN) associated with nearby (z<0.05) massive early-type galaxies. Sixty-six FR0CAT sources are in the sky regions observed by LOFAR and all of them are detected, usually showing point-like structures with sizes smaller than 3-6 kpc. However, 12 FR0s present resolved emission of low surface brightness which contributes between 5% and 40% of the total radio power at 150 MHz, usually with a jetted morphology extending between 15 and 50 kpc. No extended emission is detected around the other FR0s, with a typical luminosity limit of 5 x 10$^{22}$ W/Hz over an area of 100 kpc x 100 kpc. The spectral slopes of FR0s between 150 MHz and 1.4 GHz span a broad range (-0.7 < $α$ < 0.8) with a median value of $\overlineα\sim 0.1$; 20% of them have a steep spectrum ($α$ > 0.5), an indication of the presence of substantial extended emission confined within the spatial resolution limit. The fraction of FR0s showing evidence for the presence of jets, by including both spectral and morphological information, is at least ~40%. This study confirms that FR0s and FRIs can be interpreted as two extremes of a continuous population of jetted sources, with the FR0s representing the low end in size and radio power.
Feedback from radio jets associated with Active Galactic Nuclei (AGN) plays a profound role in the evolution of galaxies. Kinetic power of these radio jets appears to show temporal variation, but the mechanism(s) responsible for this process are not yet clear. Recently, the LOw Frequency ARray (LOFAR) has uncovered large populations of active, remnant and restarted radio jet populations. By focusing on LOFAR data in the Lockman Hole, in this work we use the Radio AGN in Semi-Analytic Environments (RAiSE) dynamical model to present the first self-consistent modelling analysis of active, remnant…
A supernova remnant (SNR) candidate SRGe~J0023+3625 = G116.6-26.1 was recently discovered in the \textit{SRG}/eROSITA all-sky X-ray survey. This large ($\sim 4$ deg in diameter) SNR candidate lacks prominent counterparts in other bands. Here we report detection of radio emission from G116.6-26.1 in the LOFAR Two-metre Sky Survey (LoTTS-DR2). Radio images show a shell-like structure coincident with the X-ray boundary of the SNR. The measured surface brightness of radio emission from this SNR is very low. Extrapolation of the observed surface brightness to 1~GHz places G116.6-26.1 well below oth…
Radio emission from 23 tidal disruption event host galaxies were searched in the 144 MHz LOFAR-LoTSS2 images. Three host galaxies are detected with diffuse radio emission, which can be interpreted as either galactic synchrotron radio emission or diffuse radio halo in dense galactic environments. Non-detection of (transient) radio emission in majority of galaxies could be due to self-absorption of radio emission associated with the tidal disruption event. The detected objects are AT2018iih, RBS 1032, and NGC 5905. Non-detection for Sw 1644+57 is also reported.
The radio pulsar and rotating radio transient populations are only known in and near the Milky Way. Investigating such populations in other galaxies requires deep pulsar and transient searches. We performed 4-h radio observations of nearby galaxies M33, M81 and M82 with LOFAR. Our main purpose was to characterise the bright end of the pulsar population in other galaxies, and compare it to that of the Milky Way. We searched for extragalactic radio pulsars through a periodic-pulse search, and for sporadic fast radio transients through a single-pulse search. We coherently combined at most 23 LOFA…
We present the first LOFAR observations of the radio jet in the quasar 4C+19.44 (a.k.a. PKS 1354+19) obtained with the long baselines. The achieved resolution is very well matched to that of archival Jansky Very Large Array (JVLA) observations at higher radio frequencies as well as the archival X-ray images obtained with {\it Chandra}. We found that, for several knots along the jet, the radio flux densities measured at hundreds of MHz lie well below the values estimated by extrapolating the GHz spectra. This clearly indicates the presence of spectral curvature. Radio spectral curvature has bee…
Giant radio galaxies (GRGs), with extended structures reaching hundreds of kpc, are among the most spectacular examples of ejection of relativistic plasma from super-massive black holes. In this work, third of a series, we present LOw Frequency ARray (LOFAR) images at 144 MHz, collected in the framework of the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS DR2), for nine sources extracted from our sample of hard X-ray selected GRGs (HXGRG, i.e. from INTEGRAL/IBIS and Swift/BAT catalogues at >20 keV). Thanks to the resolution and sensitivity of LoTSS, we could probe the complex morphology of …
We present results from LOFAR and GMRT observations of the galaxy cluster MACS$\,$J0717.5$+$3745. The cluster is undergoing a violent merger involving at least four sub-clusters, and it is known to host a radio halo. LOFAR observations reveal new sources of radio emission in the Intra-Cluster Medium: (i) a radio bridge that connects the cluster to a head-tail radio galaxy located along a filament of galaxies falling into the main cluster, (ii) a 1.9 Mpc radio arc, that is located North West of the main mass component, (iii) radio emission along the X-ray bar, that traces the gas in the X-rays …
Radio halos and radio relics are diffuse synchrotron sources that extend over Mpc-scales and are found in a number of merger galaxy clusters. They are believed to form as a consequence of the energy that is dissipated by turbulence and shocks in the intra-cluster medium (ICM). However, the precise physical processes that generate these steep synchrotron spectrum sources are still poorly constrained. We present a new LOFAR observation of the double galaxy cluster Abell 1758. This system is composed of A1758N, a massive cluster hosting a known giant radio halo, and A1758S, which is a less massiv…
The Lockman Hole Project is a wide international collaboration aimed at exploiting the multi-band extensive and deep information available for the Lockman Hole region, with the aim of better characterizing the physical and evolutionary properties of the various source populations detected in deep radio fields. Recent observations with the LOw-Frequency ARray (LOFAR) extends the multi-frequency radio information currently available for the Lockman Hole (from 350 MHz up to 15 GHz) down to 150 MHz, allowing us to explore a new radio spectral window for the faint radio source population. These LOF…
After the jets have switched off, radio galaxies undergo a fading phase which is often named the dying phase. The luminosity evolution of the remnant plasma during this period is still poorly constrained because of the paucity of objects detected. Using the new 150-MHz deep LOFAR observations of the well-known extragalactic field the Lockman Hole, we performed a systematic search of dying radio sources aiming to provide good statistics on their detection and properties. To avoid selection biases towards any specific class of dying sources we used both morphological and spectral selection crite…
Winged radio sources are a small sub-class of extragalactic radio sources which display a pair of low surface brightness radio lobes, known as `wings' aligned at a certain angle with the primary jets. Depending on the location of wings, these galaxies look like X or Z and are known as X-shaped Radio Galaxy (XRG) or Z-shaped Radio Galaxy (ZRG). We report the identification of 33 winged radio sources from the LOFAR Two-metre Sky Survey First Data Release (LoTSS DR1) out of which 21 sources are identified as X-shaped radio galaxies and 12 as Z-shaped radio galaxies. Optical counterparts are ident…
LOFAR observations at 144 MHz have revealed large-scale radio sources in the unrelaxed galaxy cluster Abell 1132. The cluster hosts diffuse radio emission on scales of $\sim$650 kpc near the cluster center and a head-tail (HT) radio galaxy, extending up to 1 Mpc, South of the cluster center. The central diffuse radio emission is not seen in NVSS, FIRST, WENSS, nor in C & D array VLA observations at 1.4 GHz, but is detected in our follow-up GMRT observations at 325 MHz. Using LOFAR and GMRT data, we determine the spectral index of the central diffuse emission to be $α=-1.75\pm0.19$ ($S\proptoν^…
Radio halos are extended ($\sim{\rm Mpc}$), steep-spectrum sources found in the central region of dynamically disturbed clusters of galaxies. Only a handful of radio halos have been reported to reside in galaxy clusters with a mass $M_{500}\lesssim5\times10^{14}\,M_\odot$. In this paper we present a LOFAR 144 MHz detection of a radio halo in the galaxy cluster Abell 990 with a mass of $M_{500}=(4.9\pm0.3)\times10^{14}\,M_\odot$. The halo has a projected size of $\sim$700$\,{\rm kpc}$ and a flux density of $20.2\pm2.2\,{\rm mJy}$ or a radio power of $1.2\pm0.1\times10^{24}\,{\rm W\,Hz}^{-1}$ at…
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 s…
We present an estimate of the optical luminosity function (OLF) of LOFAR radio-selected quasars (RSQs) at $1.4<z<5.0$ in the $9.3\:\textrm{deg}^{2}$ NOAO Deep Wide-field survey (NDWFS) of the Boötes field. The selection was based on optical/mid-ir photometry used to train three different machine learning (ML) algorithms. Objects taken as quasars by the ML algorithms are required to be detected at $5σ$ significance in deep radio maps to be classified as candidate quasars. The optical imaging came from the SDSS and the PS1 $3π$ survey; mid-ir photometry was taken from the SDWFS survey; and radio…
Compact Steep Spectrum, Gigahertz Peaked Spectrum and High Frequency Peak (CSS, GPS, HFP) sources are considered to be young radio sources but the details of their duty cycle are not well understood. In some cases they are thought to develop in large radio galaxies, while in other cases their jets may experience intermittent activity or die prematurely and remain confined within the host galaxy. By studying in a systematic way the presence and the properties of any extended emission surrounding these compact sources we can provide firmer constraints on their evolutionary history and on the tim…
The discovery of faint FR~I radio jets in the nearby elliptical galaxy NGC 5322 is reported here using 144 MHz LOFAR image. The jets have an angular extent of $\sim40$ arcmin or a projected physical extent of $\sim350$ kpc. The faint jets remain well collimated and disappear in the intergalactic medium, without any visible hotspot or radio lobes. The jets detected up to $\sim20$ kpc extent at higher frequencies are relatively bright within the optical extent of the galaxy but become faint abruptly outside, where detection is made only in the LOFAR image. The total radio luminosity of the galax…
Regular magnetic field structures can be derived from the Faraday rotation measures (RM) of polarized background sources observable at 1.4 GHz with the SKA. At lower frequencies ($<250$ MHz) polarimetry of radio sources with the Low Frequency Array (LOFAR) will allow the investigation of extremely small RM, to detect and map weak regular fields in halos and outer parts of spiral galaxies, and in the interstellar and intergalactic medium. Very little is known yet about the number density of polarized sources at low frequencies. Observed distributions of polarized sources at 350 MHz and 1.4 GHz …
Radio emission is a key indicator of star-formation activity in galaxies, but the radio luminosity-star formation relation has to date been studied almost exclusively at frequencies of 1.4 GHz or above. At lower radio frequencies the effects of thermal radio emission are greatly reduced, and so we would expect the radio emission observed to be completely dominated by synchrotron radiation from supernova-generated cosmic rays. As part of the LOFAR Surveys Key Science project, the Herschel-ATLAS NGP field has been surveyed with LOFAR at an effective frequency of 150 MHz. We select a sample from …
The radio and far-infrared luminosities of star-forming galaxies are tightly correlated over several orders of magnitude; this is known as the far-infrared radio correlation (FIRC). Previous studies have shown that a host of factors conspire to maintain a tight and linear FIRC, despite many models predicting deviation. This discrepancy between expectations and observations is concerning since a linear FIRC underpins the use of radio luminosity as a star-formation rate indicator. Using LOFAR 150MHz, FIRST 1.4 GHz, and Herschel infrared luminosities derived from the new LOFAR/H-ATLAS catalogue, …
Giant radio galaxies (GRGs) are a subclass of radio galaxies which have grown to megaparsec scales. GRGs are much rarer than normal sized radio galaxies (< 0.7 Mpc) and the reason for their gigantic sizes is still debated. Here, we report the biggest sample of GRGs identified to date. These objects were found in the LOFAR Two-metre Sky Survey (LoTSS) first data release images, which cover a 424 square degrees region. Of the 239 GRGs found, 225 are new discoveries. The GRGs in our sample have sizes ranging from 0.7 to 3.5 Mpc and have redshifts (z) between 0.1 and 2.3. Seven GRGs have sizes abo…
Diffuse radio emission at the centre of galaxy clusters has been observed both in merging clusters on scales of Mpc, called giant radio haloes, and in relaxed systems with a cool-core on smaller scales, named mini haloes. Giant radio haloes and mini haloes are thought to be distinct classes of sources. However, recent observations have revealed the presence of diffuse radio emission on Mpc scales in clusters that do not show strong dynamical activity. RX J1720.1+2638 is a cool-core cluster, presenting both a bright central mini halo and a fainter diffuse, steep-spectrum emission extending beyo…
The Odd Radio Circles are newly identified diffuse radio sources at ~1 GHz frequency, with edge-brightened nearly circular morphology, which is remarkably similar to supernova remnants although a physical association with previous population of Galactic supernova remnants is challenging due to detections of the Odd Radio Circles at high Galactic latitudes. Here, a serendipitous identification of a new source in a LOFAR 144 MHz image with similar morphology as that of Odd Radio Circles is reported. This is the first reported identification of an Odd Radio Circle at a very low frequency and with…
Active galactic nuclei (AGN) at the centres of galaxies can cycle between periods of activity and of quiescence. Characterising the duty-cycle of AGN is crucial for understanding their impact on the evolution of the host galaxy. For radio AGN, their evolutionary stage can be identified from a combination of morphological and spectral properties. We summarise the results we have obtained in the last few years by studying radio galaxies in various crucial phases of their lives, such as remnant and restarted sources. We used morphological information derived from LOw Frequency ARray (LOFAR) image…
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'…
Low radio frequency surveys are important for testing unified models of radio-loud quasars and radio galaxies. Intrinsically similar sources that are randomly oriented on the sky will have different projected linear sizes. Measuring the projected linear sizes of these sources provides an indication of their orientation. Steep-spectrum isotropic radio emission allows for orientation-free sample selection at low radio frequencies. We use a new radio survey of the Boötes field at 150 MHz made with the Low Frequency Array (LOFAR) to select a sample of radio sources. We identify 44 radio galaxies a…
Red quasi-stellar objects (QSOs) are a subset of the luminous end of the cosmic population of active galactic nuclei (AGN), most of which are reddened by intervening dust along the line-of-sight towards their central engines. In recent work from our team, we developed a systematic technique to select red QSOs from the Sloan Digital Sky Survey (SDSS), and demonstrated that they have distinctive radio properties using the Faint Images of the Radio Sky at Twenty centimeters (FIRST) radio survey. Here we expand our study using low-frequency radio data from the LOFAR Two-metre Sky Survey (LoTSS). W…
Remnant radio galaxies represent the final dying phase of radio galaxy evolution, in which the jets are no longer active. Due to their rarity in flux limited samples and the difficulty of identification, this dying phase remains poorly understood and the luminosity evolution largely unconstrained. Here we present the discovery and detailed analysis of a large (700 kpc) remnant radio galaxy with a low surface brightness that has been identified in LOFAR images at 150 MHz. By combining LOFAR data with new follow-up Westerbork observations and archival data at higher frequencies, we investigated …
We have examined the giant radio galaxy 3C~236 using LOFAR at 143 MHz down to an angular resolution of 7", in combination with observations at higher frequencies. We have used the low frequency data to derive spectral index maps with the highest resolution yet at these low frequencies. We confirm a previous detection of an inner hotspot in the north-west lobe and for the first time observe that the south-east lobe hotspot is in fact a triple hotspot, which may point to an intermittent source activity. Also, the spectral index map of 3C 236 shows that the spectral steepening at the inner region…
Measuring radio waves at low frequencies offers a new window to study cosmic magnetism, and LOFAR is the ideal radio telescope to open this window widely. The LOFAR Magnetism Key Science Project (MKSP) draws together expertise from multiple fields of magnetism science and intends to use LOFAR to tackle fundamental questions on cosmic magnetism by exploiting a variety of observational techniques. Surveys will provide diffuse emission from the Milky Way and from nearby galaxies, tracking the propagation of long-lived cosmic-ray electrons through magnetic field structures, to search for radio hal…
The correlation between radio spectral index and redshift has been exploited to discover high redshift radio galaxies, but its underlying cause is unclear. It is crucial to characterise the particle acceleration and loss mechanisms in high redshift radio galaxies to understand why their radio spectral indices are steeper than their local counterparts. Low frequency information on scales of $\sim$1 arcsec are necessary to determine the internal spectral index variation. In this paper we present the first spatially resolved studies at frequencies below 100 MHz of the $z = 2.4$ radio galaxy 4C 43…
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…
Studies of the most distant AGNs allow us to test our current understanding of the physics present in radio-jetted AGNs across a range of environments. The decrease in apparent luminosity with distance is the primary difficulty to overcome in the study of these distant AGNs, which requires highly sensitive instruments. Our goal is to employ new long wavelength radio data to better parametrise the broad-band SED of GB 1508+5714, a high-redshift (z=4.30) AGN. Its high redshift, high intrinsic luminosity, and classification as a blazar allow us to test emission models that consider the efficient …
Feedback by radio-loud active galactic nuclei (AGN) in galaxy groups is not fully understood. Open questions include the duty cycle of the AGN, the spatial extent of the radio lobes, the effect they have on the intragroup medium, and the fate of the cosmic rays. We present the discovery of a 650 kpc-radio galaxy embedded in steep diffuse emission at $z = 0.18793 \pm 5 \times 10^{-5}$ located at the center of the galaxy group MaxBCG J199.31832+51.72503 using an observation from the LOFAR Two-meter Sky Survey (LoTSS) at the central frequency of 144 MHz. Subsequently, we performed a GMRT observat…
Identifying active galactic nuclei (AGN) and isolating their contribution to a galaxy's energy budget is crucial for studying the co-evolution of AGN and their host galaxies. Brightness temperature ($T_b$) measurements from high-resolution radio observations at GHz frequencies are widely used to identify AGN. Here we investigate using new sub-arcsecond imaging at 144 MHz with the International LOFAR Telescope to identify AGN using $T_b$ in the Lockman Hole field. We use ancillary data to validate the 940 AGN identifications, finding 83 percent of sources have AGN classifications from SED fitti…
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 r…
In this Letter, we report the discovery of a radio halo in the high-redshift galaxy cluster PSZ2 G099.86+58.45 ($z=0.616$) with the LOw Frequency ARray (LOFAR) at 120-168 MHz. This is one of the most distant radio halos discovered so far. The diffuse emission extends over $\sim$ 1 Mpc and has a morphology similar to that of the X-ray emission as revealed by XMM-Newton data. The halo is very faint at higher frequencies and is barely detected by follow-up 1-2 GHz Karl G.~Jansky Very Large Array (JVLA) observations, which enable us to constrain the radio spectral index to be $α\leq 1.5-1.6$, i.e.…
New-generation radio telescopes like LOFAR are conducting extensive sky surveys, detecting millions of sources. To maximise the scientific value of these surveys, radio source components must be properly associated into physical sources before being cross-matched with their optical/infrared counterparts. In this paper, we use machine learning to identify those radio sources for which either source association is required or statistical cross-matching to optical/infrared catalogues is unreliable. We train a binary classifier using manual annotations from the LOFAR Two-metre Sky Survey (LoTSS). …
Jellyfish galaxies, characterized by long filaments of stripped interstellar medium extending from their disks, are the prime laboratories to study the outcomes of ram pressure stripping. At radio wavelengths, they often show unilateral emission extending beyond the stellar disk, and an excess of radio luminosity with respect to that expected from their current star formation rate. We present new 144 MHz images provided by the LOFAR Two-metre Sky Survey for a sample of six galaxies from the GASP survey. These galaxies are characterized by a high global luminosity at 144 MHz ($6-27\times10^{22}…
We present LOFAR observations at 120-168 MHz of 42 systems with possible X-ray cavities in their hot atmosphere, of which 17 are groups or ellipticals, 19 are nearby clusters (z<0.3), and six are higher-redshift clusters (z>0.3). The X-ray cavities, formed by the radio lobes of the central active galactic nucleus (AGN), are evidence of radio-mode AGN feedback. In the groups and ellipticals sample, more than half of the systems have X-ray cavities for which no associated lobe emission was detected. Conversely, we report the discovery of large radio lobes in NGC 6338, extending far beyond the em…
We present observations of planetary nebulae with the LOw Frequency ARray (LOFAR) between 120 and 168 MHz. The images show thermal free-free emission from the nebular shells. We have determined the electron temperatures for spatially resolved, optically thick nebulae. These temperatures are 20 to 60% lower than those estimated from collisionally excited optical emission lines. This strongly supports the existence of a cold plasma component, which co-exists with hot plasma in planetary nebulae. This cold plasma does not contribute to the collisionally excited lines, but does contribute to recom…
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-met…
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. 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. We have combined LOFAR observations at 150 MHz with public surveys at h…
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 qu…
We present LOFAR data from 110--180~MHz of the merging galaxy cluster Abell 1682, alongside archival optical, radio and X-ray data. Our 6 arc-second resolution images at low frequencies reveal new structures associated with numerous radio galaxies in the cluster. At 20 arc-second resolution we see diffuse emission throughout the cluster over hundreds of kpc, indicating particle acceleration mechanisms are in play as a result of the cluster merger event and powerful active galactic nuclei. We show that a significant part of the cluster emission is from an old radio galaxy with very steep spectr…
Low-power radio sources dominate the radio sky. They tend to be small in size and core-dominated, but the origin of their properties and the evolution of their radio plasma are not well constrained. Interestingly, there is mounting evidence that low-power radio sources can significantly impact their surrounding gaseous medium and, therefore, may be more relevant for galaxy evolution than previously thought. In this paper, we present low radio frequency observations obtained with LOFAR at 147 MHz of the radio source hosted by NGC 3998. This is a rare example of a low-power source which is extre…
We use the LOFAR Two-metre Sky Survey (LoTSS) Data Release I to identify the groups of galaxies (and individual galaxies) from the Hickson Compact Groups and Magnitude Limited Compact Groups samples that emit at the frequency of 150\,MHz, characterise their radio emission (extended or limited to the galaxies), and compare new results to earlier observations and theoretical predictions. The detection of 73 systems (and 7 more -- probably) out of 120, of which as many as 17 show the presence of extended radio structures, confirms the previous hypothesis of the common character of the magnetic fi…
Revealing the intricacies of radio galaxies and filaments in the merging galaxy cluster Abell 2255. I. Insights from deep LOFAR-VLBI sub-arcsecond resolution images
We analyse new observations with the International Low Frequency Array (LOFAR) telescope, and archival data from the Multi-Element Radio Linked Interferometer Network (MERLIN) and the Karl G. Jansky Very Large Array (VLA). We model the spatially resolved radio spectrum of Arp 220 from 150 MHz to 33 GHz. We present an image of Arp 220 at 150 MHz with resolution $0.65''\times0.35''$, sensitivity 0.15 mJy beam$^{-1}$, and integrated flux density $394\pm59$ mJy. More than 80% of the detected flux comes from extended ($6''\approx$2.2 kpc) steep spectrum ($α=-0.7$) emission, likely from star formati…
We study the properties and the origin of the radio emission in the most luminous early-type galaxies (ETGs) in the nearby Universe (MK<-25, recession velocity < 7,500 km/s) as seen by the 150 MHz Low-Frequency ARray (LOFAR) observations. LOFAR images are available for 188 of these giant ETGs (gETGs) and 146 (78%) of them are detected above a typical luminosity of ~10E21 W/Hz. They show a large spread in power, reaching up to ~10E26 W/Hz. We confirm a positive link between the stellar luminosity of gETGs and their median radio power, the detection rate, and the fraction of extended sources. Ab…
The Low Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS) is a low-frequency radio continuum survey of the Northern sky at an unparalleled resolution and sensitivity. In order to fully exploit this huge dataset and those produced by the Square Kilometre Array in the next decade, automated methods in machine learning and data-mining will be increasingly essential both for morphological classifications and for identifying optical counterparts to the radio sources. Using self-organising maps (SOMs), a form of unsupervised machine learning, we created a dimensionality reduction of the radio mor…
