Ultra-steep Spectrum Research Articles

High-redshift quasars at z ≥ 3 – I. Radio spectra

ABSTRACT We present the radio properties of optically selected quasars with z ≥ 3. The complete sample consists of 102 quasars with a flux density level S1.4 ≥ 100 mJy in a declination range −35° ≤ Dec. ≤ +49°. The observations were obtained in 2017–2020 using the radio telescope RATAN-600. We measured flux densities at six frequencies 1.2, 2.3, 4.7, 8.2, 11.2, and 22 GHz quasi-simultaneously with uncertainties of 9–31 per cent. The detection rate is 100, 89, and 46 per cent at 4.7, 11.2, and 22 GHz, respectively. We have analysed the averaged radio spectra of the quasars based on the RATAN and literature data. We classify 46 per cent of radio spectra as peaked-spectrum, 24 per cent as flat, and none as ultra-steep spectra (α ≤ −1.1). The multifrequency data reveal that a peaked spectral shape (PS) is a common feature for bright high-redshift quasars. This indicates the dominance of bright compact core emission and the insignificant contribution of extended optically thin kpc-scale components in observed radio spectra. Using these new radio data, the radio loudness log R was estimated for 71 objects with a median value of 3.5, showing that the majority of the quasars are highly radio-loud with log R > 2.5. We have not found any significant correlation between z and α. Several new megahertz- peaked spectrum (MPS) and gigahertz- peaked spectrum (GPS) candidates are suggested. Further studies of their variability and additional low-frequency observations are needed to classify them precisely.

The ultra-steep diffuse radio emission observed in the cool-core cluster RX J1720.1+2638 with LOFAR at 54 MHz

ABSTRACT 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 beyond the cluster core that resembles giant radio halo emission. In this paper, we present new observations performed with the LOw Frequency ARray Low Band Antennas (LBA) at 54 MHz. These observations, combined with data at higher frequencies, allow us to constrain the spectral properties of the radio emission. The large-scale emission presents an ultrasteep spectrum with $\alpha _{54}^{144}\sim 3.2$. The radio emission inside and outside the cluster core have strictly different properties, as there is a net change in spectral index and they follow different radio-X-ray surface brightness correlations. We argue that the large-scale diffuse emission is generated by particles re-acceleration after a minor merger. While for the central mini halo, we suggest that it could be generated by secondary electrons and positrons from hadronic interactions of relativistic nuclei with the dense cool-core gas, as an alternative to re-acceleration models.

Multi-frequency characterisation of remnant radio galaxies in the Lockman Hole field

Context.Remnant radio galaxies represent an important phase in the life-cycle of radio active galactic nuclei. It is suggested that in this phase, the jets have switched off and the extended emission is fading rapidly. This phase is not well-studied due to the lack of statistical samples observed at both low and high frequencies.Aims.In this work, we study a sample of 23 candidate remnant radio galaxies previously selected using the Low Frequency Array at 150 MHz in the Lockman Hole field. We examine their morphologies and study their spectral properties to confirm their remnant nature and revise the morphological and spectral criteria used to define the initial sample.Methods.We present new observations with theKarl G. JanskyVery Large Array at 6000 MHz at both high and low resolution. These observations allowed us to observe the presence or absence of cores and study the spectral curvature and steepness of the spectra of the total emission expected at these high frequencies for the remnant candidates.Results.We confirm 13 out of 23 candidates as remnant radio sources. This corresponds to 7% of the full sample of active, restarted, and remnant candidates from the Lockman Hole field. Surprisingly, only a minority of remnants reside in a cluster (23%). The remnant radio galaxies show a range of properties and morphologies. The majority do not show detection of the core at 6000 MHz and their extended emission often shows ultra-steep spectra (USS). However, there are also remnants with USS total emission and a detection of the core at 6000 MHz, possibly indicating a variety of evolutionary stages in the remnant phase. We confirm the importance of the combination of morphological and spectral criteria and this needs to be taken into consideration when selecting a sample of remnant radio sources.

Nonthermal phenomena in the center of Abell 1775

Context. Thermal gas in the center of galaxy clusters can show substantial motions that generate surface-brightness and temperature discontinuities known as cold fronts. The motions may be triggered by minor or off-axis mergers that preserve the cool core of the system. The dynamics of the thermal gas can also generate radio emission from the intra-cluster medium (ICM) and impact the evolution of clusters’ radio sources. Aims. We aim to study the central region of Abell 1775, a system in an ambiguous dynamical state at z = 0.072 which is known to host an extended head-tail radio galaxy, with the goal of investigating the connection between thermal and nonthermal components in its center. Methods. We made use of a deep (100 ks) Chandra observation accompanied by LOFAR 144 MHz, GMRT 235 MHz and 610 MHz, and VLA 1.4 GHz radio data. Results. We find a spiral-like pattern in the X-ray surface brightness that is mirrored in the temperature and pseudo-entropy maps. Additionally, we characterize an arc-shaped cold front in the ICM. We interpret these features in the context of a slingshot gas tail scenario. The structure of the head-tail radio galaxy “breaks” at the position of the cold front, showing an extension that is detected only at low frequencies, likely due to its steep and curved spectrum. We speculate that particle reacceleration is occurring in the outer region of this tail, which in total covers a projected size of ∼800 kpc. We also report the discovery of revived fossil plasma with ultra-steep spectrum radio emission in the cluster core together with a central diffuse radio source that is bounded by the arc-shaped cold front. Conclusions. The results reported in this work demonstrate the interplay between thermal and nonthermal components in the cluster center and the presence of ongoing particle reacceleration in the ICM on different scales.

The best of both worlds: Combining LOFAR and Apertif to derive resolved radio spectral index images

Super massive black holes at the centres of galaxies can cycle through periods of activity and quiescence. Characterising the duty cycle of active galactic nuclei (AGN) is crucial for understanding the impact of the energy they release on the host galaxy. For radio AGN, this can be done by identifying dying (remnant) and restarted radio galaxies from their radio spectral properties. Using the combination of the images at 1400 MHz produced by Apertif, the new phased-array feed receiver installed on the Westerbork Synthesis Radio Telescope, and images at 150 MHz provided by LOFAR, we have derived resolved spectral index images (at a resolution of ~15 arcsec) for all the sources within an approximately 6 deg2 area of the Lockman Hole region. In this way, we were able to select 15 extended radio sources with emission (partly or entirely) characterised by extremely steep spectral indices (steeper than 1.2). These objects represent cases of radio sources in the remnant or the restarted phases of their life cycle. Our findings confirm that these objects are not as rare as previously thought, suggesting a relatively fast cycle. They also show a variety of properties that can be relevant for modelling the evolution of radio galaxies. For example, the restarted activity can occur while the remnant structure from a previous phase of activity is still visible. This provides constraints on the duration of the “off” (dying) phase. In extended remnants with ultra-steep spectra at low frequencies, the activity likely stopped a few hundred megayears ago, and they correspond to the older tail of the age distribution of radio galaxies, in agreement with the results of simulations of radio source evolution. We find remnant radio sources with a variety of structures (from double-lobed to amorphous), possibly suggesting different types of progenitors. The present work sets the stage for exploiting the powerful tool of low-frequency spectral index studies of extended sources by taking advantage of the large areas common to the LOFAR and the Apertif surveys.

Ultra-steep-spectrum Radio “Jellyfish” Uncovered in A2877

Abstract We report on the discovery of a mysterious ultra-steep-spectrum (USS) synchrotron source in the galaxy cluster A2877. We have observed the source with the Murchison Widefield Array at five frequencies across 72–231 MHz and found the source to exhibit strong spectral curvature over this range, as well as the steepest known spectra of a synchrotron cluster source, with a spectral index across the central three frequency bands of . Higher-frequency radio observations, including a deep observation with the Australia Telescope Compact Array, fail to detect any of the extended diffuse emission. The source is approximately 370 kpc wide and bears an uncanny resemblance to a jellyfish with two peaks of emission and long tentacles descending south toward the cluster center. While the “USS Jellyfish” defies easy classification, we here propose that the phenomenon is caused by the reacceleration and compression of multiple aged electron populations from historic active galactic nucleus (AGN) activity, a so-called “radio phoenix,” by an as yet undetected weak cluster-scale mechanism. The USS Jellyfish adds to a growing number of radio phoenix in cool-core clusters with unknown reacceleration mechanisms; as the first example of a polyphoenix, however, this implies that the mechanism is on the scale of the cluster itself. Indeed, we show that in simulations, emission akin to the USS Jellyfish can be produced as a short-lived transient phase in the evolution of multiple interacting AGN remnants when subject to weak external shocks.

Diffuse galaxy cluster emission at 168 MHz within the Murchison Widefield Array Epoch of Reionization 0-h field

AbstractWe detect and characterise extended, diffuse radio emission from galaxy clusters at 168 MHz within the Epoch of Reionization 0-h field: a$45^{\circ} \times 45^{\circ}$region of the southern sky centred on R. A.${}= 0^{\circ}$, decl.${}=-27^{\circ}$. We detect 29 sources of interest; a newly detected halo in Abell 0141; a newly detected relic in Abell 2751; 4 new halo candidates and a further 4 new relic candidates; and a new phoenix candidate in Abell 2556. Additionally, we find nine clusters with unclassifiable, diffuse steep-spectrum emission as well as a candidate double relic system associated with RXC J2351.0-1934. We present measured source properties such as their integrated flux densities, spectral indices ($\alpha$, where$S_\nu \propto \nu^\alpha$), and sizes where possible. We find several of the diffuse sources to have ultra-steep spectra including the halo in Abell 0141, if confirmed, showing$\alpha \leq -2.1 \pm 0.1$with the present data making it one of the steepest-spectrum haloes known. Finally, we compare our sample of haloes with previously detected haloes and revisit established scaling relations of the radio halo power ($P_{1.4}$) with the cluster X-ray luminosity ($L_{\textrm{X}}$) and mass ($M_{500}$). We find that the newly detected haloes and candidate haloes are consistent with the$P_{1.4}$–$L_{\textrm{X}}$and$P_{1.4}$–$M_{500}$relations and see an increase in scatter in the previously found relations with increasing sample size likely caused by inhomogeneous determination of$P_{1.4}$across the full halo sample. We show that the MWA is capable of detecting haloes and relics within most of the galaxy clusters within thePlanckcatalogue of Sunyaev–Zel’dovich sources depending on exact halo or relic properties.

MWA and ASKAP observations of atypical radio-halo-hosting galaxy clusters: Abell 141 and Abell 3404

AbstractWe report on the detection of a giant radio halo in the cluster Abell 3404 as well as confirmation of the radio halo observed in Abell 141 (with linear extents$\sim\!770$and$\sim\!850$kpc, respectively). We use the Murchison Widefield Array, the Australian Square Kilometre Array Pathfinder, and the Australia Telescope Compact Array to characterise the emission and intervening radio sources from$\sim100$to 1 000 MHz; power law models are fit to the spectral energy distributions with spectral indices$\alpha_{88}^{1\,110} = -1.66 \pm 0.07$and$\alpha_{88}^{943} = -1.06 \pm 0.09$for the radio halos in Abell 3404 and Abell 141, respectively. We find strong correlation between radio and X-ray surface brightness for Abell 3404 but little correlation for Abell 141. We note that each cluster has an atypical morphology for a radio-halo-hosting cluster, with Abell 141 having been previously reported to be in a pre-merging state, and Abell 3404 is largely relaxed with only minor evidence for a disturbed morphology. We find that the radio halo powers are consistent with the current radio halo sample and$P_\nu$–Mscaling relations, but note that the radio halo in Abell 3404 is an ultra-steep–spectrum radio halo (USSRH) and, as with other USSRHs lies slightly below the best-fit$P_{1.4}$–Mrelation. We find that an updated scaling relation is consistent with previous results and shifting the frequency to 150 MHz does not significantly alter the best-fit relations with a sample of 86 radio halos. We suggest that the USSRH halo in Abell 3404 represents the faint class of radio halos that will be found in clusters undergoing weak mergers.

A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). III. Discovery of a z = 4.72 Radio Galaxy with the Lyman Break Technique

Abstract We report a discovery of a z = 4.72 radio galaxy, HSC J083913.17+011308.1, using the Lyman break technique with the Hyper Suprime-Cam Subaru Strategic Survey (HSC-SSP) catalog for Very Large Array Faint Images of the Radio Sky at Twenty centimeter radio sources. The number of known high-z radio galaxies (HzRGs) at z > 3 is quite small to constrain the evolution of HzRGs so far. The deep and wide-area optical survey by HSC-SSP enables us to apply the Lyman break technique to a large search for HzRGs. For an HzRG candidate among pre-selected r-band dropouts with a radio detection, a follow-up optical spectroscopy with the Gemini Multi-Object Spectrographs (GMOS)/Gemini has been performed. The obtained spectrum presents a clear Lyα emission line redshifted to z = 4.72. The spectral energy distribution fitting analysis with the rest-frame UV and optical photometries suggests the massive nature of this HzRG with . The small equivalent width of Lyα and the moderately red UV colors indicate its dusty host galaxy, implying a chemically evolved and dusty system. The radio spectral index does not meet a criterion for an ultra-steep spectrum, of −1.1 and of −0.9, demonstrating that the HSC-SSP survey compensates for a subpopulation of HzRGs that are missed in surveys focusing on an ultra-steep spectral index.

Revived fossil plasma sources in galaxy clusters

It is well established that particle acceleration by shocks and turbulence in the intra-cluster medium can produce cluster-scale synchrotron emitting sources. However, the detailed physics of these particle acceleration processes is still not well understood. One of the main open questions is the role of fossil relativistic electrons that have been deposited in the intracluster medium (ICM) by radio galaxies. These synchrotron-emitting electrons are very difficult to study as their radiative lifetime is only tens of Myr at gigahertz frequencies, and they are therefore a relatively unexplored population. Despite the typical steep radio spectrum due to synchrotron losses, these fossil electrons are barely visible even at radio frequencies well below the gigahertz level. However, when a pocket of fossil radio plasma is compressed, it boosts the visibility at sub-gigahertz frequencies, creating what are known as radio phoenices. This compression can be the result of bulk motion and shocks in the ICM due to merger activity. In this paper we demonstrate the discovery potential of low-frequency radio sky surveys to find and study revived fossil plasma sources in galaxy clusters. We used the 150 MHz TIFR GMRT Sky Survey and the 1.4 GHz NVSS sky survey to identify candidate radio phoenices. A subset of three candidates was studied in detail using deep multi-band radio observations (LOFAR and GMRT), X-ray obserations (Chandra or XMM-Newton), and archival optical observations. Two of the three sources are new discoveries. Using these observations, we identified common observational properties (radio morphology, ultra-steep spectrum, X-ray luminosity, dynamical state) that will enable us to identify this class of sources more easily, and will help us to understand the physical origin of these sources.

Diffuse radio emission in the galaxy cluster SPT-CL J2031−4037: a steep-spectrum intermediate radio halo?

ABSTRACT The advent of sensitive low-frequency radio observations has revealed a number of diffuse radio objects with peculiar properties that are challenging our understanding of the physics of the intracluster medium. Here, we report the discovery of a steep-spectrum radio halo surrounding the central brightest cluster galaxy (BCG) in the galaxy cluster SPT-CL J2031−4037. This cluster is morphologically disturbed yet has a weak cool core, an example of a cool-core/non-cool-core transition system, which harbours a radio halo ∼0.7 Mpc in size. The halo emission detected at 1.7 GHz is less extended compared to that in the 325 MHz observation, and the spectral index of the part of the halo visible at the 325 MHz to 1.7 GHz frequencies was found to be −1.35 ± 0.07. Also, P1.4 GHz was found to be 0.77 × 1024 W Hz−1, which falls in the region where radio mini-haloes, halo upper limits and ultra-steep-spectrum (USS) haloes are found in the P1.4 GHz–LX plane. Additionally, simulations presented in the paper provide support for the scenario of the steep spectrum. The diffuse radio emission found in this cluster may be a steep-spectrum ‘intermediate’ or ‘hybrid’ radio halo that is transitioning into a mini-halo.

Radio relic and the diffuse emission trail discovered in low-mass galaxy cluster Abell 1697

We report the discovery of a putative radio relic, 830 kpc in length, and found towards the outskirts of galaxy cluster Abell 1697 (z = 0.181), using the LOFAR Two Meter Sky Survey at 144 MHz. An X-ray-inferred mass of M500X-ray = 2.9−0.7+0.8 × 1014 M⊙ places Abell 1697 among the least massive relic hosts. The relic is also detected at 325 MHz in the Westerbork Northern Sky Survey and at 1.4 GHz in the NRAO VLA Sky Survey, with an average spectral index of α(144, 325, 1400 MHz) = − 0.98 ± 0.01, and magnetic field of Beq ∼ 0.6 μG. This relic, located in the north-east periphery of the cluster, is 300 kpc wide, exhibits a gradual spectral steepening across the width (α144 MHz1.4 GHz(inj) = −0.70 ± 0.11 to α144 MHz1.4 GHz(edge) = −1.19 ± 0.15), as well as indications of a co-spatial X-ray (ROSAT) shock and the radio relic emission. The radio power of the relic is P1.4 GHz = 8.5 ± 1.1 × 1023 W Hz−1, which is found to be in good agreement with the expected empirical correlation between the radio power and largest linear size of relics. The relic is trailed by extended (790 × 550 kpc) diffuse radio emission towards the cluster centre, which is likely an ultra-steep spectrum (α144 MHz1.4 GHz < −1.84) radio source. This structure is also found to be older by at least 190 Myr, has a very low surface brightness of 0.3 μJy arcsec−2 and magnetic field Beq ∼ 0.8 μG, similar to that of a radio phoenix. Finally, we discuss the possible mechanisms responsible for the relic and the trailing diffuse radio emission, invoking re-acceleration due to wake turbulence, as well as the revival of fossil electrons in the remnant radio lobes of active galactic nuclei by the cluster merger shocks.

LOFAR Discovery of a Radio Halo in the High-redshift Galaxy Cluster PSZ2 G099.86+58.45

Abstract 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 ∼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 observations, which enable us to constrain the radio spectral index to be α ≲ 1.5–1.6, i.e., with properties between canonical and ultra-steep spectrum radio halos. Radio halos are currently explained as synchrotron radiation from relativistic electrons that are re-accelerated in the intracluster medium by turbulence driven by energetic mergers. We show that in such a framework radio halos are expected to be relatively common at ∼150 MHz (∼30%–60%) in clusters with mass and redshift similar to PSZ2 G099.86+58.45; however, at least two-thirds of these radio halos should have a steep spectrum and thus be very faint above ∼1 GHz frequencies. Furthermore, because the luminosity of radio halos at high redshift depends strongly on the magnetic field strength in the hosting clusters, future LOFAR observations will also provide vital information on the origin and amplification of magnetic fields in galaxy clusters.

The complete local volume groups sample – III. Characteristics of group central radio galaxies in the Local Universe

ABSTRACT Using new 610 and 235 MHz observations from the giant metrewave radio telescope (GMRT) in combination with archival GMRT and very large array (VLA) survey data, we present the radio properties of the dominant early-type galaxies in the low-richness subsample of the complete local-volume groups sample (CLoGS; 27 galaxy groups) and provide results for the radio properties of the full CLoGS sample for the first time. We find a high radio detection rate in the dominant galaxies of the low-richness subsample of 82 per cent (22/27); for the full CLoGS sample the detection rate is 87 per cent (46/53). The group-dominant galaxies exhibit a wide range of radio power, 1020–1025 W Hz−1 in the 235 and 610 MHz bands, with the majority (53 per cent) presenting point-like radio emission, 19 per cent hosting currently active radio jets, 6 per cent having remnant jets, 9 per cent being diffuse, and 13 per cent having no detected radio emission. The mean spectral index of the detected radio sources in the 235−610 MHz frequency range is found to be $\alpha _{235}^{610}\sim$0.68, and $\alpha _{235}^{1400}\sim$0.59 in the 235−1400 MHz one. In agreement with earlier studies, we find that the fraction of ultrasteep spectrum sources (α > 1.3) is ∼4 per cent, mostly dependent on the detection limit at 235 MHz. The majority of point-like systems are found to reside in dynamically young groups, whereas jet systems show no preference between spiral-rich and spiral-poor group environments. The mechanical power of the jet sources in the low-richness sample groups is estimated to be ∼1042–1044 erg s−1 with their black hole masses ranging between 2 × 108 and 5 × 109 M⊙. We confirm previous findings that while radio jet sources tend to be associated with more massive black holes, black hole mass is not the decisive factor in determining jet activity or power.

Radio Sources in the Central Section of the RZF Catalog. Search for Objects with Ultra-Steep Spectra

Radio Sources in the Central Section of the RZF Catalog. Search for Objects with Ultra-Steep Spectra

Ultra-steep spectrum emission in the merging galaxy cluster Abell 1914

A number of radio observations have revealed the presence of large synchrotron-emitting sources associated with the intra-cluster medium. There is strong observational evidence that the emitting particles have been (re-)accelerated by shocks and turbulence generated during merger events. The particles that are accelerated are thought to have higher initial energies than those in the thermal pool but the origin of such mildly relativistic particles remains uncertain and needs to be further investigated. The galaxy cluster Abell 1914 is a massive galaxy cluster in which X-ray observations show clear evidence of merging activity. We carried out radio observations of this cluster with the LOw Frequency ARay (LOFAR) at 150 MHz and the Giant Metrewave Radio Telescope (GMRT) at 610 MHz. We also analysed Very Large Array (VLA) 1.4 GHz data, archival GMRT 325 MHz data, CFHT weak lensing data and Chandra observations. Our analysis shows that the ultra-steep spectrum source (4C38.39; α ≲ −2), previously thought to be part of a radio halo, is a distinct source with properties that are consistent with revived fossil plasma sources. Finally, we detect some diffuse emission to the west of the source 4C38.39 that could belong to a radio halo.

A study of spectral curvature in the radio relic in Abell 4038 using the uGMRT

The remnant radio galaxies in galaxy clusters are important sources of seed relativistic electron population in the intra-cluster medium (ICM). Their occurrence and spectral properties are poorly studied. In this work we present a broadband study of the radio relic in the galaxy cluster Abell 4038 using the Upgraded Giant Metrewave Radio Telescope (uGMRT). We present the uGMRT images in the bands 300 - 500 MHz and 1050 - 1450 MHz having rms noise $70\,\mu$Jy beam$^{-1}$ and $30\,\mu$Jy beam$^{-1}$, respectively, that are the deepest images of this field so far. A spectral analysis of the relic over 300 - 1450 MHz using images in sub-bands scaled to have constant fractional bandwidths to achieve a closely matched uv-coverage was carried out. The 100 kpc extent of the relic is divided into Loop, Arc, Bridge and North-end. The Loop has a steep spectral index of $\alpha=2.3\pm0.2$ ($S_{\nu}\propto\nu^{-\alpha}$). The North-end has ultra-steep spectra in the range $2.4 - 3.7$. The Arc is found to skirt a curved region seen in the \emph{Chandra} X-ray surface brightness image and the highest spectral curvature in it reaches $1.6\pm0.3$. We interpret the morphology and spectral properties of the relic in the scenario of an adiabatically compressed cocoon from the past activity of the Brightest Cluster Galaxy in the cluster. A comparison of the properties of the A4038 relic with a sample of 10 such relics is discussed.

A search for faint high-redshift radio galaxy candidates at 150 MHz

Ultra-steep spectrum (USS) radio sources are good tracers of powerful radio galaxies at $z > 2$. Identification of even a single bright radio galaxy at $z > 6$ can be used to detect redshifted 21cm absorption due to neutral hydrogen in the intervening IGM. Here we describe a new sample of high-redshift radio galaxy (HzRG) candidates constructed from the TGSS ADR1 survey at 150 MHz. We employ USS selection ($\alpha \le -1.3$) in $\sim10000$ square degrees, in combination with strict size selection and non-detections in all-sky optical and infrared surveys. We apply flux density cuts that probe a unique parameter space in flux density ($50 < S_{\textrm{150}} < 200$ mJy) to build a sample of 32 HzRG candidates. Follow-up Karl G. Jansky Very Large Array (VLA) observations at 1.4 GHz with an average beam size of $1.3$ arcseconds ($"$) revealed $\sim 48\%$ of sources to have a single radio component. P-band (370 MHz) imaging of 17 of these sources revealed a flattening radio SED for ten sources at low frequencies, which is expected from compact HzRGs. Two of our sources lie in fields where deeper multi-wavelength photometry and ancillary radio data are available and for one of these we find a best-fit photo-z of $4.8 \pm 2.0$. The other source has $z_{\textrm{phot}}=1.4 \pm 0.1$ and a small angular size ($3.7"$), which could be associated with an obscured star forming galaxy or with a "dead" elliptical. One USS radio source not part of the HzRG sample but observed with the VLA nonetheless is revealed to be a candidate giant radio galaxy with a host galaxy photo-z of $1.8\pm0.5$, indicating a size of 875 kpc.

LOFAR discovery of an ultra-steep radio halo and giant head–tail radio galaxy in Abell 1132

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 $\alpha=-1.75\pm0.19$ ($S\propto\nu^{\alpha}$). We classify this emission as an ultra-steep spectrum radio halo and discuss the possible implications for the physical origin of radio halos. The HT radio galaxy shows narrow, collimated emission extending up to 1 Mpc and another 300 kpc of more diffuse, disturbed emission, giving a full projected linear size of 1.3 Mpc - classifying it as a giant radio galaxy (GRG) and making it the longest HT found to date. The head of the GRG coincides with an elliptical galaxy (SDSS J105851.01$+$564308.5) belonging to Abell 1132. In our LOFAR image, there appears to be a connection between the radio halo and the GRG. The turbulence that may have produced the halo may have also affected the tail of the GRG. In turn, the GRG may have provided seed electrons for the radio halo.

Search and modelling of remnant radio galaxies in the LOFAR Lockman Hole field

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 higher frequencies to perform a complete selection and have used, for the first time, a combination of spectral criteria (e.g. the classical ultra-steep spectral index and high spectral curvature) as well as morphological criteria (e.g. low radio core prominence and relaxed shapes). Mock catalogues of radio galaxies are created based on existing spectral and dynamical evolution models combined with observed source properties. We have identified 23 candidate remnant radio galaxies which cover a variety of morphologies and spectral characteristics. We suggest that these different properties are related to different stages of the remnant evolution. We find that ultra-steep spectrum remnants represent only a fraction of our remnant sample suggesting a very rapid luminosity evolution of the radio plasma. Results from mock catalogues demonstrate the importance of dynamical evolution in the remnant phase of low-power radio galaxies to obtain fractions of remnant sources consistent with our observations. Moreover, these results confirm that ultra-steep spectrum remnants represent only a subset of the entire population ($\sim$50%) when frequencies higher than 1400 MHz are not included in the selection process, and that they are biased towards old ages.