arXiv subjects

Konstantinos Kolokythas

Publications and source records attributed to Konstantinos Kolokythas.

At least 19 recordsLinked to original sources

arXiv2026

Bridging the mass gap: Diffuse radio emission in GAMA galaxy groups using EMU and DINGO survey data

Diffuse radio emission provides a powerful probe of non-thermal processes in the large-scale structure, yet its properties in galaxy groups remain poorly constrained. Using deep 943 MHz radio continuum data from the Evolutionary Map of the Universe (EMU) and 1.37 GHz data from the Deep Investigations of Neutral Gas Origins (DINGO) survey, we investigate diffuse radio emission in 400 galaxy groups selected from the GAMA survey at $z < 0.1$. We employ a multi-resolution filtering technique to suppress compact radio sources and enhance extended, low-surface-brightness emission associated with the intra group medium. Integrated flux densities are measured within group radii, and background fluctuations are quantified using random control regions. While most systems yield non-detections, we identify 46/400 galaxy groups with candidate diffuse emission, spanning radio powers of $10^{19}-10^{24}\,\mathrm{W\,Hz^{-1}}$. Stacked measurements reveal a weak positive trend between radio power and halo mass. The observed emission levels lie above simple extrapolations of cluster scaling relations, suggesting that different physical processes dominate in the group regime. Additionally, stellar mass ratios of the most massive galaxies in the group and Early Type Galaxy fractions suggest that these galaxy groups are relatively young, evolving systems where galaxy interactions and mergers may power the emission. Comparisons with Magneto Hydrodynamical simulations indicate shock acceleration alone cannot explain the observed emission, pointing to an important role for fossil plasma re-acceleration and group-scale dynamical activity. These results demonstrate diffuse radio emission is present in a non-negligible fraction of galaxy groups, providing new constraints on non-thermal processes in low-mass environments.

astro-ph.GA↗

arXiv2026

A targeted machine learning approach for detecting diffuse radio emission with Astronomaly: Protege

Diffuse radio emission in galaxy clusters, such as radio halos, relics, and mini halos, is a key tracer of non-thermal processes, turbulence, and magnetic fields within the intra-cluster medium. However, their low surface brightness, as well as contamination from compact sources and imaging artefacts, makes their detection challenging. The sheer volume of data from instruments such as the Square Kilometre Array will render traditional manual-inspection based detection methods infeasible. This paper introduces a novel machine learning approach that uses active learning to rapidly identify diffuse emission candidates from a small, optimally-selected subset of data. We apply the self-supervised deep learning algorithm Bootstrap Your Own Latent to extract features from source cutouts in the MeerKAT Galaxy Cluster Legacy Survey (MGCLS). We then pass these features through the Astronomaly: Protege anomaly detection framework to identify the final candidates. Using a human-labelled set, we evaluate our pipeline on high-resolution (~7''), convolved (15''), and combined-feature MGCLS datasets. Interestingly, the high-resolution features identify diffuse sources more efficiently than the convolved resolution, which are in turn outperformed by the combined features. Of the top 100 sources ranked by Protege, 99% exhibit diffuse characteristics, with 55% confirmed as cluster-related emission. Our work shows that Protege can identify diffuse emission with minimal human labelling effort, offering a powerful, scalable tool capable of detecting both known and novel diffuse radio sources.

astro-ph.IM↗

arXiv2026

Mechanisms Affecting Galaxies Nearby and Environmental Trends (MAGNET)

[ABRIDGED] Galaxy evolution is shaped by internal and external mechanisms that regulate the baryon cycle and star formation activity. We present a theoretical framework based on the GAlaxy Evolution and Assembly (GAEA) semi-analytic model. We extracted portions of simulated volumes that include isolated galaxies, pairs, group, and filament members at z ~ 0, specifically avoiding massive clusters. Galaxies were classified using both intrinsic (halo-based) and observational (2D projected) parameterizations, reconstructing their environmental histories from z = 2 and identifying mergers, tidal interactions, ram pressure stripping (RPS), and starvation. 2D information decreases isolated and group fractions while doubles pairs. More than half of galaxies remain unaffected by the investigated processes since z = 2. Among affected galaxies, mergers dominate at high stellar masses (40-60% at log(M*/Msun) > 10.5). Tidal interactions are less frequent, and their incidence increases with stellar mass. RPS dominates in groups and filaments at intermediate masses (~50%), while starvation ranges from 20 to 30%. The incidence of the different mechanisms depends strongly on both mass and environment, though their imprints on global properties are often subtle. Distinct evolutionary pathways emerge: log(M*/Msun) < 9.5, galaxies in groups and filaments have a faster mass growth than galaxies in the other environments, especially those undergoing starvation, mergers and, to less extent, RPS. Differences are reduced moving to higher masses, where no clear dependence on physical mechanism emerge, even though at these masses a clear star formation suppression is evident in mergers and starved galaxies. This theoretical investigation provides essential context for the recently started multi-wavelength program Mechanisms Affecting Galaxies Nearby and Environmental Trends (MAGNET), which we introduce here.

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arXiv2025

The MeerKAT Galaxy Cluster Legacy Survey -- II. Catalogue of the diffuse radio emission in MeerKAT-GCLS clusters

We present a follow-up study focused on cluster-scale diffuse radio emissions in 115 galaxy clusters in the Southern sky, using full-resolution (7.8") and tapered low-resolution (15") images from the MeerKAT Galaxy Cluster Legacy Survey (MGCLS). In this MGCLS follow-up paper, we update and quantitatively characterise the presence of diffuse radio emission and provide detailed catalogue information on its radio properties at 1.28 GHz. As the MGCLS sample is heterogeneous and was initially used as a test bed to reveal the scientific potential of MeerKAT, the reported numbers are subject to this special case. More than half ($\sim$54%; 62/115) of the observed MGCLS clusters present diffuse cluster radio emission, with the total number of diffuse radio sources or candidates detected being 103. Including candidates, we find that radio relics are the most frequently detected diffuse sources in MGCLS at 53% (55/103), followed by halos at 32% (33/103) and mini-halos at 10% (11/103), with only 3% (3/103) being Phoenixes. The sizes of the diffuse radio structures and candidates range from 55 kpc to over 2 Mpc, with $P_{1.28GHz}$ radio power ranging from $10^{22}$ W Hz$^{-1}$ to greater than $10^{25}$ W Hz$^{-1}$. In-band radio spectral index estimates reveal revived radio plasma emissions that exhibit steep radio spectral indices down to $α^{1656}_{908} \sim -3.5$. Mini-halos and their respective candidates are detected mainly in low-mass ($M_{500}$ $\leq$ 5$\times10^{14}$ M$_\odot$) and low-power ($P_{1.4GHz}$ $\leq$ $10^{23}$ W Hz$^{-1}$) systems. We suggest the presence of a statistically significant correlation between the 1.4 GHz radio power and the cluster mass for mini-halo (and candidate) systems.

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arXiv2025

Revisiting the Group-Dominant Elliptical NGC 5044 in the Radio Band: Continuum Emission and Detection of HI Absorption

We present new MeerKAT L-band (continuum and HI) and upgraded Giant Metrewave Radio Telescope (300-850 MHz) observations of the archetypal cool-core group-dominant early-type galaxy NGC 5044. Our new continuum images reveal diffuse, steep spectrum ($α_{0.99\,\rm GHz}^{1.56\,\rm GHz}=-1.53\pm0.6$) radio emission extending about 25 kpc around the unresolved radio core. The observed radio emission overlaps with the known X-ray cavities, but is not confined to them. We also find the first direct evidence of neutral atomic gas in NGC 5044, in the form of a 3.8$σ$ significant two-component HI absorption line seen against the emission of the active nucleus. The peak velocities are well correlated with the previously reported CO(2-1) absorption, but the HI lines are moderately broader, spanning velocities from $265\,\rm \, km\,s^{-1}$ to $305\,\rm \, km\,s^{-1}$. We do not detect HI emission, but place an upper limit of $M_{HI}< 5.4 \times 10^{7} \, M_{\odot}$ in the central 15 arcsec (2.2 kpc) of the galaxy. This is significantly less than the estimated molecular gas content, and implies a molecular-to-atomic mass ratio of $\geq $1.7:1, consistent with these gas phases forming through cooling from the hot intra-group medium. We also constrain the spin temperature to $T_{\rm spin}\leq 950\,\rm K$, indicating that the detected HI is in the cold neutral phase.

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arXiv2024

A hot core in the group-dominant elliptical galaxy NGC 777

NGC 777 provides an example of a phenomenon observed in some group-central ellipticals, in which the temperature profile shows a central peak, despite the short central cooling time of the intra-group medium. We use deep Chandra X-ray observations of the galaxy, supported by uGMRT 400 MHz radio imaging, to investigate the origin of this hot core. We confirm the centrally-peaked temperature profile and find that entropy and cooling time both monotonically decline to low values (2.62 [+0.19, -0.18] keV cm$^2$ and 71.3 [+12.8, -13.1] Myr) in the central ~700 pc. Faint diffuse radio emission surrounds the nuclear point source, with no clear jets or lobes but extending to ~10 kpc on a northwest-southeast axis. This alignment and extent agree well with a previously identified filamentary H$α$+[NII] nebula. While cavities are not firmly detected, we see X-ray surface brightness decrements on the same axis at 10-20 kpc radius which are consistent with the intra-group medium having been pushed aside by expanding radio lobes. Any such outburst must have occurred long enough ago for lobe emission to have faded below detectability. Cavities on this scale would be capable of balancing radiative cooling for at least ~240 Myr. We consider possible causes of the centrally peaked temperature profile, including gravitational heating of gas as the halo relaxes after a period of AGN jet activity, and heating by particles leaking from the remnant relativistic plasma of the old radio jets.

astro-ph.GA↗

arXiv2021

The Complete Local Volume Groups Sample -- IV. Star formation and gas content in group-dominant galaxies

Using multi-band data we examine the star formation activity of the nearby group-dominant early-type galaxies of the Complete Local-volume Groups Sample (CLoGS), and the relation between star formation, gas content, and local environment. Only a small fraction of the galaxies (13%; 6/47) are found to be Far-Ultraviolet (FUV) bright, with FUV to near-infrared colours indicative of recent active star formation (NGC 252, NGC 924, NGC 940, NGC 1106, NGC 7252, and ESO 507-25). These systems are lenticulars presenting the highest FUV specific star-formation rates in the sample (sSFR FUV > 5$\times$10$^{13}$ yr$^{-1}$), significant cold gas reservoirs (M(H2)=0.5 - 61$\times$10$^8$ M$_\odot$), reside in X-ray faint groups, and none of them hosts a powerful radio AGN (P$_{1.4GHz}$<$10^{23}$ W Hz$^{-1}$). The majority of the group-dominant galaxies (87%; 41/47) are FUV faint, with no significant star formation, classified in most cases as spheroids based on their position on the infrared star-forming main sequence (87%; 46/53). Examining the relationships between radio power, SFR FUV, and stellar mass we find a lack of correlation that suggests a combination of origins for the cool gas in these galaxies, including stellar mass loss, cooling from the intra-group medium (IGrM) or galaxy halo, and acquisition through mergers or tidal interactions. X-ray bright systems, in addition to hosting radio powerful AGN, have a range of SFRs but, with the exception of NGC 315, do not rise to the highest rates seen in the FUV bright systems. We suggest that central group galaxy evolution is linked to gas mass availability, with star formation favoured in the absence of a group-scale X-ray halo, but AGN jet launching is more likely in systems with a cooling IGrM.

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arXiv2021

Searching for high-z radio galaxies with the MGCLS

We present results from a search for high-redshift radio galaxy (H$z$RG) candidates using 1.28 GHz data in the Abell 2751 field drawn from the MeerKAT Galaxy Cluster Legacy Survey (MGCLS). We use the H$z$RG criteria that a radio source is undetected in all-sky optical and infrared catalogues, and has a very steep radio spectrum. We cross-match the radio catalogue against multi-wavelength galaxy catalogues from DECaLS and AllWISE. For those radio sources with no multi-wavelength counterpart, we further implement a radio spectral index criterium of $α< -1$, using in-band spectral index measurements from the wide-band MeerKAT data. Using a 5$σ$ signal-to-noise cut on the radio flux densities, we find a total of 274 HzRG candidates: 179 ultra-steep spectrum sources, and 95 potential candidates which cannot be ruled out as they have no spectral information available. The spectral index assignments in this work are complete above a flux density of 0.3 mJy, at least an order of magnitude lower than existing studies in this frequency range or when extrapolating from lower frequency limits. Our faintest HzRG candidates with and without an in-band spectral index measurement have a 1.28\,GHz flux density of 57 $\pm$ 8 $μ$Jy and 68 $\pm$ 13 $μ$Jy, respectively. Although our study is not complete down to these flux densities, our results indicate that the sensitivity and bandwidth of the MGCLS data makes them a powerful radio resource to search for H$z$RG candidates in the Southern sky, with 20 of the MGCLS pointings having similar image quality as the Abell~2751 field and full coverage in both DECaLS and AllWISE. Data at additional radio frequencies will be needed for the faintest source populations, which could be provided in the near future by the MeerKAT UHF band (580 -- 1015 MHz) at a similar resolution ($\sim$ 8-10 arcsec).

astro-ph.GA↗

arXiv2021

The unusually weak and exceptionally steep radio relic in Abell 2108

Mergers between galaxy clusters often drive shocks into the intra cluster medium (ICM), the effects of which are sometimes visible via temperature and density jumps in the X-ray, and via radio emission from relativistic particles energized by the shock's passage. Abell2108 was selected as a likely merger system through comparing the X-ray luminosity to the Planck Sunyaev-Zeldovich signal, where this cluster appeared highly X-ray underluminous. Follow up observations confirmed it to be a merging low mass cluster featuring two distinct subclusters, both with a highly disturbed X-ray morphology. Giant Metrewave Radio Telescope (GMRT) data covering 120-750MHz show an extended radio feature resembling a radio relic, near the location of a temperature discontinuity in the X-rays. We measure a Mach number from the X-ray temperature jump. Several characteristics of radio relics are found in Abell2108, making this cluster one of the few low mass mergers likely hosting a radio relic. The radio spectrum is exceptionally steep, and the radio power is very weak (P1.4GHz=1E22W/Hz). To account for the shock/relic offset, we propose a scenario in which the shock created the relic by re-accelerating a cloud of pre-existing relativistic electrons and then moved away, leaving behind a fading relic. The electron aging timescale derived from the high-frequency steepening in the relic spectrum is consistent with the shock travel time to the observed X-ray discontinuity. However, the lower flux in GMRT band 4 data causing the steepening could be due to instrumental limitations, and deeper radio data are needed to constrain the spectral slope of the relic at high frequencies.

astro-ph.GA↗

arXiv2021

One Source, Two Source(s): Ribs and Tethers

We present the unique and challenging case of a radio galaxy in Abell 3266 observed as part of the MeerKAT Galaxy Cluster Legacy Survey. It has quasi-periodic bright patches along the tail which connect to never-before-seen thin transverse extensions, which we call `ribs', reaching up to approx. 50 kpc from the central axis of the tail. At a distance of approx. 400 kpc from the host (assuming the z=0.0594 redshift of Abell 3266) we find what appears to be a triple source with its own apparent host at a photometric redshift of 0.78. Mysteriously, the part of the tail far from the host and the triple are connected by a series of thin filaments, which we call "tethers." The far tail, tethers and triple also have similar spectra and Faraday rotation measures, suggesting that there is only one -- quite complicated -- source, with a serendipitous background AGN in the triple. We look at possible causes for the "rib" and "tether" structures, and the emerging phenomena of intracluster medium filaments associated with radio galaxies.

astro-ph.GA↗

arXiv2021

The XXL Survey: XLIII. The quasar radio loudness dichotomy exposed via radio luminosity functions obtained by combining results from COSMOS and XXL-S X-ray selected quasars

We studied a sample of 274 radio and X-ray selected quasars (XQSOs) detected in the COSMOS and XXL-S radio surveys at 3 GHz and 2.1 GHz, respectively. This sample was identified by adopting a conservative threshold in X-ray luminosity, Lx [2-10\ keV] >= 10^44 erg/s, selecting only the most powerful quasars. Using available multiwavelength data, we examined various criteria for the selection of radio-loud (RL) and radio-quiet (RQ) XQSOs, finding that the number of RL/RQ XQSOs changes significantly depending on the chosen criterion. This discrepancy arises due to the different criteria tracing different physical processes and due to our sample being selected from flux-limited radio and X-ray surveys. Another approach to study the origin of radio emission in XQSOs is via their radio luminosity function (RLF). We constructed the XQSO 1.4 GHz radio luminosity functions (RLFs) in six redshift bins at 0.5 <= z <= 3.7. The lower-1.4 GHz luminosity end shows a higher normalization than expected only from AGN contribution in all studied redshift bins. The found "bump" is mostly dominated by emission due to star-forming (SF) processes within the XQSO host galaxies. As expected, AGN-related radio emission dominates at the higher-luminosity end of RLF. The evolution of XQSO RLF was studied via combination of analytic forms from the literature to constrain the lower-luminosity "bump" and the higher-luminosity AGN part of the RLF. We defined two 1.4 GHz luminosity thresholds, L_th,SF and L_th,AGN, below and above which more than 80% of sources contributing to the RLF are dominated by SF and AGN-related activity, respectively. These thresholds evolve with redshift, most likely due to the strong evolution of SFRs of the XQSO host galaxies.

astro-ph.GA↗

arXiv2020

Evidence of AGN feedback and sloshing in the X-ray luminous NGC 1550 galaxy group

We present results from GMRT and Chandra observations of the NGC 1550 galaxy group. Although previously thought of as relaxed, we show evidence that gas sloshing and active galactic nucleus (AGN) heating have affected the structure of the system. The 610 and 235 MHz radio images show an asymmetric jet-lobe structure with a total size of $\sim$33 kpc, with a sharp kink at the base of the more extended western jet, and bending of the shorter eastern jet as it enters the lobe. The 235$-$610 MHz spectral index map shows that both radio lobes have steep spectral indices ($α_{235}^{610}\geq-1.5$) indicating the presence of an old electron population. The X-ray images reveal an asymmetric structure in the hot gas correlated with the radio structure, as well as potential cavities coincident with the radio lobes, with rims and arms of gas that may have been uplifted by the cavity expansion. The X-ray residual map reveals an arc shaped structure to the east that resembles a sloshing cold front. Radio spectral analysis suggests a radiative age of about 33 Myr for the source, comparable to the sloshing timescale and dynamical estimates of the age of the lobes. An estimate of the mechanical energy required to inflate the cavities suggests that the AGN of NGC 1550 is capable of balancing radiative losses from the intragroup medium (IGM) and preventing excessive cooling, providing that the AGN jets are efficiently coupled to the IGM gas. In conclusion, we find evidence of sloshing motions from both radio and X-ray structures, suggesting that NGC 1550 was perturbed by a minor merger or infalling galaxy about 33 Myr ago.

astro-ph.GA↗

arXiv2019

The Complete Local Volume Groups Sample -- III. Characteristics of group central radio galaxies in the Local Universe

Using new 610 MHz 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 sub-sample 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 sub-sample of 82% (22/27); for the full CLoGS sample, the detection rate is 87% (46/53). The group-dominant galaxies exhibit a wide range of radio power, 10$^{20}$ $-$ 10$^{25}$ W Hz$^{-1}$ in the 235 and 610 MHz bands, with the majority (53%) presenting point-like radio emission, 19% hosting currently active radio jets, 6% having remnant jets, 9% being diffuse and 13% 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 $α_{235}^{610}\sim$0.68, and $α_{235}^{1400}\sim$0.59 in the 235$-$1400 MHz one. In agreement with earlier studies, we find that the fraction of ultra-steep spectrum sources ($α>$1.3) is $\sim$4%, mostly dependent on the detection limit at 235 MHz. The majority of point-like systems are found to reside in dynamically young groups, whereas jet systems show no preference between spiral-rich and spiral-poor group environments. The mechanical power of the jet sources in the low$-$richness sample groups is estimated to be $\sim$10$^{42}$ $-$ 10$^{44}$ erg s$^{-1}$ with their black hole masses ranging between 2$\times$10$^{8}$ $-$ 5$\times$10$^{9}$ M$_{\odot}$. 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.

astro-ph.GA↗

arXiv2018

The Complete Local Volume Groups Sample - II. A study of the Central Radio Galaxies in the High$-$Richness Sub-sample

We present a study of the radio properties of the dominant early-type galaxies in 26 galaxy groups, the high-richness sub-sample of the Complete Local-volume Groups Sample (CLoGS). Combining new 610 MHz and 235 MHz observations of 21 groups from the Giant Metrewave Radio Telescope (GMRT) with archival GMRT and Very Large Array (VLA) survey data, we find a high detection rate, with 92% of the dominant galaxies hosting radio sources. The sources have a wide range of luminosities, 10$^{20}$ $-$ 10$^{24}$ W/Hz in the 235 and 610 MHz bands. The majority (54%) are point-like, but 23% have radio jets, and another 15% are diffuse radio sources with no clear jet/lobe morphology. The spectral index of the detected radio sources ranges from very flat values of ~0.2 to typical radio synchrotron spectra of ~0.9 with only two presenting steep radio spectra with $α_{235}^{610}$ > 1. We find that jet sources are more common in X-ray bright groups, with radio non-detections found only in X-ray faint systems. Radio point sources appear in all group environments irrespective of their X-ray properties or spiral fraction. We estimate the mechanical power (Pcav) of the jet sources in the X-ray bright groups to be 10$^{41}$ $-$ 10$^{43}$ erg/s, with the two large-scale jet systems (NGC 193 and NGC 4261) showing jet powers two orders of magnitude greater than the radiative losses from the cool cores of their groups. This suggests that central AGN are not always in balance with cooling, but may instead produce powerful periodical bursts of feedback heating.

astro-ph.GA↗

arXiv2018

The XXL Survey: XXIX. GMRT 610 MHz continuum observations

We present the 25 square-degree GMRT-XXL-N 610 MHz radio continuum survey, conducted at 50~cm wavelength with the Giant Metrewave Radio Telescope (GMRT) towards the XXL Northern field (XXL-N). We combined previously published observations of the XMM-Large Scale Structure (XMM-LSS) field, located in the central part of XXL-N, with newly conducted observations towards the remaining XXL-N area, and imaged the combined data-set using the Source Peeling and Atmospheric Modeling ({\sc SPAM}) pipeline. The final mosaic encompasses a total area of $30.4$ square degrees, with ${\rm rms}<150~μ$Jy/beam over 60\% of the area. The ${\rm rms}$ achieved in the inner 9.6 square degree area, enclosing the XMM-LSS field, is about $200~μ$Jy/beam, while that over the outer 12.66 square degree area (which excludes the noisy edges) is about $45~μ$Jy/beam. The resolution of the final mosaic is 6.5 arcsec. We present a catalogue of 5\,434 sources detected at $\geq7\times {\rm rms}$. We verify, and correct the reliability of, the catalog in terms of astrometry, flux, and false detection rate. Making use of the (to date) deepest radio continuum survey over a relatively large (2 square degree) field, complete at the flux levels probed by the GMRT-XXL-N survey, we also assess the survey's incompleteness as a function of flux density. The radio continuum sensitivity reached over a large field with a wealth of multi-wavelength data available makes the GMRT-XXL-N 610 MHz survey an important asset for studying the physical properties, environments and cosmic evolution of radio sources, in particular radio-selected active galactic nuclei (AGN).

astro-ph.GA↗

arXiv2017

The origin of the X-ray, radio and HI structures in the NGC 5903 galaxy group

The NGC 5903 galaxy group is a nearby (~30 Mpc) system of ~30 members, dominated by the giant ellipticals NGC 5903 and NGC 5898. The group contains two unusual structures, a ~110 kpc long HI filament crossing NGC 5903, and a ~75 kpc wide diffuse, steep-spectrum radio source of unknown origin which overlaps NGC 5903 and appears to be partly enclosed by the HI filament. Using a combination of Chandra, XMM-Newton, GMRT and VLA observations, we detect a previously unknown ~0.65 keV intra-group medium filling the volume within 145 kpc of NGC 5903, and find a loop of enhanced X-ray emission extending ~35 kpc southwest from the galaxy, enclosing the brightest part of the radio source. The northern and eastern parts of this X-ray structure are also strongly correlated with the southern parts of the HI filament. We determine the spectral index of the bright radio emission to be $α_{150}^{612}$=1.03$\pm$0.08, indicating a radiative age >360 Myr. We discuss the origin of the correlated radio, X-ray and HI structures, either through an interaction-triggered AGN outburst with enthalpy 1.8x10$^{57}$ erg, or via a high-velocity collision between a galaxy and the HI filament. While neither scenario provides a complete explanation, we find that an AGN outburst is the most likely source of the principal X-ray and radio structures. However, it is clear that galaxy interactions continue to play an important role in the development of this relatively highly evolved galaxy group. We also resolve the question of whether the group member galaxy ESO 514-3 hosts a double-lobed radio source, confirming that the source is a superposed background AGN.

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arXiv2017

The Complete Local Volume Groups Sample - I. Sample Selection and X-ray Properties of the High-Richness Subsample

We present the Complete Local-Volume Groups Sample (CLoGS), a statistically complete optically-selected sample of 53 groups within 80 Mpc. Our goal is to combine X-ray, radio and optical data to investigate the relationship between member galaxies, their active nuclei, and the hot intra-group medium (IGM). We describe sample selection, define a 26-group high-richness subsample of groups containing at least 4 optically bright (log L_B>=10.2 LBsol) galaxies, and report the results of XMM-Newton and Chandra observations of these systems. We find that 14 of the 26 groups are X-ray bright, possessing a group-scale IGM extending at least 65kpc and with luminosity >10^41 erg/s, while a further 3 groups host smaller galaxy-scale gas halos. The X-ray bright groups have masses in the range M_500=0.5-5x10^13 Msol, based on system temperatures of 0.4-1.4 keV, and X-ray luminosities in the range 2-200x10^41 erg/s. We find that ~53-65% of the X-ray bright groups have cool cores, a somewhat lower fraction than found by previous archival surveys. Approximately 30% of the X-ray bright groups show evidence of recent dynamical interactions (mergers or sloshing), and ~35% of their dominant early-type galaxies host AGN with radio jets. We find no groups with unusually high central entropies, as predicted by some simulations, and confirm that CLoGS is in principle capable of detecting such systems. We identify three previously unrecognized groups, and find that they are either faint (L_X,R500<10^42 erg/s) with no concentrated cool core, or highly disturbed. This leads us to suggest that ~20% of X-ray bright groups in the local universe may still be unidentified.

astro-ph.GA↗

arXiv2015

New insights into the evolution of the FR I radio galaxy 3C 270 (NGC 4261) from VLA and GMRT radio observations

We present Giant Metrewave Radio Telescope (GMRT) 240 MHz observations of the nearby luminous FR I radio source 3C 270, in the group-central elliptical NGC 4261. Combining these data with reprocessed Very Large Array (VLA) 1.55 and 4.8 GHz observations, we produce spectral index maps that reveal a constant spectral index along the jets and a gradual steepening from the ends of the jets through the lobes towards the nucleus. A Jaffe & Perola (JP) model fitted to the integrated spectrum of the source gives an asymptotic low-frequency index of $α_{inj}=0.53_{-0.02}^{+0.01}$, while JP models fitted to the observed spectral index trend along the lobes allow us to estimate radiative ages of $\sim29$ Myr and $\sim37$ Myr for the west and east lobes respectively. Our age estimates are a factor of two lower than the 75-Myr upper limit derived from X-ray data (O'Sullivan et al. 2011). We find unlikely the scenario of an early supersonic phase in which the lobe expanded into the ISM at approximately Mach 6 (3500 km s$^{-1}$), and suggest that either the source underwent multiple AGN outbursts with possible large changes in jet power, or possibly that the source age that we find is due to a backflow that transports young electrons from the jet tips through the lobes toward the nucleus relatively quickly. We calculate that in the lobes the energy ratio of non-radiating to radiating particles is $\sim4-24$ indicating significant gas entrainment. If the lobes are in pressure balance with their surroundings, the total energy required to heat the entrained material is $10^{58}$ erg, $\sim$40% of the total enthalpy of the lobes.

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