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The active galactic nucleus (AGN) feeding and feedback process in the centers of galaxy clusters and groups is still not well understood. NGC 5044 is the ideal system in which to study AGN feedback. It hosts the largest known reservoir of cold gas in any cool-core galaxy group, and features several past epochs of AGN feedback imprinted as cavities in the X-ray bright intragroup medium (IGrM), as well as parsec scale jets. We present Submillimeter Array (SMA), Karl G. Jansky Very Large Array (VLA), James Clerk Maxwell Telescope (JCMT), and Atacama Large Millimeter/submillimeter Array (ALMA) high frequency observations of NGC 5044 to assess the time variability of the mmwaveband emission from the accretion disk, and quantify the Spectral Energy Distribution (SED) from the radio to sub-millimeter band. The SED is well described by advection dominated accretion flow (ADAF) model and self-absorbed jet emission from an aging plasma with tau = 1kyr. We find a characteristic variability timescale of 150 days, which constrains the ADAF emission region to about 0.1pc, and the magnetic field to 4.7mG in the jets and and 870G in the accretion disk. A longer monitoring/sampling will allow to understand if the underlying process is truly periodic in nature., Comment: 17 pages, 7 figures, accepted for publication in ApJ

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$\alpha$+[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., Comment: 19 pages, accepted for publication in the Astrophysical Journal, v2 corrects some minor typographical errors identified at the proof stage

Ultra-steep spectrum (USS) radio sources with complex filamentary morphologies are a poorly understood subclass of diffuse radio source found in galaxy clusters. They are characterised by power law spectra with spectral indices less than -1.5, and are typically located in merging clusters. We present X-ray and radio observations of the galaxy cluster A272, containing a USS diffuse radio source. The system is an ongoing major cluster merger with an extended region of bright X-ray emission south of the core. Surface brightness analysis yields a $3\sigma$ detection of a merger shock front in this region. We obtain shock Mach numbers $M_\rho = 1.20 \pm 0.09$ and $M_T = 1.7 \pm 0.3$ from the density and temperature jumps, respectively. Optical data reveals that the system is a merger between a northern cool core cluster and a southern non-cool core cluster. We find that the USS source, with spectral index $\alpha^{\text{74 MHz}}_{\text{1.4 GHz}} = -1.9 \pm 0.1$, is located in the bright southern region. Radio observations show that the source has a double-lobed structure with complex filaments, and is centred on the brightest cluster galaxy of the southern subcluster. We provide two suggestions for the origin of this source; the first posits the source as an AGN relic that has been re-energised by the passing of a merger shock front, while the second interprets the complex structure as the result of two overlapping AGN radio outbursts. We also present constraints on the inverse Compton emission at the location of the source., Comment: 14 pages, 16 figures, submitted to MNRAS

The galaxy cluster Abell 746 (A746; $z$=0.214), featuring a double radio relic system, two isolated radio relics, a possible radio halo, disturbed V-shaped X-ray emission, and intricate galaxy distributions, is a unique and complex merging system. We present a weak-lensing analysis of A746 based on wide-field imaging data from Subaru/Hyper Suprime-Cam observations. The mass distribution is characterized by a main peak which coincides with the center of the X-ray emission. At this main peak, we detect two extensions toward the north and west, tracing the cluster galaxy and X-ray distributions. Despite the ongoing merger, our estimate of the A746 global mass $M_{500}=4.4\pm1.0\times10^{14}~M_{\odot}$ is consistent with the previous results from SZ and X-ray observations. We conclude that reconciling the distributions of mass, galaxies, and intracluster medium with the double radio relic system and other radio features remains challenging., Comment: Published in ApJ

3C295 is a bright, compact steep spectrum source with a well-studied integrated radio spectral energy distribution (SED) from 132 MHz to 15 GHz. However, spatially resolved spectral studies have been limited due to a lack of high resolution images at low radio frequencies. These frequencies are crucial for measuring absorption processes, and anchoring the overall spectral modelling of the radio SED. In this paper, we use International LOFAR (LOw-Frequency ARray) Telescope (ILT) observations of 3C295 to study its spatially resolved spectral properties with sub-arcsecond resolution at 132 MHz. Combining our new 132 MHz observation with archival data at 1.6 GHz, 4.8 GHz, and 15 GHz, we are able to carry out a resolved radio spectral analysis. The spectral properties of the hotspots provides evidence for low frequency flattening. In contrast, the spectral shape across the lobes is consistent with a JP spectral ageing model. Using the integrated spectral information for each component, we then fit low-frequency absorption models to the hotspots, finding that both free-free absorption and synchrotron self-absorption models provide a better fit to the data than a standard power law. Although we can say there is low-frequency absorption present in the hotspots of 3C295, future observations with the Low Band Antenna of the ILT at 55 MHz may allow us to distinguish the type of absorption., Comment: Submitted to a special issue of A&A on sub-arcsecond imaging with LOFAR

Observations of radio relics at very high frequency (>10 GHz) can help to understand how particles age and are (re-)accelerated in galaxy cluster outskirts and how magnetic fields are amplified in these environments. In this work, we present new single-dish 18.6 GHz Sardinia Radio Telescope and 14.25 GHz Effelsberg observations of the well known northern radio relic of CIZA J2242.8+5301. We detected the relic which shows a length of $\sim$1.8 Mpc and a flux density equal to $\rm S_{14.25\,GHz}=(9.5\pm3.9)\,mJy$ and $\rm S_{18.6\,GHz}=(7.67\pm0.90)\,mJy$ at 14.25 GHz and 18.6 GHz respectively. The resulting best-fit model of the relic spectrum from 145 MHz to 18.6 GHz is a power-law spectrum with spectral index $\alpha=1.12\pm0.03$: no evidence of steepening has been found in the new data presented in this work. For the first time, polarisation properties have been derived at 18.6 GHz, revealing an averaged polarisation fraction of $\sim40\%$ and a magnetic field aligned with the 'filaments' or 'sheets' of the relic., Comment: 10 pages, 8 figures

We examine the possibility that a claimed dependence of the FR 1/2 break value in radio luminosity on the absolute magnitude of the optical host galaxy could be due to the Malmquist bias, where a redshift-luminosity correlation appears in a flux-limited sample because of an observational selection effect. In such a sample, the redshift dependence of a phenomenon could appear as a luminosity-dependent effect and may not be really representing an intrinsic property of the radio sample. We test this on the radio complete MRC (Molonglo Reference Catalog) sample, where Spearman rank correlation and Kendall rank correlation tests show that the correlations are indeed stronger between the redshift and the optical luminosity than that between the radio luminosity and the optical luminosity, suggesting that the latter correlation perhaps arises because of the Malmquist Bias. We further show that similar effects of the Malmquist bias could also be present elsewhere in other correlations claimed in the literature between the radio luminosity and other observed properties of FR1 and FR2 sources., Comment: 5 pages, 9 figures, 1 table, to appear in MNRAS. arXiv admin note: substantial text overlap with arXiv:1212.0667