We present a catalog of 182 galaxy clusters detected through the Sunyaev-Zel'dovich effect by the Atacama Cosmology Telescope in a contiguous 987.5 deg 2 field. The clusters were detected as SZ decrements by applying a matched filter to 148 GHz maps that combine the original ACT equatorial survey with data from the first two observing seasons using the ACTPol receiver. Optical/IR confirmation and redshift measurements come from a combination of large public surveys and our own follow-up observations. Where necessary, we measured photometric redshifts for clusters using a pipeline that achieves accuracy ∆z/(1 +z) = 0.015 when tested on SDSS data. Under the assumption that clusters can be described by the so-called Universal Pressure Profile and its associated mass-scaling law, the full signal-to-noise > 4 sample spans the mass range 1.6 < M UPP 500c /10 14 M < 9.1, with median M UPP 500c = 3.1 × 10 14 M . The sample covers the redshift range 0.1 < z < 1.4 (median z = 0.49) and 28 clusters are new discoveries (median z = 0.80). We compare our catalog with other overlapping cluster samples selected using the SZ, optical,and X-ray wavelengths. We find the ratio of the UPP-based SZ mass to richness-based weak-lensing mass is M UPP 500c / M λWL 500c = 0.68 ± 0.11. After applying this calibration, the mass distribution for clusters with M 500c > 4 × 10 14 M is consistent with the number of such clusters found in the South Pole Telescope SZ survey.

Aims. We aim to investigate the bolometric L X − T relation for galaxy groups, and to study the impact of gas cooling, feedback from super-massive black holes, and the selection effects on it.Methods. With a sample of 26 galaxy groups, we obtained the best-fit L X − T relation for five different cases depending on the intracluster medium (ICM) core properties and central active galactic nuclei (AGN) radio emission, and determined the slopes, normalisations, and intrinsic and statistical scatters for both temperature and luminosity. We undertook simulations to correct for selection effects (e.g. Malmquist bias) and compared the bias-corrected relations for groups and clusters. Results. The slope of the bias-corrected L X − T relation is marginally steeper, but consistent with clusters (∼3). Groups with a central cooling time of less than 1 Gyr (SCC groups) show indications of having the steepest slope and the highest normalisation. For the groups, the bias-corrected intrinsic scatter in L X is larger than the observed scatter for most cases, and this is reported here for the first time. We see indications that the groups with an extended central radio source (CRS) have a much steeper slope than those groups that have a CRS with only core emission. Additionally, we see indications that the more powerful radio AGN are preferentially located in non strong cool core (NSCC) groups rather than strong cool core (SCC) groups.

Aims. We aim to investigate cool-core and non-cool-core properties of galaxy groups through X-ray data and compare them to the AGN radio output to understand the network of intracluster medium (ICM) cooling and feedback by supermassive black holes. We also aim to investigate the brightest cluster galaxies (BCGs) to see how they are affected by cooling and heating processes, and compare the properties of groups to those of clusters. Methods. Using Chandra data for a sample of 26 galaxy groups, we constrained the central cooling times (CCTs) of the ICM and classified the groups as strong cool-core (SCC), weak cool-core (WCC), and non-cool-core (NCC) based on their CCTs. The total radio luminosity of the BCG was obtained using radio catalogue data and/or literature, which in turn was compared to the cooling time of the ICM to understand the link between gas cooling and radio output. We determined K-band luminosities of the BCG with 2MASS data, and used a scaling relation to constrain the masses of the supermassive black holes, which were then compared to the radio output. We also tested for correlations between the BCG luminosity and the overall X-ray luminosity and mass of the group. The results obtained for the group sample were also compared to previous results for clusters. Results. The observed cool-core/non-cool-core fractions for groups are comparable to those of clusters. However, notable differences are seen: 1) for clusters, all SCCs have a central temperature drop, but for groups this is not the case as some have centrally rising temperature profiles despite very short cooling times; 2) while for the cluster sample, all SCC clusters have a central radio source as opposed to only 45% of the NCCs, for the group sample, all NCC groups have a central radio source as opposed to 77% of the SCC groups; 3) for clusters, there are indications of an anticorrelation trend between radio luminosity and CCT. However, for groups this trend is absent; 4) the indication of a trend of radio luminosity with black hole mass observed in SCC clusters is absent for groups; and 5) similarly, the strong correlation observed between the BCG luminosity and the cluster X-ray luminosity/cluster mass weakens significantly for groups. Conclusions. We conclude that there are important differences between clusters and groups within the ICM cooling/AGN feedback paradigm and speculate that more gas is fueling star formation in groups than in clusters where much of the gas is thought to feed the central AGN.

Aims. We aim to systematically investigate the cores of a sample of fossil galaxy groups and clusters ("fossil systems"), using Chandra data, to see what hints they can offer about the properties of the intracluster medium in these particular objects. Methods. We chose a sample of 17 fossil systems from literature with archival Chandra data and determined the cool-core fraction for fossils via three observable diagnostics, namely the central cooling time, cuspiness, and concentration parameter. We quantified the dynamical state of the fossils by the X-ray peak/brightest cluster galaxy (BCG) separation, and the X-ray peak/emission weighted centre separation. We also investigated the X-ray emission coincident with the brightest cluster galaxy (BCG) to detect the presence of potential thermal coronae. A deprojection analysis was performed for fossils with z < 0.05 to resolve subtle temperature structures, and to obtain the cooling time and entropy profiles. We also investigated the L X − T relation for fossils from the 400d catalogue to test whether the scaling relation deviates from what is typically observed for other groups. Results. Most fossils are identified as cool-core objects via at least two cool-core diagnostics with the population of weak cool-core fossils being the highest. All fossils have their dominant elliptical galaxy within 50 kpc of the X-ray peak, and most also have the emission weighted centre within that distance. We do not see clear indications of an X-ray corona associated with the BCG unlike coronae observed for some other clusters. Fossils lack universal temperature profiles, with some low-temperature objects generally not showing features that are expected for ostensibly relaxed objects with a cool-core. The entropy profiles of the z < 0.05 fossil systems can be described well by a power law with shallower indices than what is expected for pure gravitational processes. Finally, the fossils L X − T relation shows indications of an elevated normalisation with respect to other groups, which seems to persist even after factoring in selection effects. Conclusions. We interpret these results within the context of the formation and evolution of fossils, and speculate that nongravitational heating, and AGN feedback in particular, could have had an impact on the ICM properties of these systems.

We present the detection of a giant radio halo (GRH) in the Sunyaev-Zel'dovich (SZ)selected merging galaxy cluster ACT-CL J0256.5+0006 (z = 0.363), observed with the Giant Metrewave Radio Telescope at 325 MHz and 610 MHz. We find this cluster to host a faint (S 610 = 5.6 ± 1.4 mJy) radio halo with an angular extent of 2.6 arcmin, corresponding to 0.8 Mpc at the cluster redshift, qualifying it as a GRH. J0256 is one of the lowest-mass systems, M 500,SZ = (5.0 ± 1.2) × 10 14 M , found to host a GRH. We measure the GRH at lower significance at 325 MHz (S 325 = 10.3 ± 5.3 mJy), obtaining a spectral index measurement of α 610 325 = 1.0 +0.7 −0.9 . This result is consistent with the mean spectral index of the population of typical radio halos, α = 1.2 ± 0.2. Adopting the latter value, we determine a 1.4 GHz radio power of P 1.4GHz = (1.0 ± 0.3) × 10 24 W Hz −1 , placing this cluster within the scatter of known scaling relations. Various lines of evidence, including the ICM morphology, suggest that ACT-CL J0256.5+0006 is composed of two subclusters. We determine a merger mass ratio of 7:4, and a lineof-sight velocity difference of v ⊥ = 1880 ± 210 km s −1 . We construct a simple merger model to infer relevant time-scales in the merger. From its location on the P 1.4GHz -L X scaling relation, we infer that we observe ACT-CL J0256.5+0006 just before first core crossing.