The hot gas content of fossil galaxy clusters

We investigate the properties of the hot gas in four fossil galaxy systems detected at high significance in the Planck Sunyaev-Zeldovich (SZ) survey. XMM-Newton observations reveal overall temperatures of kT ~ 5-6 keV and yield hydrostatic masses M500,HE > 3.5 x 10e14 Msun, confirming their nature as bona fide massive clusters. We measure the thermodynamic properties of the hot gas in X-rays (out to beyond R500 in three cases) and derive their individual pressure profiles out to R ~ 2.5 R500 with the SZ data. We combine the X-ray and SZ data to measure hydrostatic mass profiles and to examine the hot gas content and its radial distribution. The average Navarro-Frenk-White (NFW) concentration parameter, c500 = 3.2 +/- 0.4, is the same as that of relaxed `normal' clusters. The gas mass fraction profiles exhibit striking variation in the inner regions, but converge to approximately the cosmic baryon fraction (corrected for depletion) at R500. Beyond R500 the gas mass fraction profiles again diverge, which we interpret as being due to a difference in gas clumping and/or a breakdown of hydrostatic equilibrium in the external regions. Overall our observations point to considerable radial variation in the hot gas content and in the gas clumping and/or hydrostatic equilibrium properties in these fossil clusters, at odds with the interpretation of their being old, evolved and undisturbed. At least some fossil objects appear to be dynamically young.Comment: 4 pages, 2 figures. Accepted for publication in A&

Homogeneity analysis with k sets of variables: An alternating least squares method with optimal scaling features

Homogeneity analysis, or multiple correspondence analysis, is usually applied to k separate variables. In this paper, it is applied to sets of variables by using sums within sets. The resulting technique is referred to as OVERALS. It uses the notion of optimal scaling, with transformations that can be multiple or single. The single transformations consist of three types: (1) nominal; (2) ordinal; and (3) numerical. The corresponding OVERALS computer program minimizes a least squares loss function by using an alternating least squares algorithm. Many existing linear and non-linear multivariate analysis techniques are shown to be special cases of OVERALS. Disadvantages of the OVERALS method include the possibility of local minima in some complicated special cases, a lack of information on the stability of results, and its inability to handle incomplete data matrices. Means of dealing with some of these problems are suggested (i.e., an alternating least squares algorithm to solve the minimization problem). An application of the method to data from an epidemiological survey is provided

A skewer survey of the Galactic halo from deep CFHT and INT images

We study the density profile and shape of the Galactic halo using deep multicolour images from the MENeaCS and CCCP projects, over 33 fields selected to avoid overlap with the Galactic plane. Using multicolour selection and PSF homogenization techniques we obtain catalogues of F stars (near-main sequence turnoff stars) out to Galactocentric distances up to 60kpc. Grouping nearby lines of sight, we construct the stellar density profiles through the halo in eight different directions by means of photometric parallaxes. Smooth halo models are then fitted to these profiles. We find clear evidence for a steepening of the density profile power law index around R=20 kpc, from -2.50 +- 0.04 to -4.85 +- 0.04, and for a flattening of the halo towards the poles with best-fit axis ratio 0.63 +- 0.02. Furthermore, we cannot rule out a mild triaxiality (w>=0.8). We recover the signatures of well-known substructure and streams that intersect our lines of sight. These results are consistent with those derived from wider but shallower surveys, and augur well for upcoming, wide-field surveys of comparable depth to our pencil beam surveys.Comment: 14 pages, 8 figures, 6 table

The dynamics of z~1 clusters of galaxies from the GCLASS survey

We constrain the internal dynamics of a stack of 10 clusters from the GCLASS survey at 0.87<z<1.34. We determine the stack cluster mass profile M(r) using the MAMPOSSt algorithm of Mamon et al., the velocity anisotropy profile beta(r) from the inversion of the Jeans equation, and the pseudo-phase-space density profiles Q(r) and Qr(r), obtained from the ratio between the mass density profile and the third power of the (total and, respectively, radial) velocity dispersion profiles of cluster galaxies. Several M(r) models are statistically acceptable for the stack cluster (Burkert, Einasto, Hernquist, NFW). The total mass distribution has a concentration c=r200/r-2=4.0-0.6+1.0, in agreement with theoretical expectations, and is less concentrated than the cluster stellar-mass distribution. The stack cluster beta(r) is similar for passive and star-forming galaxies and indicates isotropic galaxy orbits near the cluster center and increasingly radially elongated with increasing cluster-centric distance. Q(r) and Qr(r) are almost power-law relations with slopes similar to those predicted from numerical simulations of dark matter halos. Combined with results obtained for lower-z clusters we determine the dynamical evolution of galaxy clusters, and compare it with theoretical predictions. We discuss possible physical mechanisms responsible for the differential evolution of total and stellar mass concentrations, and of passive and star-forming galaxy orbits [abridged].Comment: 12 pages, 7 figures. Version accepted for publication in A&A after minor modification

Finding halo streams with a pencil-beam survey: new wraps in the Sagittarius stream

We use data from two CFHT-MegaCam photometric pencil-beam surveys in the g' and the r' bands to measure distances to the Sagittarius, the Palomar 5 and the Orphan stream. We show that, using a cross-correlation algorithm to detect the turnoff point of the main sequence, it is possible to overcome the main limitation of a two-bands pencil-beam survey, namely the lack of adjacent control-fields that can be used to subtract the foreground and background stars to enhance the signal on the colour-magnitude diagrams (CMDs). We describe the cross-correlation algorithm and its implementation. We combine the resulting main sequence turnoff points with theoretical isochrones to derive photometric distances to the streams. Our results (31 detections on the Sagittarius stream and one each for the Palomar 5 and the Orphan streams) confirm the findings by previous studies, expand the distance trend for the Sagittarius faint southern branch and, for the first time, trace the Sagittarius faint branch of the northern-leading arm out to 56 kpc. In addition, they show evidence for new substructure: we argue that these detections trace the continuation of the Sagittarius northern-leading arm into the southern hemisphere, and find a nearby branch of the Sagittarius trailing wrap in the northern hemisphere.Comment: 16 pages, 15 figures, 2 table

Evidence for strong evolution in galaxy environmental quenching efficiency between z = 1.6 and z = 0.9

Indexación: Web of Science; Scopus.We analyse the evolution of environmental quenching efficiency, the fraction of quenched cluster galaxies which would be star forming if they were in the field, as a function of redshift in 14 spectroscopically confirmed galaxy clusters with 0.87 < z < 1.63 from the Spitzer Adaptation of the Red-Sequence Cluster Survey. The clusters are the richest in the survey at each redshift. Passive fractions rise from 42-13 +10 per cent at z ~ 1.6 to 80-9 +12 per cent at z ~ 1.3 and 88-3 +4 per cent at z < 1.1, outpacing the change in passive fraction in the field. Environmental quenching efficiency rises dramatically from 16-19 +15 per cent at z ~ 1.6 to 62-15 +21 per cent at z~1.3 and 73-7 +8 per cent at z ≲ 1.1. This work is the first to show direct observational evidence for a rapid increase in the strength of environmental quenching in galaxy clusters at z ~ 1.5, where simulations show cluster-mass haloes undergo non-linear collapse and virialization.https://academic.oup.com/mnrasl/article/465/1/L104/241728