The abundance and spatial distribution of ultra-diffuse galaxies in nearby galaxy clusters

Recent observations have highlighted a significant population of faint but large (r_eff>1.5 kpc) galaxies in the Coma cluster. The origin of these Ultra Diffuse Galaxies (UDGs) remains puzzling, as the interpretation of the observational results has been hindered by the subjective selection of UDGs, and the limited study of only the Coma (and some examples in the Virgo-) cluster. We extend the study of UDGs using 8 clusters in the redshift range 0.044<z<0.063 with deep g- and r-band imaging data taken with MegaCam at the CFHT. We describe an automatic selection pipeline for quantitative identification, tested for completeness using image simulations of these galaxies. We find that the abundance of the UDGs we can detect increases with cluster mass, reaching ~200 in typical haloes of M200~10^15 Msun. The cluster UDGs have colours consistent with the cluster red sequence, and have a steep size distribution that declines as n~r_eff^-3.4. Their radial distribution is significantly steeper than NFW in the outskirts, and is significantly shallower in the inner parts. They follow the same radial distribution as the more massive quiescent galaxies in the clusters, except within the core region of r<0.15XR200 (or <300 kpc). Within this region the number density of UDGs drops and is consistent with zero. These diffuse galaxies can only resist tidal forces down to this cluster-centric distance if they are highly centrally dark-matter dominated. The observation that the radial distribution of more compact dwarf galaxies (r_eff<1.0 kpc) with similar luminosities follows the same distribution as the UDGs, but exist down to a smaller distance of 100kpc from the cluster centres, indicates that they may have similarly massive sub-haloes as the UDGs. Although several scenarios can give rise to the UDG population, our results point to differences in the formation history as the most plausible explanation.Comment: 12 pages, 11 figures. Accepted for publication in A&A after minor revisio

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&

HARD: Hard Augmentations for Robust Distillation

Knowledge distillation (KD) is a simple and successful method to transfer knowledge from a teacher to a student model solely based on functional activity. However, current KD has a few shortcomings: it has recently been shown that this method is unsuitable to transfer simple inductive biases like shift equivariance, struggles to transfer out of domain generalization, and optimization time is magnitudes longer compared to default non-KD model training. To improve these aspects of KD, we propose Hard Augmentations for Robust Distillation (HARD), a generally applicable data augmentation framework, that generates synthetic data points for which the teacher and the student disagree. We show in a simple toy example that our augmentation framework solves the problem of transferring simple equivariances with KD. We then apply our framework in real-world tasks for a variety of augmentation models, ranging from simple spatial transformations to unconstrained image manipulations with a pretrained variational autoencoder. We find that our learned augmentations significantly improve KD performance on in-domain and out-of-domain evaluation. Moreover, our method outperforms even state-of-the-art data augmentations and since the augmented training inputs can be visualized, they offer a qualitative insight into the properties that are transferred from the teacher to the student. Thus HARD represents a generally applicable, dynamically optimized data augmentation technique tailored to improve the generalization and convergence speed of models trained with KD

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

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

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

The stellar mass function of galaxies in Planck-selected clusters at 0.5 < z < 0.7: new constraints on the timescale and location of satellite quenching

We study the abundance of star-forming and quiescent galaxies in a sample of 21 massive clusters at 0.5<z<0.7, detected with the Planck satellite. We measure the cluster galaxy stellar mass function (SMF), which is a fundamental observable to study and constrain the formation and evolution of galaxies. Our measurements are based on homogeneous and deep multi-band photometry spanning u- to the Ks-band for each cluster and are supported by spectroscopic data from different programs. The galaxy population is separated between quiescent and star-forming galaxies based on their rest-frame U-V and V-J colours. The SMF is compared to that of field galaxies at the same redshifts, using data from the COSMOS/UltraVISTA survey. We find that the shape of the SMF of star-forming galaxies does not depend on environment, while the SMF of quiescent galaxies has a significantly steeper low-mass slope in the clusters compared to the field. We estimate the environmental quenching efficiency (f_EQ), i.e. the probability for a galaxy that would normally be star forming in the field, to be quenched due to its environment. The f_EQ shows no stellar-mass dependence in any environment, but it increases from 40% in the cluster outskirts to ~90% in the cluster centres. The radial signature of f_EQ provides constraints on where the dominant quenching mechanism operates in these clusters and on what timescale. Exploring these using a simple model based on galaxy orbits obtained from an N-body simulation, we find a clear degeneracy between both parameters. For example, the quenching process may either be triggered on a long (~3 Gyr) time scale at large radii (r~8R_500), or happen well within 1 Gyr at r<R_500. The radius where quenching is triggered is at least r_quench> 0.67R_500 (95%CL). The ICM density at this location suggests that ram-pressure stripping of the cold gas is a likely cause of quenching. [Abridged]Comment: 16 pages, 12 figures, accepted for publication in A&