Traditionally, galaxy clusters have been expected to retain all the materialaccreted since their formation epoch. For this reason, their matter contentshould be representative of the Universe as a whole, and thus their baryonfraction should be close to the Universal baryon fraction. We make use of thesample of the 100 brightest galaxy clusters discovered in the XXL Survey toinvestigate the fraction of baryons in the form of hot gas and stars in thecluster population. We measure the gas masses of the detected halos and use amass--temperature relation directly calibrated using weak-lensing measurementsfor a subset of XXL clusters to estimate the halo mass. We find that theweak-lensing calibrated gas fraction of XXL-100-GC clusters is substantiallylower than was found in previous studies using hydrostatic masses. Our best-fitrelation between gas fraction and mass reads $f_{\rmgas,500}=0.055_{-0.006}^{+0.007}\left(M_{\rm500}/10^{14}M_\odot\right)^{0.21_{-0.10}^{+0.11}}$. The baryon budget of galaxyclusters therefore falls short of the Universal baryon fraction by about afactor of two at $r_{\rm 500}$. Our measurements require a hydrostatic bias$1-b=M_X/M_{\rm WL}=0.72_{-0.07}^{+0.08}$ to match the gas fraction obtainedusing lensing and hydrostatic equilibrium. Comparing our gas fractionmeasurements with the expectations from numerical simulations, our resultsfavour an extreme feedback scheme in which a significant fraction of thebaryons are expelled from the cores of halos. This model is, however, incontrast with the thermodynamical properties of observed halos, which mightsuggest that weak-lensing masses are overestimated. We note that a mass bias$1-b=0.58$ as required to reconcile Planck CMB and cluster counts shouldtranslate into an even lower baryon fraction, which poses a major challenge toour current understanding of galaxy clusters. [Abridged]

In a LCDM cosmology, the baryonic Tully-Fisher relation (BTFR) is expected toshow significant intrinsic scatter resulting from the mass-concentrationrelation of dark matter halos and the baryonic-to-halo mass ratio. We study theBTFR using a sample of 118 disc galaxies (spirals and irregulars) with data ofthe highest quality: extended HI rotation curves (tracing the outer velocity)and Spitzer photometry at 3.6 $\mu$m (tracing the stellar mass). Assuming thatthe stellar mass-to-light ratio (M*/L) is nearly constant at 3.6 $\mu$m, wefind that the scatter, slope, and normalization of the BTFR systematically varywith the adopted M*/L. The observed scatter is minimized for M*/L > 0.5,corresponding to nearly maximal discs in high-surface-brightness galaxies andBTFR slopes close to ~4. For any reasonable value of M*/L, the intrinsicscatter is ~0.1 dex, below general LCDM expectations. The residuals show nocorrelations with galaxy structural parameters (radius or surface brightness),contrary to the predictions from some semi-analytic models of galaxy formation.These are fundamental issues for LCDM cosmology.

Leo P is a gas-rich dwarf galaxy with an extremely low gas-phase oxygenabundance (3% solar). The isolated nature of Leo P enables a quantitativemeasurement of metals lost solely due to star formation feedback. We present aninventory of the oxygen atoms in Leo P based on the gas-phase oxygen abundancemeasurement, the star formation history, and the chemical enrichment evolutionderived from resolved stellar populations. The star formation history alsoprovides the total amount of oxygen produced. Overall, Leo P has retained 5 %of its oxygen; 25% of the retained oxygen is in the stars while 75% is in thegas phase. This is considerably lower than the 20-25% calculated for massivegalaxies, supporting the trend for less efficient metal retention for lowermass galaxies. The retention fraction is higher than that calculated for otheralpha elements (Mg, Si, Ca) in dSph Milky Way satellites of similar stellarmass and metallicity. Accounting only for the oxygen retained in stars, ourresults are consistent with those derived for the alpha elements in dSphgalaxies. Thus, under the assumption that the dSph galaxies lost the bulk oftheir gas mass through an environmental process such as tidal stripping, theestimates of retained metal fractions represent underestimates by roughly afactor of four. Because of its isolation, Leo P provides an important datum forthe fraction of metals lost as a function of galaxy mass due to star formation.

We study the effects of filaments on galaxy properties in the Sloan DigitalSky Survey (SDSS) Data Release 12 using filaments from the `Cosmic WebReconstruction' catalogue (Chen et al. 2016), a publicly available filamentcatalogue for SDSS. Since filaments are tracers of medium-to-high densityregions, we expect that galaxy properties associated with the environment aredependent on the distance to the nearest filament. Our analysis demonstratesthat a red galaxy or a high-mass galaxy tend to reside closer to filaments thana blue or low-mass galaxy. After adjusting the effect from stellar mass, onaverage, early-forming galaxies or large galaxies have a shorter distance tofilaments than late-forming galaxies or small galaxies. For the Main galaxysample (MGS), all signals are very significant ($>6\sigma$). For the LOWZ andCMASS sample, the stellar mass and size are significant ($>2 \sigma$). Thefilament effects we observe persist until $z = 0.7$ (the edge of the CMASSsample). Comparing our results to those using the galaxy distances fromredMaPPer galaxy clusters as a reference, we find a similar result betweenfilaments and clusters. Moreover, we find that the effect of clusters on thestellar mass of nearby galaxies depends on the galaxy's filamentaryenvironment. Our findings illustrate the strong correlation of galaxyproperties with proximity to density ridges, strongly supporting the claim thatdensity ridges are good tracers of filaments.

We perform a comparative analysis of the properties of galaxies infallinginto groups classifying them accordingly to whether they are: falling alongfilamentary structures; or they are falling isotropically. For this purpose, weidentify filamentary structures connecting massive groups of galaxies in theSDSS. We perform a comparative analysis of some properties of galaxies infilaments, in the isotropic infall region, in the field, and in groups. Westudy the luminosity functions (LF) and the dependence of the specific starformation rate (SSFR) on stellar mass, galaxy type, and projected distance tothe groups that define the filaments. We find that the LF of galaxies infilaments and in the isotropic infalling region are basically indistinguishablebetween them, with the possible exception of late-type galaxies. On the otherhard, regardless of galaxy type, their LFs are clearly different from that offield or group galaxies. Both of them have characteristic absolute magnitudesand faint end slopes in between the field and group values. More significantdifferences between galaxies in filaments and in the isotropic infall regionare observed when we analyse the SSFR. We find that galaxies in filaments havea systematically higher fraction of galaxies with low SSFR as a function ofboth, stellar mass and distance to the groups, indicating a stronger quenchingof the star formation in the filaments compared to both, the isotropicinfalling region, and the field. Our results suggest that some physicalmechanisms that determine the differences observed between field galaxies andgalaxies in systems, affect galaxies even when they are not yet within thesystems.

We consider what can be learnt about the processes of gas accretion anddepletion from the kinematic misalignment between the cold/warm gas and starsin local early-type galaxies. Using simple analytic arguments and a toy modelof the processes involved, we show that the lack of objects withcounter-rotating gas reservoirs strongly constrains the relaxation, depletionand accretion timescales of gas in early-type galaxies. Standard values of theaccretion rate, star formation efficiency and relaxation rate are notsimultaneously consistent with the observed distribution of kinematicmisalignments. To reproduce that distribution, both fast gas depletion ($t_{\rmdep} <10^8$ yr; e.g. more efficient star formation) and fast gas destruction(e.g. by active galactic nucleus feedback) can be invoked, but both alsorequire a high rate of gas-rich mergers ($>1$ Gyr$^{-1}$). Alternatively, therelaxation of misaligned material could happen over very long timescales($\simeq100$ dynamical times or $\approx1$-$5$ Gyr). We explore the variousphysical processes that could lead to fast gas depletion and/or slow gasrelaxation, and discuss the prospects of using kinematic misalignments to probegas-rich accretion processes in the era of large integral-field spectroscopicsurveys.

Context. The XXL Survey is the largest survey carried out by the XMM-Newtonsatellite and covers a total area of 50 square degrees distributed over twofields. It primarily aims at investigating the large-scale structures of theUniverse using the distribution of galaxy clusters and active galactic nucleias tracers of the matter distribution. Aims. This article presents the XXL bright cluster sample, a subsample of 100galaxy clusters selected from the full XXL catalogue by setting a lower limitof $3\times 10^{-14}\,\mathrm{erg \,s^{-1}cm^{-2}}$ on the source flux within a1$^{\prime}$ aperture. Methods. The selection function was estimated using a mixture of Monte Carlosimulations and analytical recipes that closely reproduce the source selectionprocess. An extensive spectroscopic follow-up provided redshifts for 97 of the100 clusters. We derived accurate X-ray parameters for all the sources. Scalingrelations were self-consistently derived from the same sample in otherpublications of the series. On this basis, we study the number density,luminosity function, and spatial distribution of the sample. Results. The bright cluster sample consists of systems with masses between$M_{500}=7\times 10^{13}$ and $3\times 10^{14} M_\odot$, mostly located between$z=0.1$ and 0.5. The observed sky density of clusters is slightly below thepredictions from the WMAP9 model, and significantly below the predictions fromthe Planck 2015 cosmology. In general, within the current uncertainties of thecluster mass calibration, models with higher values of $\sigma_8$ and/or$\Omega_m$ appear more difficult to accommodate. We provide tight constraintson the cluster differential luminosity function and find no hint of evolutionout to $z\sim1$. We also find strong evidence for the presence of large-scalestructures in the XXL bright cluster sample and identify five newsuperclusters.