CLASH-VLT velocity anisotropy profiles in a stack of massive galaxy clusters

Abstract

We measure the velocity anisotropy profile $β(r)$ of different galaxy cluster member populations by analysing the stacked projected phase space of nine massive ($M_\mathrm{200c}>7\times10^{14}$ M$_\odot$) galaxy clusters at intermediate redshifts ($0.18 < z < 0.45$). We select our sample of galaxy clusters by choosing the most round and virialised objects among the targets of the CLASH-VLT spectroscopic program, which offers a large spectral database. Complementary MUSE observations on most of these clusters allowed us to identify an unprecedented number of cluster members, strongly enhancing the precision of our measurement with respect to previous studies. Our sample of cluster members is divided in four classes: red and blue by colour, and high- and low-mass by stellar mass. We employ two parallel techniques, namely the MAMPOSSt method (parametric in $β(r)$) and the Jeans equation inversion (non parametric in $β(r)$). The results from both techniques are found in agreement for any given cluster member population, and suggest that the orbital anisotropy in galaxy clusters grows from the centre (where $β\approx 0.2-0.4$) to the virial radius ($β\gtrsim 0.8$), and it is similar for the different cluster member populations. We also find an interesting dynamical feature in the Jeans inversion results, that is a drop in $β(r)$ at a distance of $\sim 250$ kpc from the cluster centre. We provide robust anisotropy estimates by exploring a highly significant number of model combinations: 72 with MAMPOSSt (varying the mass, surface number density, $β(r)$ model, and galaxy population) and 18 (varying total mass model and galaxy population) in the Jeans inversion. Such an extensive investigation of the $β(r)$ profile in galaxy clusters is a wide basis for future studies on cluster dynamical masses and cluster cosmology in the era of large spectroscopic surveys

Figures (12)

• Figure 1: Membership selection with CLUMPS on the PPS for the cluster M1115. The green dots represent the galaxy cluster members, while the red dots represent background and foreground galaxies.
• Figure 2: The colour subdivision for the members of M0329, both on the colour-colour plane (V$-$I vs B$-$R, left panel) and on the R-band magnitude vs B$-$R colour plane (right panel). The latter is the plot used to determine which galaxies are red and which are not, according to the recursive red sequence linear fit. The "All-matched" entry in the legend, corresponding to the yellow points, represents the objects in the photometric catalogue that have a match in the spectroscopic catalogue.
• Figure 3: Projected-phase space of the major cluster member populations in this paper, rescaled by the mean value of $R_\mathrm{200c}$ and $v_\mathrm{200}$.. The scale at the right of every plot represents the number of objects in each bin. We note that the blue galaxies, as expected, are more concentrated towards the outer regions of our ensemble cluster, while the red ones, viceversa, are mostly found in the inner regions. The high mass population instead, despite being radially distributed almost as the low mass population, is slightly less spread in velocity than the latter.
• Figure 4: Comparison between the $\beta(r)$ profiles from the two methods of the whole cluster member population. In the left panel, we plot all the MAMPOSSt outcomes with a NFW total mass profile, as well as both the JI results (NFW and Hernquist) for reference. In the right panel, similarly, we plot all the MAMPOSSt outcomes with a Hernquist total mass profile. The vertical black line represents the $R_\mathrm{200c}$ of the stacked cluster.
• Figure 5: Colour cuts operated on the AS1063 catalogues. In the left panel, we report the first cut (grey region marked by black boundaries) in the colour-colour plane V$-$I versus B$-$R. In the right panel, we report the cut (second cut), operated in the R band magnitude versus B$-$R colour plane, of the subsample extracted from the first cut. The selected objects in this panel are those above the black line. The "All-matched" entry in the legend, corresponding to the red points, indicates the objects in the photometric catalogue that have a match in the spectroscopic catalogue.