SDSS J020938.14-004856.8 , the SIMBAD biblio

Context. Determination of cluster masses is a fundamental tool for cosmology. Comparing mass estimates obtained by different probes allows to understand possible systematic uncertainties.
Aims. The cluster Abell 315 is an interesting test case, since it has been claimed to be underluminous in X-ray for its mass (determined via kinematics and weak lensing). We have undertaken new spectroscopic observations with the aim of improving the cluster mass estimate, using the distribution of galaxies in projected phase space.
Methods. We identified cluster members in our new spectroscopic sample. We estimated the cluster mass from the projected phase-space distribution of cluster members using the MAMPOSSt method. In doing this estimate we took into account the presence of substructures that we were able to identify.
Results. We identify several cluster substructures. The main two have an overlapping spatial distribution, suggesting a (past or ongoing) collision along the line-of-sight. After accounting for the presence of substructures, the mass estimate of Abell 315 from kinematics is reduced by a factor 4, down to M₂₀₀=0.8^+0.6_–0.4x10¹⁴M_☉. We also find evidence that the cluster mass concentration is unusually low, c₂₀₀∼r₂₀₀/r_–2≤1. Using our new estimate of c₂₀₀ we revise the weak lensing mass estimate down to M₂₀₀=1.8^+1.7_–0.9 x 10¹⁴M_☉. Our new mass estimates are in agreement with that derived from the cluster X-ray luminosity via a scaling relation, M₂₀₀=0.9±0.2 x 10¹⁴M_☉.
Conclusions. Abell 315 no longer belongs to the class of X-ray underluminous clusters. Its mass estimate was inflated by the presence of an undetected subcluster in collision with the main cluster. Whether the presence of undetected line-of-sight structures can be a general explanation for all X-ray underluminous clusters remains to be explored using a statistically significant sample.