The velocity modulation of galaxy properties in and near clusters: quantifying the decrease in star formation in backsplash galaxies

TL;DR

This study investigates how galaxy star formation activity varies with velocity and position in clusters, revealing that star formation is largely quenched after a single passage through the cluster environment.

Contribution

It provides a quantitative analysis of galaxy properties related to star formation and velocity in clusters, supported by cosmological simulations, highlighting the impact of cluster environment on galaxy evolution.

Findings

Star formation decreases closer to cluster centers.

High-velocity galaxies have higher ongoing star formation.

Star formation is mostly quenched after one cluster crossing.

Abstract

The efficiency of recent star formation (SF) in galaxies increases with increasing projected distance from the centre of a cluster out to several times its virial radius (R_v). Using a complete sample of galaxies in 268 clusters from the SDSS DR4, we investigate how, at a given projected radius from the cluster centre, M* and SF properties of a galaxy depend on its absolute line-of-sight velocity in the cluster rest frame, |v_LOS|. We find that for R<0.5 R_v, the fraction of high mass non-BCG galaxies increases towards the centre for low |v_LOS|. At a given projected radius, the fraction of Galaxies with Ongoing or Recent (<1-3 Gyr) Efficient Star Formation (GORES, with EW(H_delta)>2 ang & D_4000>1.5) is slightly but significantly lower for low |v_LOS| galaxies than for their high velocity counterparts. We study these observational trends with the help of a dark matter (DM) cosmological simulation. We find that the backsplash particles account for at least one-third (half) of all particles at projected radii slightly greater than the virial radius and |v_LOS|<sigma_v. The deprojection of the GORES fraction leads to a saturated linear increase with radius. We fit simple models of the fraction of GORES as a function of class only or class and distance to the cluster centre (as in our deprojected fraction). In our best-fitting model GORES account for 13% of galaxies within the virial sphere, 11% of the virial population, 34% of the distant (for projected radii R<2 R_v) infall population and 19% of the backsplash galaxies. Given the 1-3 Gyr lookback time of our GORES indicators, these results suggest that SF in a galaxy is almost completely quenched in a single passage through the cluster.