# New constraints on red-spiral galaxies from their kinematics in clusters of galaxies

## TL;DR

This study reveals a unique velocity distribution feature of red-spiral galaxies in clusters, constraining their quenching timescale and transformation radius through simulation-based modeling.

## Contribution

It introduces a phase space model explaining the velocity dip of red-spiral galaxies, linking their distribution to infall dynamics in galaxy clusters.

## Key findings

- Red-spiral galaxies show a dip at zero velocity in their line-of-sight velocity distribution.
- The velocity distribution can be explained by red-spiral galaxies residing mainly in the infall component.
- The quenching timescale for red-spiral galaxies is constrained to a few Gyrs, with a transformation radius around 0.2 h^{-1} Mpc.

## Abstract

The distributions of the pairwise line-of-sight velocity between galaxies and their host clusters are segregated according to the galaxy's colour and morphology. We investigate the velocity distribution of red-spiral galaxies, which represents a rare population within galaxy clusters. We find that the probability distribution function of the pairwise line-of-sight velocity $v_{\rm{los}}$ between red-spiral galaxies and galaxy clusters has a dip at $v_{\rm{los}} = 0$, which is a very odd feature, at 93\% confidence level. To understand its origin, we construct a model of the phase space distribution of galaxies surrounding galaxy clusters in three-dimensional space by using cosmological $N$-body simulations. We adopt a two component model that consists of the infall component, which corresponds to galaxies that are now falling into galaxy clusters, and the splashback component, which corresponds to galaxies that are on their first (or more) orbit after falling into galaxy clusters. We find that we can reproduce the distribution of the line-of-sight velocity of red-spiral galaxies with the dip with a very simple assumption that red-spiral galaxies reside predominantly in the infall component, regardless of the choice of the functional form of their spatial distribution. Our results constrain the quenching timescale of red-spiral galaxies to a few Gyrs, and the radius where the morphological transformation is effective as $r \sim 0.2 h^{-1} \rm{Mpc}$.