Heterogeneity of Inverted Calcium II H:K Ratio Cluster Galaxies

TL;DR

This study introduces a new method to measure the H:K ratio in galaxy spectra, revealing its complex relationship with galaxy properties and its potential to distinguish different galaxy evolutionary pathways in clusters.

Contribution

A novel technique for determining the H:K ratio in galaxy spectra and an analysis of its heterogeneity and implications for galaxy evolution in clusters.

Findings

H:K ratio varies within cluster galaxies and does not correlate simply with other indices.

Strongly inverted H:K galaxies are spatially and kinematically diverse within the cluster.

H:K can help identify passive or star-forming galaxies when other indicators are unavailable.

Abstract

The ratio of calcium II H plus H$\epsilon$ to calcium II K inverts as a galaxy stellar population moves from being dominated by older stars to possessing more A and B class stars. This ratio - the H:K ratio - can serve as an indicator of stellar populations younger than 200 Myr. In this work, we provide a new method to determine H:K, and apply it to spectra taken of cluster galaxies in Abell~3888. Although H:K is on average systematically lower for the cluster than for a wider field sample, we show that H:K does not have a simple relationship with other indices such as the equivalent widths of H$\delta$ and [OII]. Moreover, strongly inverted galaxies with H:K>1.1 have no preferred location within the cluster and are only slightly lower in their velocity dispersions around the cluster compared to strongly emitting [OII] galaxies. Our results indicate that selecting galaxies on H:K inversion results in a heterogeneous sample formed via a mixture of pathways that likely includes, but may not be limited to, merging spiral galaxies, and quiescent galaxies accreting lower mass, gas rich companions. In concert with other selection criteria, H:K can provide a means to select a more `pure' passive sample or to aid in the selection of highly star-forming galaxies, especially where other spectral line indicators such as H$\alpha$ may not have been observed.