Red star-forming and blue passive galaxies in clusters

TL;DR

This study investigates the complex relationship between galaxy color and star formation activity in cluster environments, revealing significant populations of red star-forming and blue passive galaxies with diverse properties.

Contribution

It provides a detailed analysis of red star-forming and blue passive galaxies, highlighting their properties and implications for environmental effects on galaxy evolution.

Findings

Blue passive galaxies resemble star-forming ones except for recent star formation truncation.

Red star-forming galaxies include massive old-core galaxies with ongoing star formation.

Metal-rich stellar populations influence the colors of some red star-forming galaxies.

Abstract

We explore the relation between colour and specific star formation rate (derived from optical spectra obtained by SDSS DR4) of over 6,000 galaxies (M_r<=-20.5) in and around low redshift (z<0.12) clusters. Even though most red galaxies have little or no ongoing star formation, and most blue galaxies are currently forming stars, there are significant populations of red star-forming (SF) and blue passive galaxies. This paper examines various properties of galaxies belonging to the latter two categories. These properties include morphological parameters, internal extinction, spectral features such as EW(H_delta) and the 4000 ang break, and metallicity. Our analysis shows that the blue passive galaxies have properties very similar to their SF counterparts, except that their large range in EW(H_delta) indicates recent truncation of star formation. The red SF galaxies fall into two broad categories, one of them being massive galaxies in cluster cores dominated by an old stellar population, but with evidence of current star formation in the core. For the remaining red SF galaxies it is evident from various metallicity measures and mean stellar ages, that their colours result from the predominance of a metal-rich stellar population. The implication of the properties of these SF galaxies on environmental studies, like that of the Butcher-Oemler effect, is discussed.