Age bimodality in the central region of pseudobulges in S0 galaxies

TL;DR

This study reveals a bimodal distribution of stellar ages in pseudobulges of S0 galaxies, linked to galaxy morphology and star formation activity, contrasting with the uniform age distribution in spiral galaxies.

Contribution

It provides the first evidence of age bimodality in S0 pseudobulges and connects this to galaxy morphology and star formation history.

Findings

Old pseudobulges are mostly in passive galaxies.

Young pseudobulges are predominantly in star-forming galaxies.

Old pseudobulges mainly found in early-type S0s.

Abstract

We present evidence for bimodal stellar age distribution of pseudobulges of S0 galaxies as probed by the $D_n(4000)$ index. We do not observe any bimodality in age distribution for pseudobulges in spiral galaxies. Our sample is flux limited and contains 2067 S0 and 2630 spiral galaxies drawn from the Sloan Digital Sky Survey. We identify pseudobulges in S0 and spiral galaxies, based on the position of the bulge on the Kormendy diagram and their central velocity dispersion. Dividing the pseudobulges of S0 galaxies into those containing old and young stellar populations, we study the connection between global star formation and pseudobulge age on the $u-r$ color-mass diagram. We find that most old pseudobulges are hosted by passive galaxies while majority of young bulges are hosted by galaxies which are star forming. Dividing our sample of S0 galaxies into early-type S0s and S0/a galaxies, we find that old pseudobulges are mainly hosted by early-type S0 galaxies while most of the pseudobulges in S0/a galaxies are young. We speculate that morphology plays a strong role in quenching of star formation in the disc of these S0 galaxies, which stops the growth of pseudobulges, giving rise to old pseudobulges and the observed age bimodality.