Decreased Frequency of Strong Bars in S0 Galaxies: Evidence for Secular Evolution?

TL;DR

This study shows that S0 galaxies have weaker bars than spiral galaxies, supporting theories that they evolve from spirals through gas depletion and internal secular processes, affecting their structure and bar strength.

Contribution

It provides the first systematic comparison of bar strengths between S0 and spiral galaxies using near-infrared data, highlighting differences linked to galaxy evolution.

Findings

S0 galaxies have significantly weaker bars than spiral galaxies.

Bar strength differences are not solely due to measurement uncertainties.

Weaker bars in S0s suggest evolution from spirals via gas depletion and secular processes.

Abstract

Using data from the Near-Infrared S0 Survey (NIRS0S) of nearby, early-type galaxies, we examine the distribution of bar strengths in S0 galaxies as compared to S0/a and Sa galaxies, and as compared to previously published bar strength data for Ohio State University Bright Spiral Galaxy Survey (OSUBSGS) spiral galaxies. Bar strengths based on the gravitational torque method are derived from 2.2 micron Ks-band images for a statistical sample of 138 (98 S0, 40 S0/a,Sa) galaxies having a mean total blue magnitude <B_T> <= 12.5 and generally inclined less than 65 degrees. We find that S0 galaxies have weaker bars on average than spiral galaxies in general, even compared to their closest spiral counterparts, S0/a and Sa galaxies. The differences are significant and cannot be due entirely to uncertainties in the assumed vertical scale-heights or in the assumption of constant mass-to-light ratios. Part of the difference is likely due simply to the dilution of the bar torques by the higher mass bulges seen in S0s. If spiral galaxies accrete external gas, as advocated by Bournaud & Combes, then the fewer strong bars found among S0s imply a lack of gas accretion according to this theory. If S0s are stripped former spirals, or else are evolved from former spirals due to internal secular dynamical processes which deplete the gas as well as grow the bulges, then the weaker bars and the prevalence of lenses in S0 galaxies could further indicate that bar evolution continues to proceed during and even after gas depletion