Bars rejuvenating bulges? Evidence from stellar population analysis

TL;DR

This study analyzes stellar ages and metallicities in 575 galaxy bulges, revealing that bars influence bulge star formation, especially in low-mass galaxies, with barred galaxies showing younger stellar populations and higher AGN activity.

Contribution

It provides new evidence that bars affect bulge stellar ages and star formation, particularly in low-mass galaxies, supporting models of bar-triggered star formation.

Findings

Barred galaxies have more young bulge populations than unbarred ones.

In low-mass galaxies, barred and unbarred bulges show similar age distributions.

Barred galaxies exhibit a bimodal age distribution in massive bulges.

Abstract

We obtained stellar ages and metallicities via spectrum fitting for a sample of 575 bulges with spectra available from the Sloan Digital Sky Survey. The structural properties of the galaxies have been studied in detail in Gadotti (2009b) and the sample contains 251 bulges in galaxies with bars. Using the whole sample, where galaxy stellar mass distributions for barred and unbarred galaxies are similar, we find that bulges in barred and unbarred galaxies occupy similar loci in the age vs. metallicity plane. However, the distribution of bulge ages in barred galaxies shows an excess of populations younger than ~ 4 Gyr, when compared to bulges in unbarred galaxies. Kolmogorov-Smirnov statistics confirm that the age distributions are different with a significance of 99.94%. If we select sub-samples for which the bulge stellar mass distributions are similar for barred and unbarred galaxies, this excess vanishes for galaxies with bulge mass log M < 10.1 M_Sun while for more massive galaxies we find a bimodal bulge age distribution for barred galaxies only, corresponding to two normal distributions with mean ages of 10.4 and 4.7 Gyr. We also find twice as much AGN among barred galaxies, as compared to unbarred galaxies, for low-mass bulges. By combining a large sample of high quality data with sophisticated image and spectral analysis, we are able to find evidence that the presence of bars affect the mean stellar ages of bulges. This lends strong support to models in which bars trigger star formation activity in the centers of galaxies.