Spatially resolved star formation histories of nearby galaxies: evidence for episodic star formation in discs

TL;DR

This study reveals that many nearby galaxies experience episodic, rather than continuous, star formation, with bursts occurring in different regions and contributing significantly to recent stellar mass, especially in low-mass, late-type galaxies.

Contribution

It provides detailed evidence of episodic star formation in galaxy disks and links these bursts to gas accretion, expanding understanding of galaxy evolution.

Findings

Many galaxies show recent short-lived star formation episodes.

Episodic bursts contribute around half of the stars formed in the last 2 Gyr.

Outer disk star formation is linked to atomic gas fraction and triggered by gas accretion.

Abstract

We use long-slit spectroscopy from Moran et al. to study the radial dependence of the recent star formation histories of nearby galaxies with stellar masses greater than 10^10M_sun. We fit stellar population models to the combination of SSFR, D4000 and Hdelta_A and show that many galaxies have Balmer absorption line equivalent widths that require recent short-lived episodes or bursts of star formation. The fraction of galaxies that have experienced episodic rather than continuous star formation is highest for late-type galaxies with low stellar masses. In these systems, bursts occur both in the inner and outer regions of the galaxy. The fraction of stars formed in a single burst episode is typically around 15% of the total stellar mass in the inner regions of the galaxy and around 5% of the mass in the outer regions. When we average over the population, we find that such bursts contribute around a half of the total mass in stars formed in the last 2 Gyr. In massive galaxies, bursts occur predominantly in the outer disk. Around a third of all massive, bulge-dominated galaxies have experienced recent star formation episodes that are fully confined to their outer (R > 0.7R_90) regions. The fraction of stars formed in a single episode is only 2 - 3 % of the underlying stellar mass, but such bursts contribute nearly all the stellar mass formed in the last 2 Gyr. Recent star formation in outer disks is strongly correlated with the global atomic gas fraction of the galaxy, but not its global molecular gas fraction. We suggest that outer episodic star formation is triggered by gas accretion events.