Constraints on feedback processes during the formation of early-type galaxies

TL;DR

This study investigates the star formation histories of early-type galaxies across a wide mass range, revealing a transition in star formation efficiency and metallicity-age relations around a characteristic mass, informing feedback process constraints.

Contribution

It provides the first detailed analysis of star formation histories in a complete sample of early-type galaxies, highlighting a mass-dependent transition in their formation processes.

Findings

High-mass galaxies formed most stars early, by redshift z~2.

Low-mass galaxies show metallicity-age relations inconsistent with gas infall, indicating outflows.

A characteristic mass (~3x10^10 Msun) marks a transition in star formation efficiency.

Abstract

Galaxies are found to obey scaling relations between a number of observables. These relations follow different trends at the low- and the high-mass ends. The processes driving the curvature of scaling relations remain uncertain. In this letter, we focus on the specific family of early-type galaxies, deriving the star formation histories of a complete sample of visually classified galaxies from SDSS-DR7 over the redshift range 0.01<z<0.025, covering a stellar mass interval from 10^9 to 3 x 10^11 Msun. Our sample features the characteristic "knee" in the surface brightness vs. mass distribution at Mstar~3 x 10^10 Msun. We find a clear difference between the age and metallicity distributions of the stellar populations above and beyond this knee, which suggests a sudden transition from a constant, highly efficient mode of star formation in high-mass galaxies, gradually decreasing towards the low-mass end of the sample. At fixed mass, our early-type sample is more efficient in building up the stellar content at early times in comparison to the general population of galaxies, with half of the stars already in place by redshift z~2 for all masses. The metallicity-age trend in low-mass galaxies is not compatible with infall of metal-poor gas, suggesting instead an outflow-driven relation.