The early-type galaxies NGC 1407 and NGC 1400 - II: star formation and chemical evolutionary history

TL;DR

This study investigates the star formation and chemical evolution of two early-type galaxies, NGC 1407 and NGC 1400, revealing a rapid, early formation process consistent with monolithic collapse and recent infall activity.

Contribution

It provides new high-quality spectroscopic data and analysis supporting a revised monolithic collapse model for galaxy formation, emphasizing early, rapid star formation and subsequent quiescent evolution.

Findings

Galaxies formed over half their mass in a short burst at z>5

Evidence of outside-in formation with supernova-driven winds

Support for revised monolithic collapse model

Abstract

We present a possible star formation and chemical evolutionary history for two early-type galaxies NGC 1407 and NGC 1400. They are the two brightest galaxies of the NGC 1407 (or Eridanus-A) group, one of the 60 groups studied as part of the Group Evolution Multi-wavelength Study (GEMS). Our analysis is based on new high signal-to-noise spatially resolved integrated spectra obtained at the ESO 3.6m telescope, out to 0.6 (NGC 1407) and 1.3 (NGC 1400) effective radii. Using Lick/IDS indices we estimate luminosity-weighted ages, metallicities and $\alpha$-element abundance ratios. Colour radial distributions from HST/ACS and Subaru Suprime-Cam multi-band wide-field imaging are compared to colours predicted from spectroscopically determinated ages and metallicities using single stellar population models. The galaxies formed over half of their mass in a single short-lived burst of star formation (> 100 M(sun)/year) at redshift z>5. This likely involved an outside-in mechanism with supernova-driven galactic winds, as suggested by the flatness of the alpha-element radial profiles and the strong negative metallicity gradients. Our results support the predictions of the revised version of the monolithic collapse model for galaxy formation and evolution. We speculate that, since formation the galaxies have evolved quiescently and that we are witnessing the first infall of NGC 1400 in the group.