Bimodality in low luminosity E and S0 galaxies

TL;DR

This study investigates low luminosity early-type galaxies, revealing their stellar populations are younger, bimodal in metallicity, and resemble spiral bulges more than luminous ellipticals, challenging traditional galaxy classification models.

Contribution

It provides new spectroscopic data and analysis showing bimodal metallicity and age characteristics in low luminosity E and S0 galaxies, linking them to spiral bulges.

Findings

LLEs fall below the luminosity-velocity dispersion correlation for ellipticals.

LLEs are similar in age to spiral bulges.

LLEs exhibit bimodal metallicity distribution.

Abstract

Stellar population characteristics are presented for a sample of low luminosity early-type galaxies (LLEs) in order to compare them with their more luminous counterparts. Long-slit spectra of a sample of 10 LLEs were taken with the ESO New Technology Telescope, selected for their low luminosities. Line strengths were measured on the Lick standard system. Lick indices for these LLEs were correlated with velocity dispersion (sigma), alongside published data for a variety of Hubble types. The LLEs were found to fall below an extrapolation of the correlation for luminous ellipticals and were consistent with the locations of spiral bulges in plots of line strengths versus sigma. Luminosity weighted average ages, metallicities and abundance ratios were estimated from chi-squared fitting of 19 Lick indices to predictions from simple stellar population models. The LLEs appear younger than luminous ellipticals and of comparable ages to spiral bulges. These LLEs show a bimodal metallicity distribution, consisting of a low metallicity group (possibly misclassified dwarf spheroidal galaxies) and a high metallicity group (similar to spiral bulges). Finally, they have low alpha-element to iron-peak abundance ratios indicative of slow, extended star formation.