Search for blue compact dwarf galaxies during quiescence II: metallicities of gas and stars, ages, and star-formation rates

TL;DR

This study investigates the metallicities, ages, and star-formation rates of quiescent Blue Compact Dwarf galaxies using SDSS data, revealing differences between gas and stellar metallicities and providing insights into their evolutionary states.

Contribution

It introduces a new spectral fitting procedure using the MILES stellar library to analyze the metallicity and age of QBCD galaxies, comparing them with active BCDs.

Findings

Gas metallicity exceeds stellar metallicity by ~0.35 dex in QBCDs.

Stellar ages in QBCDs range from 1 to 10 Gyr, while BCDs are younger than 1 Gyr.

Gas metallicity in QBCDs likely reflects a small, self-enriched gas fraction.

Abstract

We examine the metallicity and age of a large set of SDSS/DR6 galaxies that may be Blue Compact Dwarf (BCD) galaxies during quiescence (QBCDs).The individual spectra are first classified and then averaged to reduce noise. The metallicity inferred from emission lines (tracing ionized gas) exceeds by ~0.35 dex the metallicity inferred from absorption lines (tracing stars). Such a small difference is significant according to our error budget estimate. The same procedure was applied to a reference sample of BCDs, and in this case the two metallicities agree, being also consistent with the stellar metallicity in QBCDs. Chemical evolution models indicate that the gas metallicity of QBCDs is too high to be representative of the galaxy as a whole, but it can represent a small fraction of the galactic gas, self enriched by previous starbursts. The luminosity weighted stellar age of QBCDs spans the whole range between 1 and 10 Gyr, whereas it is always smaller than 1 Gyr for BCDs. Our stellar ages and metallicities rely on a single stellar population spectrum fitting procedure, which we have specifically developed for this work using the stellar library MILES.