Uncovering Blue Diffuse Dwarf Galaxies

TL;DR

This study identifies a new class of faint, blue, diffuse dwarf galaxies with extremely low metallicities, using morphology-based search methods on SDSS images, revealing their active star formation and potential link to other dwarf galaxy types.

Contribution

It introduces a novel morphology-based search technique to find diffuse, metal-poor dwarf galaxies overlooked by traditional emission-line methods.

Findings

Eight galaxies have oxygen abundances between 7.45 and 8.0.

Two galaxies are classified as extremely metal-poor (XMPs).

All galaxies show active star formation with young stellar populations.

Abstract

Extremely metal poor (XMP) galaxies are known to be very rare, despite the large numbers of low-mass galaxies predicted by the local galaxy luminosity function. This paper presents a sub-sample of galaxies that were selected via a morphology-based search on SDSS images with the aim of finding these elusive XMP galaxies. By using the recently discovered extremely metal-poor galaxy, Leo P, as a guide, we obtained a collection of faint, blue systems, each with isolated HII regions embedded in a diffuse continuum, that have remained optically undetected until now. Here we show the first results from optical spectroscopic follow-up observations of 12 of ~100 of these blue, diffuse dwarf (BDD) galaxies yielded by our search algorithm. Oxygen abundances were obtained via the direct method for eight galaxies, and found to be in the range 7.45<12+log(O/H)<8.0, with two galaxies being classified as XMPs. All BDDs were found to currently have a young star-forming population (<10 Myr) and relatively high ionisation parameters of their HII regions. Despite their low luminosities (-11< M_B < -18) and low surface brightnesses (~23-25 mag arcsec^-2), the galaxies were found to be actively star-forming, with current star-formation rates between 0.0003 and 0.078 Msol/yr. From our current subsample, BDD galaxies appear to be a population of non-quiescent dwarf irregular (dIrr) galaxies, or the diffuse counterparts to blue compact galaxies (BCDs) and as such may bridge the gap between these two populations. Our search algorithm demonstrates that morphology-based searches are successful in uncovering more diffuse metal-poor star-forming galaxies, which traditional emission-line based searches overlook.