Extremely metal-poor star-forming galaxies. New detections and general morphological and photometric properties

TL;DR

This study identifies and characterizes extremely metal-poor star-forming dwarf galaxies, revealing their structural properties, stellar populations, and morphological features, and comparing them to blue compact dwarf galaxies.

Contribution

It provides new spectroscopic and photometric data on extremely metal-poor SF galaxies, showing they are similar to BCDs but with bluer, irregular hosts, and suggests a recent star formation history.

Findings

Oxygen abundance 7.3<12+log(O/H)<7.6, among the most metal-poor SF galaxies

All galaxies have detectable stellar hosts, indicating they are not first-generation star-formers

Many exhibit cometary morphology with intense star formation at one edge

Abstract

(abridged) We present long-slit spectroscopy with the 3.6m ESO telescope of eight HII regions in seven star-forming (SF) dwarf galaxies, discovered in SDSS DR4 and 6dFGRS. In addition, we use SDSS imaging data to study the photometric structure of the sample galaxies. From the 3.6m telescope spectra, we determine the oxygen abundance of these systems to be 7.3<12+log(O/H)<7.6, placing them among the most metal-poor SF galaxies ever discovered. Our photometric analysis reveals a moderately blue, stellar host galaxy in all sample galaxies. The detection of a stellar host in all galaxies studied here and all previously studied extremely metal-deficient SF galaxies implies that they are unlikely to be forming their first generation of stars. With regard to the structural properties of their host galaxy, we demonstrate that these systems are indistinguishable from blue compact dwarf (BCD) galaxies. However, in contrast to the majority (>90%) of BCDs that are characterised by red elliptical host galaxies, extremely metal-poor SF dwarfs (hereafter XBCDs) reveal moderately blue and irregular hosts. This is consistent with a young evolutionary status and in the framework of standard star formation histories implies that several XBCDs formed most of their stellar mass in the past 2 Gyr. A large fraction of XBCDs reveal a cometary morphology due to the presence of intense SF activity at one edge of an elongated host galaxy with a gradually decreasing surface brightness towards its antipodal end.