Uncovering Blue Diffuse Dwarf Galaxies
  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.
