Blue diffuse dwarf galaxies: a clearer picture
  The search for chemically unevolved galaxies remains prevalent in the nearby
Universe, mostly because these systems provide excellent proxies for exploring
in detail the physics of high-z systems. The most promising candidates are
extremely metal-poor galaxies (XMPs), i.e., galaxies with <1/10 solar
metallicity. However, due to the bright emission line based search criteria
traditionally used to find XMPs, we may not be sampling the full XMP
population. In 2014 we reoriented this search using only morphological
properties and uncovered a population of ~150 `blue diffuse dwarf (BDD)
galaxies', and published a sub-sample of 12 BDD spectra. Here we present
optical spectroscopic observations of a larger sample of 51 BDDs, along with
their SDSS photometric properties. With our improved statistics, we use
direct-method abundances to confirm that BDDs are chemically unevolved
(7.43<12+log(O/H)<8.01), with ~20% of our sample classified as being XMP
galaxies, and find they are actively forming stars at rates of 1-33x10^-2
M_sol/yr in HII regions randomly embedded in a blue, low-surface brightness
continuum. Stellar masses are calculated from population synthesis models and
estimated to be in the range log(M_star/M_sol) ~5-9. Unlike other
low-metallicity star-forming galaxies, BDDs are in agreement with the
mass-metallicity relation at low masses, suggesting they are not accreting
large amounts of pristine gas relative to their stellar mass. BDD galaxies
appear to be a population of actively star-forming dwarf irregular (dIrr)
galaxies who fall within the class of low-surface brightness dIrr galaxies.
Their ongoing star-formation and irregular morphology make them excellent
analogues for galaxies in the early Universe.
