Mon. Not. R. Astron. Soc., 465, 3977-4015 (2017/March-2)
Blue diffuse dwarf galaxies: a clearer picture.
JAMES B.L., KOPOSOV S.E., STARK D.P., BELOKUROV V., PETTINI M., OLSZEWSKI E.W. and McQUINN K.B.W.
Abstract (from CDS):
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 Sloan Digital Sky Survey (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 per cent of our sample classified as being XMP galaxies, and find that they are actively forming stars at rates of ∼1-33 x 10–2 M☉/yr in H II 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*/M☉) ≃ 5-9. Unlike other low-metallicity star-forming galaxies, BDDs are in agreement with the mass-metallicity relation at low masses, suggesting that 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 which 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.
galaxies: abundances - galaxies: dwarf - galaxies: evolution - galaxies: irregular - galaxies: star formation - galaxies: star formation
VizieR on-line data:
<Available at CDS (J/MNRAS/465/3977): table1.dat table2.dat table3.dat table4.dat table5.dat tablea1b.dat tablea1.dat>
Table 1: JKB NNN N=39 added among (Nos 1-144).
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