Blue diffuse dwarf galaxies: a clearer picture
Monthly Notices of the Royal Astronomical Society
Oxford University Press
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James, B., Koposov, S., Stark, D., Belokurov, V., Pettini, M., Olszewski, E., & McQuinn, K. (2017). Blue diffuse dwarf galaxies: a clearer picture. Monthly Notices of the Royal Astronomical Society, 465 (4), 3977-4015. https://doi.org/10.1093/mnras/stw2962
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.
galaxies: abundances, galaxies: dwarf, galaxies: evolution, galaxies: irregular, galaxies: star formation
BLJ thanks support from the Science & Technology Facilities Council (STFC). The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 308024. SK acknowledges financial support from the ERC. DPS acknowledges support from the National Science Foundation through the grant AST-1410155. EWO is partially supported by NSF grant AST1313006.
SCIENCE & TECHNOLOGY FACILITIES COUNCIL (ST/N004493/1)
SCIENCE & TECHNOLOGY FACILITIES COUNCIL (ST/N000927/1)
European Research Council (308024)
External DOI: https://doi.org/10.1093/mnras/stw2962
This record's URL: https://www.repository.cam.ac.uk/handle/1810/262539