TY - JOUR
T1 - Blue diffuse dwarf galaxies
T2 - A clearer picture
AU - James, Bethan L.
AU - Koposov, Sergey E.
AU - Stark, Daniel P.
AU - Belokurov, Vasily
AU - Pettini, Max
AU - Olszewski, Edward W.
AU - McQuinn, Kristen B.W.
N1 - Funding Information:
The observations reported here were obtained at theMMT Observatory, a joint facility of theUniversity of Arizona and the Smithsonian Institution. The authors are sincerely grateful to Danielle Berg for discussions regarding chemical abundance calculations, Janice Lee for advice on calculating SFRs in the low-luminosity regime, and Matt Auger for assistance with SED modelling. We are grateful to the University of Arizona Observatory time assignment committee who awarded time to this programme, and thank the MMT telescope operators and stafffor technical support. 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 GrantAgreement 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. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Publisher Copyright:
© 2016 The Authors.
PY - 2017/3/11
Y1 - 2017/3/11
N2 - 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 highz 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 × 10-2 M⊙ yr-1 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*/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.
AB - 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 highz 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 × 10-2 M⊙ yr-1 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*/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.
KW - Galaxies: abundances
KW - Galaxies: dwarf
KW - Galaxies: evolution
KW - Galaxies: irregular
KW - Galaxies: star formation
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U2 - 10.1093/mnras/stw2962
DO - 10.1093/mnras/stw2962
M3 - Article
AN - SCOPUS:85014843201
SN - 0035-8711
VL - 465
SP - 3977
EP - 4015
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 4
ER -