Uncovering blue diffuse dwarf galaxies

Bethan L. James, Sergey Koposov, Daniel P. Stark, Vasily Belokurov, Max Pettini, Edward W. Olszewski

Research output: Contribution to journalArticlepeer-review

44 Scopus citations


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 subsample of galaxies that were selected via a morphology-based search on Sloan Digital Sky Survey images with the aim of finding these elusive XMP galaxies. By using the recently discovered XMP galaxy, Leo P, as a guide, we obtained a collection of faint, blue systems, each with isolated H II 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 ionization parameters of their H II regions. Despite their low luminosities (-11 ≲ MB ≲ -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 0.078Modot; yr-1. From our current subsample, BDD galaxies appear to be a population of nonquiescent dwarf irregular galaxies, or the diffuse counterparts to blue compact galaxies 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 starforming galaxies, which traditional emission-line-based searches overlook.

Original languageEnglish (US)
Pages (from-to)2687-2703
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Apr 11 2015


  • Galaxies: abundances
  • Galaxies: dwarf
  • Galaxies: evolution
  • Galaxies: irregular
  • Galaxies: star formation

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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