Quantitative Spectroscopy of Blue Supergiants in Metal-Poor Dwarf Galaxy
  NGC 3109
  We present a quantitative analysis of the low-resolution (4.5 A) spectra of
12 late-B and early-A blue supergiants (BSGs) in the metal-poor dwarf galaxy
NGC 3109. A modified method of analysis is presented which does not require use
of the Balmer jump as an independent temperature indicator, as used in previous
studies. We determine stellar effective temperatures, gravities, metallicities,
reddening, and luminosities, and combine our sample with the early-B type BSGs
analyzed by Evans et al. (2007) to derive the distance to NGC 3109 using the
Flux-weighted Gravity-Luminosity Relation (FGLR). Using primarily Fe-group
elements, we find an average metallicity of [Z] = -0.67 +/- 0.13, and no
evidence of a metallicity gradient in the galaxy. Our metallicities are higher
than those found by Evans et al. (2007) based on the oxygen abundances of
early-B supergiants ([Z] = -0.93 +/- 0.07), suggesting a low alpha/Fe ratio for
the galaxy. We adjust the position of NGC 3109 on the BSG-determined galaxy
mass-metallicity relation accordingly and compare it to metallicity studies of
HII regions in star-forming galaxies. We derive an FGLR distance modulus of
25.55 +/- 0.09 (1.27 Mpc) that compares well with Cepheid and tip of the red
giant branch (TRGB) distances. The FGLR itself is consistent with those found
in other galaxies, demonstrating the reliability of this method as a measure of
extragalactic distances.
