The H i-dominated low-surface-brightness galaxy KKR 17

We present new narrow-band (H alpha and [OIII]) images and optical spectrophotometry of H II regions for a gas-rich low-surface-brightness irregular galaxy, KKR 17. The central surface brightness of the galaxy is mu sub(0)(B)=24.15 plus or minus 0.03 mag s super(-2). The galaxy was detected by the Arecibo Legacy Fast ALFA survey (ALFALFA). Its mass is dominated by neutral hydrogen (H I) gas. In contrast, both the stellar masses of the bright H II and diffuse stellar regions are small. In addition, the fit to the spectral energy distribution to each region shows the stellar populations of H II and diffuse regions are different. The bright H II region contains a large fraction of O-type stars, revealing recent strong star formation, whereas the diffuse region is dominated by median age stars with a typical age of similar to 600 Myr. Using McGaugh's abundance model, we found that the average metallicity of KKR 17 is 12 + (O/H) = 8.0 plus or minus 0.1. The star-formation rate of KKR 17 is 0.21 plus or minus 0.4 M. yr super(-1), which is similar to 1/5 of our Milky Way's. Based on the analysis results for young stellar clusters in the H II region, the bright H II region has two sub-components with different velocities and metallicities. This may be caused by the outflow of massive stars or merging events. However, the mechanism triggering star formation in the H II region is still uncertain.