Interstellar medium abundances in the Pegasus dwarf irregular galaxy

We report H alpha imaging and optical spectrophotometry of a faint H II region in the Pegasus dwarf irregular galaxy, a low-luminosity, possible Local Group member. We find that the emission-line spectrum is consistent with excitation by a single star with a temperature of less than 35,000 K, in agreement with Aparicio & Gallart (1995). Although we cannot measure the electron temperature of the ionized gas directly, we can infer a narrow range in electron temperature by comparing the emission-line spectrum with photoionization models. Then we derive a relatively low oxygen abundance of approximately 10% of the solar value, in accordance with the well-known metallicity-luminosity relationship for dwarf irregular galaxies. We also find an N/O ratio of 6%, which is higher than the typical value of 3% found in more actively star-forming dwarf irregular galaxies. By comparing this high N/O ratio with the recent star formation history of the Pegasus dwarf irregular derived by Aparicio & Gallart, we hypothesize that this may be evidence of delayed N production.