Interstellar Medium Abundances in the Pegasus Dwarf Irregular Galaxy

Abstract

We report H alpha imaging and optical spectrophotometry of a faint H IIregion in the Pegasus dwarf irregular galaxy, a low-luminosity, possibleLocal Group member. We find that the emission-line spectrum isconsistent with excitation by a single star with a temperature of lessthan 35,000 K, in agreement with Aparicio {\amp} Gallart (1995). Althoughwe cannot measure the electron temperature of the ionized gas directly,we can infer a narrow range in electron temperature by comparing theemission-line spectrum with photoionization models. Then we derive arelatively low oxygen abundance of approximately 10% of the solar value,in accordance with the well-known metallicity-luminosity relationshipfor dwarf irregular galaxies. We also find an N/O ratio of 6%, which ishigher than the typical value of 3% found in more actively star-formingdwarf irregular galaxies. By comparing this high N/O ratio with therecent star formation history of the Pegasus dwarf irregular derived byAparicio {\amp} Gallart, we hypothesize that this may be evidence ofdelayed N production.