The Role of Starbursts in the Evolution of Galaxies

We have determined, for the first time, the nitrogen-to-oxygen abundance ratio (N/O) for a large sample of starburst nucleus galaxies (SBNGs) and compared it to the values observed in normal spiral galaxies. The N/O ratios in SBNGs are ∼ 0.2 dex higher than in normal HII regions observed in the discs of late-type spirals, but are comparable to the values found in the bulges of normal early-type spirals. The variation of the N/O ratio as a function of metallicity follows a primary + secondary relation, but the increase in nitrogen does not appear as a continuous process. Assuming that nitrogen is produced by intermediate-mass stars, we show that our observations are consistent with a model in which the bulk of the nitrogen was formed during past sequences of bursts of star formation which have probably started 2 or 3 Gyr in the past. What we observe, therefore, could be the main production of nitrogen in the bulges of these galaxies.