The abundance of nitrogen in starburst nucleus galaxies

Abstract

We show that the excess of nitrogen emission observed in a large sample of StarburstNucleusGalaxies (SBNGs) can only be explained at a givenmetallicity by an overabundance of nitrogen with respect to normal Hii regions in the disks of late–type spirals. The N/O ratios in the SBNGs are comparable to the values found in the bulges of normal early–type spirals, which suggests that what we observe could be the main production of nitrogen in the bulges of these galaxies. The variation of the N/O ratio as a function of metallicity in SBNGs follows a primary + secondary relation, but the increase of nitrogen does not appear as a continuous process. In SBNGs, nitrogen is probably produced by different populations of intermediate-mass stars,whichwere formed during past sequences of bursts of star formation. This assumption pushes the origin of the main bursts 2–3Gyrs back in the past. On a cosmological scale, this time interval corresponds to redshifts z ∼ 0.2–0.3, where a significant increase of star formation activity occurred. The origin of the SBNG phenomenon would thus have cosmological implications, it would be related to a more active phase of star formation in the Universe sometime in its recent past.