The origin of extreme N-emitters in star-forming galaxies at z < 0.5 with DESI DR1

Extreme nitrogen enhancement relative to oxygen, recently found in very high-redshift galaxies, has been seen in local star-forming galaxies (SFGs) displaying high log(N/O) values (⁠⁠>=-1.1) at relatively low O abundances, 12 + log(O/H) <8. Understanding the physical origins of these extreme N-emitters at low redshifts enables us to constrain better chemical enrichment mechanisms that drove such high log(N/O) values in the early Universe. With direct N and O abundances derived for 944 SFGs with spectroscopic observational data from the Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1), we report the discovery of 19 extreme N-emitters at low z (<0.5⁠⁠). Our sample of N-emitters represents a fivefold increase in their known number at low z with 12 + log(O/H) <8, and statistically, 2.21±0.91 per cent of DESI DR1 SFGs with reliable O and N abundances obtained directly are extreme N-emitters. The sample spans a mass range of 10^7 – 10^10 M_sun with a 12 + log(O/H) range of 7.1–8.2, and the N-emitter fraction is found to increase with increasing stellar mass and decreasing metallicity. The most extreme N-emitter in our sample has log(N/O) = -0.53±0.13⁠, while also having the lowest, 12 + log(O/H) = ⁠7.08±0.09, and the highest stellar mass, log(⁠M/M_sun⁠) = 9.95±0.13, among our sample. With galactic chemical evolution models, we show that sustained N-enhancement by asymptotic giant branch stars, in conjunction with the presence of outflows during the evolution of the galaxy, can well explain the high log(N/O) of low-z extreme N-emitters. While single starbursts with outflow are sufficient to explain lower mass N-emitters, more massive ones require a dual starburst scenario where a secondary starburst is triggered by the inflow of gas.