Souradeep Bhattacharya, Chiaki Kobayashi
ABSTRACT 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 ($\ge \!-1.1$) at relatively low O abundances, 12 + log(O/H) $\le$ 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 ($z \lt 0.5$). Our sample of N-emitters represents a fivefold increase in their known number at low z with 12 + log(O/H) $\le$ 8, and statistically, $2.21\pm 0.91$ 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 ${\sim} 10^7$–$10^{10}$ M$_{\odot }$ with a 12 + log(O/H) range of $\sim$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) = $-0.53\pm 0.13$, while also having the lowest, 12 + log(O/H) = $7.08\pm 0.09$, and the highest stellar mass, log($M_{*}$/M$_{\odot }$) = 9.95 $\pm$ 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.