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

Abstract

Extreme nitrogen enhancement relative to oxygen, recently found in very high-redshift galaxies, has been seen in local star-forming galaxies displaying high log(N/O) values ($\geq\!-1.1$) at relatively low O abundances, 12+log(O/H)$\leq$8. Understanding the physical origins of these extreme N-emitters at low redshifts enables us to better constrain chemical enrichment mechanisms that drove such high log(N/O) values in the early Universe. With direct N and O abundances derived for 944 SFGs with spectroscopic observational data from the Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1), we report the discovery of 19 extreme N-emitters at low-z (z$<$0.5). Our sample of N-emitters represents a five-fold increase in their known number at low-z with 12+log(O/H)$\leq$8, and statistically, $2.21\pm0.91$\% 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 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\pm0.13$, while also having the lowest 12+log(O/H)=$7.08\pm0.09$ and the highest stellar mass, log(M$_{*}$/M$_{\odot}$)=$9.95\pm0.13$ among our sample. With galactic chemical evolution models, we show that sustained N-enhancement by asymptotic giant branch stars, in conjunction with 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 inflow of gas.