Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

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

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 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.

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 () 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 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 (z0.5). Our sample of N-emitters represents a five-fold increase in their known number at low-z with 12+log(O/H)8, and statistically, \% of DESI DR1 SFGs with reliable O and N abundances obtained directly, are extreme N-emitters. The sample spans a mass range of --~M with 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)=, while also having the lowest 12+log(O/H)= and the highest stellar mass, log(M/M)= 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.

Paper Structure

This paper contains 19 sections, 11 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Sample Selection and Abundance determination
  3. DESI DR1 Value-added Catalogue Data
  4. Sample selection
  5. Abundance determination
  6. Consistency with abundance determinations from DESI EDR
  7. Analysis
  8. The log(N/O) vs 12+log(O/H) plane of SFGs
  9. Extreme N-emitting SFGs in DESI DR1
  10. Relationship with other physical quantities
  11. Comparison with literature extreme N-emitting Star-forming galaxies
  12. Comparison with low-redshift extreme N-emitting SFGs
  13. Comparison with high-redshift SFGs with log(N/O) determinations
  14. Probing the nature of extreme N-emitters at low redshift with GCE models
  15. Constructing GCE models
  16. ...and 4 more sections

Figures (11)

  • Figure 1: An example of the flux and wavelength calibrated spectrum (green) of a SFG observed by DESI (for the N-emitter No. 1 in Table \ref{['tab:attributes']}; its DESI ID and redshift are noted). The emission lines utilised in this work are labelled in red. The expected position of the HeII$\lambda$ 4686 Å line is marked (brown) but is not observed. The insets show the zoomed-in views around the H$\gamma$ and [OIII]$\lambda$ 4363 Å; and H$\alpha$ and [NII]$\lambda$ 6548, 6583 Å lines, respectively, both of which clearly show their detections. The uncertainty in the flux is marked in grey in these insets.
  • Figure 2: 12+log(O/H) abundances determined in this work compared to previous determinations (green squares, Zinchenko24; magenta dots, Zou24) from DESI EDR spectra. The 1:1 line is also marked. The inset shows the same for log(N/O).
  • Figure 3: 12+log(O/H) vs log(N/O) for the 944 DESI DR1 SFGs with our reliable O & N abundances. The uncertainties are marked with errorbars. The running mean in log(N/O) as a function of 12+log(O/H) is shown in red, with the 1 $\sigma$ uncertainty shaded. The local scaling relation Dopita16Nicholls17 is also marked, along with the median log(N/O) value for metal-poor galaxies computed by Zinchenko24.
  • Figure 4: 12+log(O/H) vs log(N/O) for the DESI DR1 944 SFGs with our reliable O & N abundances. 19 N-emitters (log(N/O) $\geq\!-1.1$; those above dot-dashed line) are marked in black (with their determined uncertainties) while the others are marked in grey. Different groups of literature sources are shown for comparison (see Section \ref{['sec:disc']}). Note that the upward triangle shows the lower limit for GNz11 while the downward triangle shows the upper limit for GHz2.
  • Figure 5: RGB image cutouts ($10' \times 10'$) from data release DR10 of the DESI legacy imaging surveys for the 19 SFGs identified in this work having considerably enriched N-abundances (log(N/O)$\geq$-1.1). Their sl. no. from Table \ref{['tab:attributes']} is marked. The bottom-right panel shows the DESI fiber on-sky angular extent.
  • ...and 6 more figures