Evolution of N/O Abundance Ratios and Ionization Parameters from z~0 to 2 Investigated by the Direct Temperature Method

Abstract

We present N/O abundance ratios, ionization parameters {$q_{\mathrm{ion}}$}, and oxygen abundance O/H for a total of 41 galaxies (11 individual galaxies and a 30-galaxy stack) including {Ly$\alpha$} emitters and Lyman break galaxies at $z\sim 2$, and investigate galaxy evolution from $z\sim 0$ to $2$ in conjunction with 208,529 local SDSS galaxies and 9 green pea galaxies (GPs). In contrast with most of the recent studies, we obtain the N/O ratio, {$q_{\mathrm{ion}}$}, and O/H measurements by direct $T_{\mathrm{e}}$ methods with {[O\,\textsc{iii}]} $\lambda$4363 and {O\,\textsc{iii}]} $\lambda$1665 lines. Based on these reliable measurements, we find that there exist $z\sim 2$ galaxies with an excess of N/O falling beyond the local average of N/O--O/H relation, while the majority of the $z\sim 2$ galaxies have the N/O ratios nearly comparable with $z\sim 0$ galaxies in the N/O--stellar mass relation. Our galaxies place the upper limit of N/O ratio $\log (N/O)\le -1.26$ on average, suggesting that the N/O ratio evolves, if at all, by $<0.17$ dex. Exploiting our reliable measurements free from the N/O--{$q_{\mathrm{ion}}$}--O/H degeneracies, we identify, for the first time, that $z\sim 2$ galaxies with offsets in the BPT diagram indicate either 1) only an N/O excess, 2) only a {$q_{\mathrm{ion}}$} excess, or 3) both N/O and {$q_{\mathrm{ion}}$} excesses. We argue that the BPT offsets at $z\sim 2$ are not made by one of 1)--3) galaxy populations alone, but the composite of 1)--3) populations. We confirm that these 1)--3) populations also exist at $z\sim 0$, such as GPs and SDSS low-mass and high-SFR galaxies (LMHSs).

22 pages, 13 figures, 6 tables, the data and discussions in the v2/v3 are updated in response to the latest reports from other papers. Accepted for publication in PASJ. In press

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