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_ion, and oxygen abundances O/H for a total of 41 galaxies (11 individual galaxies and a 30-galaxy stack) including Lyα emitters and Lyman break galaxies at z ∼ 2, and investigate galaxy evolution from z ∼ 0 to 2 in conjunction with 208529 local galaxies from the Sloan Digital Sky Survey (SDSS) and nine green pea galaxies (GPs). In contrast with most of the recent studies, we obtain the N/O ratio, q_ion, and O/H measurements by direct T_e methods using [O III] λ4363 and O III] λ1665 lines. Based on these reliable measurements, we find that there are z ∼ 2 galaxies with excesses of N/O falling beyond the N/O-O/H relation of the local galaxies while the majority of the z ∼ 2 galaxies have N/O ratios that are nearly comparable with those of z ∼ 0 galaxies in the N/O-stellar mass relation. Our galaxies place the upper limit of the N/O ratio log(N/O) ≤ -1.26 on average, suggesting that the N/O ratio evolves, if at all, by <0.17 dex. Obtaining the reliable measurements free from the N/O-q_ion-O/H degeneracies, we demonstrate, for the first time, that z ∼ 2 galaxies with offsets in the Baldwin-Phillips-Terlevich (BPT) diagram show (1) only an N/O excess, (2) only a q_ion excess, or (3) both N/O and q_ion excesses. We argue that the BPT offsets at z ∼ 2 are not made by one of the (1)-(3) galaxy populations alone, but a composite of (1)-(3) populations. We confirm that these (1)-(3) populations also exist at z ∼ 0, such as GPs and SDSS low-mass and high-SFR galaxies (LMHSs).