Nebular Emission Line Ratios in z~2-3 Star-Forming Galaxies with KBSS-MOSFIRE: Exploring the Impact of Ionization, Excitation, and Nitrogen-to-Oxygen Ratio

Abstract: We present a detailed study of the rest-optical (3600-7000 Angstrom) nebular spectra of ~380 star-forming galaxies at z~2-3 obtained with Keck/MOSFIRE as part of the Keck Baryonic Structure Survey (KBSS). The KBSS-MOSFIRE sample is representative of star-forming galaxies at these redshifts, with stellar masses M*=10^9-10^11.5 M_sun and star formation rates SFR=3-1000 M_sun/yr. We focus on robust measurements of many strong diagnostic emission lines for individual galaxies: [O II]3727,3729, [Ne III]3869, H-beta, [O III]4960,5008, [N II]6549,6585, H-alpha, and [S II]6718,6732. Comparisons with observations of typical local galaxies from the Sloan Digital Sky Survey (SDSS) and between subsamples of KBSS-MOSFIRE show that high-redshift galaxies exhibit a number of significant differences in addition to the well-known offset in log([O III]/H-beta) and log([N II]/H-alpha). We argue that the primary difference between H II regions in z~2.3 galaxies and those at z~0 is an enhancement in the degree of nebular excitation, as measured by [O III]/H-beta and R23=log[([O III]+[O II])/H-beta]. At the same time, KBSS-MOSFIRE galaxies are ~10 times more massive than z~0 galaxies with similar ionizing spectra and have higher N/O (likely accompanied by higher O/H) at fixed excitation. These results indicate the presence of harder ionizing radiation fields at fixed N/O and O/H relative to typical z~0 galaxies, consistent with Fe-poor stellar population models that include massive binaries, and highlight a population of massive, high-specific star formation rate galaxies at high-redshift with systematically different star formation histories than galaxies of similar stellar mass today.

Overview of Nebular Emission Line Ratios in $z\simeq2-3$ Galaxies

This paper presents an extensive study of the nebular spectra of approximately 380 star-forming galaxies at redshifts $z\simeq2-3$. Using data from the Keck Baryonic Structure Survey (KBSS) and the Multi-Object Spectrometer For InfraRed Exploration (MOSFIRE) instrument on the Keck telescope, the authors examine rest-optical emission lines to understand the ionization, excitation, and nitrogen-to-oxygen (N/O) abundance ratios in these distant galaxies. The study provides insights into the conditions in high-redshift galaxies and highlights meaningful differences from local galaxies observed by the Sloan Digital Sky Survey (SDSS).

One of the central findings is that $z\sim2.3$ galaxies show a noticeable offset in the log([\ion{O}{3}]/H$\beta$) vs. log([\ion{N}{2}]/H$\alpha$) diagnostic diagram (N2-BPT) when compared to local galaxies, which indicates different ionization conditions. The high-redshift galaxies occupy a distinct locus in this space, suggesting they experience enhanced nebular excitation characterized by these line ratios.

Key Results

The authors differentiate the high-redshift galaxies into two subsamples: "large-offset" and "small-offset," determined by their relative positions in the N2-BPT diagram. This segregation reveals that the most significant differences appear to be correlated with stellar mass (M$_{\ast}$) and specific star formation rates (sSFR), rather than electron densities. The "large-offset" galaxies tend to be less massive and exhibit higher sSFR compared to their "small-offset" counterparts.

The investigation into nitrogen-to-oxygen abundance ratios, derived using a recalibrated N2O2 index, shows that $z\sim2.3$ galaxies exhibit lower N/O at fixed stellar mass compared to SDSS, indicating a potential evolution of the N/O-M$_{\ast}$ relation with redshift. This observation suggests that the nature of nitrogen enrichment processes in galaxies may change over time; however, it does not sufficiently account for the entirety of the N2-BPT offset when compared to $z\sim0$ galaxies.

Photoionization models utilizing the BPASSv2 stellar population synthesis, which includes the presence of massive binaries, provide the framework for interpreting these nebular line ratios. The results suggest galaxies at $z\sim2.3$ predominantly have harder ionizing radiation spectra than local galaxies, which, in combination with moderately elevated N/O ratios, can explain their position in the N2-BPT diagram.

Implications

These findings imply that the spectral ionizing conditions in highly star-forming regions of the early universe are distinct from those today, driven primarily by differences in the stellar population composition and evolution. The presence of these harder radiation fields is likely linked to younger, more actively star-forming galaxies that are representative of $z\sim2.3$. This characteristic radiation affects diagnostic measurements like the BPT diagrams, emphasizing that high-redshift galaxies inherently differ in their stellar and ISM properties compared to the local universe.

This comprehensive analysis of $z\sim2.3$ galaxies not only enhances our understanding of galactic evolution but also underscores the importance of properly accounting for variations in ionization and excitation conditions when interpreting nebular emission lines at different epochs. Future research in AI and data analysis methodologies could further refine these investigations by providing more sophisticated tools for modeling complex star-forming environments in high-redshift galaxies. Such developments will undoubtedly lead to more accurate interpretations of astronomical observations, ultimately advancing our comprehension of galaxy formation and evolution across cosmic time.