Strong Nebular Line Ratios in the Spectra of z~2-3 Star-forming Galaxies: First Results from KBSS-MOSFIRE

Abstract: We present initial results of a deep near-IR spectroscopic survey covering the 15 fields of the Keck Baryonic Structure Survey (KBSS) using MOSFIRE on the Keck 1 telescope, focusing on a sample of 251 galaxies with redshifts 2.0< z < 2.6, star-formation rates 2 < SFR < 200 M_sun/yr, and stellar masses 8.6 < log(M*/M_sun) < 11.4, with high-quality spectra in both H- and K-band atmospheric windows. We show unambiguously that the locus of z~2.3 galaxies in the "BPT" nebular diagnostic diagram exhibits a disjoint, yet similarly tight, relationship between the ratios [NII]6585/Halpha and [OIII]/Hbeta as compared to local galaxies. Using photoionization models, we argue that the offset of the z~2.3 locus relative to z~ 0 is explained by a combination of harder ionizing radiation field, higher ionization parameter, and higher N/O at a given O/H than applies to most local galaxies, and that the position of a galaxy along the z~2.3 star-forming BPT locus is surprisingly insensitive to gas-phase oxygen abundance. The observed nebular emission line ratios are most easily reproduced by models in which the net ionizing radiation field resembles a blackbody with effective temperature T_eff = 50000-60000 K and N/O close to the solar value at all O/H. We critically assess the applicability of commonly-used strong line indices for estimating gas-phase metallicities, and consider the implications of the small intrinsic scatter in the empirical relationship between excitation-sensitive line indices and stellar mass (i.e., the "mass-metallicity" relation), at z~2.3.

• The paper demonstrates a significant BPT diagram offset in z~2.3 galaxies, attributed to higher ionization parameters and harder ionizing spectra.
• It employs deep near-infrared spectroscopy to uncover systematic biases in traditional strong-line metallicity calibrations at high redshift.
• The study reveals a shifted mass-metallicity relation, indicating intrinsic differences in early galaxy evolution compared to the local universe.

Overview of "Strong Nebular Line Ratios in the Spectra of $z \sim 2-3$ Star-Forming Galaxies: First Results from KBSS-MOSFIRE"

The research presented in the paper utilizes the Keck Baryonic Structure Survey (KBSS) augmented with observations from MOSFIRE on the Keck 1 telescope to explore the nebular emission lines of 251 star-forming galaxies at redshifts $2.0 < z < 2.6$. These galaxies exhibit distinct nebular diagnostic characteristics compared to local galaxies, particularly in the BPT diagram, revealing insights into the ionization parameters and chemical compositions of high-redshift galaxies.

Key Findings

1. BPT Diagram Offset: The $z \sim 2.3$ galaxies form a distinct locus on the BPT diagram compared to local star-forming galaxies, typically with higher [OIII]/\ and [NII]/\ ratios. This offset is argued to result from a combination of higher ionization parameters, a harder ionizing radiation field, and distinct nitrogen-to-oxygen ratios ($\text{N/O}$) at given oxygen-to-hydrogen ratios ($\text{O/H}$).
2. Nebular Emission Analysis: Utilizing deep near-infrared spectra, the study confirms that high-redshift galaxies exhibit a tight sequence on the BPT diagram similar to local galaxies but displaced due to intrinsic differences in their nebular environments. Higher ionization parameters and effective ionizing radiation temperatures of $50,000 - 60,000$ K are posited to account for these differences.
3. Metallicity Indicators: The paper critically assesses the reliability of using strong-line indices like N2 and O3N2 for inferring metallicities at high redshift. It reveals systematic offsets when these calibrations are applied, suggesting potential biases unless re-calibrated with direct $T_e$ methods tailored for high-redshift conditions.
4. Mass-Metallicity Relation (MZR): The study identifies a correlation between stellar mass and metallicity akin to the local universe, albeit shifted toward lower metallicities for a given stellar mass. This shallow MZR, with intrinsic scatter as low as 0.10 dex, indicates robust correlations between stellar mass and ionization conditions in the early universe.

Implications

The observations and their interpretation offer substantial implications in our understanding of galaxy evolution and the conditions prevalent in the peak epoch of galaxy formation:

• Ionizing Conditions: The harder ionizing spectra at high redshift imply significant differences in the properties and evolution of massive stars compared to the local universe.
• Star Formation and IMFs: The results suggest possible changes in the initial mass function (IMF) and stellar evolution models to account for observed differences in radiation field properties.
• Calibrations of Metallicity: This research calls for a re-evaluation of strong-line metallicity calibrations with explicit consideration of the high-redshift galaxy conditions. It emphasizes the necessity for calibrations that incorporate these systematic variations.

Future Directions

The paper highlights the need for further analysis using direct $T_e$ metallicity measurements to provide robust calibrations of strong-line indicators at high redshift. Additionally, it underscores the importance of linking massive star population synthesis models with nebular diagnostics to precisely characterize the ionizing environments in early galaxy evolution.

The juxtaposition of high-redshift star-forming galaxies with local analogs, like the green pea galaxies, offers a unique perspective and strengthens the findings by suggesting that physical processes inferred from distant galaxies can be studied in more detail through their rare local counterparts. Such studies could refine our theoretical understanding and forecast the evolution of galaxies into the contemporary universe.