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.

29 pages, 26 figures. Accepted to the Astrophysical Journal

References in corpus (16)

• Cosmic Star Formation History
• The Mass-Metallicity Relation at z~2
• Strong Nebular Line Ratios in the Spectra of z~2-3 Star-forming Galaxies: First Results from KBSS-MOSFIRE
• The KMOS^3D Survey: design, first results, and the evolution of galaxy kinematics from 0.7<z<2.7
• The Stellar, Gas and Dynamical Masses of Star-Forming Galaxies at z~2
• The MOSFIRE Deep Evolution Field (MOSDEF) Survey: Rest-Frame Optical Spectroscopy for ~1500 H-Selected Galaxies at 1.37 < z < 3.8
• Multi-Wavelength Constraints on the Cosmic Star Formation History from Spectroscopy: the Rest-Frame UV, H-alpha, and Infrared Luminosity Functions at Redshifts 1.9<z<3.4
• The MOSDEF Survey: Mass, Metallicity, and Star-formation Rate at z~2.3
• New insights into the stellar content and physical conditions of star-forming galaxies at z = 2-3 from spectral modelling
• The MOSDEF Survey: Excitation Properties of $z\sim 2.3$ Star-forming Galaxies
• AGN Emission Line Diagnostics and the Mass-Metallicity Relation up to Redshift z~2: the Impact of Selection Effects and Evolution
• Metallicities and Physical Conditions in Star-forming Galaxies at z~1.0-1.5
• The MOSDEF Survey: Optical AGN Diagnostics at z~2.3
• A Consistent Study of Metallicity Evolution at 0.8 < z < 2.6
• Carbon and oxygen abundances from recombination lines in low-metallicity star-forming galaxies. Implications for chemical evolution
• Testing the existence of non-Maxwellian electron distributions in H II regions after assessing atomic data accuracy

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