STRONG NEBULAR LINE RATIOS IN THE SPECTRA of z ∼ 2–3 STAR FORMING GALAXIES: FIRST RESULTS FROM KBSS-MOSFIRE

By C. Steidel, G. Rudie, A. Strom, M. Pettini, Naveen A. Reddy, A. Shapley, R. Trainor, D. Erb, M. Turner, N. Konidaris and 4 more. 2014

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We present initial results of a deep near-IR spectroscopic survey covering the 15 fields of the Keck Baryonic Structure Survey using the recently commissioned MOSFIRE spectrometer on the Keck 1 telescope. We focus on a sample of 251 galaxies with redshifts 2.0 < z < 2.6, star formation rates (SFRs) 2 ≲ SFR ≲ 200 M☉ yr−1, and stellar masses 8.6 < log (M*/M☉) < 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 an almost entirely disjointed, yet similarly tight, relationship between the line ratios [N ii] λ6585/Hα and [O iii]/Hβ as compared to local galaxies. Using photoionization models, we argue that the offset of the z ∼ 2.3 BPT locus relative to that at z ∼ 0 is caused by a combination of harder stellar ionizing radiation field, higher ionization parameter, and higher N/O at a given O/H compared 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 stellar ionizing radiation field resembles a blackbody with effective temperature Teff = 50, 000–60, 000 K, the gas-phase oxygen abundances lie in the range 0.2 < Z/Z☉ < 1.0, and the ratio of gas-phase N/O is close to the solar value. We critically assess the applicability at high redshift of commonly used strong line indices for estimating gas-phase metallicity, and consider the implications of the small intrinsic scatter of the empirical relationship between excitation-sensitive line indices and M* (i.e., the “mass–metallicity” relation) at z ≃ 2.3.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/b9bdf44cc51e806f290de47fce03403f93cca9fc

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Citations in the corpus

Metallicity gradients in local field star-forming galaxies: insights on inflows, outflows, and the coevolution of gas, stars and metals (2015, background): “…of metallicity out to high redshifts on large numbers of galaxies (e.g.,Savaglio et al. 2005; Erb et al. 2006; Zahid et al. 2011, 2013, 2014b; Wuyts et al. 2014; Steidel et al. 2014; Sanders et al. 2014) and laid the foundation for subsequent investigation into the physical origin of the relation.”
An extreme [O III] emitter at z = 3.2: a low metallicity Lyman continuum source (2015, result): “Emission lines are often dust corrected using the Balmer decrement (Hα/Hβ): the observed ratio between Balmer lines is compared with the expected ratio derived from theory (e.g., Hα/Hβ = 2.86; Osterbrock 1989), and assuming an attenuation curve, the dust correction is derived at any wavelength (e.g.”
SDSS-IV MaNGA : the impact of diffuse ionized gas on emission-line ratios, interpretation of diagnostic diagrams and gas metallicity measurements (2016, background): “Based on photoionization models, Steidel et al. (2014) concluded that the offset on the BPT diagram of the z∼2.3 star-forming galaxies is caused by a combination of harder stellar ionizing radiation field, higher ionization parameter, and higher N/O at a given O/H compared to most local galaxies.”
Spectroscopic detection of C iv λ1548 in a galaxy at z = 7.045: implications for the ionizing spectra of reionization-era galaxies (2015, background): “The absence of strong CIV emission in typical galaxies points to a negligible output of photons with energies greater than 47.9 eV, consistent with the radiation field expected from the blackbody models predicted for z ' 2 galaxies in Steidel et al. (2014).”
The Observer's Guide to the Gamma-Ray Burst-Supernova Connection (2016, background): “This could be caused by a harder ionization field originating from hot O-type stars [295] that emit a large number of photons capable of ionizing oxygen into [O iii].”
The difficulty of getting high escape fractions of ionizing photons from high-redshift galaxies: A view from the FIRE cosmological simulations (2015, background): “For example, Steidel et al. (2014) emphasized the importance of binary and rotating stars since these stars have high effective temperatures that are required to explain the ionization states of z = 2–3 star forming galaxies.”
AN OBJECTIVE DEFINITION FOR THE MAIN SEQUENCE OF STAR-FORMING GALAXIES (2015, methodology): “However, with the advent of near-IR multiobject spectrographs SFRs from Hα can be derived up to z ∼ 2.5, i.e., using the same indicator as used here for local galaxies (e.g., Kashino et al. 2013; Steidel et al. 2014; Wisnioski et al. 2009).”
THE MOSFIRE DEEP EVOLUTION FIELD (MOSDEF) SURVEY: REST-FRAME OPTICAL SPECTROSCOPY FOR ∼1500 H-SELECTED GALAXIES AT 1.37 ≤ z ≤ 3.8 ?> (2014, methodology): “Taking into account the integration time difference and the slit-loss correction, we find a difference of a factor of 1.5 between our optimistic 5σ depth and the 5σ depth found by Steidel et al. (2014) for the H-band.”
THE MOSDEF SURVEY: MEASUREMENTS OF BALMER DECREMENTS AND THE DUST ATTENUATION CURVE AT REDSHIFTS z ∼ 1.4–2.6 (2015, methodology): “A similar agreement has been noted in Steidel et al. (2014), who also compare Hα and SED-based SFRs, but who use a different prescription for estimating the color excess relevant for the nebular emission as they did not have Balmer decrement MOSDEF: The Dust Attenuation Curve at z ∼ 2 21”
THE MOSDEF SURVEY: ELECTRON DENSITY AND IONIZATION PARAMETER AT z ∼ 2.3 (2015, methodology): “Steidel et al. (2014) did mention the relations between ionization parameter, metallicity, and ionizing spectrum as a way to reconcile the utility of strong-line metallicity indicators in the local universe with the apparent lack of dependence of the star-forming sequence on metallicity in…”
The origin and evolution of the galaxy mass–metallicity relation (2015, background): “We emphasize that the empirical calibrations developed from the local universe are not necessarily valid for high-redshift galaxies (e.g. Steidel et al. 2014).”
The HDUV Survey: A Revised Assessment of the Relationship between UV Slope and Dust Attenuation for High-redshift Galaxies (2017, methodology): “…as the 3D-HST (van Dokkum et al. 2013), the MOSFIRE Deep Evolution Field (MOSDEF; Kriek et al. 2015), and the Keck Baryonic Structure surveys (KBSS; Steidel et al. 2014)— have enabled measurements of obscuration in individual high-redshift star-forming galaxies using Balmer recombination lines…”
GRB hosts through cosmic time - VLT/X-Shooter emission-line spectroscopy of 96 γ-ray-burst-selected galaxies at 0.1 (2015, background): “A similar offset is well-observed in many high redshift galaxy samples (e.g., Shapley et al. 2005; Kriek et al. 2007; Steidel et al. 2014) and also for galaxies hosting H-poor superluminous supernovae (Leloudas et al. 2015).”
The SINS/zC-SINF Survey of z ∼ 2 Galaxy Kinematics: SINFONI Adaptive Optics–assisted Data and Kiloparsec-scale Emission-line Properties (2018, background): “…ratio, as amply discussed in the literature 5 (e.g., Kewley & Dopita 2002; Kewley et al. 2013; Pérez-Montero & Contini 2009; Yuan et al. 2012; Andrews & Martini 2013; Steidel et al. 2014; Shapley et al. 2015; Sanders et al. 2015, 2017; Kashino et al. 2017; Strom et al. 2017; Cresci et al. 2017).”