arXiv subjects

Ryan F. Trainor

Publications and source records attributed to Ryan F. Trainor.

At least 19 recordsLinked to original sources

arXiv2026

Scaling Relations of Galactic Outflows Across Cosmic Time: New Insights from Cosmic Noon

Galactic outflows play a significant role in regulating galaxy evolution. Scaling relations between stellar mass ($M_\ast$), star formation (SFR), and outflow properties have been extensively studied at low redshifts ($z<1$) but less so beyond $z \sim 2$. We construct a joint sample of 387 galaxies from $z \sim$ 0-9, including 98 new Cosmic Noon galaxies from the Keck Baryonic Structure Survey and the Keck Lyman Continuum Spectroscopic Survey. Using high signal-to-noise emission lines (SNR $>$ 50 for H$\alpha$ or [OIII] $\lambda5007$) from Keck/MOSFIRE spectra, we detect warm-ionized outflows by decomposing lines into narrow and broad components. With the joint sample, we explore the redshift evolution of outflow scaling relations. On average, outflows at Cosmic Noon have higher maximum velocities than those at low$-z$ by up to a factor of 3 for a fixed $M_\ast$, SFR, or SFR surface density. They also have higher mass outflow rates for a fixed $M_\ast$. Despite faster and stronger outflows, there is no evolution in the mass loading factor for a fixed $M_\ast$. We find evidence for galactic fountains, as the majority of outflowing gas is recycled at a radius of $\sim$0.03 R$_\textrm{vir}$. We also constrain the overall outflow occurrence rate in our galaxy sample to be at least 30$\%$ when taking galaxy orientation and outflow geometry into account. By analyzing the largest sample of warm-ionized outflows at Cosmic Noon to date and compiling large galaxy samples across all redshifts, we present a comprehensive analysis of galactic outflows throughout cosmic time.

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arXiv2025

CECILIA: Gas-Phase Physical Conditions and Multi-Element Chemistry at Cosmic Noon

Galaxies at Cosmic Noon (z$\sim$2-3) are characterized by rapid star formation that will lead to significant metal enrichment in the interstellar medium (ISM). While much observational evidence suggests that these galaxies are chemically distinct from those in the local Universe, directly measuring the ISM chemistry in large samples of high-z galaxies is only now possible with the observational capabilities of JWST. In this first key paper of the CECILIA program, we present the direct-method physical conditions and multi-element abundances in twenty galaxies at Cosmic Noon. Using a combination of archival Keck/MOSFIRE and new $\sim$30-hr NIRSpec spectroscopy, we measure multiple electron gas densities and the temperature structure from the O$^+$ and S$^{2+}$ ions. We find that n$_e$[O II] and n$_e$[S II] are comparable but elevated with respect to n$_e$ in local star-forming galaxies, and the simultaneous T$_e$[O II] and T$_e$[S III] generally agree with photoionization model T$_e$ scaling relations. The O abundances in the CECILIA galaxies range from 12+log(O/H)$=$7.76-8.81 (12-131% solar O/H), representing some of the highest direct-method metallicities and lowest T$_e$ (T$_e$[O II]$\approx$6500 K) measured with JWST to date. The CECILIA galaxies exhibit significantly sub-solar S/O and Ar/O a signature of predominant enrichment from core collapse supernovae. The N/O-O/H trends in the CECILIA galaxies generally agree with the abundance trends in local nebulae, but the large scatter in N/O could be sensitive to the star-formation history. The CECILIA observations demonstrate that exceptionally deep JWST spectroscopy can unveil the multi-element ISM abundance patterns in typical high-z galaxies.

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arXiv2025

CECILIA: The Mass-Metallicity Relation of Low-Mass Galaxies at Cosmic Noon

A galaxy's metallicity and its relation to stellar mass encode the history of gas accretion, star formation, and outflows within cosmic ecosystems. We present new constraints on the low-mass end of the mass-metallicity relation (MZR) at $z\sim2-3$ from ultra-deep JWST/NIRSpec spectroscopy of seven continuum-faint galaxies in the Chemical Evolution Constrained using Ionized Lines in Interstellar Aurorae (CECILIA) Faint sample (Raptis et al. 2025). Our sample includes Ly$α$-selected and other low-luminosity star-forming galaxies with stellar masses $\log(M_\star / M_\odot)\sim7.2-9.7$ and moderately faint rest-UV magnitudes ($-20.7 \lesssim M_{\rm UV} \lesssim -17.3$). Gas-phase oxygen abundances, calculated using empirical calibrations of [O III]/H$β$ together with [N II]/H$α$ constraints, span $\sim0.04-0.5$ $Z_\odot$. We measure a steep MZR slope of $γ= 0.48 \pm 0.11$, suggesting a rapid increase in metal retention efficiency with mass, consistent with energy-driven outflows. Comparison with lower- and higher-redshift studies indicates an evolution in normalization from $z\sim0$ to $z\sim2$, reflecting less metal enrichment in early galaxies. We find no significant evolution in the MZR between $z\sim2$ and the Epoch of Reionization, suggesting that our galaxies may serve as useful analogs of reionization-era systems. Expanded samples and direct $T_e$-based abundance measurements will be crucial to fully trace the build-up of metals in low-mass galaxies during the peak epoch of cosmic star formation and to test the reliability of strong-line calibrations in these galaxies.

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arXiv2025

CECILIA: Ultra-Deep Rest-Optical Spectra of Faint Galaxies at Cosmic Noon

Intrinsically faint galaxies at $z\sim2-3$ offer critical insights into early galaxy formation, tracing low-metallicity, low-mass systems during Cosmic Noon and serving as analogs to reionization-era galaxies. We present ultra-deep JWST/NIRSpec spectroscopy of nine low-luminosity galaxies ($-17 \lesssim M_{\rm UV} \lesssim -20$, $M_\star \lesssim 10^9\,M_\odot$) at $z\sim2.5$ from the CECILIA program, with $\sim$29.5 hr in G235M/F170LP and 1 hr in G395M/F290LP. Our sample includes four LAEs, three rest-UV color-selected galaxies, and two serendipitous detections -- providing the most sensitive rest-optical spectra of individual faint galaxies at this epoch to date. Balmer-line measurements reveal low SFRs ($0.63 < \mathrm{SFR}/(M_\odot\,\mathrm{yr}^{-1}) < 5.43$) and a broad range of dust reddening ($0 < E(B-V) < 1$), with SFRs systematically below those of continuum-selected galaxies. Electron densities are low ($n_e \lesssim 200$cm$^{-3}$), and emission-line diagnostics indicate low [NII]/H$α$, high [OIII]/H$β$, suggesting metallicities $12+\log({\rm O/H})\lesssim8.0$. We also present the first O1-BPT constraints in such faint high-redshift galaxies. Notably, two galaxies show low [OIII]/H$β$ despite high Ly$α$ EWs and very low [NII]/H$α$, consistent with the predicted turnover in this ratio at very low metallicities -- highlighting the need for complementary diagnostics (e.g., N2, O32) to identify metal-poor systems. Direct $T_e$-based abundances and expanded samples are needed to further trace metallicity and ionization trends in low-mass galaxies.

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arXiv2025

The Lyman-alpha Halos of Galaxies at z=2-3 in the Keck Baryonic Structure Survey

We present the large-scale spatial Lya profiles of galaxies from the Keck Baryonic Structure Survey (KBSS) at 2 50 kpc, theta > 6''). We find that the scale length of the Lya halo is not strongly dependent on the properties of the central galaxy, including its net continuum luminosity or EW_Lya, although we find a possible weak tendency of continuum-faint, high-EW_Lya galaxies to exhibit larger Lya halos in contrast with previous work.

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arXiv2025

Exploring the Relationship Between Stellar Mass, Metallicity, and Star Formation Rate at $z \sim 2.3$ in KBSS-MOSFIRE

The metal enrichment of a galaxy is determined by the cycle of baryons in outflows, inflows, and star formation. The relative contribution and timescale of each process sets the relationship between stellar mass, metallicity, and the star formation rate (SFR). In the local universe, galaxies evolve in an equilibrium state where the timescales on which SFR and metallicity vary are comparable, and define a surface in mass-metallicity-SFR space known as the Fundamental Metallicity Relation (FMR). However, high-redshift observations suggest that this state of equilibrium may not persist throughout cosmic time. Using galaxies from the Keck Baryonic Structure Survey (KBSS) observed with MOSFIRE, we explore the relationship between stellar mass, gas-phase oxygen abundance, and SFR at $z \sim 2.3$. Across strong-line calibrations and SFR calculation methods, KBSS galaxies are inconsistent with the locally-defined FMR. We use both parametric and non-parametric methods of exploring a mass-metallicity-SFR relation. When using a parametric approach, we find no significant reduction mass-metallicity relation scatter when folding in SFR as a third parameter, although a non-parametric approach reveals that there could be a weak, redshift-dependent anticorrelation between residual gas-phase oxygen abundance, and SFR. Injection-recovery tests show that a significant reduction in scatter requires a stronger anticorrelation between SFR and residual metallicity. Our results suggest that the local FMR may not persist to $z \sim 2.3$, implying that $z \sim 2.3$ galaxies may not be in the equilibrium state described by the FMR and are more similar to higher redshift galaxies.

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arXiv2024

CECILIA: Direct O, N, S, and Ar Abundances in Q2343-D40, a Galaxy at $z\sim$3

Measurements of chemical abundances in high-$z$ star-forming (SF) galaxies place important constraints on the enrichment histories of galaxies and the physical conditions in the early universe. JWST is beginning to enable direct chemical abundance measurements in galaxies at $z$$>$2 via the detection of the faint T$_e$-sensitive auroral line [O III]$λ$4364. However, abundances of other elements (e.g., S and Ar) in high-$z$ galaxies remain unconstrained due to a lack of T$_e$ data and wavelength coverage. Here, we present multiple direct abundances in Q2343-D40, a galaxy at $z=$2.9628$\pm$0.0001 observed with JWST/NIRSpec as part of the CECILIA program. We report the first simultaneous measurement of T$_e$[O III] and T$_e$[S III] in a high-$z$ galaxy, finding good agreement with the temperature trends in local SF systems. We measure a gas-phase metallicity of 12+log(O/H) $=8.07\pm0.06$, and the N/O abundance, log(N/O) $=-1.37\pm0.21$, is indicative of primary nucleosynthesis. The S/O and Ar/O relative abundances, log(S/O)$=-1.88\pm0.10$ and log(Ar/O)$=-2.80\pm0.12$, are both $>$0.3 dex lower than the solar ratios. However, the relative Ar$^{2+}$/S$^{2+}$ abundance is consistent with the solar ratio, suggesting that the relative S-to-Ar abundance does not evolve significantly with redshift. Recent nucleosynthesis models find that a significant amount of S and Ar are produced in Type Ia supernovae, such that the S/O and Ar/O abundances in Q2343-D40 could be the result of predominantly core-collapse supernovae enrichment. Future JWST observations of high-$z$ galaxies will uncover whether S/O and Ar/O are sensitive to the timescales of these different enrichment mechanisms.

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arXiv2024

CECILIA: The Faint Emission Line Spectrum of z~2-3 Star-forming Galaxies

We present the first results from CECILIA, a Cycle 1 JWST NIRSpec/MSA program that uses ultra-deep ~30 hour G235M/F170LP observations to target multiple electron temperature-sensitive auroral lines in the spectra of 33 galaxies at z~1-3. Using a subset of 23 galaxies, we construct two ~600 object-hour composite spectra, both with and without the stellar continuum, and use these to investigate the characteristic rest-optical (5700-8500 Angstrom) spectrum of star-forming galaxies at the peak epoch of cosmic star formation. Emission lines of eight different elements (H, He, N, O, Si, S, Ar, and Ni) are detected, with most of these features observed to be <3% the strength of H-alpha. We report the characteristic strength of three auroral lines ([NII]5756, [SIII]6313, and [OII]7322,7332), as well as other semi-strong and faint emission lines, including forbidden [NiII]7380,7414 and the OI 8449 recombination line, some of which have never before been observed outside of the local universe. Using these measurements, we find T_e[NII]=13630+/-2540 K, representing the first measurement of electron temperature using [NII] in the high-redshift universe. We also see evidence for broad line emission with a FWHM of ~536 km/s; the broad component of H-alpha is 6.01-28.31% the strength of the narrow component and likely arises from star-formation driven outflows. Finally, we briefly comment on the feasibility of obtaining large samples of faint emission lines using JWST in the future.

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arXiv2023

The Circumgalactic Medium of Extreme Emission Line Galaxies at z ~ 2: Resolved Spectroscopy and Radiative Transfer Modeling of Spatially Extended Lyman-alpha Emission in the KBSS-KCWI Survey

The resonantly scattered Lyman-$α$ line illuminates the extended halos of neutral hydrogen in the circumgalactic medium of galaxies. We present integral field Keck Cosmic Web Imager observations of double-peaked, spatially extended Ly$α$ emission in 12 relatively low-mass ($M_{\star} \sim10^9 \, M_{\odot}$) $z\sim2$ galaxies characterized by extreme nebular emission lines. Using individual spaxels and small bins as well as radially binned profiles of larger regions, we find that for most objects in the sample the Ly$α$ blue-to-red peak ratio increases, the peak separation decreases, and the fraction of flux emerging at line center increases with radius. We use new radiative transfer simulations to model each galaxy with a clumpy, multiphase outflow with radially varying outflow velocity, and self-consistently apply the same velocity model to the low ionization interstellar absorption lines. These models reproduce the trends of peak ratio, peak separation and trough depth with radius, and broadly reconcile outflow velocities inferred from Ly$α$ and absorption lines. The galaxies in our sample are well-described by a model in which neutral, outflowing clumps are embedded in a hotter, more highly ionized inter-clump medium (ICM), whose residual neutral content produces absorption at the systemic redshift. The peak ratio, peak separation and trough flux fraction are primarily governed by the line-of-sight component of the outflow velocity, the HI column density, and the residual neutral density in the ICM respectively. Azimuthal asymmetries in the line profile further suggest non-radial gas motions at large radii and variations in the HI column density in the outer halos.

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arXiv2022

Reconciling the Results of the z~2 MOSDEF and KBSS-MOSFIRE Surveys

The combination of the MOSDEF and KBSS-MOSFIRE surveys represents the largest joint investment of Keck/MOSFIRE time to date, with ~3000 galaxies at 1.4<=z<=3.8, roughly half of which are at z~2. MOSDEF is photometric- and spectroscopic-redshift selected with a rest-optical magnitude limit, while KBSS-MOSFIRE is primarily selected based on rest-UV colors and a rest-UV magnitude limit. Analyzing both surveys in a uniform manner with consistent spectral-energy-distribution (SED) models, we find that the MOSDEF z~2 targeted sample has a higher median M_* and redder rest U-V color than the KBSS-MOSFIRE z~2 targeted sample, and a smaller median SED-based SFR and sSFR (SFR(SED) and sSFR(SED)). Specifically, MOSDEF targeted a larger population of red galaxies with U-V and V-J >=1.25, while KBSS-MOSFIRE contains more young galaxies with intense star formation. Despite these differences in the z~2 targeted samples, the subsets of the surveys with multiple emission lines detected and analyzed in previously published work are much more similar. All median host-galaxy properties with the exception of stellar population age -- i.e., M_*, SFR(SED), sSFR(SED), A_V, and UVJ colors -- agree within the uncertainties. Additionally, when uniform emission-line fitting and stellar Balmer absorption correction techniques are applied, there is no significant offset between the two samples in the [OIII]$λ$5008/H$β$ vs. [NII]$λ$6585/H$α$ diagnostic diagram, in contrast to previously-reported discrepancies. We can now combine the MOSDEF and KBSS-MOSFIRE surveys to form the largest z~2 sample with moderate-resolution rest-optical spectra and construct the fundamental scaling relations of star-forming galaxies during this important epoch.

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arXiv2021

Chemical abundance scaling relations for multiple elements in z~2-3 star-forming galaxies

The chemical abundance patterns of gas and stars in galaxies are powerful probes of galaxies' star formation histories and the astrophysics of galaxy assembly but are challenging to measure with confidence in distant galaxies. In this paper, we report the first measurements of the correlation between stellar mass and multiple tracers of chemical enrichment (including O, N, and Fe) in individual z~2-3 galaxies, using a sample of 195 star-forming galaxies from the Keck Baryonic Structure Survey (KBSS). The galaxies' chemical abundances are inferred using photoionization models capable of reconciling high-redshift galaxies' observed extreme rest-UV and rest-optical spectroscopic properties. We find that the stellar mass-O/H relation for our sample is relatively shallow, with moderately large scatter, and is offset ~0.35 dex higher than the corresponding stellar mass-Fe/H relation. The two relations have very similar slopes, indicating a high level of alpha-enhancement -- with O/Fe approximately 2.2 times higher than solar O/Fe -- across two decades in stellar mass. The stellar mass-N/H relation has the steepest slope and largest intrinsic scatter, which likely results from the fact that many z~2 galaxies are observed near or past the transition from "primary" to "secondary" N production and may reflect uncertainties in the astrophysical origin of N. Together, these results suggest that z~2 galaxies are old enough to have seen substantial enrichment from intermediate mass stars, but are still young enough that Type Ia supernovae have not had time to contribute significantly to their enrichment.

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arXiv2021

The KBSS-KCWI Survey: The connection between extended Ly$α$ halos and galaxy azimuthal angle at $z\sim 2-3$

We present the first statistical analysis of kinematically-resolved, spatially-extended Ly$α$ emission around $z = 2-3$ galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies ($z_\mathrm{med} = 2.29$) comprises the subset with typical KCWI integration times of ~5 hours and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high resolution images were used to evaluate the azimuthal dependence of the diffuse Ly$α$ emission with respect to the stellar continuum within projected galactocentric distances of $\lesssim 30$ proper kpc. We introduce cylindrically-projected 2D spectra (CP2D) that map the averaged Ly$α$ spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of Ly$α$ resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended Ly$α$ emission along the galaxy principal axes are statistically indistinguishable, with residual asymmetry of $\le$ 2% ($\sim 2 σ$) of the integrated Ly$α$ emission. The symmetry implies that the Ly$α$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of Ly$α$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak Ly$α$ emission. We speculate that this weak directional dependence of Ly$α$ emission becomes discernible only when the Ly$α$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with Ly$α$ radiative transfer modeling.

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arXiv2020

The Keck Baryonic Structure Survey: Using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at $z \sim 2$

We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ~ 2. Using the spectra of ~ 3000 galaxies with redshifts +/- 0.4 from the Keck Baryonic Structure Survey (KBSS), we assemble a sample of more than 200,000 distinct foreground-background pairs with projected angular separations of 3 - 500 arcsec and spectroscopic redshifts, with <$z_{fg}$> = 2.23 and <$z_{bg}$> = 2.57. The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess Ly$α$ optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 < $D_{tran}$/pkpc < 4000) and line-of-sight velocity. We provide information on the line-of-sight kinematics of H I gas as a function of projected distance $D_{tran}$. We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of H I. The best-fitting model suggests that galaxy-scale outflows with initial velocity $v_{out}$ ~ 600 km/s dominate the kinematics of circumgalactic H I out to $D_{tran}$ ~ 50 kpc, while H I at $D_{tran}$ > 100 kpc is dominated by infall with characteristic $v_{in}$ < $v_c$, where $v_c$ is the circular velocity of the host halo ($M_h$ ~ $10^{12} M_\odot$). Over the impact parameter range 80 < $D_{tran}$/pkpc < 200, the H I line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame Ly$α$ equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond $D_{tran}$ ~ 300 kpc, the line of sight kinematics are dominated by Hubble expansion.

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arXiv2019

Predicting Ly$α$ Emission from Galaxies via Empirical Markers of Production and Escape in the KBSS

Ly$α$ emission is widely used to detect and confirm high-redshift galaxies and characterize the evolution of the intergalactic medium. However, many galaxies do not display Ly$α$ emission in typical spectroscopic observations, and intrinsic Ly$α$-emitters represent a potentially biased set of high-redshift galaxies. In this work, we analyze a set of 703 galaxies at $2\lesssim z\lesssim3$ with both Ly$α$ spectroscopy and measurements of other rest-frame ultraviolet and optical properties in order to develop an empirical model for Ly$α$ emission from galaxies and understand how the probability of Ly$α$ emission depends on other observables. We consider several empirical proxies for the efficiency of Ly$α$ photon production as well as the subsequent escape of these photons through their local interstellar medium. We find that the equivalent width of metal-line absorption and the O3 ratio of rest-frame optical nebular lines are advantageous empirical proxies for Ly$α$ escape and production, respectively. We develop a new quantity, $X_\mathrm{LIS}^\mathrm{O3}$, that combines these two properties into a single predictor of net Ly$α$ emission, which we find describes $\sim$90% of the observed variance in Ly$α$ equivalent width when accounting for our observational uncertainties. We also construct conditional probability distributions demonstrating that galaxy selection based on measurements of galaxy properties yield samples of galaxies with widely varying probabilities of net Ly$α$ emission. The application of the empirical models and probability distributions described here may be used to infer the selection biases of current galaxy surveys and evaluate the significance of high-redshift Ly$α$ (non-)detections in studies of reionization and the intergalactic medium.

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arXiv2019

The Column Density, Kinematics, and Thermal State of Metal-Bearing Gas within the Virial Radius of z~2 Star-Forming Galaxies in the Keck Baryonic Structure Survey

We present results from the Keck Baryonic Structure Survey (KBSS) including the first detailed measurements of the column densities, kinematics, and internal energy of metal-bearing gas within the virial radius (35-100 physical kpc) of eight ~$L^*$ galaxies at $z\sim2$. From our full sample of 130 metal-bearing absorbers, we infer that halo gas is kinematically complex when viewed in singly, doubly, and triply ionized species. Broad OVI and CIV absorbers are detected at similar velocities to the lower-ionization gas but with very different kinematic structure indicating that the circumgalactic medium (CGM) is multi-phase. There is a high covering fraction of metal-bearing gas within 100 kpc including highly ionized gas such as OVI; however, observations of a single galaxy probed by a lensed background QSO suggest the size of metal-bearing clouds is small (<400 pc for all but the OVI-bearing gas). The mass in metals found within the halo is substantial, equivalent to $\gtrsim$ 25% of the metal mass within the interstellar medium. The gas kinematics unambiguously show that 70% of galaxies with detected metal absorption have some unbound metal-enriched gas, suggesting galactic winds may commonly eject gas from halos at $z\sim2$. Significant thermal broadening is detected in CGM absorbers which dominates the internal energy of the gas. 40% of the detected gas has temperatures in the range $10^{4.5-5.5}$ K where cooling times are short, suggesting the CGM is dynamic, with constant heating and/or cooling to produce this short-lived thermal phase.

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arXiv2018

Measuring the Physical Conditions in High-Redshift Star-Forming Galaxies: Insights from KBSS-MOSFIRE

We use photoionization models designed to reconcile the joint rest-UV-optical spectra of high-z star-forming galaxies to self-consistently infer the gas chemistry and nebular ionization and excitation conditions for ~150 galaxies from the Keck Baryonic Structure Survey (KBSS), using only observations of their rest-optical nebular spectra. We find that the majority of z~2-3 KBSS galaxies are moderately O-rich, with an interquartile range in 12+log(O/H)=8.29-8.56, and have significantly sub-solar Fe enrichment, with an interquartile range of [Fe/H]=[-0.79,-0.53], contributing additional evidence in favor of super-solar O/Fe in high-z galaxies. Model-inferred ionization parameter and N/O are strongly correlated with common strong-line indices (such as O32 and N2O2), with the latter exhibiting similar behavior to local extragalactic HII regions. In contrast, diagnostics commonly used for measuring gas-phase O/H (such as N2 and O3N2) show relatively large scatter with the overall amount of oxygen present in the gas and behave differently than observed at z~0. We provide a new calibration for using R23 to measure O/H in typical high-z galaxies, although it is most useful for relatively O-rich galaxies; combining O32 and R23 does not yield a more effective calibration. Finally, we consider implications for the intrinsic correlations between physical conditions across the galaxy sample and find that N/O varies with O/H in high-z galaxies in a manner almost identical to local HII regions. However, we do not find a strong anti-correlation between ionization parameter and metallicity (O/H or Fe/H) in high-z galaxies, which is one of the principal bases for using strong-line ratios to infer oxygen abundance.

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arXiv2018

Dust Attenuation, Star Formation, and Metallicity in z ~ 2-3 Galaxies from KBSS-MOSFIRE

We present a detailed analysis of 317 $2.0 \leq z \leq 2.7$ star-forming galaxies from the Keck Baryonic Structure Survey (KBSS). Using complementary spectroscopic observations with Keck/LRIS and Keck/MOSFIRE, as well as spectral energy distribution (SED) fits to broadband photometry, we examine the joint rest-UV and rest-optical properties of the same galaxies, including stellar and nebular dust attenuation, metallicity, and star formation rate (SFR). The inferred parameters of the stellar population (reddening, age, SFR, and stellar mass) are strongly dependent on the details of the assumed stellar population model and the shape of the attenuation curve. Nebular reddening is generally larger than continuum reddening, but with large scatter. Compared to local galaxies, high-redshift galaxies have lower gas-phase metallicities (and/or higher nebular excitation) at fixed nebular reddening, and higher nebular reddening at fixed stellar mass, consistent with gas fractions that increase with redshift. We find that continuum reddening is correlated with $12+\log(\mathrm{O}/\mathrm{H})_{\mathrm{O3N2}}$ at $3.0σ$ significance, whereas nebular reddening is correlated with only $1.1σ$ significance. This may reflect the dependence of both continuum reddening and O3N2 on the shape of the ionizing radiation field produced by the massive stars. Finally, we show that H$α$-based and SED-based estimates of SFR exhibit significant scatter relative to one another, and agree on average only for particular combinations of spectral synthesis models and attenuation curves. We find that the SMC extinction curve predicts consistent SFRs if we assume the sub-solar ($0.14Z_{\odot}$) binary star models that are favored for high-redshift galaxies.

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arXiv2018

Imaging Spectroscopy of Ionized Gaseous Nebulae around Optically Faint AGN at Redshift z ~ 2

We present Keck/OSIRIS laser guide-star assisted adaptive optics (LGSAO) integral field spectroscopy of [O III] 5007 Angstrom nebular emission from twelve galaxies hosting optically faint (R = 20 - 25; nuLnu ~ 10^44 -10^46 erg/s) active galactic nuclei (AGN) at redshift z ~ 2 - 3. In combination with deep Hubble Space Telescope Wide Field Camera 3 rest-frame optical imaging, Keck/MOSFIRE rest-optical spectroscopy, and Keck/KCWI rest-UV integral field spectroscopy we demonstrate that both the continuum and emission-line structure of these sources exhibit a wide range of morphologies from compact isolated point sources to double-AGN merging systems with extensive ~ 50 kpc tidal tails. One of the twelve galaxies previously known to exhibit a proximate damped LyA system coincident in redshift with the galaxy shows evidence for both an extended [O III] narrow-line emission region and spatially offset LyA emission (with morphologically distinct blueshifted and redshifted components) indicative of large scale gas flows photoionized by the central AGN. We do not find widespread evidence of star formation in the host galaxies surrounding these AGN; the [O III] velocity dispersions tend to be high (sigma = 100 - 500 km/s), the continuum morphologies are much more compact than a mass-matched star forming comparison sample, and the diagnostic nebular emission line ratios are dominated by an AGN-like ionizing spectrum. The sample is most consistent with a population of AGN that radiate at approximately their Eddington limit and photoionize extended [O III] nebulae whose characteristic sizes scale approximately as the square root of the AGN luminosity.

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