arXiv subjects

Naveen A. Reddy

Publications and source records attributed to Naveen A. Reddy.

At least 91 records · Page 5Linked to original sources

arXiv2021

The MOSDEF Survey: Environmental dependence of the gas-phase metallicity of galaxies at $1.4 \leq z \leq 2.6$

Using the near-IR spectroscopy of the MOSFIRE Deep Evolution Field (MOSDEF) survey, we investigate the role of local environment in the gas-phase metallicity of galaxies. The local environment measurements are derived from accurate and uniformly calculated photometric redshifts with well-calibrated probability distributions. Based on rest-frame optical emission lines, [NII]$\lambda6584$ and H$α$, we measure gas-phase oxygen abundance of 167 galaxies at $1.37\leq z\leq1.7$ and 303 galaxies at $2.09\leq z\leq2.61$, located in diverse environments. We find that at $z\sim1.5$, the average metallicity of galaxies in overdensities with $M_*\sim10^{9.8}M_\odot, 10^{10.2}M_\odot$ and $10^{10.8}M_\odot$ is higher relative to their field counterparts by $0.094\pm0.051$, $0.068\pm0.028$ and $0.052\pm0.043$ dex, respectively. However, this metallicity enhancement does not exist at higher redshift, $z\sim2.3$, where, compared to the field galaxies, we find $0.056\pm0.043$, $0.056\pm0.028$ and $0.096\pm 0.034$ dex lower metallicity for galaxies in overdense environments with $M_*\sim10^{9.8}M_\odot, 10^{10.2}M_\odot$ and $10^{10.7}M_\odot$, respectively. Our results suggest that, at $1.37\leq z\leq2.61$, the variation of mass-metallicity relation with local environment is small ($<0.1$dex), and reverses at $z\sim2$. Our results support the hypothesis that, at the early stages of cluster formation, owing to efficient gas cooling, galaxies residing in overdensities host a higher fraction of pristine gas with prominent primordial gas accretion, which lowers their gas-phase metallicity compared to their coeval field galaxies. However, as the Universe evolves to lower redshifts ($z\lesssim2$), shock-heated gas in overdensities cannot cool down efficiently, and galaxies become metal-rich rapidly due to the suppression of pristine gas inflow and re-accretion of metal-enriched outflows in overdensities.

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arXiv2020

The MOSDEF Survey: Differences in SFR and Metallicity for Morphologically-Selected Mergers at z~2

We study the properties of 55 morphologically-identified merging galaxy systems at z~2. These systems are flagged as mergers based on features such as tidal tails, double nuclei, and asymmetry. Our sample is drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey, along with a control sample of isolated galaxies at the same redshift. We consider the relationships between stellar mass, star formation rate (SFR), and gas-phase metallicity for both merging and non-merging systems. In the local universe, merging systems are characterized by an elevated SFR and depressed metallicity compared to isolated systems at a given mass. Our results indicate SFR enhancement and metallicity deficit for merging systems relative to non-merging systems for a fixed stellar mass at z~2, though larger samples are required to establish these preliminary results with higher statistical significance. In future work, it will be important to establish if the enhanced SFR and depressed metallicity in high-redshift mergers deviate from the "fundamental metallicity relation," as is observed in mergers in the local universe, and therefore shed light on gas flows during galaxy interactions.

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arXiv2020

The First Robust Constraints on the Relationship Between Dust-to-Gas Ratio and Metallicity in Luminous Star-forming Galaxies at High Redshift

We present rest-optical spectroscopic properties of a sample of four galaxies in the Atacama Large Millimeter/submillimeter Array Hubble Ultra Deep Field (ALMA HUDF). These galaxies span the redshift range $1.41 \leq z \leq 2.54$ and the stellar mass range $10.36\leq\log(M_*/{\rm M}_{\odot})\leq10.91$. They have existing far-infrared and radio measurements of dust-continuum and molecular gas emission from which bolometric star-formation rates (SFRs), dust masses, and molecular gas masses have been estimated. We use new $H$- and $K$-band near-infrared spectra from the Keck/MOSFIRE spectrograph to estimate SFRs from dust-corrected H$α$ emission (SFR(H$α$)) and gas-phase oxygen abundances from the ratio [NII]$λ6584$/H$α$. We find that the dust-corrected SFR(H$α$) is systematically lower than the bolometric SFR by a factor of several, and measure gas-phase oxygen abundances in a narrow range, $12+\log(\mbox{O/H})=8.59-8.69$ ($0.8-1.0\: (\mbox{O/H})_{\odot}$). Relative to a large $z\sim 2$ comparison sample from the MOSDEF survey, the ALMA HUDF galaxies scatter roughly symmetrically around the best-fit linear mass-metallicity relation, providing tentative evidence for a flattening in the SFR dependence of metallicity at high stellar mass. Combining oxygen abundances with estimates of dust and molecular gas masses, we show that there is no significant evolution in the normalization of the dust-to-gas ratio DGR vs. metallicity relation from $z\sim0$ to $z\sim2$. This result is consistent with some semi-analytic models and cosmological simulations describing the evolution of dust in galaxies. Tracing the actual form of the DGR vs. metallicity relation at high redshift now requires combined measurements of dust, gas, and metallicity over a significantly wider range in metallicity.

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arXiv2020

The MOSDEF-LRIS Survey: The Connection Between Massive Stars and Ionized Gas in Individual Galaxies at $z\sim2$

We present constraints on the massive star and ionized gas properties for a sample of 62 star-forming galaxies at $z\sim2.3$. Using BPASS stellar population models, we fit the rest-UV spectra of galaxies in our sample to estimate age and stellar metallicity which, in turn, determine the ionizing spectrum. In addition to the median properties of well-defined subsets of our sample, we derive the ages and stellar metallicities for 30 high-SNR individual galaxies -- the largest sample of individual galaxies at high redshift with such measurements. Most galaxies in this high-SNR subsample have stellar metallicities of $0.001<Z_*<0.004$. We then use Cloudy+BPASS photoionization models to match observed rest-optical line ratios and infer nebular properties. Our high-SNR subsample is characterized by a median ionization parameter and oxygen abundance, respectively, of $\log(U)_{\textrm{med}}=-2.98\pm0.25$ and $12+\log(\textrm{O/H})_{\textrm{med}}=8.48\pm0.11$. Accordingly, we find that all galaxies in our sample show evidence for $α$-enhancement. In addition, based on inferred $\log(U)$ and $12+\log(\textrm{O/H})$ values, we find that the local relationship between ionization parameter and metallicity applies at $z\sim2$. Finally, we find that the high-redshift galaxies most offset from the local excitation sequence in the BPT diagram are the most $α$-enhanced. This trend suggests that $α$-enhancement resulting in a harder ionizing spectrum at fixed oxygen abundance is a significant driver of the high-redshift galaxy offset on the BPT diagram relative to local systems. The ubiquity of $α$-enhancement among $z\sim2.3$ star-forming galaxies indicates important differences between high-redshift and local galaxies that must be accounted for in order to derive physical properties at high redshift.

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arXiv2020

The MOSDEF Survey: Neon as a Probe of ISM Physical Conditions at High Redshift

We present results on the properties of neon emission in $z\sim2$ star-forming galaxies drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Doubly-ionized neon ([NeIII]3869) is detected at $\geq3σ$ in 61 galaxies, representing $\sim$25% of the MOSDEF sample with H$α$, H$β$, and [OIII]$5007$ detections at similar redshifts. We consider the neon emission-line properties of both individual galaxies with [NeIII]3869 detections and composite $z\sim2$ spectra binned by stellar mass. With no requirement of [NeIII]3869 detection, the latter provide a more representative picture of neon emission-line properties in the MOSDEF sample. The [NeIII]3869/[OII]3727 ratio (Ne3O2) is anti-correlated with stellar mass in $z\sim2$ galaxies, as expected based on the mass-metallicity relation. It is also positively correlated with the [OIII]$5007$/[OII]$3727$ ratio (O32), but $z\sim2$ line ratios are offset towards higher Ne3O2 at fixed O32, compared with both local star-forming galaxies and individual H~II regions. Despite the offset towards higher Ne3O2 at fixed O32 at $z\sim2$, biases in inferred Ne3O2-based metallicity are small. Accordingly, Ne3O2 may serve as an important metallicity indicator deep into the reionization epoch. Analyzing additional rest-optical line ratios including [NeIII]$3869$/[OIII]$5007$ (Ne3O3) and [OIII]$5007$/H$β$ (O3H$β$), we conclude that the nebular emission-line ratios of $z\sim2$ star-forming galaxies suggest a harder ionizing spectrum (lower stellar metallicity, i.e., Fe/H) at fixed gas-phase oxygen abundance, compared to systems at $z\sim0$. These new results based on neon lend support to the physical picture painted by oxygen, nitrogen, hydrogen, and sulfur emission, of an ionized ISM in high-redshift star-forming galaxies irradiated by chemically young, $α$-enhanced massive stars.

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arXiv2020

The MOSDEF Survey: The First Direct Measurements of the Nebular Dust Attenuation Curve at High Redshift

We use a sample of 532 star-forming galaxies at redshifts $z\sim 1.4-2.6$ with deep rest-frame optical spectra from the MOSFIRE Deep Evolution Field (MOSDEF) survey to place the first constraints on the nebular attenuation curve at high redshift. Based on the first five low-order Balmer emission lines detected in the composite spectra of these galaxies (${\rm Hα}$ through ${\rm Hε}$), we derive a nebular attenuation curve that is similar in shape to that of the Galactic extinction curve, suggesting that the dust covering fraction and absorption/scattering properties along the lines-of-sight to massive stars at high redshift are similar to those of the average Milky Way sightline. The curve derived here implies nebular reddening values that are on average systematically larger than those derived for the stellar continuum. In the context of stellar population synthesis models that include the effects of stellar multiplicity, the difference in reddening of the nebular lines and stellar continuum may imply molecular cloud crossing timescales that are a factor of $\gtrsim 3\times$ longer than those inferred for local molecular clouds, star-formation rates that are constant or increasing with time such that newly-formed and dustier OB associations always dominate the ionizing flux, and/or that the dust responsible for reddening the nebular emission may be associated with non-molecular (i.e., ionized and neutral) phases of the ISM. Our analysis points to a variety of investigations of the nebular attenuation curve that will be enabled with the next generation of ground- and space-based facilities.

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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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arXiv2020

The MOSDEF Survey: Calibrating the relationship between H$α$ star-formation rate and radio continuum luminosity at $1.4 < z < 2.6$

The observed empirical relation between the star-formation rates (SFR) of low-redshift galaxies and their radio continuum luminosity offers a potential means of measuring SFR in high redshift galaxies that is unaffected by dust obscuration. In this study, we make the first test for redshift evolution in the SFR-radio continuum relation at high redshift using dust-corrected H$α$ SFR. Our sample consists of 178 galaxies from the MOSFIRE Deep Evolution Field (MOSDEF) Survey at $1.4 < z < 2.6$ with rest-frame optical spectroscopy and deep 1.5 GHz radio continuum observations from the Karl G. Jansky Very Large Array (VLA) GOODS North field. Using a stacking analysis we compare the observed radio continuum luminosities with those predicted from the dust-corrected H$α$ SFR assuming a range of $z\sim0$ relations. We find no evidence for a systematic evolution with redshift, when stacking the radio continuum as a function of dust-corrected H$α$ SFR and when stacking both optical spectroscopy and radio continuum as a function of stellar mass. We conclude that locally calibrated relations between SFR and radio continuum luminosity remain valid out to $z\sim 2$.

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arXiv2020

The MOSDEF-LRIS Survey: The Interplay Between Massive Stars and Ionized Gas in High-Redshift Star-Forming Galaxies

We present a joint analysis of rest-UV and rest-optical spectra obtained using Keck/LRIS and Keck/MOSFIRE for a sample of 62 star-forming galaxies at $z\sim2.3$. We divide our sample into 2 bins based on their location in the [OIII]/Hb vs. [NII]/Ha BPT diagram, and perform the first differential study of the rest-UV properties of massive ionizing stars as a function of rest-optical emission-line ratios. Fitting BPASS stellar population synthesis models, including nebular continuum emission, to our rest-UV spectra, we find that high-redshift galaxies offset towards higher [OIII]/Hb and [NII]/Ha have younger ages (log(Age/yr)=$7.20^{+0.57}_{-0.20}$) and lower stellar metallicities ($Z_*=0.0010^{+0.0011}_{-0.0003}$) resulting in a harder ionizing spectrum, compared to the galaxies in our sample that lie on the local BPT star-forming sequence (log(Age/yr)=$8.57^{+0.88}_{-0.84}$, $Z_*=0.0019\pm0.0006$). Additionally, we find that the offset galaxies have an ionization parameter of log(U)=$-3.04^{+0.06}_{-0.11}$ and nebular metallicity of 12+log(O/H)=$8.40^{+0.06}_{-0.07}$, and the non-offset galaxies have log(U)=$-3.11\pm0.08$ and 12+log(O/H)=$8.30^{+0.05}_{-0.06}$. The stellar and nebular metallicities derived for our sample imply that the galaxies offset from the local BPT relation are more alpha-enhanced ($7.28^{+2.52}_{-2.82}$ O/Fe$_\odot$) compared to those consistent with the local sequence ($3.04^{+0.95}_{-0.54}$ O/Fe$_\odot$). However, even galaxies that are entirely consistent with the local nebular excitation sequence appear to be alpha-enhanced -- in contrast with typical local systems. Such differences must be considered when estimating gas-phase O/H at high redshift based on strong emission-line ratios. Specifically, a similarity in the location of high-redshift and local galaxies in the BPT diagram may not be indicative of a similarity in their physical properties.

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arXiv2020

The MOSDEF Survey: Kinematic and Structural Evolution of Star-Forming Galaxies at $1.4\leq z\leq 3.8$

We present ionized gas kinematics for 681 galaxies at $z\sim 1.4-3.8$ from the MOSFIRE Deep Evolution Field survey, measured using models which account for random galaxy-slit misalignments together with structural parameters derived from CANDELS Hubble Space Telescope (HST) imaging. Kinematics and sizes are used to derive dynamical masses. Baryonic masses are estimated from stellar masses and inferred gas masses from dust-corrected star formation rates (SFRs) and the Kennicutt-Schmidt relation. We measure resolved rotation for 105 galaxies. For the remaining 576 galaxies we use models based on HST imaging structural parameters together with integrated velocity dispersions and baryonic masses to statistically constrain the median ratio of intrinsic ordered to disordered motion, $V/σ_{V,0}$. We find that $V/σ_{V,0}$ increases with increasing stellar mass and decreasing specific SFR (sSFR). These trends may reflect marginal disk stability, where systems with higher gas fractions have thicker disks. For galaxies with detected rotation we assess trends between their kinematics and mass, sSFR, and baryon surface density ($Σ_{\mathrm{bar},e}$). Intrinsic dispersion correlates most with $Σ_{\mathrm{bar},e}$ and velocity correlates most with mass. By comparing dynamical and baryonic masses, we find that galaxies at $z\sim 1.4-3.8$ are baryon dominated within their effective radii ($R_E$), with Mdyn/Mbaryon increasing over time. The inferred baryon fractions within $R_E$, $f_{\mathrm{bar}}$, decrease over time, even at fixed mass, size, or surface density. At fixed redshift, $f_{\mathrm{bar}}$ does not appear to vary with stellar mass but increases with decreasing $R_E$ and increasing $Σ_{\mathrm{bar},e}$. For galaxies at $z\geq2$, the median inferred baryon fractions generally exceed 100%. We discuss possible explanations and future avenues to resolve this tension.

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arXiv2020

The Redshift Evolution of Rest-UV Spectroscopic Properties to z~5

We perform a comprehensive analysis of the redshift evolution of the rest-UV spectra of star-forming galaxies out to z~5. We combine new z~5 measurements of HI Ly$α$ and low- and high-ionization interstellar metal absorption features with comparable measurements at z~2-4. We measure the equivalent widths of interstellar absorption features using stacked spectra in bins of Ly$α$ equivalent width, performing corrections to Ly$α$ strengths based on a model for the transmission of the intergalactic medium. We find a strong correlation between decreasing low-ionization absorption strength and increasing Ly$α$ emission strength over the redshift range z~2-5, suggesting that both of these quantities are fundamentally linked to neutral gas covering fraction. At the highest Ly$α$ equivalent widths, we observe evolution at $z\sim5$ towards greater Ly$α$ emission strength at fixed low-ionization absorption strength. If we interpret the non-evolving relationship of Ly$α$ emission strength and low-ionization line strength at z~2-4 as primarily reflecting the radiative transfer of Ly$α$ photons, this evolution at z~5 suggests a higher intrinsic production rate of Ly$α$ photons than at lower redshift. Our conclusion is supported by the joint evolution of the relationships among Ly$α$ emission strength, interstellar absorption strength, and dust reddening. We perform additional analysis in bins of stellar mass, star-formation rate, UV luminosity, and age, examining how the relationships between galaxy properties and Ly$α$ emission evolve towards higher redshift. We conclude that increasing intrinsic Ly$α$ photon production and strong detection of nebular CIV emission (signaling lower metallicity) at z~5 indicate an elevated ionized photon production efficiency ($ξ_{\rm ion}$).

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arXiv2019

The MOSDEF Survey: [SIII] as a New Probe of Evolving ISM Conditions

We present measurements of [SIII]$λλ$9069,9531 for a sample of $z\sim1.5$ star-forming galaxies, the first sample with measurements of these lines at z>0.1. We employ the line ratio S$_{32}$$\equiv$[SIII]$λλ$9069,9531/[SII]$λλ$6716,6731 as a novel probe of evolving ISM conditions. Since this ratio includes the low-ionization line [SII], it is crucial that the effects of diffuse ionized gas (DIG) on emission-line ratios be accounted for in $z\sim0$ integrated galaxy spectra, or else that comparisons be made to samples of local HII regions in which DIG emission is not present. We find that S$_{32}$ decreases with increasing stellar mass at both $z\sim1.5$ and $z\sim0$, but that the dependence is weak suggesting S$_{32}$ has a very shallow anticorrelation with metallicity, in contrast with O$_{32}$ that displays a strong metallicity dependence. As a result, S$_{32}$ only mildly evolves with redshift at fixed stellar mass. The $z\sim1.5$ sample is systematicallty offset towards lower S$_{32}$ and higher [SII]/H$α$ at fixed [OIII]/H$β$ relative to $z=0$ HII regions. By comparing to photoionization model grids, we find that such trends can be explained by a scenario in which the ionizing spectrum is harder at fixed O/H with increasing redshift, but are inconsistent with an increase in ionization parameter at fixed O/H. This analysis demonstrates the advantages of expanding beyond the strongest rest-optical lines for evolutionary studies, and the particular utility of [SIII] for characterizing evolving ISM conditions and stellar compositions. These measurements provide a basis for estimating [SIII] line strengths for high-redshift galaxies, a line that the James Webb Space Telescope will measure out to z~5.5.

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arXiv2019

The MOSDEF Survey: Direct-Method Metallicities and ISM Conditions at $z\sim1.5-3.5$

We present detections of [OIII]$λ$4363 and direct-method metallicities for star-forming galaxies at $z=1.7-3.6$. We combine new measurements from the MOSFIRE Deep Evolution Field (MOSDEF) survey with literature sources to construct a sample of 18 galaxies with direct-method metallicities at $z>1$, spanning $7.5<1$2+log(O/H$)<8.2$ and log(M$_*$/M$_{\odot})=7-10$. We find that strong-line calibrations based on local analogs of high-redshift galaxies reliably reproduce the metallicity of the $z>1$ sample on average. We construct the first mass-metallicity relation at $z>1$ based purely on direct-method O/H, finding a slope that is consistent with strong-line results. Direct-method O/H evolves by $\lesssim0.1$ dex at fixed M$_*$ and SFR from $z\sim0-2.2$. We employ photoionization models to constrain the ionization parameter and ionizing spectrum in the high-redshift sample. Stellar models with super-solar O/Fe and binary evolution of massive stars are required to reproduce the observed strong-line ratios. We find that the $z>1$ sample falls on the $z\sim0$ relation between ionization parameter and O/H, suggesting no evolution of this relation from $z\sim0$ to $z\sim2$. These results suggest that the offset of the strong-line ratios of this sample from local excitation sequences is driven primarily by a harder ionizing spectrum at fixed nebular metallicity compared to what is typical at $z\sim0$, naturally explained by super-solar O/Fe at high redshift caused by rapid formation timescales. Given the extreme nature of our $z>1$ sample, the implications for representative $z\sim2$ galaxy samples at $\sim10^{10}$ M$_{\odot}$ are unclear, but similarities to $z>6$ galaxies suggest that these conclusions can be extended to galaxies in the epoch of reionization.

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arXiv2019

The MOSDEF Survey: The Metallicity Dependence of X-ray Binary Populations at $z\sim2$

Population synthesis models predict that high-mass X-ray binary (HMXB) populations produced in low metallicity environments should be more X-ray luminous, a trend supported by studies of nearby galaxies. This trend may be responsible for the observed increase of the X-ray luminosity ($L_{\mathrm{X}}$) per star formation rate (SFR) with redshift due to the decrease of metallicity ($Z$) at fixed stellar mass as a function of redshift. To test this hypothesis, we use a sample of 79 $z\sim2$ star-forming galaxies with oxygen abundance measurements from the MOSDEF survey, which obtained rest-frame optical spectra for $\sim1500$ galaxies in the CANDELS fields at $1.37<z<3.80$. Using Chandra data from the AEGIS-X Deep, Deep Field North, and Deep Field South surveys, we stack the X-ray data at the galaxy locations in bins of redshift and $Z$ because the galaxies are too faint to be individually detected. In agreement with previous studies, the average $L_{\mathrm{X}}$/SFR of our $z\sim2$ galaxy sample is enhanced by $\approx0.4-0.8$ dex relative to local HMXB $L_{\mathrm{X}}$-SFR scaling relations. Splitting our sample by $Z$, we find that $L_{\mathrm{X}}$/SFR and $Z$ are anti-correlated with 97% confidence. This observed $Z$ dependence for HMXB-dominated galaxies is consistent both with the local $L_{\mathrm{X}}$-SFR-$Z$ relation and a subset of population synthesis models. Although the statistical significance of the observed trends is weak due to the low X-ray statistics, these results constitute the first direct evidence connecting the redshift evolution of $L_{\mathrm{X}}$/SFR and the $Z$ dependence of HMXBs.

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arXiv2019

The MOSDEF Survey: Sulfur Emission-line Ratios Provide New Insights into Evolving ISM Conditions at High Redshift

We present results on the emission-line properties of 1.3<=z<=2.7 galaxies drawn from the complete MOSFIRE Deep Evolution Field (MOSDEF) survey. Specifically, we use observations of the emission-line diagnostic diagram of [OIII]5007/Hb vs. [SII]6717,6731/Ha, i.e., the "[SII] BPT diagram," to gain insight into the physical properties of high-redshift star-forming regions. High-redshift MOSDEF galaxies are offset towards lower [SII]6717,6731/Ha at fixed [OIII]5007/Hb, relative to local galaxies from the Sloan Digital Sky Survey (SDSS). Furthermore, at fixed [OIII]5007/Hb, local SDSS galaxies follow a trend of decreasing [SII]6717,6731/Ha as the surface density of star formation (Sigma_SFR) increases. We explain this trend in terms of the decreasing fractional contribution from diffuse ionized gas (f_DIG) as Sigma_SFR increases in galaxies, which causes galaxy-integrated line ratios to shift towards the locus of pure HII-region emission. The z~0 relationship between f_DIG and Sigma_SFR implies that high-redshift galaxies have lower f_DIG values than typical local systems, given their significantly higher typical Sigma_SFR. When an appropriate low-redshift benchmark with zero or minimal f_DIG is used, high-redshift MOSDEF galaxies appear offset towards higher [SII]6717,6731/Ha and/or [OIII]5007/Hb. The joint shifts of high-redshift galaxies in the [SII] and [NII] BPT diagrams are best explained in terms of the harder spectra ionizing their star-forming regions at fixed nebular oxygen abundance (expected for chemically-young galaxies), as opposed to large variations in N/O ratios or higher ionization parameters. The evolving mixture of HII regions and DIG is an essential ingredient to our description of the ISM over cosmic time.

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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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arXiv2019

The MOSDEF Survey: No Significant Enhancement in Star Formation or Deficit in Metallicity in Merging Galaxy Pairs at 1.5<=z<=3.5

We study the properties of 30 spectroscopically-identified pairs of galaxies observed during the peak epoch of star formation in the universe. These systems are drawn from the MOSFIRE Deep Evolution Field (MOSDEF) Survey at $1.4 \leq z \leq 3.8$, and are interpreted as early-stage galaxy mergers. Galaxy pairs in our sample are identified as two objects whose spectra were collected on the same Keck/MOSFIRE spectroscopic slit. Accordingly, all pairs in the sample have projected separations $R_{\rm proj}\leq 60$ kpc. The velocity separation for pairs was required to be $Δv \leq 500 \mbox{ km s}^{-1}$, which is a standard threshold for defining interacting galaxy pairs at low redshift. Stellar mass ratios in our sample range from 1.1 to 550, with 12 ratios closer than or equal to 3:1, the common definition of a "major merger." Studies of merging pairs in the local universe indicate anenhancement in star-formation activity and deficit in gas-phase oxygen abundance relative to isolated galaxies of the same mass. We compare the MOSDEF pairs sample to a control sample of isolated galaxies at the same redshift, finding no measurable SFR enhancement or metallicity deficit at fixed stellar mass for the pairs sample. The lack of significant difference between the average properties of pairs and control samples appears in contrast to results from low-redshift studies, although the small sample size and lower signal-to-noise of the high-redshift data limit definitive conclusions on redshift evolution. These results are consistent with some theoretical works suggesting a reduced differential effect of pre-coalescence mergers on galaxy properties at high redshift -- specifically that pre-coalescence mergers do not drive strong starbursts.

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arXiv2018

The MOSDEF Survey: Significant Evolution in the Rest-Frame Optical Emission Line Equivalent Widths of Star-Forming Galaxies at z=1.4-3.8

We use extensive spectroscopy from the MOSFIRE Deep Evolution Field (MOSDEF) survey to investigate the relationships between rest-frame optical emission line equivalent widths ($W$) and a number of galaxy and ISM characteristics for a sample of $1134$ star-forming galaxies at redshifts $1.4\lesssim z\lesssim 3.8$. We examine how the equivalent widths of [OII]$λλ3727, 3730$, H$β$, [OIII]$λλ4960, 5008$, [OIII]$+$H$β$, H$α$, and H$α$+[NII]$λλ6550, 6585$, depend on stellar mass, UV slope, age, star-formation rate (SFR) and specific SFR (sSFR), ionization parameter and excitation conditions (O32 and [OIII]/H$β$), gas-phase metallicity, and ionizing photon production efficiency ($ξ_{\rm ion}$). The trend of increasing $W$ with decreasing stellar mass is strongest for [OIII] (and [OIII]+H$β$). More generally, the equivalent widths of all the lines increase with redshift at a fixed stellar mass or fixed gas-phase metallicity, suggesting that high equivalent width galaxies are common at high redshift. This redshift evolution in equivalent widths can be explained by the increase in SFR and decrease in metallicity with redshift at a fixed stellar mass. Consequently, the dependence of $W$ on sSFR is largely invariant with redshift, particularly when examined for galaxies of a given metallicity. Our results show that high equivalent width galaxies, specifically those with high $W({\rm [OIII]})$, have low stellar masses, blue UV slopes, young ages, high sSFRs, ISM line ratios indicative of high ionization parameters, high $ξ_{\rm ion}$, and low metallicities. As these characteristics are often attributed to galaxies with high ionizing escape fractions, galaxies with high $W$ are likely candidates for the population that dominates cosmic reionization.

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