arXiv subjects

Brian Siana

Publications and source records attributed to Brian Siana.

At least 37 records · Page 2Linked to original sources

arXiv2023

The Direct-Method Oxygen Abundance of Typical Dwarf Galaxies at Cosmic High-Noon

We present a Keck/MOSFIRE, rest-optical, composite spectrum of 16 typical, gravitationally-lensed, star-forming, dwarf galaxies at $1.7 \lesssim z \lesssim 2.6$ ($z_{\rm{mean}}=2.30$), all chosen independent of emission-line strength. These galaxies have a median stellar mass of $\log(M_\ast/\rm{M_\odot})_{\rm{med}} = 8.29^{+0.51}_{-0.43}$ and a median star formation rate of $\rm{SFR_{Hα}^{med} = 2.25^{+2.15}_{-1.26}\ M_\odot\ yr^{-1}}$. We measure the faint, electron-temperature-sensitive, [O III] $λ$4363 emission line at $2.5σ$ ($4.1σ$) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of $12+\log(\rm{O/H})_{\rm{direct}}=7.88^{+0.25}_{-0.22}$ ($0.15^{+0.12}_{-0.06}\ \rm{Z_\odot}$). We investigate the applicability at high-$z$ of locally-calibrated, oxygen-based, strong-line metallicity relations, finding that the local reference calibrations of arXiv:1805.08224 best reproduce ($\lesssim 0.12$ dex) our composite metallicity at fixed strong-line ratio. At fixed $M_\ast$, our composite is well-represented by the $z \sim 2.3$ direct-method stellar mass$\,-\,$gas-phase metallicity relation (MZR) of arXiv:1907.00013. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixed $M_\ast$ and SFR, of the locally-defined fundamental metallicity relation. We measure the doublet ratio [O II] $λ$3729/[O II] $\lambda3726 = 1.56 \pm 0.32$ ($1.51 \pm 0.12$) and a corresponding electron density of $n_e = 1^{+215}_{-0}\ \rm{cm^{-3}}$ ($n_e = 1^{+74}_{-0}\ \rm{cm^{-3}}$) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies at $z \sim 2$.

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arXiv2022

The MOSDEF Survey: Probing Resolved Stellar Populations at $z\sim2$ Using a New Bayesian-defined Morphology Metric Called Patchiness

We define a new morphology metric called "patchiness" ($P$) that is sensitive to deviations from the average of a resolved distribution, does not require the galaxy center to be defined, and can be used on the spatially-resolved distribution of any galaxy property. While the patchiness metric has a broad range of applications, we demonstrate its utility by investigating the distribution of dust in the interstellar medium of 310 star-forming galaxies at spectroscopic redshifts $1.36<z<1.66$ observed by the MOSFIRE Deep Evolution Field (MOSDEF) survey. The stellar continuum reddening distribution, derived from high-resolution multi-waveband CANDELS/3D-HST imaging, is quantified using the patchiness, Gini, and $M_{20}$ coefficients. We find that the reddening maps of high-mass galaxies, which are dustier and more metal-rich on average, tend to exhibit patchier distributions (high $P$) with the reddest components concentrated within a single region (low $M_{20}$). Our results support a picture where dust is uniformly distributed in low-mass galaxies ($\lesssim$10$^{10}$ $M_\odot$), implying efficient mixing of dust throughout the interstellar medium. On the other hand, the dust distribution is patchier in high-mass galaxies ($\gtrsim$10$^{10}$ $M_\odot$). Dust is concentrated near regions of active star formation and dust mixing timescales are expected to be longer in high-mass galaxies, such that the outskirt regions of these physically larger galaxies remain relatively unenriched. This study presents direct evidence for patchy dust distributions on scales of a few kpc in high-redshift galaxies, which previously has only been suggested as a possible explanation for the observed differences between nebular and stellar continuum reddening, SFR indicators, and dust attenuation curves.

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arXiv2022

The MOSDEF Survey: A New View of a Remarkable z=1.89 Merger

We present a detailed study of a galaxy merger taking place at $z=1.89$ in the GOODS-S field. Here we analyze Keck/MOSFIRE spectroscopic observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey along with multi-wavelength photometry assembled by the 3D-HST survey. The combined dataset is modeled to infer the past star-formation histories (SFHs) of both merging galaxies. They are found to be massive, with log$_{10}(M_{\ast}/M_{\odot}) > 11$, with a close mass ratio satisfying the typical major-merger definition. Additionally, in the context of delayed-$τ$ models, GOODS-S 43114 and GOODS-S 43683 have similar SFHs and low star-formation rates (log$_{10}$(SFR(SED)/$M_{\odot}/\rm{yr}^{-1}$) $<$ 1.0) compared to their past averages. The best-fit model SEDs show elevated H$δ_{\rm{A}}$ values for both galaxies, indicating that their stellar spectra are dominated by A-type stars, and that star formation peaked $\sim0.5-1$ Gyr ago and has recently declined. Additionally, based on SED fitting both merging galaxies turned on and shut off star formation within a few hundred Myr of each other, suggesting that their bursts of star formation may be linked. Combining the SFHs and H$δ_{\rm{A}}$ results with recent galaxy merger simulations, we infer that these galaxies have recently completed their first pericentric passage and are moving apart. Finally, the relatively low second velocity moment of GOODS-S 43114 given its stellar mass, suggests a disk-like structure. However, including the geometry of the galaxy in the modeling does not completely resolve the discrepancy between the dynamical and stellar masses. Future work is needed to resolve this inconsistency in mass.

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arXiv2022

The MOSDEF Survey: Towards a Complete Census of the z ~ 2.3 Star-forming Galaxy Population

We analyze the completeness of the MOSDEF survey, in which z ~ 2 galaxies were selected for rest-optical spectroscopy from well-studied HST extragalactic legacy fields down to a fixed rest-optical magnitude limit (H_AB = 24.5). The subset of z ~ 2 MOSDEF galaxies with high signal-to-noise (S/N) emission-line detections analyzed in previous work represents a small minority (<10%) of possible z ~ 2 MOSDEF targets. It is therefore crucial to understand how representative this high S/N subsample is, while also more fully exploiting the MOSDEF spectroscopic sample. Using spectral-energy-distribution (SED) models and rest-optical spectral stacking, we compare the MOSDEF z ~ 2 high S/N subsample with the full MOSDEF sample of z ~ 2 star-forming galaxies with redshifts, the latter representing an increase in sample size of more than a factor of three. We find that both samples have similar emission-line properties, in particular in terms of the magnitude of the offset from the local star-forming sequence on the [N II] BPT diagram. There are small differences in median host galaxy properties, including the stellar mass (M_*), star-formation rate (SFR) and specific SFR (sSFR), and UVJ colors; however, these offsets are minor considering the wide spread of the distributions. Using SED modeling, we also demonstrate that the sample of z ~ 2 star-forming galaxies observed by the MOSDEF survey is representative of the parent catalog of available such targets. We conclude that previous MOSDEF results on the evolution of star-forming galaxy emission-line properties were unbiased relative to the parent z ~ 2 galaxy population.

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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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arXiv2022

CO Emission, Molecular Gas, and Metallicity in Main-Sequence Star-Forming Galaxies at $z\sim2.3$

We present observations of CO(3-2) in 13 main-sequence $z=2.0-2.5$ star-forming galaxies at $\log(M_*/M_{\odot})=10.2-10.6$ that span a wide range in metallicity (O/H) based on rest-optical spectroscopy. We find that CO(3-2)/SFR decreases with decreasing metallicity, implying that the CO luminosity per unit gas mass is lower in low-metallicity galaxies at $z\sim2$. We constrain the CO-to-H$_2$ conversion factor ($α_{\text{CO}}$) and find that $α_{\text{CO}}$ inversely correlates with metallicity at $z\sim2$. We derive molecular gas masses ($M_{\text{mol}}$) and characterize the relations among $M_*$, SFR, $M_{\text{mol}}$, and metallicity. At $z\sim2$, $M_{\text{mol}}$ increases and molecular gas fraction ($M_{\text{mol}}$/$M_*$) decrease with increasing $M_*$, with a significant secondary dependence on SFR. Galaxies at $z\sim2$ lie on a near-linear molecular KS law that is well-described by a constant depletion time of 700 Myr. We find that the scatter about the mean SFR-$M_*$, O/H-$M_*$, and $M_{\text{mol}}$-$M_*$ relations is correlated such that, at fixed $M_*$, $z\sim2$ galaxies with larger $M_{\text{mol}}$ have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at $z\sim2$ where O/H is inversely correlated with both SFR and $M_{\text{mol}}$ at fixed $M_*$. These results suggest that the scatter of the $z\sim2$ star-forming main sequence, mass-metallicity relation, and $M_{\text{mol}}$-$M_*$ relation are primarily driven by stochastic variations in gas inflow rates. We place constraints on the mass loading of galactic outflows and perform a metal budget analysis, finding that massive $z\sim2$ star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium.

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arXiv2022

The MOSDEF-LRIS Survey: Connection between Galactic-scale Outflows and the Properties of $z$~2 star-forming Galaxies

We investigate the conditions that facilitate galactic-scale outflows using a sample of 155 typical star-forming galaxies at $z$~2 drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. The sample includes deep rest-frame UV spectroscopy from the Keck Low-Resolution Imaging Spectrometer (LRIS), which provides spectral coverage of several low-ionisation interstellar (LIS) metal absorption lines and Ly$α$ emission. Outflow velocities are calculated from the centroids of the LIS absorption and/or Ly$α$ emission, as well as the highest-velocity component of the outflow from the blue wings of the LIS absorption lines. Outflow velocities are found to be marginally correlated or independent of galaxy properties, such as star-formation rate (SFR) and star-formation rate surface density ($Σ_{\rm SFR}$). Outflow velocity scales with SFR as a power-law with index 0.24, which suggests that the outflows may be primarily driven by mechanical energy generated by supernovae explosions, as opposed to radiation pressure acting on dusty material. On the other hand, outflow velocity and $Σ_{\rm SFR}$ are not significantly correlated, which may be due to the limited dynamic range of $Σ_{\rm SFR}$ probed by our sample. The relationship between outflow velocity and $Σ_{\rm SFR}$ normalised by stellar mass ($Σ_{\rm sSFR}$), as a proxy for gravitational potential, suggests that strong outflows (e.g., > 200 km s$^{-1}$) appear ubiquitous above a threshold of log($Σ_{\rm sSFR}/\rm{yr}^{-1}\ \rm{kpc}^{-2}$) ~ -11.3, and that above this threshold, outflow velocity uncouples from $Σ_{\rm sSFR}$. These results highlight the need for higher resolution spectroscopic data and spatially resolved imaging to test the driving mechanisms of outflows predicted by theory.

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arXiv2022

IR SED and Dust Masses of Sub-solar Metallicity Galaxies at z~2.3

We present results from ALMA 1.2mm continuum observations of a sample of 27 star-forming galaxies at z=2.1-2.5 from the MOSFIRE Deep Evolution Field (MOSDEF) survey. These galaxies have gas-phase metallicity and star-formation rate measurements from Hb, [OIII], Ha, and [NII]. Using stacks of Spitzer, Herschel, and ALMA photometry (rest-frame ~ 8-400$μ$m), we examine the IR SED of high-redshift subsolar metallicity (~0.5 $Z_{\odot}$) LIRGs. We find that the data agree well with an average SED template of higher luminosity local low-metallicity dwarf galaxies (reduced $χ^2$ of 1.8). When compared with the commonly used templates for solar-metallicity local galaxies or high-redshift LIRGs and ULIRGs, even in the most favorable case (with reduced $χ^2$ of 2.8), the templates are rejected at >98% confidence level. The broader and hotter IR SED of both the local dwarfs and high-redshift subsolar metallicity galaxies may result from different grain properties, a clumpy dust geometry, or a harder/more intense ionizing radiation field that heats the dust to higher temperatures. The obscured SFR indicated by the FIR emission of the subsolar metallicity galaxies is only ~ 60% of the total SFR, which is considerably lower than that of the local LIRGs with ~ 96-97% obscured fractions. Due to the evolving IR SED shape, the local LIRG templates fit to mid-IR data can overestimate the Rayleigh-Jeans tail measurements at z~2 by a factor of 2-20, and these templates underestimate IR luminosities if fit to the observed ALMA fluxes by >0.4dex. At a given stellar mass or metallicity, dust masses at z~2.3 are an order of magnitude higher than those at z~0. Given the predicted molecular gas mass fractions, the observed z~2.3 dust-to-stellar mass ratios suggest lower dust-to-molecular gas masses than in local galaxies at the same metallicity.

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arXiv2022

The MOSDEF Survey: Implications of the Lack of Evolution in the Dust Attenuation-Mass Relation to z~2

We investigate the relationship between dust attenuation and stellar mass ($M_*$) in star-forming galaxies over cosmic time. For this analysis, we compare measurements from the MOSFIRE Deep Evolution Field (MOSDEF) survey at $z\sim2.3$ and the Sloan Digital Sky Survey (SDSS) at $z\sim0$, augmenting the latter optical dataset with both UV Galaxy Evolution Explorer (GALEX) and mid-infrared Wide-field Infrared Survey Explorer (WISE) photometry from the GALEX-SDSS-WISE Catalog. We quantify dust attenuation using both spectroscopic measurements of H$α$ and H$β$ emission lines, and photometric measurements of the rest-UV stellar continuum. The H$α$/H$β$ ratio is used to determine the magnitude of attenuation at the wavelength of H$α$, $A_{{\rm H}α}$. Rest-UV colors and spectral-energy-distribution fitting are used to estimate $A_{1600}$, the magnitude of attenuation at a rest wavelength of 1600Å. As in previous work, we find a lack of significant evolution in the relation between dust attenuation and $M_*$ over the redshift range $z\sim0$ to $z\sim2.3$. Folding in the latest estimates of the evolution of $M_{\rm dust}$, $({M_{\rm dust}}/{M_{\rm gas}})$, and gas surface density at fixed $M_*$, we find that the expected $M_{\rm dust}$ and dust mass surface density are both significantly higher at $z\sim2.3$ than at $z\sim0$. These differences appear at odds with the lack of evolution in dust attenuation. To explain the striking constancy in attenuation vs. $M_*$, it is essential to determine the relationship between metallicity and $({M_{\rm dust}}/{M_{\rm gas}})$, the dust mass absorption coefficient, and dust geometry, and the evolution of these relations and quantities from $z\sim0$ to $z\sim2.3$.

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arXiv2021

The MOSDEF Survey: The Dependence of H$α$-to-UV SFR Ratios on SFR and Size at $z\sim2$

We perform an aperture-matched analysis of dust-corrected H$α$ and UV SFRs using 303 star-forming galaxies with spectroscopic redshifts $1.36<z_\text{spec}<2.66$ from the MOSFIRE Deep Evolution Field (MOSDEF) survey. By combining H$α$ and H$β$ emission line measurements with multi-waveband resolved CANDELS/3D-HST imaging, we directly compare dust-corrected H$α$ and UV SFRs, inferred assuming a fixed attenuation curve shape and constant SFHs, within the spectroscopic aperture. Previous studies have found that H$α$ and UV SFRs inferred with these assumptions generally agree for typical star-forming galaxies, but become increasingly discrepant for galaxies with higher SFRs ($\gtrsim$100 M$_\odot$ yr$^{-1}$), with H$α$-to-UV SFR ratios being larger for these galaxies. Our analysis shows that this trend persists even after carefully accounting for the apertures over which H$α$ and UV-based SFRs (and the nebular and stellar continuum reddening) are derived. Furthermore, our results imply that H$α$ SFRs may be higher in the centers of large galaxies (i.e., where there is coverage by the spectroscopic aperture) compared to their outskirts, which could be indicative of inside-out galaxy growth. Overall, we suggest that the persistent difference between nebular and stellar continuum reddening and high H$α$-to-UV SFR ratios at the centers of large galaxies may be indicative of a patchier distribution of dust in galaxies with high SFRs.

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arXiv2021

Testing the Relationship Between Bursty Star Formation and Size Fluctuations of Local Dwarf Galaxies

Stellar feedback in dwarf galaxies plays a critical role in regulating star formation via galaxy-scale winds. Recent hydrodynamical zoom simulations of dwarf galaxies predict that the periodic outward flow of gas can change the gravitational potential sufficiently to cause radial migration of stars. To test the effect of bursty star formation on stellar migration, we examine star formation observables and sizes of 86 local dwarf galaxies. We find a correlation between the R-band half-light radius (R$_e$) and far-UV luminosity (L$_{FUV}$) for stellar masses below 10$^8 $ M$_{\odot}$ and a weak correlation between the R$_e$ and H$α$ luminosity (L$_{Hα}$). We produce mock observations of eight low-mass galaxies from the FIRE-2 cosmological simulations and measure the similarity of the time sequences of R$_e$ and a number of star formation indicators with different timescales. Major episodes of R$_e$ time sequence align very well with the major episodes of star formation, with a delay of $\sim$ 50 Myrs. This correlation decreases towards SFR indicators of shorter timescales such that $R_e$ is weakly correlated with L$_{FUV}$ (10-100 Myr timescale) and is completely uncorrelated with L$_{Hα}$ (a few Myr timescale), in agreement with the observations. Our findings based on FIRE-2 suggest that the R-band size of a galaxy reacts to star formation variations on a $\sim50$ Myr timescale. With the advent of a new generation of large space telescopes (e.g., JWST), this effect can be examined explicitly in galaxies at higher redshifts where bursty star formation is more prominent.

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arXiv2021

The MOSDEF-LRIS Survey: Probing ISM/CGM Structure of Star-Forming Galaxies at z~2 Using Rest-UV Spectroscopy

The complex structure of gas, metals, and dust in the interstellar and circumgalactic medium (ISM and CGM, respectively) in star-forming galaxies can be probed by Ly$α$ emission and absorption, low-ionization interstellar (LIS) metal absorption, and dust reddening E(B-V). We present a statistical analysis of the mutual correlations among Ly$α$ equivalent width (EW$_{Lyα}$), LIS equivalent width (EW$_{LIS}$), and E(B-V) in a sample of 157 star-forming galaxies at $z\sim2.3$. With measurements obtained from individual, deep rest-UV spectra and spectral-energy distribution (SED) modeling, we find that the tightest correlation exists between EW$_{LIS}$ and E(B-V), although correlations among all three parameters are statistically significant. These results signal a direct connection between dust and metal-enriched HI gas, and that they are likely co-spatial. By comparing our results with the predictions of different ISM/CGM models, we favor a dusty ISM/CGM model where dust resides in HI gas clumps and Ly$α$ photons escape through the low HI covering fraction/column density intra-clump medium. Finally, we investigate the factors that potentially contribute to the intrinsic scatter in the correlations studied in this work, including metallicity, outflow kinematics, Ly$α$ production efficiency, and slit loss. Specifically, we find evidence that scatter in the relationship between EW$_{Lyα}$ and E(B-V) reflects the variation in metal-to-HI covering fraction ratio as a function of metallicity, and the effects of outflows on the porosity of the ISM/CGM. Future simulations incorporating star-formation feedback and the radiative transfer of Ly$α$ photons will provide key constraints on the spatial distributions of neutral hydrogen gas and dust in the ISM/CGM structure.

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arXiv2021

The MOSDEF Survey: The Mass-Metallicity relationship and the existence of the FMR at z~1.5

We analyze the rest-optical emission-line ratios of z~1.5 galaxies drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Using composite spectra we investigate the mass-metallicity relation (MZR) at z~1.5 and measure its evolution to z=0. When using gas-phase metallicities based on the N2 line ratio, we find that the MZR evolution from z~1.5 to z=0 depends on stellar mass, evolving by $Δ\rm log(\rm O/H)\sim0.25$ dex at $M_*<10^{9.75}M_{\odot}$ down to $Δ\rm log(\rm O/H)\sim0.05$ at $M_*>10^{10.5}M_{\odot}$. In contrast, the O3N2-based MZR shows a constant offset of $Δ\rm log(\rm O/H)\sim0.30$ across all masses, consistent with previous MOSDEF results based on independent metallicity indicators, and suggesting that O3N2 provides a more robust metallicity calibration for our z~1.5 sample. We investigated the secondary dependence of the MZR on SFR by measuring correlated scatter about the mean $M_*$-specific SFR and $M_*-\log(\rm O3N2)$ relations. We find an anti-correlation between $\log(\rm O/H)$ and sSFR offsets, indicating the presence of a $M_*$-SFR-Z relation, though with limited significance. Additionally, we find that our z~1.5 stacks lie along the z=0 metallicity sequence at fixed $μ=\log(M_*/M_{\odot})-0.6\times\log(\rm SFR / M_{\odot} yr^{-1})$ suggesting that the z~1.5 stacks can be described by the z=0 fundamental metallicity relation (FMR). However, using different calibrations can shift the calculated metallicities off of the local FMR, indicating that appropriate calibrations are essential for understanding metallicity evolution with redshift. Finally, understanding how [NII]/H$α$ scales with galaxy properties is crucial to accurately describe the effects of blended [NII] and H$α$ on redshift and H$α$ flux measurements in future large surveys utilizing low-resolution spectra such as with Euclid and the Roman Space Telescope.

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arXiv2021

The MOSDEF Survey: The Evolution of the Mass-Metallicity Relation from $z=0$ to $z\sim3.3$

We investigate the evolution of galaxy gas-phase metallicity (O/H) over the range $z=0-3.3$ using samples of $\sim300$ galaxies at $z\sim2.3$ and $\sim150$ galaxies at $z\sim3.3$ from the MOSDEF survey. This analysis crucially utilizes different metallicity calibrations at $z\sim0$ and $z>1$ to account for evolving ISM conditions. We find significant correlations between O/H and stellar mass ($M_*$) at $z\sim2.3$ and $z\sim3.3$. The low-mass power law slope of the mass-metallicity relation is remarkably invariant over $z=0-3.3$, such that $\textrm{O/H}\propto M_*^{0.30}$ at all redshifts in this range. At fixed $M_*$, O/H decreases with increasing redshift as dlog(O/H)/d$z=-0.11\pm0.02$. We find no evidence that the fundamental metallicity relation between $M_*$, O/H, and star-formation rate (SFR) evolves out to $z\sim3.3$, with galaxies at $z\sim2.3-3.3$ having O/H within 0.04~dex of local galaxies matched in $M_*$ and SFR on average. We employ analytic chemical evolution models to place constraints on the mass and metal loading factors of galactic outflows. The efficiency of metal removal increases toward lower $M_*$ at fixed redshift, and toward higher redshift at fixed $M_*$. These models suggest that the slope of the mass-metallicity relation is set by the scaling of the metal loading factor of outflows with $M_*$, not by the change in gas fraction as a function of $M_*$. The evolution toward lower O/H at fixed $M_*$ with increasing redshift is driven by both higher gas fraction (leading to stronger dilution of ISM metals) and higher metal removal efficiency, with models suggesting that both effects contribute approximately equally to the observed evolution. These results suggest that the processes governing the smooth baryonic growth of galaxies via gas flows and star formation hold in the same form over at least the past 12~Gyr.

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arXiv2021

The MOSDEF Survey: A Comprehensive Analysis of the Rest-optical Emission-line Properties of $z\sim 2.3$ Star-forming Galaxies

We analyze the rest-optical emission-line spectra of $z\sim2.3$ star-forming galaxies in the complete MOSFIRE Deep Evolution Field (MOSDEF) survey. In investigating the origin of the well-known offset between the sequences of high-redshift and local galaxies in the [O III]5008/H$β$ vs. [N II]6585/H$α$ ("[N II] BPT") diagram, we define two populations of $z\sim2.3$ MOSDEF galaxies. These include the "high" population that is offset towards higher [O III]5008/H$β$ and/or [N II]6585/H$α$ with respect to the local SDSS sequence and the "low" population that overlaps the SDSS sequence. These two groups are also segregated within the [O III]5008/H$β$ vs. [S II]6718,6733/H$α$ and the [O III]4960,5008/[O II]3727,3730 (O$_{32}$) vs. ([O III]4960,5008+[O II]3727,3730)/H$β$ (R$_{23}$) diagram, which suggests qualitatively that star-forming regions in the more offset galaxies are characterized by harder ionizing spectra at fixed nebular oxygen abundance. We also investigate many galaxy properties of the split sample and find that the "high" sample is on average smaller in size and less massive, but has higher specific star-formation rate and star-formation-rate surface density values and is slightly younger compared to the "low" population. From Cloudy+BPASS photoionization models, we estimate that the "high" population has a lower stellar metallicity (i.e., harder ionizing spectrum) but slightly higher nebular metallicity and higher ionization parameter compared to the "low" population. While the "high" population is more $α$-enhanced (i.e., higher $α$/Fe) than the "low" population, both samples are significantly more $α$-enhanced compared to local star-forming galaxies with similar rest-optical line ratios. These differences must be accounted for in all high-redshift star-forming galaxies -- not only those "offset" from local excitation sequences.

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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 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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