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Bahram Mobasher

Publications and source records attributed to Bahram Mobasher.

At least 127 records · Page 7Linked to original sources
arXiv2020

The SFR-radius connection: data and implications for wind strength and halo concentration

This paper is one in a series that explores the importance of radius as a second parameter in galaxy evolution. The topic investigated here is the relationship between star formation rate (SFR) and galaxy radius ($R_{\rm e}$) for main-sequence star-forming galaxies. The key observational result is that, over a wide range of stellar mass and redshift in both CANDELS and SDSS, there is little trend between SFR and $R_{\rm e}$ at fixed stellar mass. The Kennicutt-Schmidt law, or any similar density-related star formation law, then implies that smaller galaxies must have lower gas fractions than larger galaxies (at fixed $M_{\ast}$), and this is supported by observations of local star-forming galaxies. We investigate the implication by adopting the equilibrium "bathtub" model: the ISM gas mass is assumed to be constant over time and the net star formation rate is the difference between the accretion rate of gas onto the galaxy from the halo and the outflow rate due to winds. To match the observed null correlation between SFR and radius, the bathtub model requires that smaller galaxies at fixed mass have weaker galactic winds. Our hypothesis is that galaxies are a 2-dimensional family whose properties are set mainly by halo mass and concentration. Galaxy radius and accretion rate plausibly both depend on halo concentration, which predicts how wind strength should vary with $R_{\rm e}$ and SFR.

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arXiv2020

The MOSDEF Survey: the Variation of the Dust Attenuation Curve with Metallicity

We derive the UV-optical stellar dust attenuation curve of galaxies at z=1.4-2.6 as a function of gas-phase metallicity. We use a sample of 218 star-forming galaxies, excluding those with very young or heavily obscured star formation, from the MOSFIRE Deep Evolution Field (MOSDEF) survey with H$α$, H$β$, and [NII]$λ6585$ spectroscopic measurements. We constrain the shape of the attenuation curve by comparing the average flux densities of galaxies sorted into bins of dust obscuration using Balmer decrements, i.e., H$α$-to-H$β$ luminosities. The average attenuation curve for the high-metallicity sample (12+log(O/H)>8.5, corresponding to $M_*\gtrsim10^{10.4}\,M_{\odot}$) has a shallow slope, identical to that of the Calzetti local starburst curve, and a significant UV 2175A extinction bump that is $\sim 0.5\times$ the strength of the Milky Way bump. On the other hand, the average attenuation curve of the low-metallicity sample (12+log(O/H) $\sim 8.2-8.5$) has a steeper slope similar to that of the SMC curve, only consistent with the Calzetti slope at the $3σ$ level. The UV bump is not detected in the low-metallicity curve, indicating the relative lack of the small dust grains causing the bump at low metallicities. Furthermore, we find that on average the nebular reddening (E(B-V)) is a factor of 2 times larger than that of the stellar continuum for galaxies with low metallicities, while the nebular and stellar reddening are similar for galaxies with higher metallicities. The latter is likely due to a high surface density of dusty clouds embedding the star forming regions but also reddening the continuum in the high-metallicity galaxies.

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arXiv2020

Resolved Multi-element Stellar Chemical Abundances in the Brightest Quiescent Galaxy at z $\sim$ 2

Measuring the chemical composition of galaxies is crucial to our understanding of galaxy formation and evolution models. However, such measurements are extremely challenging for quiescent galaxies at high redshifts, which have faint stellar continua and compact sizes, making it difficult to detect absorption lines and nearly impossible to spatially resolve them. Gravitational lensing offers the opportunity to study these galaxies with detailed spectroscopy that can be spatially resolved. In this work, we analyze deep spectra of MRG-M0138, a lensed quiescent galaxy at z = 1.98 which is the brightest of its kind, with an H-band magnitude of 17.1. Taking advantage of full spectral fitting, we measure $[{\rm Mg/Fe}]=0.51\pm0.05$, $[\rm{Fe/H}]=0.26\pm0.04$, and, for the first time, the stellar abundances of 6 other elements in this galaxy. We further constrained, also for the first time in a $z\sim2$ galaxy, radial gradients in stellar age, [Fe/H], and [Mg/Fe]. We detect no gradient in age or [Mg/Fe] and a slightly negative gradient in [Fe/H], which has a slope comparable to that seen in local early-type galaxies. Our measurements show that not only is MRG-M0138 very Mg-enhanced compared to the centers of local massive early-type galaxies, it is also very iron rich. These dissimilar abundances suggest that even the inner regions of massive galaxies have experienced significant mixing of stars in mergers, in contrast to a purely inside-out growth model. The abundance pattern observed in MRG-M0138 challenges simple galactic chemical evolution models that vary only the star formation timescale and shows the need for more elaborate models.

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arXiv2020

Similar Scaling Relations for the Gas Content of Galaxies across Environments to z ~ 3.5

We study the effects of the local environment on the molecular gas content of a large sample of log($M_{*}$/$M_{\odot}$) $\gtrsim$ 10 star-forming and starburst galaxies with specific star-formation rates (sSFRs) on and above the main sequence (MS) to $z$ $\sim$ 3.5. ALMA observations of the dust continuum in the COSMOS field are used to estimate molecular gas masses at $z$ $\approx$ 0.5-3.5. We also use a local universe sample from the ALFALFA HI survey after converting it into molecular masses. The molecular mass ($M_{ISM}$) scaling relation shows a dependence on $z$, $M_{*}$, and sSFR relative to the MS, but no dependence on environmental overdensity $Δ$ ($M_{ISM}$ $\propto$ $Δ^{0.03}$). Similarly, gas mass fraction (f$_{gas}$) and depletion timescale ($τ$) show no environmental dependence to $z$ $\sim$ 3.5. At $\langle z\rangle$ $\sim$ 1.8, the average $\langle M_{ISM}\rangle$,$\langle$f$_{gas}\rangle$, and $\langle τ\rangle$ in densest regions is (1.6$\pm$0.2)$\times$10$^{11}$ $M_{\odot}$, 55$\pm$2%, and 0.8$\pm$0.1 Gyr, respectively, similar to those in the lowest density bin. Independent of the environment, f$_{gas}$ decreases and $τ$ increases with increasing cosmic time. Cosmic molecular mass density ($ρ$) in the lowest density bins peaks at $z$ $\sim$ 1-2, and this peak happens at $z$ $<$ 1 in densest bins. This differential evolution of $ρ$ across environments is likely due to the growth of the large-scale structure with cosmic time. Our results suggest that the molecular gas content and the subsequent star-formation activity of log($M_{*}$/$M_{\odot}$) $\gtrsim$ 10 star-forming and starburst galaxies is primarily driven by internal processes, and not by their local environment since $z$ $\sim$ 3.5.

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arXiv2020

Spectroscopic Confirmation of a Coma Cluster Progenitor at z ~ 2.2

We report the spectroscopic confirmation of a new protocluster in the COSMOS field at $z$ $\sim$ 2.2, COSMOS Cluster 2.2 (CC2.2), originally identified as an overdensity of narrowband selected H$α$ emitting candidates. With only two masks of Keck/MOSFIRE near-IR spectroscopy in both $H$ ($\sim$ 1.47-1.81 $μ$m) and $K$ ($\sim$ 1.92-2.40 $μ$m) bands ($\sim$ 1.5 hour each), we confirm 35 unique protocluster members with at least two emission lines detected with S/N $>$ 3. Combined with 12 extra members from the zCOSMOS-deep spectroscopic survey (47 in total), we estimate a mean redshift and a line-of-sight velocity dispersion of $z_{mean}$=2.23224 $\pm$ 0.00101 and $σ_{los}$=645 $\pm$ 69 km s$^{-1}$ for this protocluster, respectively. Assuming virialization and spherical symmetry for the system, we estimate a total mass of $M_{vir}$ $\sim$ $(1-2) \times$10$^{14}$ $M_{\odot}$ for the structure. We evaluate a number density enhancement of $δ_{g}$ $\sim$ 7 for this system and we argue that the structure is likely not fully virialized at $z$ $\sim$ 2.2. However, in a spherical collapse model, $δ_{g}$ is expected to grow to a linear matter enhancement of $\sim$ 1.9 by $z$=0, exceeding the collapse threshold of 1.69, and leading to a fully collapsed and virialized Coma-type structure with a total mass of $M_{dyn}$($z$=0) $\sim$ 9.2$\times$10$^{14}$ $M_{\odot}$ by now. This observationally efficient confirmation suggests that large narrowband emission-line galaxy surveys, when combined with ancillary photometric data, can be used to effectively trace the large-scale structure and protoclusters at a time when they are mostly dominated by star-forming galaxies.

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arXiv2020

Effects of Stellar Feedback on Stellar and Gas Kinematics of Star-Forming Galaxies at 0.6<z<1.0

Recent zoom-in cosmological simulations have shown that stellar feedback can flatten the inner density profile of the dark matter halo in low-mass galaxies. A correlation between the stellar/gas velocity dispersion ($σ_{star}$, $σ_{gas}$) and the specific star formation rate (sSFR) is predicted as an observational test of the role of stellar feedback in re-shaping the dark matter density profile. In this work we test the validity of this prediction by studying a sample of star-forming galaxies at $0.6<z<1.0$ from the LEGA-C survey, which provides high signal-to-noise measurements of stellar and gas kinematics. We find that a weak but significant correlation between $σ_{star}$ (and $σ_{gas}$) and sSFR indeed exists for galaxies in the lowest mass bin (M$_\ast\sim10^{10}\,$M$_\odot$). This correlation, albeit with a $\sim$35% scatter, holds for different tracers of star formation, and becomes stronger with redshift. This result generally agrees with the picture that at higher redshifts star formation rate was generally higher, and galaxies at M$_\ast\lesssim10^{10}\,$M$_\odot$ have not yet settled into a disk. As a consequence, they have shallower gravitational potentials more easily perturbed by stellar feedback. The observed correlation between $σ_{star}$ (and $σ_{gas}$) and sSFR supports the scenario predicted by cosmological simulations, in which feedback-driven outflows cause fluctuations in the gravitation potential that flatten the density profiles of low-mass galaxies.

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arXiv2020

Selection of massive evolved galaxies at $3 \leq z \leq 4.5$ in the CANDELS fields

Using the CANDELS photometric catalogs for the HST/ACS and WFC3, we identified massive evolved galaxies at $3 < z < 4.5$, employing three different selection methods. We find the comoving number density of these objects to be $\sim 2 \times 10^{-5}$ and $8 \times 10^{-6}Mpc^{-3}$ after correction for completeness for two redshift bins centered at $z=3.4, 4.7$. We quantify a measure of how much confidence we should have for each candidate galaxy from different selections and what are the conservative error estimates propagated into our selection. Then we compare the evolution of the corresponding number densities and their stellar mass density with numerical simulations, semi-analytical models, and previous observational estimates, which shows slight tension at higher redshifts as the models tend to underestimate the number and mass densities. By estimating the average halo masses of the candidates ($M_h \approx 4.2, 1.9, 1.3 \times 10^{12} M_\odot$ for redshift bins centered at $z=3.4, 4.1, 4.7$), we find them to be consistent with halos that were efficient in turning baryons to stars and were relatively immune to the feedback effects and on the verge of transition into hot-mode accretion. This can suggest the relative cosmological starvation of the cold gas followed by an overconsumption phase in which the galaxy consumes the available cold gas rapidly as one of the possible drivers for the quenching of the massive evolved population at high redshift.

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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 CatWISE Preliminary Catalog: Motions from ${\it WISE}$ and ${\it NEOWISE}$ Data

CatWISE is a program to catalog sources selected from combined ${\it WISE}$ and ${\it NEOWISE}$ all-sky survey data at 3.4 and 4.6 $μ$m (W1 and W2). The CatWISE Preliminary Catalog consists of 900,849,014 sources measured in data collected from 2010 to 2016. This dataset represents four times as many exposures and spans over ten times as large a time baseline as that used for the AllWISE Catalog. CatWISE adapts AllWISE software to measure the sources in coadded images created from six-month subsets of these data, each representing one coverage of the inertial sky, or epoch. The catalog includes the measured motion of sources in 8 epochs over the 6.5 year span of the data. From comparison to ${\it Spitzer}$, the SNR=5 limits in magnitudes in the Vega system are W1=17.67 and W2=16.47, compared to W1=16.96 and W2=16.02 for AllWISE. From comparison to ${\it Gaia}$, CatWISE positions have typical accuracies of 50 mas for stars at W1=10 mag and 275 mas for stars at W1=15.5 mag. Proper motions have typical accuracies of 10 mas yr$^{-1}$ and 30 mas yr$^{-1}$ for stars with these brightnesses, an order of magnitude better than from AllWISE. The catalog is available in the WISE/NEOWISE Enhanced and Contributed Products area of the NASA/IPAC Infrared Science Archive.

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arXiv2020

The BUFFALO HST Survey

The Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) is a 101 orbit + 101 parallel Cycle 25 Hubble Space Telescope Treasury program taking data from 2018-2020. BUFFALO will expand existing coverage of the Hubble Frontier Fields (HFF) in WFC3/IR F105W, F125W, and F160W and ACS/WFC F606W and F814W around each of the six HFF clusters and flanking fields. This additional area has not been observed by HST but is already covered by deep multi-wavelength datasets, including Spitzer and Chandra. As with the original HFF program, BUFFALO is designed to take advantage of gravitational lensing from massive clusters to simultaneously find high-redshift galaxies which would otherwise lie below HST detection limits and model foreground clusters to study properties of dark matter and galaxy assembly. The expanded area will provide a first opportunity to study both cosmic variance at high redshift and galaxy assembly in the outskirts of the large HFF clusters. Five additional orbits are reserved for transient followup. BUFFALO data including mosaics, value-added catalogs and cluster mass distribution models will be released via MAST on a regular basis, as the observations and analysis are completed for the six individual clusters.

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arXiv2020

Bridging between the integrated and resolved main sequence of star formation

The position of galaxies on the stellar mass, star formation rate plane with respect to the star-forming main sequence at each redshift is a convenient way to infer where the galaxy is in its evolution compared to the rest of the population. We use Hubble Space Telescope high resolution images in the GOODS-S field from the CANDELS survey and fit multi wavelength lights in resolution elements of galaxies with stellar population synthesis models. We then construct resolved kpc-scale stellar mass, star formation rate surface density curves for galaxies at z~1. Fitting these resolved main sequence curves with Schechter functions, we parameterize and explain the multi-wavelength structure of galaxies with three variables: phi*, alpha, and M*. For quenched galaxies below the main sequence, we find an average high mass slope (alpha) of the resolved main sequence curves to be ~ -0.4. The scatter of this slope is higher among the lower mass star forming galaxies and those above the main sequence compared to quenched galaxies, due to lack of an evolved bulge. Our findings agree well with an inside-out quenching of star-formation. We find that the knee of the Schechter fits (M*) for galaxies below the main sequence occurs at lower stellar mass surface densities compared to star forming galaxies, which hints at how far quenching has proceeded outwards.

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arXiv2020

A comprehensive study of H$α$ emitters at $z \sim$ 0.62 in the DAWN survey: the need for deep and wide regions

We present new estimates of the luminosity function (LF) and star formation rate density (SFRD) for an H$α$ selected sample at $z\sim0.62$ from the Deep And Wide Narrow-band (DAWN) survey. Our results are based on a new H$α$ sample in the extended COSMOS region (compared to Coughlin et al. 2018) with the inclusion of flanking fields, resulting in a total area coverage of $\sim$1.5 deg$^2$. A total of 241 H$α$ emitters were selected based on robust selection criteria using spectro-photometric redshifts and broadband color-color classification. We explore the effect of different dust correction prescriptions by calculating the LF and SFRD using a constant dust extinction correction, A{$_{\textrm{H}α}=1$} mag, a luminosity-dependent correction, and a stellar-mass dependent correction. The resulting H$α$ LFs are well fitted using Schechter functions with best-fit parameters: L$^*=10^{42.24}$ erg s$^{-1}$, $ϕ^*=10^{-2.85}$ Mpc$^{-3}$, $α= -1.62$ for constant dust correction, L$^*=10^{42.31}$ erg s$^{-1}$, $ϕ^*=10^{-2.8}$ Mpc$^{-3}$, $α=-1.39$ for luminosity-dependent dust correction, and L$^*=10^{42.36}$ erg s$^{-1}$, $ϕ^*=10^{-2.91}$ Mpc$^{-3}$, $α= -1.48$, for stellar mass-dependent dust correction. The deep and wide nature of the DAWN survey effectively samples H$α$ emitters over a wide range of luminosities, thereby providing better constraints on both the faint and bright end of the LF. Also, the SFRD estimates $ρ_{\textrm{SFR}}=10^{-1.39}$ M$_{\odot}$yr$^{-1}$Mpc$^{-3}$ (constant dust correction), $ρ_{\textrm{SFR}}=10^{-1.47}$ M$_{\odot}$yr$^{-1}$Mpc$^{-3}$ (luminosity-dependent dust correction), and $ρ_{\textrm{SFR}}=10^{-1.49}$ M$_{\odot}$yr$^{-1}$Mpc$^{-3}$ (stellar mass-dependent dust correction) are in good agreement with the evolution of SFRD across redshifts ($0 < z < 2$) seen from previous H$α$ surveys.

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arXiv2020

Searching for z > 6.5 Analogs Near the Peak of Cosmic Star Formation

Strong [OIII]$λλ$4959,5007+H$β$ emission appears to be typical in star-forming galaxies at z > 6.5. As likely contributors to cosmic reionization, these galaxies and the physical conditions within them are of great interest. At z > 6.5, where Ly$α$ is greatly attenuated by the intergalactic medium, rest-UV metal emission lines provide an alternative measure of redshift and also constraints on the physical properties of star-forming regions and massive stars. We present the first statistical sample of rest-UV line measurements in z $\sim$ 2 galaxies selected as analogs of those in the reionization era based on [OIII]$λλ$4959,5007 EW or rest-frame U-B color. Our sample is drawn from the 3D-HST Survey and spans the redshift range 1.36 $\leqslant$ z $\leqslant$ 2.49. We find that the median Ly$α$ and CIII]$λλ$1907,1909 EWs of our sample are significantly greater than those of z $\sim$ 2 UV-continuum-selected star-forming galaxies. Measurements from both individual and composite spectra indicate a monotonic, positive correlation between CIII] and [OIII], while a lack of trend is observed between Ly$α$ and [OIII] at [OIII] EW < 1000$\unicode{x212B}$. At higher [OIII] EW, extreme Ly$α$ emission starts to emerge. Using stacked spectra, we find that Ly$α$ and CIII] are significantly enhanced in galaxies with lower metallicity. Two objects in our sample appear comparable to z > 6.5 galaxies with exceptionally strong rest-UV metal line emission. These objects have significant CIV$λλ$1548,1550, HeII$λ$1640, and OIII]$λλ$1661,1665 emission in addition to intense Ly$α$ or CIII]. Detailed characterization of these lower-redshift analogs provides unique insights into the physical conditions in z > 6.5 star-forming regions, motivating future observations of reionization-era analogs at lower redshifts.

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arXiv2019

Large Scale Structures in the CANDELS Fields: The Role of the Environment in Star Formation Activity

We present a robust method, weighted von Mises kernel density estimation, along with boundary correction to reconstruct the underlying number density field of galaxies. We apply this method to galaxies brighter than $\rm HST/F160w\le 26$ AB mag at the redshift range of $0.4\leq z \leq 5$ in the five CANDELS fields (GOODS-N, GOODS-S, EGS, UDS, and COSMOS). We then use these measurements to explore the environmental dependence of the star formation activity of galaxies. We find strong evidence of environmental quenching for massive galaxies ($\rm M \gtrsim 10^{11} \rm {M}_\odot$) out to $z\sim 3.5$ such that an over-dense environment hosts $\gtrsim 20\%$ more massive quiescent galaxies compared to an under-dense region. We also find that environmental quenching efficiency grows with stellar mass and reaches $\sim 60\%$ for massive galaxies at $z\sim 0.5$. The environmental quenching is also more efficient in comparison to the stellar mass quenching for low mass galaxies ($\rm M \lesssim 10^{10} \rm {M}_\odot$) at low and intermediate redshifts ($z\lesssim 1.2$). Our findings concur thoroughly with the "over-consumption" quenching model where the termination of cool gas accretion (cosmological starvation) happens in an over-dense environment and the galaxy starts to consume its remaining gas reservoir in depletion time. The depletion time depends on the stellar mass and could explain the evolution of environmental quenching efficiency with the stellar mass.

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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

Spatially resolved properties of galaxies from CANDELS+MUSE: Radial extinction profile and insights on quenching

Studying the internal processes of individual galaxies at kilo-parsec scales is crucial in enhancing our understanding of galaxy formation and evolution processes. In this work, we investigate the distribution of star formation rate (SFR), specific SFR (sSFR), and dust attenuation across individual galaxies for a sample of 32 galaxies selected from the MUSE-Wide Survey at 0.1 $< \textit{z} <$ 0.42 with a dynamic range in stellar masses between $10^{7.7}$ and $10^{10.3}$ M$_{\odot}$. We take advantage of the high spatial resolution of the MUSE integral-field spectrograph and measure reliable spatially resolved H$α$ and H$β$ emission line maps for individual galaxies. We also derive resolved stellar mass, SFR and dust maps using pixel-by-pixel SED fitting on high resolution multi-band HST/ACS and HST/WFC3 data from the CANDELS survey. By combining these, we analyze the radial profile of various physical parameters across these galaxies. We observe a radial dependence in both stellar and nebular color excess profiles peaking at the inner regions of galaxies. We also find the color excess profiles to most strongly correlate with the integrated sSFRs of galaxies. The median sSFR$_{\mathrm{H}α}$ radial profiles of galaxies in our sample show a 0.8 dex increase from the central regions outward. This increase compared to the almost flat median radial profile of sSFR$_{\mathrm{SED}}$, which traces longer timescales of star formation, is in favor of the inside-out quenching of star formation. We bring further evidence for this quenching scenario from the locus of different subregions of galaxies on the SFR-M$_{*}$ and sSFR-M$_{*}$ relations.

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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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