arXiv subjects

Alison L. Coil

Publications and source records attributed to Alison L. Coil.

At least 37 records · Page 2Linked to original sources

arXiv2023

Ionized Gas Extended Over 40 kpc in an Odd Radio Circle Host Galaxy

A new class of extragalactic astronomical sources discovered in 2021, named Odd Radio Circles (ORCs, Norris et al. 2021), are large rings of faint, diffuse radio continuum emission spanning ~1 arcminute on the sky. Galaxies at the centers of several ORCs have photometric redshifts of z~0.3-0.6, implying physical scales of several 100 kiloparsecs in diameter for the radio emission, the origin of which is unknown. Here we report spectroscopic data on an ORC including strong [OII] emission tracing ionized gas in the central galaxy of ORC4 at z=0.4512. The physical extent of the [OII] emission is ~40 kpc in diameter, larger than expected for a typical early-type galaxy (Pandya et al, 2017) but an order of magnitude smaller than the large-scale radio continuum emission. We detect a ~200 km/s velocity gradient across the [OII] nebula, as well as a high velocity dispersion of ~180 km/s. The [OII] equivalent width (EW, ~50 Ang) is extremely high for a quiescent galaxy. The morphology, kinematics, and strength of the [OII] emission are consistent with the infall of shock ionized gas near the galaxy, following a larger-scale, outward moving shock driven by a galactic wind. Both the extended optical and radio emission, while observed on very different scales, may therefore result from the same dramatic event.

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arXiv2023

The MOSDEF-LRIS Survey: Detection of Inflowing Gas Towards Three Star-forming Galaxies at z ~ 2

We report on the discovery of cool gas inflows towards three star-forming galaxies at $\left \sim$ 2.30. Analysis of Keck Low-Resolution Imaging Spectrometer spectroscopy reveals redshifted low-ionisation interstellar (LIS) metal absorption lines with centroid velocities of 60 - 130 km $\rm{s}^{-1}$. These inflows represent some of the most robust detections of inflowing gas into isolated, star-forming galaxies at high redshift. Our analysis suggests that the inflows are due to recycling metal-enriched gas from previous ejections. Comparisons between the galaxies with inflows and a larger parent sample of 131 objects indicate that galaxies with detected inflows may have higher specific star-formation rates (sSFR) and star-formation-rate surface densities. However, when additional galaxies without robustly detected inflows based on centroid velocity but whose LIS absorption line profiles indicate large red-wing velocities are considered, galaxies with inflows do not show unique properties relative to those lacking inflows. Additionally, we calculate the covering fraction of cool inflowing gas as a function of red-wing inflow velocity, finding an enhancement in high sSFR binned galaxies, likely due to an increase in the amount of recycling gas. Together, these results suggest that the low detection rate of galaxies with cool inflows is primarily related to the viewing angle rather than the physical properties of the galaxies.

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arXiv2023

An Updated Dust-to-Star Geometry: Dust Attenuation Does Not Depend on Inclination in $1.3\leq z\leq 2.6$ Star-Forming Galaxies from MOSDEF

We investigate dust attenuation and its dependence on viewing angle for 308 star-forming galaxies at $1.3\leq z\leq2.6$ from the MOSFIRE Deep Evolution Field (MOSDEF) survey. We divide galaxies with a detected H$α$ emission line and coverage of H$β$ into eight groups by stellar mass, star formation rate (SFR), and inclination (i.e., axis ratio), then stack their spectra. From each stack, we measure Balmer decrement and gas-phase metallicity, then we compute median \AV and UV continuum spectral slope ($β$). First, we find that none of the dust properties (Balmer decrement, \AV, $β$) vary with axis ratio. Second, both stellar and nebular attenuation increase with increasing galaxy mass, showing little residual dependence on SFR or metallicity. Third, nebular emission is more attenuated than stellar emission, and this difference grows even larger at higher galaxy masses and SFRs. Based on these results, we propose a three-component dust model where attenuation predominantly occurs in star-forming regions and large, dusty star-forming clumps, with minimal attenuation in the diffuse ISM. In this model, nebular attenuation primarily originates in clumps, while stellar attenuation is dominated by star-forming regions. Clumps become larger and more common with increasing galaxy mass, creating the above mass trends. Finally, we argue that a fixed metal yield naturally leads to mass regulating dust attenuation. Infall of low-metallicity gas increases SFR and lowers metallicity, but leaves dust column density mostly unchanged. We quantify this idea using the Kennicutt-Schmidt and fundamental metallicity relations, showing that galaxy mass is indeed the primary driver of dust attenuation.

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arXiv2023

Kinematics, Structure, and Mass Outflow Rates of Extreme Starburst Galactic Outflows

We present results on the properties of extreme gas outflows in massive ($\rm M_* \sim$10$^{11} \ \rm M_{\odot}$), compact, starburst ($\rm SFR \sim$$200 \, \rm M_{\odot} \ yr^{-1}$) galaxies at z = $0.4-0.7$ with very high star formation surface densities ($\rm Σ_{SFR} \sim$$2000 \,\rm M_{\odot} \ yr^{-1} \ kpc^{-2}$). Using optical Keck/HIRES spectroscopy of 14 HizEA starburst galaxies we identify outflows with maximum velocities of $820 - 2860$ \kmps. High-resolution spectroscopy allows us to measure precise column densities and covering fractions as a function of outflow velocity and characterize the kinematics and structure of the cool gas outflow phase (T $\sim$10$^4$ K). We find substantial variation in the absorption profiles, which likely reflects the complex morphology of inhomogeneously-distributed, clumpy gas and the intricacy of the turbulent mixing layers between the cold and hot outflow phases. There is not a straightforward correlation between the bursts in the galaxies' star formation histories and their wind absorption line profiles, as might naively be expected for starburst-driven winds. The lack of strong \mgii \ absorption at the systemic velocity is likely an orientation effect, where the observations are down the axis of a blowout. We infer high mass outflow rates of $\rm \sim$50 $-$ 2200 $\rm M_{\odot} \, yr^{-1}$, assuming a fiducial outflow size of 5 kpc, and mass loading factors of $η\sim$5 for most of the sample. %with $η\sim$20 for two galaxies. While these values have high uncertainties, they suggest that starburst galaxies are capable of ejecting very large amounts of cool gas that will substantially impact their future evolution.

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arXiv2023

The Ionization and Dynamics of the Makani Galactic Wind

The Makani galaxy hosts the poster child of a galactic wind on scales of the circumgalactic medium. It consists of a two-episode wind in which the slow, outer wind originated 400 Myr ago (Episode I; R_I = 20-50 kpc) and the fast, inner wind is 7 Myr old (Episode II; R_II = 0-20 kpc). While this wind contains ionized, neutral, and molecular gas, the physical state and mass of the most extended phase--the warm, ionized gas--is unknown. Here we present Keck optical spectra of the Makani outflow. These allow us to detect hydrogen lines out to r = 30-40 kpc and thus constrain the mass, momentum, and energy in the wind. Many collisionally-excited lines are detected throughout the wind, and their line ratios are consistent with 200-400 km/s shocks that power the ionized gas, with v_shock = $σ$_wind. Combining shock models, density-sensitive line ratios, and mass and velocity measurements, we estimate that the ionized mass and outflow rate in the Episode II wind could be as high as that of the molecular gas: M_II(HII) ~ M_II(H_2) = (1-2)x10^9 Msun and dM/dt_II(HII) ~ dM/dt_II(H_2) = 170-250 Msun/yr. The outer wind has slowed, so that dM/dt_I(HII) ~ 10 Msun/yr, but it contains more ionized gas: M_I(HII) = 5x10^9 Msun. The momentum and energy in the recent Episode II wind imply a momentum-driven flow (p ``boost" ~ 7) driven by the hot ejecta and radiation pressure from the Eddington-limited, compact starburst. Much of the energy and momentum in the older Episode I wind may reside in a hotter phase, or lie further into the CGM.

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arXiv2023

The Impact of Star-Formation-Rate Surface Density on the Electron Density and Ionization Parameter of High-Redshift Galaxies

We use the large spectroscopic dataset of the MOSFIRE Deep Evolution Field (MOSDEF) survey to investigate some of the key factors responsible for the elevated ionization parameters (U) inferred for high-redshift galaxies, focusing in particular on the role of star-formation-rate surface density (Sigma_SFR). Using a sample of 317 galaxies with spectroscopic redshifts z~1.9-3.7, we construct composite rest-frame optical spectra in bins of Sigma_SFR and infer electron densities, n_e, using the ratio of the [OII] 3727, 3730 doublet. Our analysis suggests a significant (~3 sigma) correlation between n_e and Sigma_SFR. We further find significant correlations between U and Sigma_SFR for composite spectra of a subsample of 113 galaxies, and for a smaller sample of 25 individual galaxies with inferences of U. The increase in n_e -- and possibly also the volume filling factor of dense clumps in HII regions -- with Sigma_SFR appear to be important factors in explaining the relationship between U and Sigma_SFR. Further, the increase in n_e and SFR with redshift at a fixed stellar mass can account for most of the redshift evolution of U. These results suggest that the gas density, which sets n_e and the overall level of star-formation activity, may play a more important role than metallicity evolution in explaining the elevated ionization parameters of high-redshift galaxies.

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arXiv2022

The dust-to-gas mass ratio of luminous galaxies as a function of their metallicity at cosmic noon

We aim to quantify the relation between the dust-to-gas mass ratio (DTG) and gas-phase metallicity of $z=$2.1-2.5 luminous galaxies and contrast this high-redshift relation against analogous constraints at z$=$0. We present a sample of ten star-forming main-sequence galaxies in the redshift range $2.1<z<2.5$ with rest-optical emission-line information available from the MOSDEF survey and with ALMA 1.2 millimetre and CO J$=$3-2 follow-up observations. The galaxies have stellar masses ranging from $10^{10.3}$ to $10^{10.6}\,\rm{M}_\odot$ and cover a range in star-formation rate from 35 to 145 $\rm{M}_\odot\,\rm{yr}^{-1}$. We calculated the gas-phase oxygen abundance of these galaxies from rest-optical nebular emission lines (8.4 < $12 + \log{(\rm{O/H})} < 8.8$, corresponding to 0.5 - 1.25 Z$_\odot$). We estimated the dust and H$_2$ masses of the galaxies (using a metallicity-dependent CO-to-H$_2$ conversion factor) from the 1.2~mm and CO J$=$3-2 observations, respectively, from which we estimated a DTG. We find that the galaxies in this sample follow the trends already observed between CO line luminosity and dust-continuum luminosity from $z=0$ to $z=3$, extending such trends to fainter galaxies at $2.1<z<2.5$ than observed to date. We find no second-order metallicity dependence in the CO - dust-continuum luminosity relation for the galaxies presented in this work. The DTGs of main-sequence galaxies at $2.1<z<2.5$ are consistent with an increase in the DTG with gas-phase metallicity. The metallicity dependence of the DTG is driven by the metallicity dependence of the CO-to-H$_2$ conversion factor. Galaxies at z$=$2.1-2.5 are furthermore consistent with the DTG-metallicity relation found at z$=$0 (i.e. with no significant evolution), providing relevant constraints for galaxy formation models.

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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 Space Density of Intermediate Redshift, Extremely Compact, Massive Starburst Galaxies

We present a measurement of the intrinsic space density of intermediate redshift ($z\sim0.5$), massive ($M_{*} \sim 10^{11} \ \text{M}_{\odot}$), compact ($R_{e} \sim 100$ pc) starburst ($Σ_{SFR} \sim 1000 \ \text{M}_{\odot} \ \text{yr}^{-1} \text{kpc}^{-1}$) galaxies with tidal features indicative of them having undergone recent major mergers. A subset of them host kiloparsec scale, $>1000 \ \text{km}\ \text{s}^{-1}$ outflows and have little indication of AGN activity, suggesting that extreme star formation can be a primary driver of large-scale feedback. The aim for this paper is to calculate their space density so we can place them in a better cosmological context. We do this by empirically modeling the stellar populations of massive, compact starburst galaxies. We determine the average timescale for which galaxies that have recently undergone an extreme nuclear starburst would be targeted and included in our spectroscopically selected sample. We find that massive, compact starburst galaxies targeted by our criteria would be selectable for $\sim 148 ^{+27}_{-24}$ Myr and have an intrinsic space density $n_{\text{CS}} \sim (1.1^{+0.5}_{-0.3}) \times 10^{-6} \ \ \text{Mpc}^{-3}$. This space density is broadly consistent with our $z\sim0.5$ compact starbursts being the most extremely compact and star forming low redshift analogs of the compact star forming galaxies in the early Universe as well as them being the progenitors to a fraction of intermediate redshift post starburst and compact quiescent galaxies.

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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

AGN accretion and black hole growth across compact and extended galaxy evolution phases

The extent of black hole growth during different galaxy evolution phases and the connection between galaxy compactness and AGN activity remain poorly understood. We use Hubble Space Telescope imaging of the CANDELS fields to identify star-forming and quiescent galaxies at z=0.5-3 in both compact and extended phases and use Chandra X-ray imaging to measure the distribution of AGN accretion rates and track black hole growth within these galaxies. Accounting for the impact of AGN light changes ~20% of the X-ray sources from compact to extended galaxy classifications. We find that ~10-25% of compact star-forming galaxies host an AGN, a mild enhancement (by a factor ~2) compared to extended star-forming galaxies or compact quiescent galaxies of equivalent stellar mass and redshift. However, AGN are not ubiquitous in compact star-forming galaxies and this is not the evolutionary phase, given its relatively short timescale, where the bulk of black hole mass growth takes place. Conversely, we measure the highest AGN fractions (~10-30%) within the relatively rare population of extended quiescent galaxies. For massive galaxies that quench at early cosmic epochs, substantial black hole growth in this extended phase is crucial to produce the elevated black hole mass-to-galaxy stellar mass scaling relation observed for quiescent galaxies at z~0. We also show that AGN fraction increases with compactness in star-forming galaxies and decreases in quiescent galaxies within both the compact and extended sub-populations, demonstrating that AGN activity depends closely on the structural properties of galaxies.

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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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arXiv2021

The Effects of Stellar Population and Gas Covering Fraction on the Emergent Lyman Alpha Emission of High-Redshift Galaxies

We perform joint modeling of the composite rest-frame far-UV (FUV) and optical spectra of redshift 1.85<z<3.49 star-forming galaxies to deduce key properties of the massive stars, ionized ISM, and neutral ISM, with the aim of investigating the principal factors affecting the production and escape of Ly-alpha (Lya) photons. Our sample consists of 136 galaxies with deep Keck/LRIS and MOSFIRE spectra covering, respectively, Ly-beta through CIII] 1907, 1909; and [OII], [NeIII], H-beta, [OIII], H-alpha, [NII], and [SII]. Spectral and photoionization modeling indicate that the galaxies are uniformly consistent with stellar population synthesis models that include the effects of stellar binarity. Over the dynamic range of our sample, there is little variation in stellar and nebular abundance with Lya equivalent width, W(Lya), and only a marginal anti-correlation between age and W(Lya). The inferred range of ionizing spectral shapes is insufficient to solely account for the variation in W(Lya). Rather, the covering fraction of optically-thick HI appears to be the principal factor modulating the escape of Lya, with most of the Lya photons in down-the-barrel observations of galaxies escaping through low-column-density or ionized channels in the ISM. Our analysis shows that a high star-formation-rate surface density, Sigma_SFR, particularly when coupled with a low galaxy potential (i.e., low stellar mass), can aid in reducing the covering fraction and ease the escape of Lya photons. We conclude with a discussion of the implications of our results for the escape of ionizing radiation at high redshift.

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arXiv2021

Physical Properties of Massive Compact Starburst Galaxies with Extreme Outflows

We present results on the nature of extreme ejective feedback episodes and the physical conditions of a population of massive ($\rm M_* \sim 10^{11} M_{\odot}$), compact starburst galaxies at z = 0.4-0.7. We use data from Keck/NIRSPEC, SDSS, Gemini/GMOS, MMT, and Magellan/MagE to measure rest-frame optical and near-IR spectra of 14 starburst galaxies with extremely high star formation rate surface densities (mean $\rm Σ_{SFR} \sim 3000 \,M_{\odot} yr^{-1} kpc^{-2}$) and powerful galactic outflows (maximum speeds v$_{98} \sim$ 1000-3000 km s$^{-1}$). Our unique data set includes an ensemble of both emission [OII]$λλ$3726,3729, H$β$, [OIII]$λλ$4959,5007, H$α$, [NII]$λλ$6548,6583, and [SII]$λλ$6716,6731) and absorption MgII$λλ$2796,2803, and FeII$λ$2586) lines that allow us to investigate the kinematics of the cool gas phase (T$\sim$10$^4$ K) in the outflows. Employing a suite of line ratio diagnostic diagrams, we find that the central starbursts are characterized by high electron densities (median n$_e \sim$ 530 cm$^{-3}$), and high metallicity (solar or super-solar). We show that the outflows are most likely driven by stellar feedback emerging from the extreme central starburst, rather than by an AGN. We also present multiple intriguing observational signatures suggesting that these galaxies may have substantial Lyman continuum (LyC) photon leakage, including weak [SII] nebular emission lines. Our results imply that these galaxies may be captured in a short-lived phase of extreme star formation and feedback where much of their gas is violently blown out by powerful outflows that open up channels for LyC photons to escape.

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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

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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