arXiv subjects

Ryan Sanders

Publications and source records attributed to Ryan Sanders.

At least 19 recordsLinked to original sources

arXiv2026

The ALPINE-CRISTAL-JWST Survey: Gas-phase abundance gradients of main sequence star-forming galaxies and their kinematics at $4 < z < 6$

We present gas-phase radial metallicity profiles for 20 main-sequence galaxies at $4 0.05{\rm dexkpc^{-1}}$ at $1\sigma$, and none have a significant negative gradient. We investigated the correlation between the metallicity gradients and the intrinsic gas velocity dispersion $\sigma_0$ as well as the $V_{\rm rot}/\sigma_0$ ratio of the disks. Combining our sample with mass-matched literature samples at $3<z<7$, we found a negative shallow correlation between $V_{\rm rot}/\sigma_0$ and the metallicity gradients, but no strong relationships with $\sigma_0$. As $V_{\rm rot}/\sigma_0$ increases towards later cosmic times, the observed negative trend with $V_{\rm rot}/\sigma_0$ is consistent with the overall cosmic evolution of metallicity gradients from high to low redshifts. This suggests that disk maturity plays a crucial role in shaping the radial metallicity gradients. [Abridged abstract]

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arXiv2026

Consistent Gas-Phase Temperatures and Metallicities from UV and Optical Nebular Emission: A Reliable Foundation from z=0 to Cosmic Dawn

The rest-frame UV spectra of star-forming galaxies are increasingly important as they become one of the primary windows to probe the physical properties of cosmic dawn (z>8) galaxies with the James Webb Space Telescope. However, the systematic discrepancies between UV and optical gas-phase metallicity measurements remain poorly understood in the local universe, partly due to challenges in achieving precise comparisons between UV and optical spectra for the same objects. In this work, we introduce a novel method that leverages the HeII 1640 and HeII 4686 nebular emission lines to achieve accurate aperture and reddening corrections between UV and optical spectra. Here we apply this method to three nearby Blue Compact Dwarf (BCD) galaxies. Our results demonstrate that this approach enables precise measurements, with electron temperatures ($T_e$) derived from UV and optical spectra exhibiting closer agreement compared to previous studies, and O/H abundance agreeing within 0.1 dex. However, two BCDs appear to have lower UV-based electron temperatures $T_{e~1666} < T_{e~4363}$, in contrast to expectations from the temperature fluctuation model. We consider a variety of possible explanations for these unphysical temperatures - differential dust attenuation, aperture differences, and spatial extent of emission lines - but no suitable cause is identified. These findings suggest a complex gaseous environment associated with star formation, and underscore the need for additional observations to further investigate the nature of HeII nebular emission and address the systematic issues between UV and optical nebular properties. Nonetheless, the close empirical agreement of these results indicates that UV- and optical-based nebular temperature and abundance measurements can be reliably compared within 0.1 dex, providing a solid foundation for evolutionary studies from the local Universe to cosmic dawn.

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arXiv2025

The AURORA Survey: Constraining Chemical Enrichment Pathways at Cosmic Noon with Argon Abundances

We present argon abundances from a sample of 46 star-forming galaxies at $z=2-3.5$ from the Assembly of Ultradeep Rest-Optical Observations Revealing Astrophysics (AURORA) program. Although argon is an $\alpha-$element produced by Core Collapse Supernovae (CCSNe), the latest supernova yield models suggest additional argon production and enrichment by Type Ia supernovae (SNe Ia), unlike other $\alpha-$elements such as oxygen. To study the relationship between argon and oxygen abundances, we construct two median-stacked composite spectra for separate $z=2.0-2.6$ and $z=2.8-3.5$ redshift bins, presenting more representative measurements than previous samples that require individual detection of faint lines. Abundance ratios were determined using an empirical calibration based on the strength of the [ArIII]$\lambda7137$ emission line relative to the [OIII]$\lambda5008$ emission line. With this calibration, we estimate argon abundances (Ar/O) of $0.42^{+0.12}_{-0.10}\mbox{(Ar/O)}_{\odot}$ for the $\langle z \rangle = 2.26$ bin and $0.42^{+0.12}_{-0.11}\mbox{(Ar/O)}_{\odot}$ for the $\langle z \rangle = 3.15$ bin, suggesting minimal SNe Ia and dominant CCSNe enrichment in this sample. Comparison of our abundance measurements of $z\sim 2-3$ AURORA galaxies with chemical evolution modeling of Milky Way stars shows consistency with the Milky Way Bulge component, suggesting a rapid star-formation timescale. However, even larger samples of actively star-forming galaxies with available argon abundances, as well as comparisons between argon abundance and other critical galaxy properties (e.g., sSFR) and models (e.g., one tuned specifically to this redshift range) are needed to draw stronger conclusions on the role of argon in galactic chemical enrichment at Cosmic Noon.

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arXiv2025

The AURORA Survey: Ionizing Photon Production Efficiency with Minimal Nebular Dust Attenuation Systematics

We present ionizing photon production efficiencies (${\xi}_{\rm ion}$) for 63 z=1.5-6.9 star-forming galaxies using precise nebular dust attenuation corrections from the JWST/AURORA survey. A subset of objects within AURORA have individually-determined nebular dust attenuation curves, which vary significantly in shape and normalization, resulting in reduced systematic uncertainty when constraining the total attenuation of H${\alpha}$ luminosity, and thus the intrinsic ionizing output within our sample. We find evidence for positive correlations between ${\xi}_{\rm ion}$ and redshift, equivalent width of [OIII]${\lambda}$5007, and O32=[OIII]${\lambda}$5007/[OII]${\lambda}$3726,3729, and negative correlations between ${\xi}_{\rm ion}$ and stellar attenuation, UV luminosity (L$_{\rm UV}$), stellar mass, and direct-method metallicity. We test alternate dust prescriptions within this sample, and find that the total attenuation is lower when using the commonly-assumed Galactic extinction curve or when assuming that stellar attenuation is equal to nebular attenuation. We also find that assuming either of these alternate dust prescriptions can change the slope of relationships between ${\xi}_{\rm ion}$ and galaxy property, notably inducing a flat trend between ${\xi}_{\rm ion}$ and L$_{\rm UV}$ within AURORA. While the novel nebular dust curves derived from AURORA spectroscopy reveal obscured ionizing photon production within star-forming galaxies at these redshifts, a more complete understanding of stellar attenuation is required to fully reduce dust systematics on ${\xi}_{\rm ion}$ for inclusion in reionization models.

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arXiv2025

JWST Spectroscopic Insights Into the Diversity of Galaxies in the First 500 Myr: Short-Lived Snapshots Along a Common Evolutionary Pathway

We investigate the nature and spectroscopic diversity of early galaxies from a sample of 41 sources at z>10 with JWST/NIRSpec prism observations. We compare the properties of strong UV line emitters, traced by intense CIV emission, with those of more "typical" sources with weak or undetected CIV. The more typical (or "CIV-weak") sources reveal significant scatter in their CIII] line strengths, UV continuum slopes, and physical sizes, spanning CIII] equivalent widths of ~1-51 \r{A}, UV slopes of $\beta$~-1.6 to -2.6, and half-light radii of ~50-1000 pc. In contrast, CIV-strong sources occupy the tail of these distributions, with CIII] EWs of 16-51 \r{A}, UV slopes $\beta$<-2.5, compact morphologies ($r_{50}$<100 pc), and elevated star formation surface densities ($\Sigma_{SFR}$>100 $M_{\odot}yr^{-1}kpc^{-2}$). These properties suggest concentrated starbursts that temporarily outshine the host galaxy. Comparing average properties from composite spectra, we find the diversity of the sample is primarily driven by bursty star formation on very short timescales (<3 Myr), with strong CIV emitters observed at the apex of the bursts and sources devoid of emission lines during relative inactivity. An apparent association between strong CIV and enhanced nitrogen abundance suggests both may be modulated by the same duty cycle, reflecting a generic mode of star formation. We show that AGN are unlikely to contribute significantly to this duty cycle based on UV line diagnostics and photoionisation models. Our results support a picture whereby brief bursts and lulls can explain the spectral diversity and early growth of bright galaxies in the first 500 Myr.

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arXiv2025

Resolved Stellar and Nebular Kinematics of a Star-forming Galaxy at $z\sim2$

The kinematics of star-forming galaxy populations at high redshifts are integral to our understanding of disk properties, merger rates, and other defining characteristics. Nebular gas emission is a common tracer of galaxies' gravitational potentials and angular momenta, but is sensitive to non-gravitational forces as well as galactic outflows, and thus might not accurately trace the host galaxy dynamics. We present kinematic maps of young stars from rest-ultraviolet photospheric absorption in the star-forming galaxy CASSOWARY 13 (a.k.a. SDSS J1237+5533) at $z=1.87$ using the Keck Cosmic Web Imager, alongside nebular emission measurements from the same observations. Gravitational lensing magnification of the galaxy enables good spatial sampling of multiple independent lensed images. We find close agreement between the stellar and nebular velocity fields. We measure a mean local velocity dispersion of $\sigma = 64\pm12$ km$\,$s$^{-1}$ for the young stars, consistent with that of the H II regions traced by nebular C III] emission ($52\pm9$ km$\,$s$^{-1}$). The $\sim20$ km$\,$s$^{-1}$ average difference in line-of-sight velocity is much smaller than the local velocity width and the velocity gradient ($\gtrsim 100$ km$\,$s$^{-1}$). We find no evidence of asymmetric drift nor evidence that outflows bias the nebular kinematics, and thus conclude that nebular emission appears to be a reasonable dynamical tracer of young stars in the galaxy. These results support the picture of star formation in thick disks with high velocity dispersion at $z\sim2$, and represent an important step towards establishing robust kinematics of early galaxies using collisionless tracers.

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arXiv2024

Detection of gas inflow during the onset of a starburst in a low-mass galaxy at z=2.45

The baryon cycle is crucial for understanding galaxy formation, as gas inflows and outflows vary throughout a galaxy's lifetime and affect its star formation rate. Despite the necessity of accretion for galaxy growth at high redshifts, direct observations of inflowing gas have proven elusive especially at $z\gtrsim2$. We present spectroscopic analysis of a galaxy at redshift $z=2.45$ which exhibits signs of inflow in several ultraviolet interstellar absorption lines, with no clear outflow signatures. The absorption lines are redshifted by $\sim$250 km sec$^{-1}$ with respect to the systemic redshift, and C IV shows a prominent inverse P-Cygni profile. Simple stellar population models suggest that this galaxy has a low metallicity ($\sim$5% solar), with a very young starburst of age $\sim$4 Myr dominating the ultraviolet luminosity. The gas inflow velocity and nebular velocity dispersion suggest an approximate halo mass of order $\sim 10^{11}M_{\odot}$, a regime in which simulations predict that bursty star formation is common at this redshift. We conclude that this system is likely in the beginning of a cycle of bursty star formation, where inflow and star formation rates are high, but where supernovae and other feedback processes have not yet launched strong outflows. In this scenario, we expect the inflow-dominated phase to be observable (e.g., with net redshifted ISM absorption) for only a short timescale after a starburst onset. This result represents a promising avenue for probing the full baryon cycle, including inflows, during the formative phases of low-mass galaxies at high redshifts.

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arXiv2024

MSA-3D: dissecting galaxies at z~1 with high spatial and spectral resolution

Integral field spectroscopy (IFS) is a powerful tool for understanding the formation of galaxies across cosmic history. We present the observing strategy and first results of MSA-3D, a novel JWST program using multi-object spectroscopy in a slit-stepping strategy to produce IFS data cubes. The program observed 43 normal star-forming galaxies at redshifts $0.5 \lesssim z \lesssim 1.5$, corresponding to the epoch when spiral thin-disk galaxies of the modern Hubble sequence are thought to emerge, obtaining kpc-scale maps of rest-frame optical nebular emission lines with spectral resolution $R\simeq2700$. Here we describe the multiplexed slit-stepping method which is $>15$ times more efficient than the NIRSpec IFS mode for our program. As an example of the data quality, we present a case study of an individual galaxy at $z=1.104$ (stellar mass $M_{*} = 10^{10.3}~M_{\odot}$, star formation rate~$=3~M_{\odot}$ yr$^{-1}$) with prominent face-on spiral structure. We show that the galaxy exhibits a rotationally supported disk with moderate velocity dispersion ($\sigma = 36^{+5}_{-4}$~\kms), a negative radial metallicity gradient ($-0.020\pm0.002$~dex\,kpc$^{-1}$), a dust attenuation gradient, and an exponential star formation rate density profile which closely matches the stellar continuum. These properties are characteristic of local spirals, indicating that mature galaxies are in place at $z\sim1$. We also describe the customized data reduction and original cube-building software pipelines which we have developed to exploit the powerful slit-stepping technique. Our results demonstrate the ability of JWST slit-stepping to study galaxy populations at intermediate to high redshifts, with data quality similar to current surveys of the $z\sim0.1$ universe.

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arXiv2024

A spectroscopic analysis of the ionizing photon production efficiency in JADES and CEERS: implications for the ionizing photon budget

We have used a combined sample of JADES and CEERS objects in order to constrain ionizing photon production efficiency ($\xi_{\rm ion}$) from JWST/NIRSpec and JWST/NIRCam data. We examine 163 objects at 1.06 < z < 6.71 with significant (3$\sigma$) spectroscopic detections of H$\alpha$ and H$\beta$ in order to constrain intrinsic H$\alpha$ luminosities corrected from nebular dust attenuation via Balmer decrements. We constrain dust-corrected UV luminosities from best-fit spectral-energy distribution modeling. We find a sample median log$_{10}$($\xi{\rm ion,0}$/erg Hz$^{-1}$) = $25.29^{+0.29}_{-0.37}$, assuming f$_{\rm esc}$=0 for the escape fraction of Lyman continuum emission. We find significant correlation between $\xi_{\rm ion,0}$ and z, with 17 objects at z > 4.64 having median log$_{10}$($\xi_{\rm ion,0}$/erg Hz$^{-1}$) = $25.38^{+0.38}){-0.38}$, with those below having log$_{10}$($\xi_{\rm ion,0}$/erg Hz$^{-1}$) = $25.24^{+0.30}_{-0.33}$. We also find significant, positive correlations between $\xi_{\rm ion,0}$ and LUV; W{\lambda}([O iii]); [O iii]{\lambda}5007/[O ii]{\lambda}{\lambda}3726, 3729; and inverse correlations with metallicity. In contrast with some previous results, we find no trends between $\xi_{\rm ion,0}$ and stellar mass, stellar dust attenuation, or UV slope. Applying a multivariate fit to $\xi_{\rm ion,0}$, z, and MUV to an empirically-motivated model of reionization, and folding in f$_{\rm esc}$ estimates from direct observations of the Lyman continuum at z ~ 3 from the Keck Lyman Continuum Spectroscopic survey, we find that the number of ionizing photons entering the IGM causes reionization to end at z ~ 5 - 7.

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arXiv2024

Reply to: Effects of density and temperature variations on the metallicity of Mrk 71

In Chen et al., 2023 (C23; arXiv:2304.09898), we introduced a new method to directly measure temperature fluctuations and applied it to a nearby dwarf galaxy, Mrk 71, finding a temperature fluctuation parameter $t^2 = 0.008\pm 0.043$. This result is lower by $\sim 2\sigma$ than the value required to explain the abundance discrepancy (AD) in this object. In the Matters Arising article submitted by Mendez-Delgado et al. (arXiv:2310.01197), the authors claim that using the same data presented in C23 in a different way, it is possible to conclude that the measurements are consistent with a larger $t^2 \simeq 0.1$ inferred indirectly from recombination lines (RLs). However, this requires a higher density such that the infrared [O III] 52 $\mu$m and [O III] 88 $\mu$m lines -- which form the basis of the direct measurement method -- are mutually inconsistent. Moreover, to reach agreement between the direct $t^2$ measurement and the larger $t^2$ value inferred from RLs requires systematically varying four parameters by $\sim 1\sigma$ from their best-determined values, which collectively amount to a $\sim2\sigma$ difference, consistent with the significance ($\sim 2 \sigma$) originally reported in C23. Therefore, we conclude that the results of C23 hold, and that the combined optical and infrared [O III] data disfavour $t^2 \simeq 0.1$ at the $\approx2\sigma$ level in Mrk 71. Future work is nonetheless warranted to better understand the AD associated with both optical and infrared emission line analysis.

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arXiv2023

Unravelling the Dust Attenuation Scaling Relations and their Evolution

We explore the dependence of dust attenuation, as traced by the $\rm H_{\alpha}/\rm H_{\beta}$ Balmer decrement, on galactic properties by using a large sample of SDSS spectra. We use both Partial Correlation Coefficients (PCC) and Random Forest (RF) analysis to distinguish those galactic parameters that directly and primarily drive dust attenuation in galaxies, from parameters that are only indirectly correlated through secondary dependencies. We find that, once galactic inclination is controlled for, dust attenuation depends primarily on stellar mass, followed by metallicity and velocity dispersion. Once the dependence on these quantities is taken into account, there is no dependence on star formation rate. While the dependence on stellar mass and metallicity was expected based on simple analytical equations for the interstellar medium, the dependence on velocity dispersion was not predicted and we discuss possible scenarios to explain it. We identify a projection of this multi-dimensional parameters space which minimises the dispersion in terms of the Balmer decrement and which encapsulates the primary and secondary dependences of the Balmer decrement into a single parameter defined as the reduced mass $\mu = \log {\rm M}_{\star} +3.67 [{\rm O/H}] + 2.96 \log (\sigma_v/100~km~s^{-1})$. We show that the dependence of the Balmer decrement on this single parameter also holds at high redshift, suggesting that the processes regulating dust production and distribution do not change significantly through cosmic epochs at least out to z$\sim$2.

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arXiv2023

Accurate Oxygen Abundance of Interstellar Gas in Mrk 71 from Optical and Infrared Spectra

The heavy element content ("metallicity") of the Universe is a record of the total star formation history. Gas-phase metallicity in galaxies, as well as its evolution with time, is of particular interest as a tracer of accretion and outflow processes. However, metallicities from the widely-used electron temperature ($T_e$) method are typically ~2x lower than the values based on the recombination line method. This "abundance discrepancy factor" (ADF) is well known and is commonly ascribed to bias due to temperature fluctuations. We present a measurement of oxygen abundance in the nearby (3.4 Mpc) system, Mrk 71, using a combination of optical and far-IR emission lines to measure and correct for temperature fluctuation effects. Our far-IR result is inconsistent ($> 2 \sigma$ significance) with the metallicity from recombination lines and instead indicates little to no bias in the standard $T_e$ method, ruling out the long-standing hypothesis that the ADF is explained by temperature fluctuations for this object. Our results provide a framework to accurately measure metallicity across cosmic history, including with recent data reaching within the first billion years with JWST and the Atacama Large Millimeter Array (ALMA).

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arXiv2023

Early Results from GLASS-JWST. XXI: Rapid assembly of a galaxy at z=6.23 revealed by its C/O abundance

The abundance of carbon relative to oxygen (C/O) is a promising probe of star formation history in the early universe, as the ratio changes with time due to production of these elements by different nucleosynthesis pathways. We present a measurement of $\log{\mathrm{(C/O)}} = -1.01\pm0.12$ (stat) $\pm0.15$ (sys) in a $z=6.23$ galaxy observed as part of the GLASS-JWST Early Release Science Program. Notably, we achieve good precision thanks to the detection of the rest-frame ultraviolet O III], C III], and C IV emission lines delivered by JWST/NIRSpec. The C/O abundance is $\sim$0.8 dex lower than the solar value and is consistent with the expected yield from core-collapse supernovae, indicating that longer-lived intermediate mass stars have not fully contributed to carbon enrichment. This in turn implies rapid buildup of a young stellar population with age $\lesssim100$ Myr in a galaxy seen $\sim$900 million years after the Big Bang. Our chemical abundance analysis is consistent with spectral energy distribution modeling of JWST/NIRCam photometric data, which indicates a current stellar mass $\log\,\mathrm{M}_* / \mathrm{M_{sun}} = 8.4^{+0.4}_{-0.2}$ and specific star formation rate sSFR $\simeq 20$ Gyr$^{-1}$. These results showcase the value of chemical abundances and C/O in particular to study the earliest stages of galaxy assembly.

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arXiv2022

The Art of Measuring Physical Parameters in Galaxies: A Critical Assessment of Spectral Energy Distribution Fitting Techniques

The study of galaxy evolution hinges on our ability to interpret multi-wavelength galaxy observations in terms of their physical properties. To do this, we rely on spectral energy distribution (SED) models which allow us to infer physical parameters from spectrophotometric data. In recent years, thanks to the wide and deep multi-waveband galaxy surveys, the volume of high quality data have significantly increased. Alongside the increased data, algorithms performing SED fitting have improved, including better modeling prescriptions, newer templates, and more extensive sampling in wavelength space. We present a comprehensive analysis of different SED fitting codes including their methods and output with the aim of measuring the uncertainties caused by the modeling assumptions. We apply fourteen of the most commonly used SED fitting codes on samples from the CANDELS photometric catalogs at z~1 and z~3. We find agreement on the stellar mass, while we observe some discrepancies in the star formation rate (SFR) and dust attenuation results. To explore the differences and biases among the codes, we explore the impact of the various modeling assumptions as they are set in the codes (e.g., star formation histories, nebular, dust, and AGN models) on the derived stellar masses, SFRs, and A_V values. We then assess the difference among the codes on the SFR-stellar mass relation and we measure the contribution to the uncertainties by the modeling choices (i.e., the modeling uncertainties) in stellar mass (~0.1dex), SFR (~0.3dex), and dust attenuation (~0.3mag). Finally, we present some resources summarizing best practices in SED fitting.

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arXiv2022

CLASSY V: The impact of aperture effects on the inferred nebular properties of local star-forming galaxies

Strong nebular emission lines are an important diagnostic tool for tracing the evolution of star-forming galaxies across cosmic time. However, different observational setups can affect these lines, and the derivation of the physical nebular properties. We analyze 12 local star-forming galaxies from the COS Legacy Spectroscopy SurveY (CLASSY) to assess the impact of using different aperture combinations on the determination of the physical conditions and gas-phase metallicity. We compare optical spectra observed with the SDSS aperture, which has a 3" of diameter similar to COS, to IFU and longslit spectra, including new LBT/MODS observations of five CLASSY galaxies. We calculate the reddening, electron densities and temperatures, metallicities, star formation rates, and equivalent widths (EWs). We find that measurements of the electron densities and temperatures, and metallicity remained roughly constant with aperture size, indicating that the gas conditions are relatively uniform for this sample. However, using the IFU observations of 3 galaxies, we find that the E(B-V) values derived from the Balmer ratios decrease ( by up to 53%) with increasing aperture size. The values change most significantly in the center of the galaxies, and level out near the COS aperture diameter of 2.5". We examine the relative contributions from the gas and stars using the H$\alpha$ and [OIII] $\lambda$5007 EWs as a function of aperture light fraction, but find little to no variations within a given galaxy. These results imply that the optical spectra provide nebular properties appropriate for the FUV CLASSY spectra, even when narrow 1.0" long-slit observations are used.

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arXiv2022

CLASSY II: A technical Overview of the COS Legacy Archive Spectroscopic SurveY

The COS Legacy Archive Spectroscopic SurveY (CLASSY) is designed to provide the community with a spectral atlas of 45 nearby star-forming galaxies which were chosen to cover similar properties as those seen at high-z (z>6). The prime high level science product of CLASSY is accurately coadded UV spectra, ranging from ~1000-2000A, derived from a combination of archival and new data obtained with HST's Cosmic Origins Spectrograph (COS). This paper details the multi-stage technical processes of creating this prime data product, and the methodologies involved in extracting, reducing, aligning, and coadding far-ultraviolet (FUV) and near-ultraviolet (NUV) spectra. We provide guidelines on how to successfully utilize COS observations of extended sources, despite COS being optimized for point sources, and best-practice recommendations for the coaddition of UV spectra in general. Moreover, we discuss the effects of our reduction and coaddition techniques in the scientific application of the CLASSY data. In particular, we find that accurately accounting for flux calibration offsets can affect the derived properties of the stellar populations, while customized extractions of NUV spectra for extended sources are essential for correctly diagnosing the metallicity of galaxies via CIII] nebular emission. Despite changes in spectral resolution of up to ~25% between individual datasets (due to changes in the COS line spread function), no adverse affects were observed on the difference in velocity width and outflow velocities of isolated absorption lines when measured in the final combined data products, owing in-part to our signal-to-noise regime of S/N<20.

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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$\alpha$ emission. Outflow velocities are calculated from the centroids of the LIS absorption and/or Ly$\alpha$ 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 ($\Sigma_{\rm SFR}$). 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 $\Sigma_{\rm SFR}$ are not significantly correlated, which may be due to the limited dynamic range of $\Sigma_{\rm SFR}$ probed by our sample. The relationship between outflow velocity and $\Sigma_{\rm SFR}$ normalised by stellar mass ($\Sigma_{\rm sSFR}$), as a proxy for gravitational potential, suggests that strong outflows (e.g., > 200 km s$^{-1}$) appear ubiquitous above a threshold of log($\Sigma_{\rm sSFR}/\rm{yr}^{-1}\ \rm{kpc}^{-2}$) ~ -11.3, and that above this threshold, outflow velocity uncouples from $\Sigma_{\rm sSFR}$. 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

The COS Legacy Archive Spectroscopy SurveY (CLASSY) Treasury Atlas

Far-ultraviolet (FUV; ~1200-2000 angstroms) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of JWST will soon usher in a new era, pushing the UV spectroscopic frontier to higher redshifts than ever before, however, its success hinges on a comprehensive understanding of the massive star populations and gas conditions that power the observed UV spectral features. This requires a level of detail that is only possible with a combination of ample wavelength coverage, signal-to-noise, spectral-resolution, and sample diversity that has not yet been achieved by any FUV spectral database. We present the COS Legacy Spectroscopic SurveY (CLASSY) treasury and its first high level science product, the CLASSY atlas. CLASSY builds on the HST archive to construct the first high-quality (S/N_1500 >~ 5/resel), high-resolution (R~15,000) FUV spectral database of 45 nearby (0.002 < z < 0.182) star-forming galaxies. The CLASSY atlas, available to the public via the CLASSY website, is the result of optimally extracting and coadding 170 archival+new spectra from 312 orbits of HST observations. The CLASSY sample covers a broad range of properties including stellar mass (6.2 < logM_star(M_sol) < 10.1), star formation rate (-2.0 < log SFR (M_sol/yr) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O_32 < 38.0), reddening (0.02 < E(B-V < 0.67), and nebular density (10 < n_e (cm^-3) < 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a sample that is consistent with z~0 mass-metallicity relationship, but is offset to higher SFRs by roughly 2 dex, similar to z >~2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.

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