arXiv subjects

Cody Carr

Publications and source records attributed to Cody Carr.

27 records · Page 2Linked to original sources

arXiv2023

CLASSY VII Lyα Profiles: The Structure and Kinematics of Neutral Gas and Implications for LyC Escape in Reionization-Era Analogs

Lyman-alpha line profiles are a powerful probe of ISM structure, outflow speed, and Lyman continuum escape fraction. In this paper, we present the Ly$α$ line profiles of the COS Legacy Archive Spectroscopic SurveY, a sample rich in spectroscopic analogs of reionization-era galaxies. A large fraction of the spectra show a complex profile, consisting of a double-peaked Ly$α$ emission profile in the bottom of a damped, Ly$α$ absorption trough. Such profiles reveal an inhomogeneous interstellar medium (ISM). We successfully fit the damped Ly$α$ absorption (DLA) and the Ly$α$ emission profiles separately, but with complementary covering factors, a surprising result because this approach requires no Ly$α$ exchange between high-$N_\mathrm{HI}$ and low-$N_\mathrm{HI}$ paths. The combined distribution of column densities is qualitatively similar to the bimodal distributions observed in numerical simulations. We find an inverse relation between Ly$α$ peak separation and the [O III]/[O II] flux ratio, confirming that the covering fraction of Lyman-continuum-thin sightlines increases as the Ly$α$ peak separation decreases. We combine measurements of Ly$α$ peak separation and Ly$α$ red peak asymmetry in a diagnostic diagram which identifies six Lyman continuum leakers in the CLASSY sample. We find a strong correlation between the Ly$α$ trough velocity and the outflow velocity measured from interstellar absorption lines. We argue that greater vignetting of the blueshifted Ly$α$ peak, relative to the redshifted peak, is the source of the well-known discrepancy between shell-model parameters and directly measured outflow properties. The CLASSY sample illustrates how scattering of Ly$α$ photons outside the spectroscopic aperture reshapes Ly$α$ profiles as the distances to these compact starbursts span a large range.

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arXiv2023

CLASSY VI: Density, Structure and Size of Galactic Outflows

Galaxy formation and evolution are regulated by the feedback from galactic winds. Absorption lines provide the most widely available probe of winds. However, since most data only provide information integrated along the line-of-sight, they do not directly constrain the radial structure of the outflows. In this paper, we present a method to directly measure the gas electron density in outflows (ne), which in turn yields estimates of outflow cloud properties (e.g., density, volume filling-factor, and sizes/masses). We also estimate the distance (r) from the starburst at which the observed densities are found. We focus on 22 local star-forming galaxies primarily from the COS Legacy Archive Spectroscopic SurveY (CLASSY). In half of them, we detect absorption lines from fine structure excited transitions of Si II (i.e., Si II*). We determine ne from relative column densities of Si II and Si II*, given Si II* originates from collisional excitation by free electrons. We find that the derived ne correlates well with the galaxy's star-formation rate per unit area. From photoionization models or assuming the outflow is in pressure equilibrium with the wind fluid, we get r ~ 1 to 2 * rstar or ~ 5 * rstar, respectively, where rstar is the starburst radius. Based on comparisons to theoretical models of multi-phase outflows, nearly all of the outflows have cloud sizes large enough for the clouds to survive their interaction with the hot wind fluid. Most of these measurements are the first-ever for galactic winds detected in absorption lines and, thus, will provide important constraints for future models of galactic winds.

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arXiv2022

A Semi-Analytical Line Transfer (SALT) Model III: Galactic Inflows

We present calculations of ultraviolet spectra resulting from the scattering of photons by gas in-falling onto an isotropically emitting source of radiation. The model is based on an adaptation of the semi-analytical line transfer (SALT) code of Scarlata & Panagia (2015), and designed to interpret the inverse P-Cygni profiles observed in the spectra of partially ionized galactic inflows. In addition to presenting the model, we explore the parameter space of the inflowing SALT model and recreate various physically motivated scenarios including spherical inflows, inflows with covering fractions less than unity, and galactic fountains (i.e., galactic systems with both an inflowing and outflowing component). The resulting spectra from inflowing gas show spectral features that could be misinterpreted as ISM features in low resolution spectroscopy ($σ\approx 120$ $\rm{km }$ $\rm{s}^{-1}$), suggesting that the total number of galactic systems with inflows is undercounted. Our models suggest that observations at medium resolution ($R = 6000$ or $σ\approx 50$ $\rm{km }$ $\rm{s}^{-1}$) that can be obtained with 8m-class telescopes will be able to resolve the characteristic inverse P Cygni profiles necessary to identify inflows.

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arXiv2022

Testing SALT Approximations with Numerical Radiation Transfer Code Part 1: Validity and Applicability

Absorption line spectroscopy offers one of the best opportunities to constrain the properties of galactic outflows and the environment of the circumgalactic medium. Extracting physical information from line profiles is difficult, however, for the physics governing the underlying radiation transfer is complicated and depends on many different parameters. Idealized analytical models are necessary to constrain the large parameter spaces efficiently, but are typically plagued by model degeneracy and systematic errors. Comparison tests with idealized numerical radiation transfer codes offer an excellent opportunity to confront both of these issues. In this paper, we present a detailed comparison between SALT, an analytical radiation transfer model for predicting UV spectra of galactic outflows, with the numerical radiation transfer software, RASCAS. Our analysis has lead to upgrades to both models including an improved derivation of SALT and a customizable adaptive mesh refinement routine for RASCAS. We explore how well SALT, when paired with a Monte Carlo fitting procedure, can recover flow parameters from non-turbulent and turbulent flows. When the velocity and density gradients are excluded, we find that flow parameters are well recovered from high resolution (20 $\rm{km}$ $\rm{s}^{-1}$) data and moderately well from medium resolution (100 $\rm{km}$ $\rm{s}^{-1}$) data without turbulence at a S/N = 10, while derived quantities (e.g., mass outflow rates, column density, etc.) are well recovered at all resolutions. In the turbulent case, biased errors emerge in the recovery of individual parameters, but derived quantities are still well recovered.

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arXiv2022

Tracing Lyman-alpha and Lyman Continuum Escape in Galaxies with Mg II Emission

Star-forming galaxies are considered the likeliest source of the H I ionizing Lyman Continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above z >~ 6, the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of indirect indicators for LyC that can be calibrated at lower redshifts and applied in the Epoch of Reionization. Emission from Mg II \ly\ly 2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for 8 LyC emitter candidates, selected to have strong Mg II emission lines. We securely detect LyC emission in 50% (4/8) galaxies with 2$σ$ significance. This high detection rate suggests that strong Mg II emitters might be more likely to leak LyC than similar galaxies without strong Mg II. Using photoionization models, we constrain the escape fraction of Mg II as ~ 15 -- 60%. We confirm that the escape fraction of Mg II correlates tightly with that of Lyman-alpha (LyA), which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg II emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg II emission can be adopted to estimate the escape fraction of LyA and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the Epoch of Reionization.

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arXiv2022

The Low-Redshift Lyman Continuum Survey II: New Insights into LyC Diagnostics

The Lyman continuum (LyC) cannot be observed at the epoch of reionization (z {\gtrsim} 6) due to intergalactic H I absorption. To identify Lyman continuum emitters (LCEs) and infer the fraction of escaping LyC, astronomers have developed various indirect diagnostics of LyC escape. Using measurements of the LyC from the Low-redshift Lyman Continuum Survey (LzLCS), we present the first statistical test of these diagnostics. While optical depth indicators based on Lyα, such as peak velocity separation and equivalent width, perform well, we also find that other diagnostics, such as the [O III]/[O II] flux ratio and star formation rate surface density, predict whether a galaxy is a LCE. The relationship between these galaxy properties and the fraction of escaping LyC flux suggests that LyC escape depends strongly on H I column density, ionization parameter, and stellar feedback. We find LCEs occupy a range of stellar masses, metallicities, star formation histories, and ionization parameters, which may indicate episodic and/or different physical causes of LyC escape.

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arXiv2022

The Low-Redshift Lyman Continuum Survey I: New, Diverse Local Lyman-Continuum Emitters

The origins of Lyman continuum (LyC) photons responsible for the reionization of the universe are as of yet unknown and highly contested. Detecting LyC photons from the epoch of reionization is not possible due to absorption by the intergalactic medium, which has prompted the development of several indirect diagnostics to infer the rate at which galaxies contribute LyC photons to reionize the universe by studying lower-redshift analogs. We present the Low-redshift Lyman Continuum Survey (LzLCS) comprising measurements made with HST/COS for a z=0.2-0.4 sample of 66 galaxies. After careful processing of the FUV spectra, we obtain a total of 35 Lyman continuum emitters (LCEs) detected with 97.725% confidence, nearly tripling the number of known local LCEs. We estimate escape fractions from the detected LyC flux and upper limits on the undetected LyC flux, finding a range of LyC escape fractions up to 50%. Of the 35 LzLCS LCEs, 12 have LyC escape fractions greater than 5%, more than doubling the number of known local LCEs with cosmologically relevant LyC escape.

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arXiv2020

The Effect of Bi-conical Outflows on Ly$α$ Escape From Green Peas

We analyze the spectra of $10$ Green Pea galaxies, previously studied by Henry et al. (2015), using a semi-analytical line transfer (SALT) model to interpret emission and absorption features observed in UV galactic spectra. We focus our analysis on various ionization states of silicon, associated with the cool ($\sim 10^4$ K) and warm ($\sim 10^5$ K) gas. By analyzing low-ionization lines, we study the relationships between the distribution and kinematics of the outflowing H I gas and the observed Ly$α$ escape fraction, $f_{esc}^{Lyα}$, as well as the Ly$α$ emission peak separation, $Δ_{peak}$. We find that outflow geometries which leave a portion of the source uncovered along the line of sight create the best conditions for Ly$α$ escape and have narrow peak separations, while geometries which block the observer's view of the source create the worst conditions for Ly$α$ escape and have large peak separations. To isolate the effects of outflow kinematics, we restricted our testing set to galaxies with spherical outflows and found that $f_{esc}^{Lyα}$ and the Ly$α$ luminosity both increase with the extent of the galactic winds. A simple estimate suggests that the collisional excitation of neutral hydrogen by free electrons in the cool gas of the winds can account for the Ly$α$ luminosity observed in these objects. Finally, we speculate on the relationship between outflows and the escape of ionizing radiation from the CGM.

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arXiv2018

A Semi-Analytical Line Transfer (SALT) Model II: the effects of a Bi-Conical geometry

We generalize the semi-analytical line transfer (SALT) model recently introduced by Scarlata \& Panagia (2015) for modeling galactic outflows, to account for bi-conical geometries of various opening angles and orientations with respect to the line-of-sight to the observer, as well as generalized velocity fields. We model the absorption and emission component of the line profile resulting from resonant absorption in the bi-conical outflow. We show how the outflow geometry impacts the resulting line profile. We use simulated spectra with different geometries and velocity fields to study how well the outflow parameters can be recovered. We find that geometrical parameters (including the opening angle and the orientation) are always well recovered. The density and velocity field parameters are reliably recovered when both an absorption and an emission component are visible in the spectra. This condition implies that the velocity and density fields for narrow cones oriented perpendicular to the line of sight will remain unconstrained.

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