CLASSY. VI. The Density, Structure, and Size of Absorption-line Outflows in Starburst Galaxies

Abstract

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 (n e ), which in turn yields estimates of outflow cloud properties (e.g., density, volume filling factor, and sizes/masses). We also estimate the distance (r n ) 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 n e 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 n e 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 n ∼ 1–2r * or ∼5r *, respectively, where r * is the starburst radius. Based on comparisons to theoretical models of multiphase 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.

CLusters in the Uv as EngineS (CLUES). II. Subkiloparsec-scale Outflows Driven by Stellar Feedback

“…However, the lack of a clear correlation between v out and age in our sample does not allow us to apply a similar method in this work. The radii derived here are larger than the inner radii determined for the large-scale outflows in Chisholm et al (2016Chisholm et al ( , 2017 using photoionization models, while they overlap with the smaller sizes reported for the CLASSY sample (Xu et al 2023) using the nonresonant absorption lines.…”

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

“…We present a uniform analysis of the Lyα profiles. The outflow properties have been determined from the blueshifted components of the LIS resonance lines (Xu et al 2022) and the excited fine-structure lines (Xu et al 2023). The results provide new insight into the clumpiness of the ISM, as described by the relative covering fractions of high-N H I and low-N H I gas, yet also strongly suggest that the discrepancies between shell-model parameters and LIS absorption lines arise from aperture vignetting.…”

CLASSY. VIII. Exploring the Source of Ionization with UV Interstellar Medium Diagnostics in Local High-z Analogs*

“…As discussed in Section 2. scatter) increases at larger stellar masses, with only objects with M å  10 7 M e needing a two-component fit. We highlight here that all the galaxies characterized by a broad component in optical emission lines (apart Mrk 996 and SBS 0335-052 E) were found to have an outflowing component in rest-UV absorption lines analyzed by Xu et al (2022Xu et al ( , 2023 (CLASSY Paper III, Paper VI). Recent studies have shown that low-mass systems (M å < 10 9 M e ) such as those of the CLASSY survey can have irregular velocity fields in the ionized gas studied through optical emission lines, indicating the presence of noncircular motions (e.g., Marasco et al 2023).…”