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

Abstract

Lyα line profiles are a powerful probe of interstellar medium (ISM) structure, outflow speed, and Lyman-continuum escape fraction. In this paper, we present the Lyα line profiles of the Cosmic Origins Spectrograph (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 Lyα emission profile in the bottom of a damped, Lyα absorption trough. Such profiles reveal an inhomogeneous ISM. We successfully fit the damped Lyα absorption and the Lyα emission profiles separately, but with complementary covering factors, a surprising result because this approach requires no Lyα exchange between high-N H i and low-N H i paths. The combined distribution of column densities is qualitatively similar to the bimodal distributions observed in numerical simulations. We find an inverse relation between Lyα peak separation and the [O iii]/[O ii] flux ratio, confirming that the covering fraction of Lyman-continuum-thin sightlines increases as the Lyα peak separation decreases. We combine measurements of Lyα peak separation and Lyα red peak asymmetry in a diagnostic diagram, which identifies six Lyman-continuum leakers in the COS Legacy Archive Spectrocopy SurveY (CLASSY) sample. We find a strong correlation between the Lyα trough velocity and the outflow velocity measured from interstellar absorption lines. We argue that greater vignetting of the blueshifted Lyα 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 Lyα photons outside the spectroscopic aperture reshapes Lyα profiles because the distances to these compact starbursts span a large range.

Resolving the Mechanical and Radiative Feedback in J1044+0353 with Keck Cosmic Web Imager Spectral Mapping

“…This high-energy ionizing photon production problem has been observed in other local galaxies (Shirazi & Brinchmann 2012;Jaskot & Oey 2013;Stasińska et al 2015;Senchyna et al 2017;Schaerer et al 2019;Mingozzi et al 2022Mingozzi et al , 2023 and is a common feature of reionization-era galaxies (Stark et al 2015;Mainali et al 2017;Bunker et al 2023;Senchyna et al 2023;Topping et al 2024). The damped Lyα profile in the same FUV spectrum of J1044 +0353 (Hu et al 2023), however, indicates a low f esc value along our sight line. In known LyC leakers, the intensity of He II relative to Hβ appears to be driven by metallicity rather than f esc (Marques-Chaves et al 2022).…”

“…The stretching method is adapted from Lupton et al (2004) but generalized to more than three filters. The red circle of diameter 2 5 (≈350 pc) identifies the HST COS aperture used to study the starburst previously (Berg et al 2022;Xu et al 2022;Hu et al 2023). The three slicer pointings (white) outline the footprint of the KCWI mosaic cube.…”

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

“…In Figure 1, the CLASSY targets with at least one UV emission line (i.e., C III] λλ1907,9) are highlighted by a thick black edge: they all have generally low stellar masses and 12 + log(O/ H) 8.3. C III] λλ1907,9 is the most common UV emission line in CLASSY galaxies (observed in 28 objects) without considering Lyα (see Hu et al 2023 or Paper VII) because it is usually seen in low-mass SFGs (e.g., Rigby et al 2015;Du et al 2017;Maseda et al 2017). Specifically, all the CLASSY galaxies showing either He II λ1640 (19) or O III] λ1666 (22) also have C III] emission (see Paper IV).…”

Absorption Troughs of Lyα Emitters in HETDEX

“…The shape of the middle profile differs from the HETDEX stack, as it more closely follows a typical Voigt profile, rather than the "boxy" troughs seen in Figure 1. For these particular CLASSY profiles, Hu et al (2023) reports that they are indicative of an inhomogeneous ISM with outflows and can be modeled by splitting the H I covering factor between low-and high-density channels. Additionally, while several Lyα absorption profiles in CLASSY are saturated, none are negative.…”

“…If we instead restrict b < 200 km s −1 to reflect typical conditions of the ISM, the modeled profile more closely resembles the stack of the four CLASSY galaxies with Lyα absorption and emission (see the middle panel of Figure 3). The Lyα profiles in these CLASSY galaxies have been previously fitted in this manner (Hu et al 2023).…”