The Effects of Stellar Population and Gas Covering Fraction on the Emergent Lyα Emission of High-redshift Galaxies*

Abstract

We perform joint modeling of the composite rest-frame far-UV and optical spectra of redshift 1.85 ≤ z ≤ 3.49 star-forming galaxies to deduce key properties of the massive stars, ionized interstellar medium (ISM), and neutral ISM, with the aim of investigating the principal factors affecting the production and escape of Lyα photons. Our sample consists of 136 galaxies with deep Keck/LRIS and MOSFIRE spectra covering, respectively, Lyβ through C iii] λλ1907, 1909 and [O ii], [Ne iii], Hβ, [O iii], Hα, [N ii], and [S ii]. Spectral and photoionization modeling indicates 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 Lyα equivalent width, W λ (Lyα), and only a marginal anticorrelation between age and W λ (Lyα). The inferred range of ionizing spectral shapes is insufficient to solely account for the variation in W λ (Lyα); rather, the covering fraction of optically thick H i appears to be the principal factor modulating the escape of Lyα, with most of the Lyα 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, ΣSFR, 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 Lyα photons. We conclude with a discussion of the implications of our results for the escape of ionizing radiation at high redshift.

The Low-redshift Lyman Continuum Survey. II. New Insights into LyC Diagnostics

“…These galaxies are likely analogous to LBGs, suggesting that LBGs are only weakly leaking, if at all. Reddy et al (2022) find that these galaxies have older burst ages (continuous star formation ages of ∼100 Myr), moderate ionization parameters ( U log 3 ~-), and relatively higher metallicities (∼30%−40% solar), which could readily account for the observed O 32 values. These same properties may be associated with low levels of LyC escape.…”

“…We did find a significant trend between f esc LyC and sSFR in Section 3.8, which may indicate that strong feedback better facilitates LyC escape in weaker gravitational potentials. At z ∼ 2.3 (Sanders et al 2016), z ∼ 3 (Reddy et al 2022), and z = 7 (Endsley et al 2021), galaxies tend to have sSFR in the 1-10 Gyr −1 range, which corresponds to weaker LCEs and nonemitters at z ∼ 0.3. Taking the Section 3.8 results at face value, these higher-redshift galaxies might not be prodigious LCEs.…”

“…With these physical and empirical caveats in mind, recent cosmological hydrodynamic simulations have predicted where high-redshift LCEs should appear in the O 32 −R 32 diagnostic (e.g., Katz et al 2020). We show the distribution of LzLCS (Izotov et al 2016a(Izotov et al , 2016b(Izotov et al , 2018a(Izotov et al , 2018b(Izotov et al , 2021Wang et al 2019) and published LCEs (de Barros et al 2016;Nakajima et al 2020) and LAEs (Reddy et al 2022) at z ∼ 2−3 in Figure 26, along with z ä [0.2, 0.4] star-forming galaxies taken from the Thomas et al (2013) SDSS catalog for comparison. Simulations by Barrow et al (2020) suggest that the entire distribution of LCEs shifts 0.5 dex lower in R 23 at high redshift owing to decreases in metallicity while maintaining high O 32 through high ionization parameter.…”

“…Other properties may readily persist at higher redshifts (e.g., Hβ EW, [O III] λ5007 EW, O 32 , R 23 ; see Nakajima et al 2020;Boyett et al 2021;Endsley et al 2021;Reddy et al 2022;Saxena et al 2022) but are difficult to observe at z  6 without space-based facilities like JWST. However, as we enter the JWST era, indirect indicators of f esc LyC will prove immensely useful via the trends presented in Section 3.…”

Lyα Halos around [O iii]-selected Galaxies in HETDEX

“…Galaxies selected via other methods such as LBGs or via their rest-frame optical emission lines may have similar physical properties, but with smaller Lyα escape fractions than LAEs. Erb et al (2016), Hathi et al (2016), Trainor et al (2016Trainor et al ( , 2019, and Reddy et al (2022) argue that LAEs have different properties from other star-forming galaxies, such as less dust and metal content, lower star formation rates (SFR) and stellar masses, and higher H I covering fractions. Conversely, Hagen et al (2016) and Shimakawa et al (2017) report no statistical difference between the properties of the samples of LAEs and rest-frame optical emission-line galaxies except at high stellar masses.…”