Low-z Lyman Continuum Survey (LzLCS)

• LzLCS is a systematic survey of low-redshift (z ~ 0.3) star-forming galaxies aimed at detecting escaping Lyman continuum photons and investigating interstellar and circumgalactic feedback.
• It utilizes HST/COS observations and high-resolution Mg II spectroscopy, analyzed with SALT and CLOUDY, to model multiphase outflows and neutral gas properties.
• The study reveals that radiation-dominated feedback in young starbursts temporarily suppresses neutral outflows, enhancing LyC escape and offering insights into cosmic reionization.

The Low-redshift Lyman Continuum Survey (LzLCS) is a comprehensive, systematically selected paper of star-forming galaxies at $0.23 \lesssim z \lesssim 0.37$, designed to directly detect escaping Lyman continuum (LyC; $\lambda < 912$ Å) photons using the Hubble Space Telescope/COS and to determine how the multiphase interstellar and circumgalactic medium (ISM/CGM), stellar feedback, and galaxy morphology regulate the escape fraction ($f_{esc}^{\rm LyC}$). Recent analysis of high-resolution follow-up spectroscopy in the Mg II $\lambda\lambda2796,2803$ lines, supplemented by photoionization modeling, has yielded the most detailed observational constraints to date on the connection between feedback processes—radiation pressure, stellar winds, and supernovae—and LyC escape in the local universe (Carr et al., 8 Sep 2024).

1. Survey Design, Sample Selection, and Observational Data

The LzLCS parent sample consists of 89 UV-bright, non-AGN star-forming galaxies at $z \sim 0.3$ observed with HST/COS G140L for LyC measurements [Flury et al. 2022a, 2022b]. Galaxies were selected based on (i) secure LyC detection or stringent upper limits, (ii) high GALEX FUV brightness (yielding S/N $\gtrsim$ 10 per resolution element at Mg II), and (iii) the availability of diagnostic emission lines (O II, O III, H$\beta$). The Mg II follow-up focused on 29 galaxies, with eight observed using VLT/X-Shooter ($R \sim 5400$–8900, 3000–5600 Å) and 23 using the MMT Blue Spectrograph ($R \sim 3300$, 3100–4100 Å), including two overlaps.

Mg II $\lambda2796,\lambda2803$ lines were extracted after continuum normalization, and velocity-resolved profiles at instrumental FWHM $\sim$55–90 km s$^{-1}$ were analyzed for outflow kinematics. High-ionization tracers (notably [O III] 5007 Å) from archival HST/STIS and ground-based spectroscopy were used to probe warm ($T\sim10^4$–$10^5$ K) winds, facilitating an investigation of the multiphase structure of galactic outflows.

2. Multiphase Outflow Modeling and Photoionization Analysis

The outflow structure was modeled using the Semi-Analytic Line Transfer (SALT) framework, which treats galactic winds as biconical flows characterized by half-opening angle $\alpha$, inclination $\psi$, density profile $n(r)=n_0(r/R_{\rm SF})^{-\delta}$, and velocity law $v(r)$ parameterized by $v_0$, exponent $\gamma$, and terminal velocity $v_\infty$ beyond radius $R_W$.

Key physical relations include:

• Base density determined by the observed Mg II optical depth:

$$\tau = \frac{\pi e^2}{m_e c} n_0 \frac{R_{\rm SF}}{v_0}$$

• Monte Carlo integration over the wind yields the Mg$^+$ column density $N_{\rm Mg^+}$.
• Photoionization modeling with CLOUDY computes a typical neutral hydrogen to Mg$^+$ column density ratio $\alpha_{\rm H^0/Mg^+} \sim 10^{4.3 \pm 0.3}$.
• The bulk LyC escape fraction is predicted from a porosity-weighted, multiphase ISM model:

$$f_{esc}^{\rm LyC} = \left\langle 1 - f_c (1 - e^{-\sigma_{\rm UV} N_{\rm H^0,i}}) \right\rangle_i\, e^{-0.4\,E(B-V)_{\rm UV} k(912)}$$

with $f_c$ the porosity, $\sigma_{\rm UV} = 6.3 \times 10^{-18}$ cm$^2$, and $k(912)$ the dust attenuation curve at 912 Å.

Mass ($\dot{M}$), momentum ($\dot{P}$), and energy ($\dot{E}$) outflow rates are evaluated at $r = R_{\rm SF}$ (set to the observed UV half-light radius), and compared to star-formation-driven input rates [Murray et al. 2005]: $$\eta = \frac{\dot{M}_{\rm out}}{\rm SFR}, \quad \zeta = \frac{\dot{P}_{\rm out}}{\dot{P}_*}, \quad \epsilon = \frac{\dot{E}_{\rm out}}{\dot{E}_*}$$

3. Stellar Population Ages and Feedback Modes

The rest-frame far-UV SED (950–1250 Å) of each galaxy was fitted with STARBURST99 templates, assuming continuous star formation and a Salpeter IMF, to decompose the stellar light into four age bins:

• $t<3$ Myr: Pre-supernova (radiation-dominated) phase
• $3
• $6
• $8

A galaxy is classified “radiation-feedback dominated” if $>50\%$ of the UV light arises from $t<6$ Myr stars; “supernova-feedback dominated” if $f_\star(t>6\,\rm Myr)$ is large.

4. Feedback Dependence of LyC Escape: Observational Results

The principal findings indicate a dichotomy in feedback mode and LyC leakage:

• Radiation Feedback Dominance and Efficient LyC Escape:
  • Galaxies with high $f_{esc}^{\rm LyC} > 5\%$ are almost exclusively in the radiation-dominated phase ($t<6$ Myr), showing no blueshifted Mg II absorption (i.e., no detectable cool neutral outflow).
  • These systems exhibit broad wings in the high-ionization [O III] 5007 Å line, with terminal velocities $v_{\rm term} \gtrsim 500$ km s$^{-1}$.
  • The outflow rates and loading factors ($\eta$, $\zeta$, $\epsilon$) in neutral gas are exceptionally low: $10^{-2} < \eta < 1$, $10^{-3} < \zeta <0.1$, $10^{-4} < \epsilon < 10^{-2}$.
  • This regime is interpreted as the “catastrophic cooling” phase: radiation-driven winds in compact, high-SFR, low-metallicity clusters rapidly cool, fragment into dense clumps, open low covering fraction ionized channels, and enable transient but high LyC escape. The absence of Mg II absorption reflects the suppression of neutral, outflowing material.
• Supernova Feedback and Quenched LyC Escape:
  • Galaxies dominated by older ($t > 6$ Myr) populations (“supernovae feedback dominated”) display strong, blueshifted Mg II absorption—indicative of cool, neutral outflows—and typically lower $f_{esc}^{\rm LyC}$.
  • Both neutral (Mg II) and warm, ionized outflows ([O III]) are present, but the increased neutral covering fraction suppresses LyC leakage.
  • The neutral wind loading factors span the lower envelope of predictions for cool-phase outflows in galactic feedback models.

A temporal sequence emerges: strong LyC escape only occurs during the brief, pre-supernova, radiation-driven phase of a starburst; as the stellar population matures, mechanical feedback from supernovae reinstates cool neutral outflows, raising the covering fraction and shutting down LyC channels.

5. Theoretical Interpretation and Implications

The observed transition from high $f_{esc}^{\rm LyC}$, radiation-dominated, Mg II emission-only systems to low $f_{esc}^{\rm LyC}$, supernova-driven, Mg II absorption systems is consistent with numerical predictions of feedback “catastrophic cooling” in low-metallicity, compact star-forming environments [Silich et al. 2003; Gray et al. 2019; Pandya et al. 2021]. The absence of neutral outflow signatures in strong leakers implies that radiation pressure—rather than supernovae—is responsible for clearing low-column sightlines during the brief phase when LyC photons can escape efficiently.

In the context of $\Sigma_{\rm SFR}$ and $r_{1/2}$ (“feedback-efficient” parameter space), neutral outflow rates and velocities plummet, and Mg II kinematics decouple from those of the ionized wind. This reinforces the idea that feedback mode (radiative vs. mechanical) and stellar age dictate the multiphase porosity of the ISM and regulate LyC leakage.

A plausible implication for cosmic reionization is that transient, radiation-dominated phases in compact, low-$Z$ starbursts provide the most conducive conditions for LyC escape. This time-dependent feedback paradigm is critical for interpreting the physical nature of reionization-era “leakers” seen with JWST, as it predicts that LyC escape is both episodic and short-lived.

6. Methodological Considerations and Limitations

While the SALT+Cloudy model predicts the direction of trends between Mg II, neutral hydrogen, and LyC escape, it systematically overestimates $f_{esc}^{\rm LyC}$ relative to direct COS detections by a factor of a few. This discrepancy is reconciled by including a modest optical depth ($\tau_{\rm thin}\sim0.4$–1.3) in the transparent sightlines, consistent with a clumpy ISM in which Mg II traces only the optically thick regime. The adoption of multiphase/partial covering models is thus necessary for reliable LyC escape predictions.

The cooling rates, outflow geometries, and wind loading factors are derived under assumptions about metallicity, SFR, and ISM distribution, with normalization to empirical, star formation–based feedback recipes. The timing of feedback transitions and the degree of catastrophic cooling are sensitive to initial cluster compactness and metallicity, underscoring the need for multiwavelength and spatially resolved observational constraints.

7. Conclusions and Future Directions

The LzLCS reveals a clear phenomenological link between feedback mode and LyC escape in local star-forming galaxies. Efficient, high $f_{esc}^{\rm LyC}$ is achieved only during early, radiation-pressure dominated feedback, in starbursts with high $\Sigma_{\rm SFR}$, small $r_{1/2}$, and low metallicity—conditions that briefly suppress the presence of cool neutral winds and open ionized escape routes. As the stellar population ages, mechanical feedback from supernovae generates renewed neutral outflows, increases covering fractions, and curtails LyC leakage.

This evolutionary pathway serves as an empirically grounded template for understanding the time dependence of LyC escape, the physical origin of “leaker” diagnostics, and the regulatory mechanisms that shaped the timeline of cosmic reionization. These findings are immediately relevant for interpreting LyC leakage in high-redshift galaxies, where the interplay of feedback modes, star formation history, and ISM geometry drive the efficiency of ionizing photon escape (Carr et al., 8 Sep 2024).