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Resolving the Origins and Pathways of Ionizing Radiation Escape with UV Integral Field Spectroscopy

Cody Carr, Renyue Cen, Brian Fleming, Sophia Flury, Stephan McCandliss, Sally Oey, Allison Strom

TL;DR

The paper defines a science case for resolving the origins and pathways of Lyman continuum escape with a UV integral field spectrograph on a Habitable Worlds Observatory. It links LyC escape to multiscale feedback in star-forming galaxies by detailing diagnostic avenues across the LyC, Ly$\alpha$, Lyman series, LIS, and WIS metal lines, plus FUV SED modeling, and prescribes instrument requirements (e.g., $R \sim 5{,}000$, diffraction-limited $<30$ mas, $3^{\prime\prime} \times 3^{\prime\prime}$ FOV) with MOS options to sample multiple SSCs per galaxy. The work argues that such capabilities would enable direct, cluster-scale mapping of feedback-driven escape channels and CGM structure, thereby clarifying the dominant mechanisms of LyC leakage and informing reionization-era galaxy models. Overall, it outlines a concrete observational pathway to connect local LyC leakage signatures to high-redshift reionization contributions using next-generation UV instrumentation.

Abstract

The Epoch of Reionization marks the last major phase transition in the early Universe, during which the majority of neutral hydrogen once filling the intergalactic medium was ionized by the first galaxies. The James Webb Space Telescope (JWST) is now identifying promising galaxy candidates capable of producing sufficient ionizing photons to drive this transformation. However, the fraction of these photons that escape into intergalactic space--the escape fraction--remains highly uncertain. Stellar feedback is thought to play a critical role in carving low-density channels that allow ionizing radiation to escape, but the dominant mechanisms, their operation, and their connection to observable signatures are not well understood. Local analogs of high-redshift galaxies offer a powerful alternative for studying these processes, since ionizing radiation is unobservable at high redshift due to intergalactic absorption. However, current UV space-based instrumentation lacks the spatial resolution and sensitivity required to fully address this problem. The core challenge lies in the multiscale nature of LyC escape: ionizing photons are generated on scales of 1--100 pc in super star clusters but must traverse the circumgalactic medium which can extend beyond 100 kpc. The proposed Habitable Worlds Observatory (HWO) will provide a platform for future UV instruments capable of resolving these scales. In this article, we present a science case for understanding how LyC photons escape from star-forming galaxies and define the observational requirements for future instruments aboard HWO, including a UV integral field spectrograph (IFS).

Resolving the Origins and Pathways of Ionizing Radiation Escape with UV Integral Field Spectroscopy

TL;DR

The paper defines a science case for resolving the origins and pathways of Lyman continuum escape with a UV integral field spectrograph on a Habitable Worlds Observatory. It links LyC escape to multiscale feedback in star-forming galaxies by detailing diagnostic avenues across the LyC, Ly, Lyman series, LIS, and WIS metal lines, plus FUV SED modeling, and prescribes instrument requirements (e.g., , diffraction-limited mas, FOV) with MOS options to sample multiple SSCs per galaxy. The work argues that such capabilities would enable direct, cluster-scale mapping of feedback-driven escape channels and CGM structure, thereby clarifying the dominant mechanisms of LyC leakage and informing reionization-era galaxy models. Overall, it outlines a concrete observational pathway to connect local LyC leakage signatures to high-redshift reionization contributions using next-generation UV instrumentation.

Abstract

The Epoch of Reionization marks the last major phase transition in the early Universe, during which the majority of neutral hydrogen once filling the intergalactic medium was ionized by the first galaxies. The James Webb Space Telescope (JWST) is now identifying promising galaxy candidates capable of producing sufficient ionizing photons to drive this transformation. However, the fraction of these photons that escape into intergalactic space--the escape fraction--remains highly uncertain. Stellar feedback is thought to play a critical role in carving low-density channels that allow ionizing radiation to escape, but the dominant mechanisms, their operation, and their connection to observable signatures are not well understood. Local analogs of high-redshift galaxies offer a powerful alternative for studying these processes, since ionizing radiation is unobservable at high redshift due to intergalactic absorption. However, current UV space-based instrumentation lacks the spatial resolution and sensitivity required to fully address this problem. The core challenge lies in the multiscale nature of LyC escape: ionizing photons are generated on scales of 1--100 pc in super star clusters but must traverse the circumgalactic medium which can extend beyond 100 kpc. The proposed Habitable Worlds Observatory (HWO) will provide a platform for future UV instruments capable of resolving these scales. In this article, we present a science case for understanding how LyC photons escape from star-forming galaxies and define the observational requirements for future instruments aboard HWO, including a UV integral field spectrograph (IFS).
Paper Structure (10 sections, 2 figures, 1 table)

This paper contains 10 sections, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. Science Case: Linking Stellar Feedback to LyC Escape and UV Observables
  3. Desired Observations and Current Limitations
  4. Lyman Continuum
  5. Lyman Alpha
  6. Lyman Series
  7. LIS Metal Lines
  8. WIS Metal Lines
  9. FUV spectral energy distribution
  10. Discussion and Requirements

Figures (2)

  • Figure 1: ACS/SBC synthetic filter images of two nearby SF galaxies McCandliss2007. Blue subplots show regions of diffuse Ly$\alpha$ emission. We count roughly 20 (left panel) and 30 (right panel) distinct star clusters in each galaxy. Spatially resolving such features will be critical for understanding how LyC photons escape from galaxies and linking them to observational signatures.
  • Figure 2: LyC escape according to the [O III] 5007Å / [O II] 3726,9Å ratio and the slope of the FUV continuum slope ($\beta$). The left panel shows values drawn from individual regions of the Sunburst Arc galaxy. Figure taken from Kim2023, with permission. The black points correspond to regions with LyC emission and the red points without. The purple point represents the average value measured over the whole galaxy. The left panel shows values derived from HST COS integrated spectra for entire galaxies selected from LzLCS. Data were taken from Flury2022aFlury2022_erratum. To understand the relationship between $f_{\mathrm{esc}}^{\mathrm{LyC}}$ and observables, we must resolve galaxies on the cluster scale.