Searching for the connection between ionizing-photon escape and the surface density of star formation at z~3

Abstract

The connection between the escape fraction of ionizing photons ($f_{\mathrm{esc}}$) and star-formation rate surface density (${\Sigma }_{\mathrm{SFR}}$) is a key input for reionization models, but remains untested at high redshift. We analyse 35 z~3 galaxies from the Keck Lyman Continuum Survey (KLCS) covered by deep, rest far-UV spectra of the Lyman continuum (LyC) and high-resolution HST V${}_{606}$ imaging, enabling estimates of both $f_{\mathrm{esc}}$ and rest-UV sizes. Using Sérsic profile fits to HST images and spectral-energy distribution fits to multi-band photometry, we measure effective sizes and star-formation rates for the galaxies in our sample, and separate the sample into two bins of ${\Sigma }_{\mathrm{SFR}}$. Based on composite spectra, we estimate <$f_{\mathrm{esc}}$> for both ${\Sigma }_{\mathrm{SFR}}$ subsamples, finding no significant difference in <$f_{\mathrm{esc}}$> between the two. To test the representativeness of the KLCS HST sample and the robustness of this result, we attempt to recover the well-established correlation between $f_{\mathrm{esc}}$ and Ly$\alpha$ equivalent width. This correlation is not significant within the KLCS HST sample, indicating that the sample is not sufficient for correlating $f_{\mathrm{esc}}$ and galaxy properties such as ${\Sigma }_{\mathrm{SFR}}$. We perform stacking simulations using the KLCS parent sample to determine the optimal sample size for robust probes of the $f_{\mathrm{esc}}$-${\Sigma }_{\mathrm{SFR}}$ connection to inform future HST LyC observing programs. For a program with a selection independent of ionizing properties, >= 90 objects are required; for one preferentially observing strongly-leaking LyC sources, >= 58 objects are required. More generally, measuring the connection between $f_{\mathrm{esc}}$ and ${\Sigma }_{\mathrm{SFR}}$ requires a larger, representative sample spanning a wide dynamic range in galaxies properties such as ${\Sigma }_{\mathrm{SFR}}$.

13 pages, 5 figures, accepted for publication in MNRAS

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• The THESAN project: ionizing escape fractions of reionization-era galaxies
• Inferring the HII region escape fraction of ionizing photons from infrared emission lines in metal-poor star-forming dwarf galaxies
• The connection between the escape of ionizing radiation and galaxy properties at z~3 in the Keck Lyman Continuum Spectroscopic Survey
• The AURORA Survey: Tracing Galactic Outflows at $z\gtrsim 2.5$ with JWST/NIRSpec NUV Absorption Lines