HST Imaging of the Ionizing Radiation from a Star-forming Galaxy at z = 3.794

Abstract

We report on the HST detection of the Lyman-continuum (LyC) radiation emitted by a galaxy at redshift z = 3.794, dubbed Ion1 (Vanzella et al. 2012). The LyC from Ion1 is detected at restframe wavelength λλ 820∼890Å with HST WFC3/UVIS in the F410M band (m 410 = 27.60 ± 0.36 m AB , peak SNR = 4.17 in a circular aperture with radius r = 0.12") and at λλ 700∼830Å with the VLT/VIMOS in the U-band (m U = 27.84 ± 0.19 m AB , peak SNR = 6.7 with a r = 0.6" aperture). A 20-hr VLT/VIMOS spectrum shows low-and high-ionization interstellar metal absorption lines, the P-Cygni profile of C IV and Lyα in absorption. The latter spectral feature differs from what observed in known LyC emitters, which show strong Lyα emission. An HST far-UV color map reveals that the LyC emission escapes from a region of the galaxy that is bluer than the rest, presumably because of lower dust obscuration. The F410M image shows that the centroid of the LyC emission is offset from the centroid of the non-ionizing UV emission by 0.12"±0.03", corresponding to 0.85±0.21 kpc (physical), and that its morphology is likely moderately resolved. These morphological characteristics favor a scenario where the LyC photons produced by massive stars escape from low H I columndensity "cavities" in the ISM, possibly carved by stellar wind and/or supernova. We also collect the VIMOS U-band images of a sample of 107 Lyman-break galaxies with spectroscopic redshifts at 3.40 < z < 3.95, i.e. sampling the LyC, and stack them with inverse-variance weights. No LyC emission is detected in the stacked image, resulting in a 32.5 m AB flux limit (1σ) and an upper limit of absolute LyC escape fraction f abs esc ≤ 0.63%. LyC emitters like Ion1 are very likely at the bright-end of the LyC luminosity function.

An uncontaminated measurement of the escaping Lyman continuum at z ∼ 3

“…f c and Lyα escape fraction have also been determined to predict LyC escape fractions at low redshift (Chisholm et al 2018;Gazagnes et al 2018). Additionally, HST analysis of a spatially resolved FUV colour map has shown that bluer colours are coincident with LyC escape, demonstrating that favorable H I and dust configurations lead to LyC escape (Ji et al 2020). Finally, the Lyα kinematics of a strongly-leaking z ∼ 3 galaxy have demonstrated the consistent geometry of LyC and Lyα escape (Vanzella et al 2020).…”

The synchrony of production and escape: half the bright Lyα emitters at z ≈ 2 have Lyman continuum escape fractions ≈50

“…In recent years, direct 𝑓 esc studies have largely concentrated on two redshift windows set by available UV instrumentation -one at 𝑧 ≈ 0.3 where Lyman continuum (LyC) is accessible to HST/COS (e.g., Izotov et al 2016aIzotov et al , 2018bIzotov et al , 2021aWang et al 2019Wang et al , 2021, and another at 𝑧 ≈ 2 − 4 accessible to ground-based facilities and HST/WFC3 UVIS (e.g., Jones et al 2018;Smith et al 2020;Ji et al 2020;Meštrić et al 2020;Marques-Chaves et al 2021;Davis et al 2021;Prichard et al 2021). The 𝑧 ≈ 0.3 COS efforts were first undertaken at a time when only a handful of robust LyC leakers had been identified, and it was unclear whether LyC leakage even occurred among the 0.5𝐿 * galaxies for which 𝑓 esc measurements were feasible (e.g., Izotov et al 2016a,b).…”

No missing photons for reionization: moderate ionizing photon escape fractions from the FIRE-2 simulations

“…Strong LyC leakage has been confirmed from a number of galaxies at z ∼ 0-4, with inferred fesc from a few per cent to over 50 per cent (e.g. Vanzella et al 2012Vanzella et al , 2016Vanzella et al , 2020Izotov et al 2016bIzotov et al ,a, 2018Fletcher et al 2019;Rivera-Thorsen et al 2019;Ji et al 2020). Most of them are compact, extreme starburst galaxies that are thought to be analogs of typical star-forming galaxies in the reionization era.…”