J1154+2443: a low-redshift compact star-forming galaxy with a 46 per cent leakage of Lyman continuum photons

Abstract

We report the detection of the Lyman continuum (LyC) radiation of the compact\nstar-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins\nSpectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a\nredshift of z=0.3690, is characterized by a high emission-line flux ratio\nO32=[OIII]5007/[OII]3727=11.5. The escape fraction of the LyC radiation\nfesc(LyC) in this galaxy is 46 per cent, the highest value found so far in\nlow-redshift SFGs and one of the highest values found in galaxies at any\nredshift. The narrow double-peaked Lya emission line is detected in the\nspectrum of J1154+2443 with a separation between the peaks Vsep of 199 km/s,\none of the lowest known for Lya-emitting galaxies, implying a high fesc(Lya).\nComparing the extinction-corrected Lya/Hb flux ratio with the case B value we\nfind fesc(Lya) = 98 per cent. Our observations, combined with previous\ndetections in the literature, reveal an increase of O32 with increasing\nfesc(LyC). We also find a tight anticorrelation between fesc(LyC) and Vsep. The\nsurface brightness profile derived from the COS acquisition image reveals a\nbright star-forming region in the centre and an exponential disc in the\noutskirts with a disc scale length alpha=1.09 kpc. J1154+2443, compared to\nother known low-redshift LyC leakers, is characterized by the lowest\nmetallicity, 12+logO/H=7.65+/-0.01, the lowest stellar mass M*=10^8.20 Msun, a\nsimilar star formation rate SFR=18.9 Msun/yr and a high specific SFR of\n1.2x10^-7 yr^-1.\n