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 star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a redshift of z = 0.3690, is characterized by a high emission-line flux ratio O<SUB>32</SUB> = [O III] λ5007/[O II] λ3727 = 11.5. The escape fraction of the LyC radiation f<SUB>esc</SUB>(LyC) in this galaxy is 46 per cent, the highest value found so far in low- redshift SFGs and one of the highest values found in galaxies at any redshift. The narrow double-peaked Ly α emission line is detected in the spectrum of J1154+2443 with a separation between the peaks V<SUB>sep</SUB> of 199 km s<SUP>-1</SUP>, one of the lowest known for Ly α-emitting galaxies, implying a high f<SUB>esc</SUB>(Ly α). Comparing the extinction-corrected Ly α/H β flux ratio with the case B value, we find f<SUB>esc</SUB>(Ly α) = 98 per cent. Our observations, combined with previous detections in the literature, reveal an increase of O<SUB>32</SUB> with increasing f<SUB>esc</SUB>(LyC). We also find a tight anticorrelation between f<SUB>esc</SUB>(LyC) and V<SUB>sep</SUB>. The surface brightness profile derived from the COS acquisition image reveals a bright star-forming region in the centre and an exponential disc in the outskirts with a disc scale length α = 1.09 kpc. J1154+2443, compared to other known low-redshift LyC leakers, is characterized by the lowest metallicity, 12+log O/H = 7.65 ± 0.01, the lowest stellar mass M<SUB>⋆</SUB> = 10<SUP>8.20</SUP> M<SUB>☉</SUB>, a similar star formation rate SFR = 18.9 M<SUB>☉</SUB> yr<SUP>-1</SUP>, and a high specific SFR of 1.2 × 10<SUP>-7</SUP> yr<SUP>-1</SUP>.