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 _ { 32 } = [ O iii ] \lambda 5007/ [ O ii ] \lambda 3727 = 11.5 .
The escape fraction of the LyC radiation f _ { esc } ( 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 \alpha emission line is detected in the spectrum of J1154 + 2443 with a separation between the peaks V _ { sep } of 199 km s ^ { -1 } , one of the lowest known for Ly \alpha -emitting galaxies , implying a high f _ { esc } ( Ly \alpha ) .
Comparing the extinction-corrected Ly \alpha /H \beta flux ratio with the case B value we find f _ { esc } ( Ly \alpha ) = 98 per cent .
Our observations , combined with previous detections in the literature , reveal an increase of O _ { 32 } with increasing f _ { esc } ( LyC ) .
We also find a tight anticorrelation between f _ { esc } ( LyC ) and V _ { sep } .
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 \alpha = 1.09 kpc .
J1154 + 2443 , compared to other known low-redshift LyC leakers , is characterized by the lowest metallicity , 12+logO/H = 7.65 \pm 0.01 , the lowest stellar mass M _ { \star } = 10 ^ { 8.20 } M _ { \odot } , a similar star formation rate SFR = 18.9 M _ { \odot } yr ^ { -1 } and a high specific SFR of 1.2 \times 10 ^ { -7 } yr ^ { -1 } .