We analyze the properties of a multiply-imaged Lyman- \alpha ( Ly \alpha ) emitter at z =5.75 identified through SHARDS Frontier Fields intermediate-band imaging of the Hubble Frontier Fields ( HFF ) cluster Abell 370 .
The source , A370-L57 , has low intrinsic luminosity ( M _ { UV } \sim -16.5 ) , steep UV spectral index ( \beta =-2.4 \pm 0.1 ) , and extreme rest-frame equivalent width of Ly \alpha ( EW _ { 0 } ( Ly \alpha ) =420 ^ { +180 } _ { -120 } Ã ) .
Two different gravitational lens models predict high magnification ( \mu \sim 10–16 ) for the two detected counter-images , separated by 7 ” , while a predicted third counter-image ( \mu \sim 3–4 ) is undetected .
We find differences of \sim 50 % in magnification between the two lens models , quantifying our current systematic uncertainties .
Integral field spectroscopy of A370-L57 with MUSE shows a narrow ( FWHM =204 \pm 10 km s ^ { -1 } ) and asymmetric Ly \alpha profile with an integrated luminosity L ( Ly \alpha ) \sim 10 ^ { 42 } erg s ^ { -1 } .
The morphology in the HST bands comprises a compact clump ( r _ { e } < 100 pc ) that dominates the Ly \alpha and continuum emission and several fainter clumps at projected distances \lesssim 1 kpc that coincide with an extension of the Ly \alpha emission in the SHARDS F823W17 and MUSE observations .
The latter could be part of the same galaxy or an interacting companion .
We find no evidence of contribution from AGN to the Ly \alpha emission .
Fitting of the spectral energy distribution with stellar population models favors a very young ( t < 10 Myr ) , low mass ( M _ { * } \sim 10 ^ { 6.5 } M _ { \odot } ) , and metal poor ( Z \lesssim 4 \times 10 ^ { -3 } ) stellar population .
Its modest star formation rate ( SFR \sim 1.0 M _ { \odot } yr ^ { -1 } ) implies high specific SFR ( sSFR \sim 2.5 \times 10 ^ { -7 } yr ^ { -1 } ) and SFR density ( \Sigma _ { SFR } \sim 7–35 M _ { \odot } yr ^ { -1 } kpc ^ { -2 } ) .
The properties of A370-L57 make it a good representative of the population of galaxies responsible for cosmic reionization .