SHARDS Frontier Fields: Physical Properties of a Low-mass Lyα Emitter at z=5.75

We analyze the properties of a multiply imaged Ly 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 similar to -16.5), steep UV spectral index (beta = -2.4 +/- 0.1), and extreme rest-frame equivalent width of Ly alpha (EW0 (Ly alpha) = 420(-120)(+180) angstrom). Two different gravitational lens models predict high magnification (mu similar to 10-16) for the two detected counterimages, separated by 7 '', while a predicted third counterimage (mu similar to 3-4) is undetected. We find differences of similar to 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 +/- 10 km s(-1)) and asymmetric Ly alpha profile with an integrated luminosity L(Ly alpha) similar to 10(42) erg s(-1). The morphology in the Hubble Space Telescope bands comprises a compact clump (r(e) < 100 pc) that dominates the Ly alpha and continuum emission and several fainter clumps at projected distances. 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 a contribution from active galactic nuclei 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-* 10(6.5) M circle dot), and metal-poor (Z less than or similar to 4 x 10(-3)) stellar population. Its modest star formation rate (SFR similar to 1.0 M circle dot yr(-1)) implies high specific SFR (sSFR similar to 2.5 x 10(-7) yr(-1)) and SFR density (SSFR similar to 7-35 M circle dot yr(-1) kpc(-2)). The properties of A370-L57 make it a good representative of the population of galaxies responsible for cosmic reionization.