SHARDS frontier fields: Physical properties of a low-mass Lyα emitter at z = 5.75

arXiv:1710.00826v1We analyze the properties of a multiply imaged Lyα (Lyα) 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 ∼ -16.5), steep UV spectral index (β = -2.4 ± 0.1), and extreme rest-frame equivalent width of Lyα. Two different gravitational lens models predict high magnification (μ ∼ 10-16) for the two detected counterimages, separated by 7″, while a predicted third counterimage (μ ∼ 3-4) is undetected. We find differences of ∼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) and asymmetric Lyα profile with an integrated luminosity L(Lyα) ∼ 10 erg s. The morphology in the Hubble Space Telescope bands comprises a compact clump (r < 100 pc) that dominates the Lyα and continuum emission and several fainter clumps at projected distances ≲1 kpc that coincide with an extension of the Lyα 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 Lyα emission. Fitting of the spectral energy distribution with stellar population models favors a very young (t < 10 Myr), low-mass (), and metal-poor (Z ≲ 4 10) stellar population. Its modest star formation rate (SFR ∼ 1.0 yr) implies high specific SFR (sSFR ∼ 2.5 10 yr) and SFR density ( yr kpc). The properties of A370-L57 make it a good representative of the population of galaxies responsible for cosmic reionization.A.H.-C. and P.G.P.-G. acknowledge funding by the Spanish Ministry of Economy and Competitiveness (MINECO) under grants AYA2012-31277, AYA2015-70815-ERC, and AYA2015-63650-P. J.M.D. acknowledges support of the projects AYA2015-64508-P (MINECO/FEDER, UE), AYA2012-39475-C02-01, and the consolider project CSD2010-00064 funded by MINECO. D.J.L. and J.R. acknowledge support from the ERC starting grant 336736-CALENDS. R.A.M. acknowledges support by the Swiss National Science Foundation. A.A.-H. acknowledges funding by the MINECO grant AYA2015-64346-C2-1-P, which is partly funded by the FEDER program. H.D.S. acknowledges funding from the program ANR-T-ERC ASTROBRAIN. P.S. acknowledges funding from the Swiss National Science Foundation under grant P2GEP2_165426. J.M.R.E. acknowledges funding by the MINECO grant AYA2015-70498-C2-1-R. H.D. acknowledges financial support from the Spanish 2014 Ramón y Cajal program MINECO RYC-2014-15686.Peer Reviewe