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

Abstract

We analyze the properties of a multiply imaged Ly α (Ly α ) 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  ∼ −16.5), steep UV spectral index ( β  = −2.4 ± 0.1), and extreme rest-frame equivalent width of Ly α ( Å). 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 −1 ) and asymmetric Ly α profile with an integrated luminosity L (Ly α ) ∼ 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 α and continuum emission and several fainter clumps at projected distances ≲1 kpc that coincide with an extension of the Ly α 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 α 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 −3 ) stellar population. Its modest star formation rate (SFR ∼ 1.0 yr −1 ) implies high specific SFR (sSFR ∼ 2.5 × 10 −7 yr −1 ) and SFR density ( yr −1 kpc −2 ). The properties of A370-L57 make it a good representative of the population of galaxies responsible for cosmic reionization.