2016A&A...592L...7T

ID11 is an actively star-forming, extremely compact galaxy and Lyα emitter at z=3.117 that is gravitationally magnified by a factor of ∼17 by the cluster of galaxies Hubble Frontier Fields AS1063. The observed properties of this galaxy resemble those of low luminosity HII galaxies or giant HII regions such as 30 Doradus in the Large Magellanic Cloud. Using the tight correlation correlation between the Balmer-line luminosities and the width of the emission lines (typically L(Hβ)-σ(Hβ)), which are valid for HII galaxies and giant HII regions to estimate their total luminosity, we are able to measure the lensing amplification of ID11. We obtain an amplification of 23±11 that is similar within errors to the value of ∼17 estimated or predicted by the best lensing models of the massive cluster Abell S1063. We also compiled, from the literature, luminosities and velocity dispersions for a set of lensed compact star-forming regions. There is more scatter in the L-σ correlation for these lensed systems, but on the whole the results tend to support the lensing model estimates of the magnification. Our result indicates that the amplification can be independently measured using the L-σ relation in lensed giant HII regions or HII galaxies. It also supports the suggestion, even if lensing is model dependent, that the L-σ relation is valid for low luminosity high-z objects. Ad hoc observations of lensed star-forming systems are required to determine the lensing amplification accurately.