A bright lensed galaxy at $z=5.4$ with strong Ly$\alpha$ emission

We present a detailed study of a unusually bright, lensed galaxy at $z=5.424$ discovered within the CFHTLS imaging survey. With an observed flux of $i_{\rm AB}=23.0$, J141446.82+544631.9 is one of the brightest galaxies known at $z>5$. It is characterized by strong Ly$\alpha$ emission, reaching a peak in (observed) flux density of $>10^{-16}~{\rm erg}~{\rm s}^{-1}~{\rm cm}^{-2}~{\rm \AA}^{-1}$. A deep optical spectrum from the LBT places strong constraints on NV and CIV emission, disfavouring an AGN source for the emission. However, a detection of the NIV]~$\lambda$1486 emission line indicates a hard ionizing continuum, possibly from hot, massive stars. Resolved imaging from HST deblends the galaxy from a foreground interloper; these observations include narrowband imaging of the Ly$\alpha$ emission, which is marginally resolved on $\sim$few~kpc scales and has EW$_0~\sim$ 260\AA. The Ly$\alpha$ emission extends over ~2000~${\rm km}~{\rm s}^{-1}$ and is broadly consistent with expanding shell models. SED fitting that includes Spitzer/IRAC photometry suggests a complex star formation history that include both a recent burst and an evolved population. J1414+5446 lies 30" from the centre of a known lensing cluster in the CFHTLS; combined with the foreground contribution this leads to a highly uncertain estimate for the lensing magnification in the range $5 \leq \mu \leq 25$. Because of its unusual brightness J1414+5446 affords unique opportunities for detailed study of an individual galaxy near the epoch of reionization and a preview of what can be expected from upcoming wide-area surveys that will yield hundreds of similar objects.