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.
