Abstract
We present a detailed study of a unusually bright 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 also one of the strongest Ly$\alpha$-emitting galaxies known, with an
observed flux of $f_Ly\alpha 10^-15~erg~\rm
s^-1~cm^-2$. A deep optical spectrum from the LBT places strong
constraints on NV and CIV emission, disfavoring an AGN source for the emission.
However, a detection of the NIV~$łambda$1486 emission line indicates a hard
ionizing continuum, possibly from hot, massive stars. Resolved imaging from HST
deblends the galaxy from a foreground interloper; these observations include
narrowband imaging of the Ly$\alpha$ emission, which is marginally resolved on
$\sim4$ kpc scales and has EW$_0~\sim$ 260\AA. The Ly$\alpha$ emission extends
over $>1000~km~s^-1$ 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 center 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 $5 łeq
25$. 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.
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