A bright lensed galaxy at z = 5.4 with strong Lyα 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_AB = 23.0, J141446.82+544631.9 is one of the brightest galaxies known at z > 5. It is characterized by strong Lyα emission, reaching a peak in (observed) flux density of > 10⁻¹⁶ erg s⁻¹ cm⁻² Å⁻¹. A deep optical spectrum from the LBT places strong constraints on N V and C IV emission, disfavouring an AGN source for the emission. However, a detection of the N IV] λ1486 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 Lyα emission, which is marginally resolved on ∼few kpc scales and has EW₀ ∼ 260Å. The Lyα emission extends over ∼ 2000 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 30⁰⁰ 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 ^<_∼ µ ^<_∼ 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.