Near-Infrared Imaging of a z=6.42 Quasar Host Galaxy With the Hubble
  Space Telescope Wide Field Camera 3
  We report on deep near-infrared F125W (J) and F160W (H) Hubble Space
Telescope Wide Field Camera 3 images of the z=6.42 quasar J1148+5251 to attempt
to detect rest-frame near-ultraviolet emission from the host galaxy. These
observations included contemporaneous observations of a nearby star of similar
near-infrared colors to measure temporal variations in the telescope and
instrument point spread function (PSF). We subtract the quasar point source
using both this direct PSF and a model PSF.
  Using direct subtraction, we measure an upper limit for the quasar host
galaxy of m_J>22.8, m_H>23.0 AB mag (2 sigma). After subtracting our best model
PSF, we measure a limiting surface brightness from 0.3"-0.5" radius of mu_J >
23.5, mu_H > 23.7 AB magarc (2 sigma). We test the ability of the model
subtraction method to recover the host galaxy flux by simulating host galaxies
with varying integrated magnitude, effective radius, and S\'ersic index, and
conducting the same analysis. These models indicate that the surface brightness
limit (mu_J > 23.5 AB magarc) corresponds to an integrated upper limit of m_J >
22 - 23 AB mag, consistent with the direct subtraction method. Combined with
existing far-infrared observations, this gives an infrared excess log(IRX) >
1.0 and corresponding ultraviolet spectral slope beta > -1.2\pm0.2. These
values match those of most local luminous infrared galaxies, but are redder
than those of almost all local star-forming galaxies and z~6 Lyman break
galaxies.
