HST WFC3/Grism Observations of the Candidate Ultra-High-Redshift Radio
  Galaxy GLEAM J0917-0012
  We present Hubble Space Telescope Wide Field Camera 3 photometric and grism
observations of the candidate ultra-high-redshift (z>7) radio galaxy, GLEAM
J0917-0012. This radio source was selected due to the curvature in its 70-230
MHz, low-frequency Murchison Widefield Array radio spectrum and its faintness
in K-band. Follow-up spectroscopic observations of this source with the VLA and
ALMA were inconclusive as to its redshift. Our F105W and F0986M imaging
observations detect the host of GLEAM J0917-0012 and a companion galaxy, ~one
arcsec away. The G102 grism observations reveal a single weak line in each of
the spectra of the host and the companion. To help identify these lines we
utilised several photometric redshift techniques including template fitting to
the grism spectra, fitting the UV-to-radio photometry with galaxy templates
plus a synchrotron model, fitting of the UV-to-near-infrared photometry with
EAZY, and fitting the radio data alone with RAiSERed. For the host of GLEAM
J0917-0012 we find a line at 1.12 micron and the UV-to-radio spectral energy
distribution fitting favours solutions at z~2 or z~8. While this fitting shows
a weak preference for the lower redshift solution, the models from the higher
redshift solution are more consistent with the strength of the spectral line.
The redshift constraint by RAiSERed of z>6.5 also supports the interpretation
that this line could be Lyman-alpha at z=8.21; however EAZY favours the z~2
solution. We discuss the implications of both solutions. For the companion
galaxy we find a line at 0.98 micron and the spectral energy distribution
fitting favours solutions at z<3 implying that the line could be the [OII]3727
doublet at z=1.63 (although the EAZY solution is z~2.6+/-0.5). Further
observations are still required to unambiguously determine the redshift of this
intriguing candidate ultra-high-redshift radio galaxy (abridged).
