<i>HST</i> WFC3/Grism observations of the candidate ultra-high-redshift radio galaxy GLEAM J0917–0012

Abstract We present Hubble Space Telescope Wide Field Camera 3 photometric and grism observations of the candidate ultra-high-redshift ( $z&gt;7$ ) radio galaxy, GLEAM J0917–0012. This source was selected due to curvature in its 70–230 MHz, low-frequency Murchison Widefield Array spectrum faintness K -band. Follow-up spectroscopic this with Jansky Very Large Atacama Millimetre were inconclusive as redshift. Our F105W F0986M imaging detect host J0917–0012 a companion $\sim$ one arcsec away. The G102 reveal single weak line each spectra companion. To help identify these lines we utilised several redshift techniques including template fitting spectra, ultraviolet (UV)-to-radio photometry galaxy templates plus synchrotron model, UV-to-near-infrared EAZY , data alone RAiSERed . For find at $1.12\,\mu$ m UV-to-radio spectral energy distribution (SED) favours solutions $z\sim 2$ or 8$ While shows preference for lower solution, models from higher solution are more consistent strength line. constraint by $&gt;6.5$ also supports interpretation that could be Lyman $-\alpha$ $z=8.21$ ; however solution. discuss implications both solutions. $0.98\,\mu$ SED $z&lt;3$ implying [OII] $\lambda3727$ doublet $z=1.63$ (although is 2.6\pm 0.5$ ). Further still required unambiguously determine intriguing galaxy.