16662

Powerful radio galaxies have been unique probes of the distant Universe for many decades. This proposal requests WFC3 G141 grism with F140W pre-imaging observations to confirm an emission line observed at 1.15 microns. A high continuum flux longward of this line (seen with the G141 grism and broad-band photometry) would confirm that this line is Lyman-alpha and hence this radio galaxy at z=8.45. This detection would be the most distant super-massive black hole (SMBH) and active galactic nucleus (AGN) known. At this redshift the radio galaxy would lie at the centre of a highly ionised Stromgren sphere with contributions to the ionising flux from the accretion onto the SMBH comma inverse Compton emission from the jets comma and star-formation. As well as reaffirming the redshift of the host and challenging models of black hole formation comma these observations will potentially reveal the complex interaction of this radio-loud AGN with its local environment. Furthermore comma confirmation of the existence of a luminous radio galaxy within the Epoch of Reionisation would allow studies of neutral hydrogen from absorption of the 21 cm line. Due to the transmission of the Earth.s atmosphere at 1.15 microns comma this observation would take >40 times longer if conducted from the ground (eight times longer if the spectrum is binned to the resolution of the G141 grism).