Keck Observations of the Most Distant Galaxy: 8C1435+63 at z=4.25

Abstract

We report on Keck observations and confirm the redshift of the most distant galaxy known: 8C1435+63 at z=4.25. The spectrum shows a strong Ly$α$ line, a Ly$α$ forest continuum break and a continuum break at $λ_{rest}=912$Å. The Ly$α$ emission is spatially extended and roughly aligned with the radio source. The galaxy shows a double structure in the $I$-band ($λ_{rest}\approx$1500Å) which is aligned with the radio axis; the two $I$-band components spatially coincide with the nuclear and southern radio components. Some fraction of the $I$ band emission could be due to a nonthermal process such as inverse compton scattering. In the $K$-band ($λ_{rest}\approx$4200Å), which may be dominated by starlight, the galaxy has a very low surface brightness, diffuse morphology. The $K$ morphology shows little relationship to the radio source structure, although the major axis of the $K$ emission is elongated roughly in the direction of the radio source axis. The galaxian continuum is very red ($I-K>4$) and if the $K$ continuum is due to starlight, implies a formation redshift of $z_f > 5$. We speculate that this galaxy may be the progenitor of a present day cD galaxy.