Radio detections of neutral hydrogen at redshift<i>Z</i>=3.4

Abstract

We have used the Very Large Array to search for ``21-cm'' narrow-band signals from neutral hydrogen at redshift Z=3.4. We detect absorption at Z=3.3968\ifmmode\pm\else\textpm\fi{}0.0004 against a radio source that has been previously identified with a ``normal'' galaxy at Z=3.395\ifmmode\pm\else\textpm\fi{}0.005. We detect emission at Z=3.3970\ifmmode\pm\else\textpm\fi{}0.0003 separated from the absorption by 33' (7--24 Mpc, depending on the cosmological model). This narrow-band emission signal is due to about 3\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathit{M}}_{\mathrm{\ensuremath{\bigodot}}}$ of neutral hydrogen as expected from a protocluster of galaxies. We identify it as the first example of a Zel'dovich ``pancake.''