Warm neutral gas at redshift z=3.4

Radio spectroscopy at 323 MHz using the Arecibo Telescope(2) and the VLA(3) has produced a tentative detection of the 21-cm line of neutral hydrogen at z(abs) = 3.38716 +/- 0.00007 in absorption against the radio continuum of the QSO MG 0201 + 113 (z(em) = 3.61). This redshift roughly agrees with one determined by the Westerbork Synthesis Radio Telescope and reported earlier; however the observations at different telescopes produce different results for width and optical depth of the line. If the detection holds, it provides evidence for a high column density of neutral gas that is confined to a dynamically cold layer with velocity dispersion similar to 10 km s(-1). Although the interpretation is uncertain due to a lack of detailed knowledge of the extended radio structure of the background quasar and the relative quantities of neutral gas in the cold and turbulent components, the observations specify high spin temperatures, T-s greater than or equal to 1000 K for both this 21-cm line absorbing cloud and the turbulent component, which together have N-H I approximate to 10(21.4) cm(-2). Results of optical spectroscopy require the additional presence of metal-enriched clouds of still broader velocity dispersion than the 21-cm line, creating a picture which is consistent with this system being a young disk galaxy that is sufficiently evolved by z = 3.4 to have collapsed to a flattened system and produced a population of stars that have aged to pollute a turbulent halo. The observations constrain the neutral gas mass of a possible 21-cm line emitter associated with the intervening absorber to be less than similar to (Delta V/200 km s(-1)) 10(13)M. for velocity widths Delta V (FWHM) in the range 200 to 1200 km s(-1) (H-0 = 100 km s(-1) Mpc(-1), Ohm(0) = 1). (C) 1997 American Astronomical Society.