WARM NEUTRAL GAS AT REDSHIFT Z=3.4

Radio spectroscopy at 323 MHz using the Arecibo Telescope(2) and the V LA(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 rad io continuum of the QSO MG 0201 + 113 (z(em) = 3.61). This redshift ro ughly agrees with one determined by the Westerbork Synthesis Radio Tel escope and reported earlier; however the observations at different tel escopes produce different results for width and optical depth of the l ine. If the detection holds, it provides evidence for a high column de nsity of neutral gas that is confined to a dynamically cold layer with velocity dispersion similar to 10 km s(-1). Although the interpretati on is uncertain due to a lack of detailed knowledge of the extended ra dio structure of the background quasar and the relative quantities of neutral gas in the cold and turbulent components, the observations spe cify high spin temperatures, T-s greater than or equal to 1000 K for b oth 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 optica l spectroscopy require the additional presence of metal-enriched cloud s of still broader velocity dispersion than the 21-cm line, creating a picture which is consistent with this system being a young disk galax y that is sufficiently evolved by z = 3.4 to have collapsed to a flatt ened system and produced a population of stars that have aged to pollu te a turbulent halo. The observations constrain the neutral gas mass o f a possible 21-cm line emitter associated with the intervening absorb er to be less than similar to (Delta V/200 km s(-1)) 10(13)M. for velo city widths Delta V (FWHM) in the range 200 to 1200 km s(-1) (H-0 = 10 0 km s(-1) Mpc(-1), Ohm(0) = 1). (C) 1997 American Astronomical Societ y.