Warm Neutral Gas at Redshift 3.4

Radio spectroscopy at 323 MHz using the Arecibo Telescope and the VLA has produced a tentative detection of the 21cm line of neutral hydrogen at Z_abs=3.38716+-0.00007 in absorption against the radio continuum of the QSO MG0201+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 ~10 km/s. 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 > 1000 K for both this 21cm line absorbing cloud and the turbulent component, which together have N_HI ~ 10^21.4 cm^-2. Results of optical spectroscopy require the additional presence of metal-enriched clouds of still broader velocity dispersion than the 21cm 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 21cm line emitter associated with the intervening absorber to be less than ~(Delta V/200 km/s) 10^13 Msolar for velocity widths Delta V (FWHM) in the range 200 to 1200 km/s (H_o=100 km/s/Mpc, Omega_o=1).