Cold Neutral Gas in a z = 4 . 2 Damped Lyman - α System : The Fuel for Star Formation

We introduce a new method for determining the temperature of interstellar gas based on observations of the the upper P3/2 fine structure levels of Si II and C II. The excitation energies of the fine structure levels above ground are significantly different (Tex ∼ 413 and 92 K for Si II and C II, respectively); their ratio is, therefore, sensitive to temperature. We show how observations of the P3/2 fine structure levels of Si II and C II are used to limit the interstellar gas temperature and apply this this method to the z = 4.224 damped Lyman-α system toward the quasar PSS 1443+27. The lack of significant absorption out of the Si II P3/2 level and the presence of strong P3/2 C II provides an upper limit to the temperature in this system. Assuming solar relative Si/C abundances, the observations imply a 2σ upper limit T < 1096 K for this absorber; super-solar gas-phase Si/C abundances result in stricter limits, T < 575 K. The observations require the presence of a cold neutral medium phase, i.e., the fuel for star formation in this young galaxy. Subject headings: ISM: evolution – ISM: atoms – galaxies: evolution – quasars: absorption lines