Cold neutral gas in a z=4.2 damped Lya system:: Fuel for star formation

We discuss interstellar temperature determinations using the excitation equilibrium of the P-2 levels of Si II and C II. We show how observations of the P-2(3/2) fine-structure levels of Si II and C II ( which have significantly different excitation energies, corresponding to similar to 413 and 92 K, respectively) can be used to limit gas kinetic temperatures. We apply this method to the z = 4.224 damped Ly alpha system toward the quasar PSS 1443 +/- 27. The lack of significant absorption out of the Si II P-2(3/2) level and the presence of very strong C II P-2(3/2) provide an upper limit to the temperature of the C ii* - bearing gas in this system. Assuming a solar Si/ C ratio, the observations imply a 2 sigma limit T < 954 K for this absorber; a supersolar Si/C ratio gives stricter limits, T < 524 K. The observations suggest the presence of a cold neutral medium; such cold gas may serve as the fuel for star formation in this young galaxy.