COLD NEUTRAL GAS IN A z = 4.2 DAMPED Lyα SYSTEM: FUEL FOR STAR FORMATION

摘要

We discuss interstellar temperature determinations using the excitation equilibrium of the ~2P levels of Si II and C II. We show how observations of the ~2P_(3/2) fine-structure levels of Si II and C II (which have significantly different excitation energies, corresponding to ～413 and 92 K, respectively) can be used to limit gas kinetic temperatures. We apply this method to the z = 4.224 damped Lyα system toward the quasar PSS 1443 + 27. The lack of significant absorption out of the Si II ~2P_(3/2) level and the presence of very strong C n ~2P_(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 σ 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.