Physical conditions in the neutral interstellar medium at z=2.43 toward Q2348-011

Abstract

We obtained a high-resolution VLT-UVES spectrum of the quasar Q2348-011 over a wavelength range that covers most of the prominent metal and molecular absorption lines from the log N(HI)=20.50+-0.10 damped Lyman-alpha system at zabs=2.43. From the column density ratios and the relative populations of H2 rotational and CI fine-structure levels, we derive the physical conditions (relative abundances, dust-depletion, particle density, kinetic temperature and ionizing flux) and discuss physical conditions in the neutral phase. Molecular hydrogen is detected in seven components in the first four rotational levels (J = 0-3) of the vibrational ground state. Absorption lines of H2 J = 4 (resp. J = 5) are also detected in six (resp. two) of these components. This leads to a total molecular fraction of log f = -1.69+0.37-0.58. We confirm the earlier findings that there is a correlation between N(FeII)/N(SII) and N(SiII)/N(SII) indicative of a dust-depletion pattern. Surprisingly, however, the depletion of metals onto dust in the H2 components is not large in this system: [Fe/S] = -0.8 to -0.1. The gas in H2-bearing components is found to be cold but still hotter than similar gas in our Galaxy (T > 130 K, instead of typically 80 K). and dense (n=100-200 cm^-3). There is an anti-correlation between the logarithm of the photo-absorption rate, and log N(H2)/N(CI) derived for each H2 component. We show that this is mostly due to shielding effects and implies that the photo- absorption rate is a good indicator of the physical conditions in the gas. We find that the gas is immersed in a intense UV field, about one order of magnitude higher than in the solar vicinity. The results suggest that the gas in H2-bearing DLAs is clumpy, and star-formation occurs in the associated object.