X-shooter observations of strong H2-bearing DLAs at high redshift

Abstract

We present results from spectroscopic observations with X-shooter at the Very Large Telescope of seven H 2 -bearing DLAs at high redshifts (z abs ∼ 2.5 − 3). These DLAs were originally selected from the presence of strong H 2 lines directly seen at the DLA redshift in low-resolution, low S/N SDSS spectra. We confirm the detection of molecular hydrogen in all of them. We measure the column densities of H i, H 2 in various rotational levels, and metal species, and associated dust extinction. The metallicities, obtained from undepleted species, are in the range log Z = −0.8 to −0.2. We discuss the chemical enrichment in these clouds and compare their properties with that of other molecular-rich systems selected by other means. In particular, we show that three different methods of pre-selection of H 2 -bearing DLAs in the SDSS have their own biases but complement each other mostly in terms of chemical enrichment. We use the rotational excitation of H 2 molecules together with the fine-structure energy levels of neutral carbon to constrain the physical conditions in the gas with the help of numerical modelling as well as analytical expressions for the surface density at which atomic to molecular conversion happens. We find that the H 2 -bearing medium revealed by the studied DLAs has typical values for the kinetic temperature, hydrogen density, and UV radiation field of, respectively, T ∼ 100 K, n H ∼ 100 cm −3 , and I UV about twice the intensity of the Draine field. Detailed studies combining different selections should therefore bring important clues to understand the H i-H 2 transition at high redshift.

Nature of the DLA towards Q 0528−250: High pressure and strong UV field revealed by excitation of C i, H2, and Si ii

“…the technique, see, e.g., Balashev et al 2019). We find that the total neutral-hydrogen column density is log N(H i) = 21.37 ± 0.01 centered on z abs = 2.811379(20) in agreement with the value reported by Ledoux et al (2006)…”

“…One can see that the UV flux in both components of the DLA towards Q 0528−250, as well as the thermal pressure (shown by inclined dashed lines in the figure), are higher than the average of other H 2 -bearing DLAs. Balashev et al (2017Balashev et al ( , 2019 also reported tentative evidence of an increase of thermal pressure with increasing hydrogen column density. This suggests that high H i column-density systems have smaller impact parameters than typical DLAs and hence probe the inner regions of galaxies where the thermal pressure may be enhanced by high UV flux.…”

“…In Figs. 2 and 3, we give estimates of number density, temperature, and UV flux in a sample of H 2 /C i-bearing DLAs, which we also determined using our code from the excitation of C i fine-structure and H 2 rotational levels (i.e., the compilation of data from Balashev et al 2019). One can see that the UV flux in both components of the DLA towards Q 0528−250, as well as the thermal pressure (shown by inclined dashed lines in the figure), are higher than the average of other H 2 -bearing DLAs.…”

“…3 the constraints on number density and UV flux in components A and B based on the excitation of C i and Si ii assuming T kin = T 01 in our calculations. We also use the measured excitation of high H 2 rotational levels (taken from Klimenko et al 2015) to further restrain the solutions (see Balashev et al 2019). Indeed, H 2 rotational levels are populated by collisions and radiative pumping in resonant lines in a UV wavelength range similar to C i and Si ii.…”

HD/H2 ratio in the diffuse interstellar medium

“…Fortunately, in many cases the metallicity can be well constrained using the associated metal lines. Additionally, the number density and UV flux also can be estimated for some absorption systems from associated finestructure levels of C i and rotational levels of H2 (Balashev et al 2019). In Fig.…”