Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions

TL;DR

This study investigates the physical conditions and excitation mechanisms of molecular hydrogen in low molecular fraction damped Lyman-alpha systems at high redshift, revealing complex multi-component structures and excitation processes.

Contribution

It presents new detections of H2 in low molecular fraction systems and analyzes their excitation, proposing a two-component model for the observed rotational levels.

Findings

Detected three new H2 systems with low molecular fractions.

Found that H2 rotational excitation requires multi-component models.

Observed correlation between Doppler width and rotational energy levels.

Abstract

We probe the physical conditions in high-redshift damped Ly-alpha systems using the observed molecular fraction and the rotational excitation of molecular hydrogen. We report two new detections of H2 at z = 2.402 and 1.989 toward, respectively, HE 0027-1836 and HE 2318-1107. We also present a detailed analysis of our recent H2 detection toward Q2343+125. All three systems have low molecular fractions, log f < -4, with f = 2N(H2)/(2N(H2) + N(HI)). Only one such H2 system was known previously. The depletion patterns for Si, S, Ti, Cr, Mn, Fe and Ni in the three systems are found to be very similar to what is observed in diffuse gas of the Galactic halo. H2 absorption from rotational levels up to J = 5 is observed in a single component toward HE 0027-1836. We show that the width (Doppler parameter) of the H2 lines increases with increasing J and that the kinetic energy derived from the Doppler parameter is linearly dependent on the relative energy of the rotational levels. The excitation temperature is found to be 90 K for J = 0 to J = 2 and ~500 K for higher J levels. Single isothermal PDR models fail to reproduce the observed rotational excitations. A two-component model is needed: one component of low density (~50 cm-3) with weak illumination (chi = 1) to explain the J <= 2 rotational levels and another of high density (~500 cm-3) with strong illumination (chi = 30) for J >= 3 levels. However, the juxtaposition of these two PDR components may be ad-hoc and the multicomponent structure could result either from turbulent dissipation or C-shocks.