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

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.

16 pages, 25 figures, accepted for publication in A&A

References in corpus (4)

• The penetration of FUV radiation into molecular clouds
• Hot Halos around High Redshift Protogalaxies: Observations of O VI and N V Absorption in Damped Lyman Alpha systems
• Metallicity as a criterion to select H2 bearing Damped Lyman-alpha systems
• Physical conditions in the neutral interstellar medium at z=2.43 toward Q2348-011

Cited by in corpus (5)

• Molecular hydrogen in high-redshift Damped Lyman-alpha systems: The VLT/UVES database
• Directional Radiation and Photodissociation Regions in Molecular Hydrogen Clouds
• Relationships between the HI 21-cm line strength, MgII equivalent width and metallicity in damped Lyman-alpha absorption systems
• HE 0515-4414 - an unusual sub-damped Ly alpha system revisited
• A search for damped Lyman-alpha systems towards radio-loud quasars I: The optical survey