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

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

2007-07-30

arxiv.org/abs/0707.4479v1

Astronomy and Astrophysics

Astrophysics

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

Scientific paper

10.1051/0004-6361:20078021

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.

Affiliated with

Also associated with

No associations

Rating

Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.

If you have personal experience with Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFWR-SCP-O-221778