1. Spatial Distribution of Molecules in Damped Lya Clouds
　　 　 Hiroyuki Hirashita　
　 (Nagoya University, Japan / SISSA, Italy)
　 A. Ferrara, K. Wada, P. Richter　

2. Contents:
H2 and Dust in DLAs
Spatial H2 Distribution (Theory)
Summary

3. 1. H2 and Dust in DLAs Molecular hydrogen (H2)
The most abundant molecule in the universe: a tracer of cool environments
Molecular clouds are the site of star formation.
Dust
H2 formation takes place.
Shielding of UV and reprocess into IR

4. Damped Lya Cloud (DLA) > High H I column density (～1021 cm-2)
QSO
Damped Lya cloud
Lya absorption
>
High H I column density (～1021 cm-2)
High redshift sample with detailed information on ISM.
How about H2 and dust? (e.g., Petitjean et al. 2002)

5. Correlation :Dust and H2 in DLAs
Ledoux, Petitjean, & Srianand (2003)
Correlation between dust abundance and molecular fraction.
Large scatter
log (molecular fraction)
H2 is not detected.
metal depletion
log (dust/gas)

6. How can we explain it? Dust poor Dust rich
◆ Strongly inhomogeneous H2 distribution?
H2 rich regions
Dust poor
Dust rich
UV background
UV background
Hard to detect H2 rich regions
Large change of H2 detection
(with large scatter in abundance)

7. 2. Spatial H2 distribution
Hirashita et al. (2003)
◆Numerical calculation (vcir = 100 km/s, zform = 3)
Density
Temperature
1 kpc

8. Molecular Fraction Map
Included physics on H2:
(1) Formation on dust grains
(2) Dissociation by UV bg
(self-shielding included)
(1) = (2)
i21 = 0.1, D = 0.1 Dsun
The H2 distribution is highly inhomogeneous (confined in small clumpy regions).
⇒ Low chance to detect　H2 from DLAs

9. H2 distribution (small scale)
50 pc
H2 rich regions are confined in small regions.
The area with fH2 > 10–6 is only 10% of the surface.

10. H2 and Dust dust-to-gas ratio
Random 5 lines of sight through the disc for each
dust-to-gas ratio
Overall correlation
Rapid increase of fH2 around log (D/Dsun) ~ –1.5.
Large scatter for high D
log (molecular fraction)
×: Ledoux et al. (2003)
◆: our simulation
log (dust-to-gas ratio)

11. 3. Summary
The paucity of H2 detection for dust-poor DLAs is explained by the small area covered by H2 rich regions.
The correlation between dust-to-gas ratio and H2 abundance has been explained.
The large variety in H2 fraction for relatively dust-rich DLAs is naturally explained by strong inhomogeneity of H2 distribution.