Spatial Distribution of Molecules in Damped Lya Clouds

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

• H2 and Dust in DLAs
• Spatial H2 Distribution (Theory)
• Summary

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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

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Damped Lya Cloud (DLA)
How about H2 and dust? (e.g., Petitjean et
al. 2002)
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Correlation Dust and H2 in DLAs
Ledoux, Petitjean, Srianand (2003)
Correlation between dust abundance and molecular
fraction.
H2 is not detected.
log (molecular fraction)
metal depletion
log (dust/gas)
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How can we explain it?
? 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)
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2. Spatial H2 distribution
Hirashita et al. (2003)
?Numerical calculation (vcir 100 km/s, zform
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Temperature
Density
1 kpc
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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
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H2 distribution (small scale)
50 pc

• H2 rich regions are confined in small regions.
• The area with fH2 gt 106 is only 10 of the
  surface.

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H2 and Dust
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)
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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.