H2 in Damped Lya Clouds Theoretical Modeling

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H2 in Damped Lya Clouds (Theoretical Modeling)
Hiroyuki Hirashita
(SISSA, Italy)
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Contents

• Damped Lya Clouds (DLAs)
• Physical State of Gas
• Getting a Realistic H2 Distribution
• Summary

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1. Damped Lya Clouds (DLAs)
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Search for H2 and Dust in DLAs

• Molecular hydrogen (H2)
• The most abundant molecule in the universe
• Tracer of star-forming places

• Dust
• Formation of H2 on the surface
• Shielding of UV and reprocess into IR

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H2 Absorption Lines in DLAs
Ledoux et al. (2002)
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Evidence of Dust
Depletion relative to the solar abundance ratio
Solar
Ledoux et al. (2002)
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Physics on H2 and Dust
Molecular hydrogen (H2)
self-shielding
dissociation
formation
UV
Dust
shielding
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Correlation Dust and H2 in DLAs
Ledoux, Petitjean, Srianand (2003)
Correlation between dust abundance and molecular
fraction.
log (molecular fraction)
log (molecular fraction)
H2 is not detected.
metal depletion
log (dust/gas)
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2. Physical State of Gas
Analysis of H2 detected DLAs
J 0, 1 J 4, 5 Dust-to-gas ratio H2 fraction
T
Pumping rate
UV field
H2 formation rate
n
30 lt n lt 300 cm2 30 lt T lt 300 K 3 lt
UV/UV(Galactic) lt 30
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Equilibrium Molecular Fraction
Hirashita Ferrara (2004)
H2 formation on dust 41017(D/0.01) S (Tgas,
Tdust) cm3 s1
Assumption H2 abundance is in equilibrium
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Equilibrium Molecular Fraction
NH 1021 cm2 UV G0 (Galactic)
(n cm3, T K)
(33, 300)
(10, 1000)
log (molecular fraction)
(3.3, 3000)
? Ledoux et al. (2003)
log (dust/gas)
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Likelihood
30 lt n lt 300 cm2 30 lt T lt 300 K 3 lt
UV/UV(Galactic) lt 30
Cold phase
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Physical State of Gas
High density and low UV
Low density and high UV
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Star Formation Rate
3 lt UV/UV(Galactic) lt 30
SFR surface density 0.005 0.05 Msun/yr/kpc2
Typical radius 3 kpc (e.g. Kulkarni et al.
2000)
SFR 0.1 1 Msun/yr
Mild star formation rate (spiral or dwarf?)
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Comparison with C II Method
Wolfe et al. (2003)
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3. Getting a Realistic H2 distribution
Hirashita et al. (2003)
?Numerical calculation (2D, vcir 100 km/s,
zform 3)
Temperature
Density
1 kpc
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Spatial Distribution of H2
log (molecular fraction)
Included physics on H2 (1) Formation on
grains (2) Dissociation by UV
(self-shielding) (1) (2) i21
0.1, D 0.1 Dsun
50 pc
Highly inhomogeneous (confined in clumpy regions)
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Qualitative Picture
? Strongly inhomogeneous H2 distribution?
H2 rich regions
Dust poor
Dust rich
UV background
UV background
Hard to detect H2 rich regions
Large chance of H2 detection (with large scatter
in abundance)
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Observations of Simulated Galaxies
Select random lines of sight

• Overall correlation
• Rapid increase of f H2 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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4. Summary

• Our simulations of H2 distribution reproduce
• Overall correlation between dust/gas ratio and H2
  fraction
• Clumpy H2 rich regions (? lack of H2 detection)
• Effect of self-shielding (? large variation of H2
  fraction)
• Our likelihood analysis shows
• The cold phase suggested by H2 detected objects
  covers all the data in the likely range.
• The upper limit data are consistent also with the
  warm phase.
• DLAs are objects with SFR 0.1 1 Msun/yr.