Star Formation and H2 in Damped Lya Clouds

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Star Formation and H2 in Damped Lya Clouds
Hiroyuki Hirashita
(University of Tsukuba, Japan)
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Contents

1. Damped Lya Clouds (DLAs)
2. Physical State of Gas
3. Getting a Realistic H2 Distribution
4. Summary

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1. Damped Lya Clouds (DLAs)

• High H I column density (gt 21020 cm2)
• Reservoir of a large amount of H I
• ?progenitors of nearby large galaxies?
• Unique objects at high z for detailed study ISM
  by using various species.

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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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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
Hirashita Ferrara (2005)
Analysis of H2 detected DLAs
J 0, 1 J 4, 5 Dust-to-gas ratio H2 fraction
T
n
H2 formation rate

H2 destruction rate
UV field
30 lt n lt 300 cm3 30 lt T lt 300 K 3 lt
UV/UV(Galactic) lt 30
cold phase
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H2 Formation and Destruction
H2 formation on dust 41017(D/0.01) S (Tgas,
Tdust) cm3 s1
Hollenbach McKee (1979)
Assumption H2 abundance is in equilibrium
i.e. molecular fraction ? D, j(UV)
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Likelihood of Cold Phase
30 lt n lt 300 cm3 3 lt c lt 30 30 lt T lt 1000 K
High density and low UV
Low density and high UV
H2 forms in gas phase.
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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
Similar to spirals or dwarfs
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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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Observations of Simulated Galaxies
Select random lines of sight

• Overall correlation
• Rapid increase of f H2 around log(D/Dsun)
  1.5.
• (?self-shielding)
• Large scatter for high D

log (molecular fraction)
Ledoux et al. (2003) ? our simulation
log (dust-to-gas ratio)
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Search for NIR H2 Lines in GRBs
Hirashita et al. (2005)
Less affected by extinction Dense molecular
clouds may be directly detected
Typical flux of GRB afterglows
Dust-to-gas ratio 1/100 of MW
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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.
• Star formation rate is 0.1 1 Msun/yr.