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

2. Contents: Damped Lya Clouds (DLAs) Physical State of Gas
Getting a Realistic H2 Distribution
Summary

3. 1. Damped Lya Clouds (DLAs)
QSO
Damped Lya cloud
Lya absorption
High H I column density (> 2×1020 cm–2)
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.

4. 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

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

6. 2. Physical State of Gas Analysis of H2 detected DLAs J = 0, 1
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 < n < 300 cm–3
30 < T < 300 K
3 < UV/UV(Galactic) < 30
“cold phase”

7. H2 Formation and Destruction
H2 formation on dust
4×10–17(D/0.01) S (Tgas, Tdust) cm3 s–1
Hollenbach & McKee (1979)
H2 destruction (dissociation)
s–1
Abel et al. (1998)
self-shielding effect included
Assumption: H2 abundance is in equilibrium
i.e. molecular fraction ∝ D, j(UV)

8. Likelihood of Cold Phase
30 < n < 300 cm–3
3 < c < 30
30 < T < 1000 K
log (dust-to-gas ratio)
log (H2 fraction)
High density and low UV
Low density and high UV
H2 forms
in gas phase.

9. Star Formation Rate 3 < UV/UV(Galactic) < 30
SFR surface density = – 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

10. 3. Getting a Realistic H2 distribution
Hirashita et al. (2003)
◆Numerical calculation (2D, vcir = 100 km/s, zform = 3)
Density
Temperature
1 kpc

11. 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)

12. “Observations” of Simulated Galaxies
Select random lines of sight
Severe dust extinction?
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)

13. 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

14. 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.