1. 原始高柱密度ガスのガンマ線バースト残光による探査法
　　 平下 博之 (H. Hirashita)　
　 　　　　(筑波大学)
　　　　　　　　　芝井 広、竹内 努

2. Contents: High Column Density Clouds? Absorption Intensity of H2
Infrared Extinction
Summary
Primeval (原始)
= not evolved (low metallicity/low dust content)

3. 1. High Column Density Clouds?
これまでのQSO absorption line systemの研究
up to log N(H I) [cm–2] ~ 22
Possible bias due to dust extinction for log N(H I) > 22 (Fall et al. 1989; Vladilo & Péroux 2005).
より高い柱密度の系を観測する意義
Possibly related to star-forming places
Contribution to the total baryons in the Universe
Crucial to avoid dust extinction
→ Near-infrared H2 lines are useful (Shibai et al. 2001).

4. Why H2 lines? Dust-to-gas ratio ~ 1/100 of MW
Hirashita & Ferrara (2005)
Probability of 2N(H2)/N(H)
log N(H2)/N(H)
large molecular fraction for log N(H) > 23

5. 2. Absorption Intensity of H2
S Dn [1 – exp(–tline)] exp(–tdust)
Continuum flux
Competition between the line optical depth and dust optical depth

6. Various Dust-to-Gas Ratio
Galactic extinction curve
0.001
0.01
0.1
Dust abundance 1

7. Various Source Fluxes Typical flux of GRB afterglows
Dust-to-gas ratio ~ 1/100 of MW

8. 3. Infrared Extinction
17 mm/2.12 mm line ratio

9. 4. Summary Infrared H2 lines are useful for log N(H) > 22–23,
because we can reduce the extinction effect.
because a large fraction of H is in the form of H2.
The H2 absorption in GRB is observable by SPICA.
The extinction curves could be constrained by taking line ratio.