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

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

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).

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

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

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

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

3. Infrared Extinction 17 mm/2.12 mm line ratio

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.