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Star Formation and H 2 in Damped Ly a Clouds

Star Formation and H 2 in Damped Ly a Clouds. Hiroyuki Hirashita (University of Tsukuba, Japan). Contents:. Damped Ly a Clouds (DLAs) Physical State of Gas Getting a Realistic H 2 Distribution Summary. Damped Ly a cloud Ly a absorption. QSO. 1. Damped Ly a Clouds (DLAs).

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Star Formation and H 2 in Damped Ly a Clouds

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  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. Damped Lya cloud Lya absorption QSO 1. Damped Lya Clouds (DLAs) • 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. Large scatter 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)

  6. 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 < n < 300 cm–3 30 < T < 300 K 3 < UV/UV(Galactic) < 30 “cold phase”

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

  8. log (H2 fraction) log (dust-to-gas ratio) 30 < n < 300 cm–3 3 < c < 30 30 < T < 1000 K Likelihood of Cold Phase 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 = 0.005 – 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

  10. 3. Getting a Realistic H2 distribution Hirashita et al. (2003) ◆Numerical calculation (2D, vcir = 100 km/s, zform = 3) Temperature Density 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 = 0.1Dsun 50 pc Highly inhomogeneous (confined in clumpy regions)

  12. Severe dust extinction? “Observations” of Simulated Galaxies Select random lines of sight • Overall correlation • Rapid increase of f H2around 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 H2distribution 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.

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