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The HI – H 2 Transition in the Interstellar Medium

The HI – H 2 Transition in the Interstellar Medium. Mark Krumholz, UC Santa Cruz STScI May Symposium, 2012

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The HI – H 2 Transition in the Interstellar Medium

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  1. The HI – H2 Transition in the Interstellar Medium Mark Krumholz, UC Santa Cruz STScI May Symposium, 2012 Collaborators: A. Dekel (HU Jerusalem), M. Fumagalli (UCSC), M. Kuhlen (UCB), A. Leroy (NRAO), J. X. Prochaska (UCSC), P. Madau (UCSC), C. McKee (UCB), B. Smith (Michigan State), J. Tumlinson (STScI), J. Wise (Georgia Tech)

  2. What you get if you google image search “interstellar medium”… “The black light of the interstellar medium of deep space illumines all that is not, to be seen and not to ‘Be’. To ‘Be’ and not seen – this is the path of the Panther woman. From the hydrogen oceans of Deep Space she condenses rain. Water precipitates from Her Vast Womb, falling onto planets open to Her Life–giving moisture.” From the hydrogen oceans of Deep Space she condenses rain.

  3. Outline • Why do we care about H2? • A minimal theoretical model • Low metallicity implications • CO versus H2 • Dwarf galaxy mass function • SF history of the universe • The outer disk / low metallicity frontier

  4. Maps of HI, H2, and SFR Leroy+ 2008

  5. SF Follows H2, not HI 10 M pc−2 Bigiel+ 2008

  6. SF Follows H2, not HI Bigiel+ 2008

  7. Minimal Theoretical Model • SFR ≈ εff fH2 M / tff, with εff ≈ 0.01 • Model requirements • Compute tff from observables • Explain why SF follows H2, not some other phase • Compute fH2 from observables

  8. Computing tff • Density hard to measure; must estimate • In MW-like galaxies, GMCs have ΣGMC ~ 100 M pc−2, MGMC ~ σ4 / G2 Σgal; this gives • In SB / high-z galaxies, Toomre stability gives • Ansatz: ρ = max(ρT, ρGMC)

  9. Estimated tff versus Reality Krumholz, Dekel, & McKee 2011

  10. Why Does SF Follow H2? HI Interstellar UV photons H2 T ~ 10-20 K T ~ 300 K

  11. Chemical and Thermal Balance H2 formation H2 photodissociation Absorption by dust, H2 Decrease in rad. intensity Line cooling Photoelectric heating Decrease in rad. intensity Absorption by dust

  12. Calculating Molecular Fractions To good approximation, solution only depends on two numbers: An approximate analytic solution can be given from these parameters. R depends only on galaxy , Z  can be measured directly Analytic solution for location of HI / H2 transition vs. exact numerical result

  13. BE Mass Follows H2 Krumholz, Leroy, & McKee 2011

  14. Computing fH2 in Galaxies(Krumholz, McKee, & Tumlinson2008, 2009; McKee & Krumholz 2010) Allowed nCNM • Dust opacity d and H2 formation rate R both  Z, so d / R ~ const • CNM dominates shielding, so n is the CNM density What is  (d /R) (E0*/ n)? FGH curves for MW (Wolfire et al. 2003) • CNM density set by pressure balance with WNM, and nCNM E0*, with weak Z dependence. •  (d /R) (E0*/ n) ~ 1 in all galaxies!

  15. Results for fH2 • Approximate solution: • Qualitative effect: fH2 goes from ~0 to ~1 when Z ~ 10 M pc–2 Krumholz, McKee, & Tumlinson (2008, 2009); McKee & Krumholz (2010)

  16. Observational Test:Individual Clouds in the MW KMT09 PREdiction KMT09 PREdiction Lee+ 2012

  17. The Star Formation Law KMT09

  18. Pushing to Low Metallicity Bolatto+ 2011 Fumagalli+ 2010 KMT09 PREdiction

  19. H2 and CO HI, CII Interstellar UV photons H2, CII H2, CO T ~ 10 K T ~ 20 K T ~ 300 K

  20. SF Follows H2, not CO Krumholz, Leroy, & McKee 2011; see also Bolatto+ 2011, Genzel+ 2011, Leroy+ 2011

  21. Implications for Dwarf Galaxies Gas Stars Galaxy in a ~1010 M halo at z ~ 5, from a cosmological simulation using the standard (stars form regardless of H2 fraction) star formation recipe Star formation laws with metallicity-dependent SF Same galaxy, but in a simulation where stars only form in molecular gas (using KMT star formation recipe) Temp Metals Kuhlen+ 2012

  22. Mass Function at High z

  23. Implications for theSFR Density at High z SF history from cosmological simulation (Springel & Hernquist 2003) Observed SF history of the universe (Bouwens+ 2010) SF history from a SAM (Baugh+ 2005)

  24. Results of PS-Based Model Krumholz & Dekel 2012

  25. Observed vs. Model SF History

  26. H2 and SF at Low Column / Z Total gas: Bigiel+ 2010 H2 only: Schruba+ 2011 H2 and SF do not go to 0 at low column; instead, there seems to be a floor: fH2 ~ 0.02, tdep,H2 ~ 2 Gyr

  27. Fixing the Two-Phase Assumption Hypothesis: H2 fraction cannot fall so low that warm HI phase is lost HSE pressure

  28. Preliminary Result Gray: SMC data (Bolatto+ 2011) Blue: base KMT model Red: KMT model with minimum H2 fraction, pressure computed using Ostriker, McKee, & Leroy (2010) model

  29. Summary • HI / H2 transition coincides with warm / cold transition, and is driven by competition between radiation and shielding • Shielding depends on metallicity, resulting in H2 fraction and SF rate that depend on metallicity • This may have dramatic effects for galaxy formation at the dwarf end

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