1. How Well Do We Know Stellar Populations? Nick Gnedin

2. Co-starring Andrey Kravtsov (Chicago) Kostas Tassis (Crete) Oleg Gnedin (Michigan) Sasha Muratov (Michigan)

3. Where Do Stars Form? F. Walter & The HI Nearby Galaxy Survey

4.  In the local universe stars only form in molecular gas.  We know of no example of a single star forming in atomic gas.  On theoretical grounds, molecular gas is expected to be a good tracer of star-forming gas. (Krumholz, Leroy, McKee 2011) Where Do Stars Form?

5. How Molecular Clouds Form  Molecular hydrogen is fragile: it is destroyed by UV radiation in the Lyman-Werner band (11.3 – 13.6 eV).  Molecular clouds only exist because of shielding.  Both, shielding by H 2 (self- shielding) and shielding by dust are important.

6. Atomic-To-Molecular Transition 101  Dust shielding for hydrogen molecules is like a castle wall for defenders: without the wall, they are not able to withstand the assault of the UV radiation.  But without the defenders, the wall is useless.

7. Putting It All Together  ART (Adaptive Refinement Tree) Code  N-body + gas + SF + RT + NLTE chemistry  50 pc spatial resolution in the ISM with mesh refinement  Star formation recipe from Krumholz & Tan 07  Optically Thin Variable Eddington Tensor Approximation (OTVET) for RT  Non-equilibrium cooling rates and ionization & chemical balance are computed “on the fly”  Realistic galaxies in cosmological simulations in a 35 comoving Mpc box (dynamic range of >10 5 )

8. Sub-cell Model  Radiative transfer in LW bands can not be done exactly in realistic simulations: 3D, adaptive in space, time-dependent.  Spatial scales over which absorption lines are coherent are sub-parsec (= unresolvable).  Unless aliens give us a super-duper hypercomputer to solve all this, we need to use a “sub-cell” model.  RT in LW bands is done in Sobolev- like approximation (“like” because the velocity field is not resolved).

9. Training the Model  H 2 fractions in translucent clouds have been measured by Copernicus & FUSE space missions (Tumlinson et al 2002, Rachford et al 2002, Gillmon et al 2006, Wolfire et al 2008).  HI fractions are measured in a handful of lines of sight by Goldsmith & Li (2005) in the MW and by Leroy et al. 2007 in SMC.  On large scales Wong & Blitz (2002) measured surface densities of HI and H 2 in many galaxies.

10. Training the Model: Milky Way The agreement is by construction!

11. Training the Model: LMC+SMC

12. Multi-phase ISM All 3 main ISM phases are there: hot coronal gas warm diffuse gas cold HI / H 2 gas

13.  Picture of M51

14. Atomic-To-Molecular Transition  Transition between atomic and molecular phases scales non-trivially with the dust-to-gas ratio D MW and the interstellar radiation field U MW. Smaller D MW Larger U MW

15. Kennicutt-Schmidt Relation  Local galaxies Atomic gas Molecular gas All neutral gas

16. Kennicutt-Schmidt Relation  Just like atomic-to-molecular transition scales non- trivially with D MW and U MW, so does star formation.

17. Does It Work In Real Life? LBG measurement from Rafelski (2011) cB58 (Baker et al. 2004) Galaxies at z=3 Local galaxies (THINGS)

18. Does It Work In Real Life?  Gas in nearby, low metallicity dwarfs appears to be inert to star formation. UGC 5288NGC 2915

19. Bimodality Prediction Lot’s of dust, efficient SF Little dust, inefficient SF

20. Can’t Kill All Birds With One Stone Molecule Oops – observations don’t show any bimodality Sloan galaxiesLocal Group dwarfs

21. A Mystery of the “Metallicity Floor” All those heavy elements came from dust-unrelated star formation process.

22. A Mystery of the “Metallicity Floor”  There is no place known in the whole universe (except 2 stars and 2 LL systems) that has metallicity less than about 0.2% solar. Most metal-poor galaxies. Lowest metallicity stars in the Milky Way. Damped Lyman-  Absorbers.  These heavy elements might have come from the metal-free generation of stars (Pop III stars), but why is it so universal?  Could there be another population of stars (call it Pop A)?

23. Genealogy Tree for Stars Primordial gas, no metals Pop III stars Metal-enriched gas Dust is present, Pop M (Pop I+II) stars No dust, Pop A stars

24. From Pop III to Other Pops  Pop III episode is brief: Wise & Abel 2008Muratov et all 2011

25. Key Questions #1  Can dust form fast right after a Pop III episode? Little dust is observed in z>5 galaxies & GRBs (Bouwens et al 2010, Zafar et al 2011). Dust mostly forms in the ISM by nucleation; seeds come from AGB and SNe. Nucleation time is (Inoue 2011) long!

26. Key Questions #2  Have all metals in dwarf galaxies (M * <10 8 M  ) come from Pop III stars? Pop III episode is brief. Chemical abundances of the most metal poor stars in the MW and stars in LG dwarfs look normal, unlike those of Pop III ejecta. (Caffau 2011)

27. Conclusions  The existence of the “metallicity floor” is a hard to understand puzzle in early galaxy formation.  Metals in galaxies with M * <10 8 M  came from a dust-unrelated episode of star formation.  Pop III  Pop Whatever transition cannot be properly modelled without understanding how dust forms at z>10.  If the dust formation time-scale is long, then high-z galaxies contain little dust (as observed) and a population of stars forming in atomic gas (Pop A) can not be excluded.

28. The End