Studying the Atomic-Molecular Transition in the Local Group

1. Studying the Atomic-Molecular Transition in the Local Group Erik Rosolowsky Radio Astronomy Lab, UC Berkeley Ringberg - May 19, 2004

2. Collaborators The Boss: Leo Blitz Collaborators: Dick Plambeck Greg Engargiola Julianne Dalcanton (UW)

3. Star Formation • A fundamental problem • Solution required for a time evolution of stellar populations in disk. • With fundamental complications:

4. The Schmidt Law Kennicutt (1998)

5. Resolved Schmidt Law Studies Wong & Blitz (2002) studied CO and star formation in a sample of 7 galaxies.

6. A slight case of déjà vu. CO Only CO & HI Kormendy & Kennicutt (2004)

7. Molecular Clouds Star Forming Regions The Gas Cycle in the ISM Stars Supernova Remnants Stellar Ejecta Atomic ISM Photo Credits: R. Gendler ,the FORS Team, D. Malin, SAO/Chandra, D. Thilker

8. Schmidt Law This Talk Stellar Evolution & Turbulent ISM Infall Molecular Clouds Stars Toward a simple model…. Atomic ISM Photo Credits: R. Gendler, D. Malin, D. Thilker

9. What is a GiantMolecular Cloud? • Large cloud of molecular gas: M > 104Msun • Self gravitating [?] • -T ~ Ugrav/2 • In MW, nearly all the molecular mass is in GMCs. • Since SFR scales with MH2, then GMCs populations set star formation history. The Orion Molecular Cloud in 12CO(1-0) PASJ, vol 56, no. 3, cover

10. How do GMCs vary across the Local Group? (which is secretly a question about how GMCs form…)

11. Macroscopic Cloud Properties • Resolved observations give cloud radius (R) • Correct for beam convolution! • Get linewidth (DV) from spectral lines • Luminous mass from MCOXLCO • Virial Mass for resolved observations

12. Constant X Factor? • Comparing Virial and CO masses over a range of galactic radii in M33 • No significant trend with radius • No change in X due only to: • Metallicity (0.6 dex) • ISRF (1 dex) • Midplane hydrostatic pressure (1 dex)

13. Larson’s Laws • Larson (1981) noted correlations among the simplest characteristics of molecular clouds. • The linewidth-size relationship is expected for turbulent motions. • If the clouds are virialized, the mass-linewidth relationship follows from linewidth-size and V.T. • Caveat: How well do these characterize GMC properties?

14. The Linewidth-Size Relationship

15. The Linewidth-Mass Relationship

16. The LG-GMC Population • Individual GMCs in MW, LMC, M31, M33 are consistent with being drawn from the SAME statistical population • 1 Parameter Clouds • These macroscopic properties of GMCs set average internal properties (r, Pint, tdyn) • A constant IMF would not be surprising for a common population of molecular clouds.

17. The GMC Mass Distribution • Parameterize with cumulative mass distribution: • Binned approximations are only accurate for sample sizes larger than ~300 (only 1 sample of GMCs) M33

18. The Local Group GMC Mass Distribution.

19. Re-fit all catalogs of GMCs available that have reliable data Changing index is likely the signature of different formation mechanisms. Enter: the importance of dynamics. Mass Spectra are different! Increasing HD Stability

20. Inferences about GMC formation • Physics intrinsic to GMCs establishes their macroscopic properties (e.g. self gravity). • GMCs appear to unify the star formation process across a variety of environments. • Suggests important factor in SF is the conversion of gas into GMCs. • Conversion efficiency (and process?) varies across environments.

21. Where does H2 form? (and what physics makes that so?) (and is this the same as making GMCs?)

22. Why go extragalactic? • Top-down perspective • No blending! • Association with other components in the ISM • Spatially complete studies • Wide range of galactic radii From Dame, Elmegreen, Cohen & Thaddeus (1986)

23. M33 in Ha • 850 kpc distant • Sc spiral • 1 of 3 Local Group spirals Cheng et al. (1993)

24. The D-array Survey

25. The GMCs in M33

26. Correlation with HI Deul & van der Hulst (1987)

27. S*=120 Msun / pc2 What determines fmol(R)? • BIMA SONG (Helfer et al., 2003) • SCO(a,d) • Literature Maps of HI • SHI (single value) • 2MASS K-band maps (Jarret et al. 2003) • S*(a,d)

28. The Physics of S*=120 Msun/pc2 • Constant value of ISRF • Sets H2 dissociation rate • Constant Midplane Pressure • Constant volume density (nH) • Sets H2 formation rate

29. Work in Progress • Include spatial distribution of HI • Include rotation curves

30. Assembling a Big Picture • Filaments of HI (H2) collected by [M]HD processes • Another factor [f(R)] determines what fraction of these clouds are converted to molecular gas • Different environments create different mass distributions of bound molecular clouds. • Self-gravity (or other physics) establishes uniform Larson Law scalings across environments. • Macroscopic properties of GMCs set their internal properties, which are the initial conditions of star formation.

31. Future Efforts: NGC 4826 • Extreme surface density of molecular gas. • No sign of discrete 12CO clouds. • 13CO clouds have similar properties as MW GMCs and show signs of star formation.

32. Dwarf Ellipicals • CO emission seen in dEs NGC 185 and NGC 205. • Gas appears to be intrinsic, not from infall or stripping • Presence of cool ISM and star formation without: • Spiral arms • Ordered B-field • Shearing disks • High HI column densities NGC 185 - L. Young (2001)

33. Requirements for Formation • Consider a 106 Msun GMC with D=80 pc • Requires enhancing the surface gas density from Sgas=10 Msun pc-2 (ISM) to SGMC = 200 Msunpc-2 • Implies accumulation scale of l >350 pc. • If atomic, the conversion to molecular gas is reasonably quick for typical densities (3-10 Myr).