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

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

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

The Schmidt Law Kennicutt (1998)

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

A slight case of déjà vu. CO Only Kormendy & Kennicutt (2004) CO & H I

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

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

What is a Giant Molecular Cloud? Large cloud of molecular gas: M > 10 4 M sun Self gravitating [?] –-T ~ U grav /2 In MW, nearly all the molecular mass is in GMCs. Since SFR scales with M H2, then GMCs populations set star formation history. PASJ, vol 56, no. 3, cover The Orion Molecular Cloud in 12 CO(1-0)

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

Macroscopic Cloud Properties Resolved observations give cloud radius (R) –Correct for beam convolution! Get linewidth (  V) from spectral lines Luminous mass from M CO  XL CO Virial Mass for resolved observations

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)

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?

The Linewidth-Size Relationship

The Linewidth-Mass Relationship

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 ( , P int, t dyn ) A constant IMF would not be surprising for a common population of molecular clouds.

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

The Local Group GMC Mass Distribution.

Mass Spectra are different! 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. Object  Inner MW-1.60 to LMC-1.63 to Outer MW-1.91 to M to Increasing HD Stability

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.

Where does H 2 form? (and what physics makes that so?) (and is this the same as making GMCs?)

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)

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

The D-array Survey

The GMCs in M33

Correlation with H I Deul & van der Hulst (1987)

Determining f mol (R) Molecular surface mass density –BIMA SONG (Helfer et al., 2003) HI surface mass density (single value) –Literature maps (various) Stellar surface mass density –2MASS, LGA (Jarrett et al., 2003)

BIMA SONG (Helfer et al., 2003)  CO ( ,  ) Literature Maps of H I  HI (single value) 2MASS K-band maps (Jarret et al. 2003)  * ( ,  ) What determines f mol (R)?  * =120 M sun / pc 2

The Physics of  * =120 M sun /pc 2 Constant value of ISRF –Sets H 2 dissociation rate Constant Midplane Pressure Constant volume density (n H ) –Sets H 2 formation rate

Work in Progress 1.Include spatial distribution of H I 2.Include rotation curves

Assembling a Big Picture 1.Filaments of H I (H 2 ) collected by [M]HD processes 2.Another factor [f(R)] determines what fraction of these clouds are converted to molecular gas 3.Different environments create different mass distributions of bound molecular clouds. 4.Self-gravity (or other physics) establishes uniform Larson Law scalings across environments. 5.Macroscopic properties of GMCs set their internal properties, which are the initial conditions of star formation.

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

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 H I column densities NGC L. Young (2001)

Requirements for Formation Consider a 10 6 M sun GMC with D=80 pc Requires enhancing the surface gas density from  gas =10 M sun pc -2 (ISM) to  GMC = 200 M sun pc -2 Implies accumulation scale of l >350 pc. If atomic, the conversion to molecular gas is reasonably quick for typical densities (3-10 Myr).

Thilker, Private Communication The Importance of Filamentary Structure All GMCs are found on filaments Not all filaments show GMCs (esp. at large R) Is filament formation the first step of star formation?

Surface Density Criterion In a pixel-to-pixel comparison of H I vs. CO maps, CO only found at high H I column densities. Not all H I pixels at high column have CO emission. Same cutoff threshold as seen in MW. What sets molecular gas fraction as a function of galactic radius?