studying the atomic-molecular transition in the local group erik rosolowsky radio astronomy lab, uc...
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Studying the Atomic-Molecular Transition in the Local Group
Erik Rosolowsky
Radio Astronomy Lab, UC Berkeley
Ringberg - May 19, 2004
CollaboratorsCollaboratorsThe BossThe Boss: Leo Blitz: Leo BlitzCollaboratorsCollaborators::
Dick PlambeckDick PlambeckGreg EngargiolaGreg EngargiolaJulianne Dalcanton (UW)Julianne Dalcanton (UW)
Star Formation
• A fundamental problem– Solution required for a time evolution of stellar
populations in disk.
• With fundamental complications:
Resolved Schmidt Law Studies
Wong & Blitz (2002) studied CO and star formation in a sample of 7 galaxies.
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 RemnantsStellar Ejecta
Atomic ISM
Photo Credits: R. Gendler, D. Malin, D. Thilker
Molecular Clouds
Stars
Atomic ISM
This TalkThis Talk
Schmidt LawSchmidt Law
Stellar EvolutionStellar Evolution& &
Turbulent ISMTurbulent ISM
InfallInfall
Toward a simple model….
What is a Giant Molecular Cloud?• Large cloud of molecular
gas: M > 104 Msun
• 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.
PASJ, vol 56, no. 3, cover
The Orion Molecular Cloud in 12CO(1-0)
Macroscopic Cloud Properties
• Resolved observations give cloud radius (R)– Correct for beam convolution!Correct for beam convolution!
• Get linewidth (V) from spectral lines
• Luminous mass from MCOXLCO
• 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 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 (, Pint, tdyn)
• 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
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 -1.72
LMC -1.63 to -1.92
Outer MW -1.91 to -2.11
M33 -2.10 to -2.60Inc r
e asi
ng H
D S
tab i
lity
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.
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)
• BIMA SONG (Helfer et al., 2003) CO(,)
• Literature Maps of HI HI (single value)
• 2MASS K-band maps (Jarret et al. 2003) *(,)
What determines fmol(R)?
*=120 Msun / pc2
The Physics of *=120 Msun/pc2
• Constant value of ISRF– Sets H2 dissociation rate
• Constant Midplane Pressure
• Constant volume density (nH)
– Sets H2 formation rate
Assembling a Big Picture1. Filaments of HI (H2) collected by [M]HD
processes2. Another factor [f(R)] determines what fraction of
these clouds are converted to molecular gas3. 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 12CO clouds.
• 13CO 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 HI column densities
NGC 185 - L. Young (2001)
Requirements for Formation
• Consider a 106 Msun GMC with D=80 pc
• Requires enhancing the surface gas density from gas=10 Msun pc-2 (ISM) to
GMC = 200 Msun 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).