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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
CO & HI
Kormendy & Kennicutt (2004)
Molecular
Clouds
Star Forming Regions
The Gas
Cycle in
the ISM
Supernova Remnants
Stellar Ejecta
Atomic ISM
Photo Credits: R. Gendler ,the FORS Team, D. Malin, SAO/Chandra, D. Thilker
Stars
Schmidt Law
Molecular
Clouds
Toward a simple
model….
This Talk
Stellar Evolution
&
Turbulent ISM
Infall
Atomic ISM
Photo Credits: R. Gendler, D. Malin, D. Thilker
Stars
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.
The Orion Molecular Cloud in 12CO(1-0)
PASJ, vol 56, no. 3, cover
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 (DV) from spectral lines
• Luminous mass from MCOXLCO
• 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 (r, Pint, tdyn)
• A constant IMF would not be surprising for
a common population of molecular clouds.
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
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
a
-1.60 to -1.72
LMC
-1.63 to -1.92
Outer MW
-1.91 to -2.11
M33
-2.10 to -2.60
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 H2 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 Ha
• 850 kpc distant
• Sc spiral
• 1 of 3 Local Group
spirals
Cheng et al. (1993)
The D-array Survey
The GMCs in M33
Correlation with HI
Deul & van der Hulst (1987)
What determines fmol(R)?
• BIMA SONG (Helfer et al., 2003)
 SCO(a,d)
• Literature Maps of HI
 SHI (single value)
S*=120 Msun / pc2
• 2MASS K-band maps (Jarret et al. 2003)
 S*(a,d)
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
Work in Progress
1. Include spatial distribution of HI
2. Include rotation curves
Assembling a Big Picture
1. Filaments of HI (H2) 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
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:
–
–
–
–
NGC 185 - L. Young (2001)
Spiral arms
Ordered B-field
Shearing disks
High HI column densities
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 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).