Studying the Atomic-Molecular Transition in the Local Group

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Studying the Atomic-Molecular Transition in the
Local Group

• Erik Rosolowsky
• Radio Astronomy Lab, UC Berkeley
• Ringberg - May 19, 2004

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Collaborators

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

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Star Formation

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

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The Schmidt Law
Kennicutt (1998)
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Resolved Schmidt Law Studies
Wong Blitz (2002) studied CO and star formation
in a sample of 7 galaxies.
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A slight case of déjà vu.
CO Only
CO HI
Kormendy Kennicutt (2004)
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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
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Molecular Clouds
Stars
Toward a simple model.
Atomic ISM
Photo Credits R. Gendler, D. Malin, D. Thilker
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What is a Giant Molecular Cloud?

• Large cloud of molecular gas M gt 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
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How do GMCs vary across the Local Group?
(which is secretly a question about how GMCs
form)
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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

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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)

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Larsons 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?

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The Linewidth-Size Relationship
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The Linewidth-Mass Relationship
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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.

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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
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The Local Group GMC Mass Distribution.
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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 a
Inner MW -1.60 to -1.72
LMC -1.63 to -1.92
Outer MW -1.91 to -2.11
M33 -2.10 to -2.60
Increasing HD Stability
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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.

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Where does H2 form?

• (and what physics makes that so?)
• (and is this the same as making GMCs?)

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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)
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M33 in Ha

• 850 kpc distant
• Sc spiral
• 1 of 3 Local Group spirals

Cheng et al. (1993)
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The D-array Survey
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The GMCs in M33
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Correlation with HI
Deul van der Hulst (1987)
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Determining fmol(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)

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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)

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The Physics of S120 Msun/pc2

• Constant value of ISRF
• Sets H2 dissociation rate
• Constant Midplane Pressure
• Constant volume density (nH)
• Sets H2 formation rate

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Work in Progress

1. Include spatial distribution of HI
2. Include rotation curves

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Assembling a Big Picture

1. Filaments of HI (H2) collected by MHD 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.

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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.

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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)
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Requirements for Formation

• Consider a 106 Msun GMC with D80 pc
• Requires enhancing the surface gas density from
  Sgas10 Msun pc-2 (ISM) to SGMC 200 Msun
  pc-2
• Implies accumulation scale of l gt350 pc.
• If atomic, the conversion to molecular gas is
  reasonably quick for typical densities (3-10 Myr).

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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?

Thilker, Private Communication
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Surface Density Criterion

• In a pixel-to-pixel comparison of HI vs. CO maps,
  CO only found at high HI column densities.
• Not all HI 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?