Atomic and Molecular Gas in Galaxies

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Atomic and MolecularGas in Galaxies

• Mark Krumholz
• UC Santa Cruz
• The EVLA Galaxies Through Cosmic Time
• December 18, 2008
• Collaborators

Sara Ellison (U. Victoria) Chris Matzner (U.
Toronto) Chris McKee (UC Berkeley)
Xavier Prochaska (UC Santa Cruz) Jonathan Tan (U.
Florida) Jason Tumlinson (STScI)
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Stars Do Not Form in Gas
SINGS GALEX THINGS SONG (animation borrowed
from N. Gnedin)
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Stars Form in Molecular Gas
The SFR in a galaxy correlates with its molecular
gas surface density. Atomic gas is important
primarily for producing molecules.
SFR vs. surface densities of HI (blue asterisks)
and H2 (black and green triangles) in M51a
(Kennicutt et al. 2007)
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Implications

• The relationship between HI content and star
  formation is not straightforward, particularly at
  low surface density
• A theory of star formation must be able to
  predict the atomic and molecular fractions in a
  galaxys ISM from physical principles

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Anatomy of An Atomic-Molecular Complex

• Molecules reside in giant molecular clouds (GMCs)
  that are the inner parts of atomic-molecular
  complexes
• The outer parts are dissociated by interstellar
  Lyman-Werner photons
• Goal compute HI and H2 mass fractions

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Dissociation Balance in Atomic-Molecular
Complexes(Krumholz, McKee, Tumlinson 2008a)
The basic equations for this system are chemical
equilibrium and radiative transfer.
Formation on grains Photodissociation

• Idealized problem spherical cloud of radius R,
  density n, dust opacity ?d, H2 formation rate
  coefficient R, immersed in radiation field with
  LW photon number density E0, find fraction of
  mass in HI and H2.

Decrease in radiation intensity Absorptions by
H2 molecules dust grains
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Calculating Molecular Fractions

• To good approximation, solution only depends on
  two dimensionless numbers
• Approximate solution

Top analytic solution for location of HI / H2
transition vs. exact numerical result Bottom H2
volume fraction vs. ?, ?R
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Shielding Layers in Galaxies(Krumholz, McKee,
Tumlinson 2008b, in press)

• What is ? ? (?d /R) (E0/ n)?

Allowed nCNM

• Dust opacity ?d and H2 formation rate R both ? Z,
  so ?d / R const
• CNM dominates shielding, so n is the CNM density

FGH curves for MW (Wolfire et al. 2003)

• CNM density set by pressure balance with WNM, and
  nCNM ? E0, with weak Z dependence.
• ? ? ? (?d /R) (E0/ n) 1 in all galaxies!

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Reality Check
Compare model to BIMA SONG (Blitz Rosolowsky
2006) and HERA / THINGS (Leroy et al. 2008)
surveys, with galaxies binned by metallicity
HI
H2
Matches observed saturation of HI, with higher
?HI at low metallicity!
Matches column needed for molecules to appear,
with higher ? at lower metallicity!
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Another Application DLAs(Krumholz, Ellison,
Prochaska, McKee, Tumlinson, 2009, in
preparation)
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What Does this Imply for SF?
tdep 10 tff
tdep 10 tff
tdep 1000 tff
tdep 1000 tff
Depletion time as a function of ?H2 for 2 local
galaxies (left, Wong Blitz 2002) and as a
function of LHCN for a sample of local and z 2
galaxies (right, Gao Solomon 2004, Gao et al.
2007)
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There is a Universal SFR

• Clouds convert 1 of their mass to stars per
  tff, regardless of density or environment
  (Krumholz McKee 2005 Tan, Krumholz, McKee
  2006 Krumholz Tan 2007)

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A Remark on GMCs

• SFR is simply 0.01 ?Mmol / tff-mol !
• In low ? galaxies, GMCs all have ? 100 M? pc2
  (Bolatto et al. 2008) due to internal regulation
  (Krumholz, Matzner, McKee 2006)
• In high ? galaxies, ?GMC must be ?gal to
  maintain hydrostatic balance

Luminosity (?mass) vs. radius for galactic and
extragalactic GMCs (Bolatto et al. 2008)
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Putting it Together The Total Gas Star Formation
Law(Krumholz, McKee, Tumlinson, 2009, in
preparation)
Lines theory Contours THINGS, Bigiel et al.
2008 Symbols literature data compiled by Bigiel
et al. 2008
Super-linear from HI ?H2 conversion
Linear from universal GMC properties
Super-linear from rising GMC density
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Atomic and Molecular Star Formation Laws
HI
H2
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A Project for the EVLA
Density of star-forming DLAs (points) and
Kennicutt law prediction (blue line) (Wolfe
Chen 2006)
SF law in LSB galaxies (points) and Kennicutt
law (line) (Wyder et al. 2009)

• In low metallicity, low ? gas, the SFR is below
  Kennicutt law prediction. This may be an effect
  of low H2 content. The EVLA can check this
  directly.

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Summary

• Star formation is a 2-step process (1) convert
  HI in the ISM into H2, (2) turn H2 into stars.
• Making molecules depends on gas ?, Z
• Making stars from molecules depends on tff
• Either step can be the rate-limiting one,
  depending on the environment
• This has important implications for SF and galaxy
  evolution at high z, low metallicity