Atomic and MolecularAtomic and MolecularGas in GalaxiesGas 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)

Stars Do Not Form in Gas

SINGS + GALEX + THINGS + SONG (animation borrowed from N. Gnedin)

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)

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 galaxy’s ISM from physical principles

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

Dissociation Balance in Atomic-Molecular Complexes

(Krumholz, McKee, & Tumlinson 2008a)

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 =Decrease in radiation intensity =

Absorptions by H2 molecules + dust grainsAbsorptions by H2 molecules + dust grains

The basic equations for this system are chemical equilibrium and radiative transfer.Formation on grains = PhotodissociationFormation on grains = Photodissociation

Calculating Molecular FractionsTo 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

Shielding Layers in Galaxies(Krumholz, McKee, & Tumlinson 2008b, in press)

What is (d /R) (E0*/ n)?

• Dust opacity d and H2 formation rate R both Z, so d / R ~ const

• CNM dominates shielding, so n is the CNM density

• CNM density set by pressure balance with WNM, and nCNM E0

*, with weak Z dependence.

(d /R) (E0*/ n) ~ 1 in all galaxies!

Allowed nCNM

FGH curves for MW (Wolfire et al. 2003)

Reality Check…

Matches observed saturation of HI, with higher HI at low metallicity!

Compare model to BIMA SONG (Blitz & Rosolowsky 2006) and HERA / THINGS (Leroy et al. 2008) surveys, with galaxies binned by metallicity

HI H2

Matches column needed for molecules to appear, with higher at lower metallicity!

Another Application: DLAs(Krumholz, Ellison, Prochaska, McKee, & Tumlinson, 2009, in preparation)

What Does this Imply for SF?

t dep =

10

t fft de

p =

100

0 t ff

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)

t dep =

10

t ff

t dep =

100

0 t ff

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)

A Remark on GMCs• SFR is simply 0.01

Mmol / tff-mol !

• In low galaxies, GMCs all have ~ 100 M pc–2 (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)

Putting it Together: The Total Gas Star Formation Law

(Krumholz, McKee, & Tumlinson, 2009, in preparation)

Super-linear from HI H2 conversionLinear from universal GMC propertiesSuper-linear from rising GMC density

Lines: theory

Contours: THINGS, Bigiel et al. 2008

Symbols: literature data compiled by Bigiel et al. 2008

Atomic and Molecular Star Formation Laws

HI H2

A Project for the EVLA

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.

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)

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