Galactic Scale

• HI column density "hits a wall"
• molecular gas correlated with SF
• "depletion time" ~ 2 Gyr
• power law index of molecular clouds: -1.5 to -2 -> most mass in big clouds
  • only in gas-rich regions
  • steeper in outer regions of galaxies: -2 to -2.5
• typical mean surface density 100 Msun/pc^2
• PDF of surface densities "lognormal" with powerlaw tail

Theory

Virial Parameter: \(\sigma_v = (\alpha_G \pi G \Sigma R / 5)^{1/2}\)

\(\alpha_G = -2 T/ W\)

• \(\alpha_G = 2\) -> free-fall collapse
• 3 options: pressure confinement, virial balance, or free-fall collapse
• Most velocity dispersion on largest scale
• median \(\alpha_G \sim 0.1\)

\(M/M_{crit} = \frac{M}{\phi/(4\pi G)^{1/2}} \approx 2\)

• B-fields not strong enough to support against gravity in mol. clds (Crutcher)

Lifetime of Molecular Clouds

Spatial distribution + galactic dynamics.

Method 1: compare dynamics to distribution

• Formed in the arms and survive passage between arms? (implies lifetime ~100 Myr)

Method 2: compare star cluster ages to spatial cloud distribution

• lifetimes closer to 30 Myr

Method 3: look at T Tau vs post T Tau (Hartmann 2009)

• 3 Myr for Taurus

Why difference?

• size matters
• should compare lifetimes to cloud dynamical times

\(t_{ff} = \left(\frac{3\pi}{32 G \rho}\right)^{3/2}\)

Efficiency

The rate at which gas converts into stars.

\(\epsilon_{ff} = SFR / (M_{gas}/t_{ff}) \sim 0.01\)

• if pure free-fall, \(\epsilon_{ff} = 1\)
• Low star formation rate
• Lifetime << 100 \(t_{ff}\)
• Lifetime * efficiency < 1, so low overall star formation efficiency

Metallicity

Break between HI and CO:

• 10 Msun in Galaxy, local spirals
• 50 Msun in SMC

Converging Flows

• Collision -> cooling
• vazquez-semadeni et al
• "local turbulence" triggers collision of warm HI
• Can't make big clouds: limited to mean surface density * scale height? [needs more]

Cloud Collision in Spiral Arms

• Spiral arms are convergence points
• Collisions unlikely in most of galaxy
• orbits crowd in arms: collisions more likely
• mechanism probably can't work in flocculent galaxies
• explains counter-orbiting GMCs [are these really a common feature?]

Gravitational Magneto-Jeans (Kim & Ostriker 2006)

• Toomre Q instability at Q < 1.5
• makes 10^7-10^8 Msun clouds; smaller ones must fragment
• produces low spin clouds. Mol. Clds apparently have much less angular momentum than expected
• easily explains "beads on a string" morphology [why?]

Parker + Thermal instability (Mouschovias et al 2005)

• Buoyancy draws B-fields out of plane
• may not work with turbulence

Chemistry: Forming H2 and CO

• H2 dominance begins at ~10 Msun/pc^2
• Glover and Clark: Is the molecular transition a necessary prerequisite to star formation?
• Extincting FUV allows cooling, H2 formation

Mol Cld Disruption

• Dont' survive, must be disrupted....

Disruption Scenarios

• ionization can disrupt up to 10^5, may not work above that mass
• turbulence slows down SF
• turbulence must be maintained

Scales

• "Normal" from 10-100 msun/pc^2 (all clouds are the same; cloud counting)
• extreme above 100 msun/pc^2 (densities in mol clds very high; mean surface density of galaxy matches individual clouds)
• "all hell break loose" < 10 msun/pc^2

Citations

• Leroy 2008
• Roman-Duval 2010
• Gratier 2012
• Narayanan 2008: Taurus cloud CO 3-color by velocity
• Crutcher 201
• Koda 2009 M51
• Kawamura 2009 LMC
• Rosolowsky & Blitz 2005