Turbulence, Feedback, and Slow Star

Formation

Turbulence, Feedback, and Slow Star

FormationMark Krumholz

Princeton University / UC Santa CruzGas Accretion and Star Formation in

GalaxiesMPA/ESO/MPE/USM Conference

11 September 2007Collaborators:

Tom Gardiner, Jim Stone (Princeton) Todd Thompson (Princeton / Ohio State)

Chris McKee (UC Berkeley) Chris Matzner (U. Toronto)Jonathan Tan (U. Florida)

Mark KrumholzPrinceton University / UC Santa CruzGas Accretion and Star Formation in

GalaxiesMPA/ESO/MPE/USM Conference

11 September 2007Collaborators:

Tom Gardiner, Jim Stone (Princeton) Todd Thompson (Princeton / Ohio State)

Chris McKee (UC Berkeley) Chris Matzner (U. Toronto)Jonathan Tan (U. Florida)

OutlineOutline

Embarrassing observational facts

Turbulence-regulated star formation

Star formation-regulated turbulence

Conclusions

Embarrassing observational facts

Turbulence-regulated star formation

Star formation-regulated turbulence

Conclusions

ObservationsObservations

Star Formation is Slow…

(Zuckerman & Evans 1974; Zuckerman & Palmer 1974; Rownd & Young 1999; Wong & Blitz 2002)

Star Formation is Slow…

(Zuckerman & Evans 1974; Zuckerman & Palmer 1974; Rownd & Young 1999; Wong & Blitz 2002)The Milky Way contains Mmol ~ 109

M of gas in GMCs (Bronfman et al. 2000), with n ~ 100 H cm–3 (Solomon et al. 1987), free-fall time tff ~ 4 Myr

This suggests a star formation rate ~ Mmol / tff ~ 250 M / yr

Observed SFR is ~ 3 M / yr (McKee &

Williams 1997)

Numbers similar in nearby disks

The Milky Way contains Mmol ~ 109 M of gas in GMCs (Bronfman et al. 2000), with n ~ 100 H cm–3 (Solomon et al. 1987), free-fall time tff ~ 4 Myr

This suggests a star formation rate ~ Mmol / tff ~ 250 M / yr

Observed SFR is ~ 3 M / yr (McKee &

Williams 1997)

Numbers similar in nearby disks

…even in starbursts……even in starbursts… Example: Arp 220 Measured properties: n ~ 104 H cm–3, tff ~ 0.4 Myr, Mmol ~ 2 109 M (Downes & Solomon 1998)

Suggested SFR ~ Mmol / tff ~ 5000 M / yr

Observed SFR is ~ 50 M / yr (Downes & Solomon 1998): still too small by a factor of ~100

Example: Arp 220 Measured properties: n ~ 104 H cm–3, tff ~ 0.4 Myr, Mmol ~ 2 109 M (Downes & Solomon 1998)

Suggested SFR ~ Mmol / tff ~ 5000 M / yr

Observed SFR is ~ 50 M / yr (Downes & Solomon 1998): still too small by a factor of ~100

HST/NICMOS image of Arp 220, Thompson et al. 1997

HST/NICMOS image of Arp 220, Thompson et al. 1997

…even in dense gas… (Gao & Solomon 2004, Wu et al. 2005, Krumholz & Tan, 2007, ApJ, 654, 304)

…even in dense gas… (Gao & Solomon 2004, Wu et al. 2005, Krumholz & Tan, 2007, ApJ, 654, 304)

Example: HCN observations show LIR/L ≈ 900 LHCN/K km s–1 pc–2

This implies a SFR ~ MHCN / 30 Myr

Critical density ~ 105 cm–3 tff ~ 0.2 Myr

Again, SFR too small by factor of ~100

Example: HCN observations show LIR/L ≈ 900 LHCN/K km s–1 pc–2

This implies a SFR ~ MHCN / 30 Myr

Critical density ~ 105 cm–3 tff ~ 0.2 Myr

Again, SFR too small by factor of ~100

Observed IR-HCN correlation, (Wu et al. 2005)

Observed IR-HCN correlation, (Wu et al. 2005)

Galaxies

Galaxies

MW Clumps

MW Clumps

All Observed Star Formation is Slow!

(Tan, Krumholz, & McKee, 2006, ApJL, 641, 121; Krumholz & Tan, 2007, ApJ, 654, 304)

All Observed Star Formation is Slow!

(Tan, Krumholz, & McKee, 2006, ApJL, 641, 121; Krumholz & Tan, 2007, ApJ, 654, 304)

Rant #1:(with apologies to R.

Kennicutt)

Rant #1:(with apologies to R.

Kennicutt)The oft-repeated claim that CO doesn’t really trace the “actively” star forming gas but HCN or other dense gas tracers does is misleading.

The HCN isn’t all turning into stars any more than the CO is. All the SF inside the HCN clouds occurs in a few active regions containing a few percent of the mass, exactly as for the CO clouds.

The oft-repeated claim that CO doesn’t really trace the “actively” star forming gas but HCN or other dense gas tracers does is misleading.

The HCN isn’t all turning into stars any more than the CO is. All the SF inside the HCN clouds occurs in a few active regions containing a few percent of the mass, exactly as for the CO clouds.In other words: it’s turtles all the way down!In other words: it’s turtles all the way down!

Implications of Slow Star FormationImplications of

Slow Star FormationFor galaxy modelers:

Bad news: unless you can resolve > 105 cm–3, you cannot avoid subgrid models.

Good news: setting SFR ~1% / tff in cold gas is a good model for any maximum .

For SF theorists:A big question is: why SF is so slow?The answer for the SF rate (as opposed to threshold) must be at small scales (it’s not cloud formation, spiral arms, etc.).

For galaxy modelers:Bad news: unless you can resolve > 105 cm–3, you cannot avoid subgrid models.

Good news: setting SFR ~1% / tff in cold gas is a good model for any maximum .

For SF theorists:A big question is: why SF is so slow?The answer for the SF rate (as opposed to threshold) must be at small scales (it’s not cloud formation, spiral arms, etc.).

Turbulence-Regulated SF(Krumholz & McKee, 2005, ApJ, 630, 250)

Turbulence-Regulated SF(Krumholz & McKee, 2005, ApJ, 630, 250)

Overdense regions can have PE(l ) ~ KE(l )

PE = KE implies J ≈ s, where

Overdense regions can have PE(l ) ~ KE(l )

PE = KE implies J ≈ s, where LL

ll

ll

Whole cloud: PE(L) ~ KE(L), (i.e. vir ~ 1)

Linewidth-size relation: = cs (l/s)1/2

Whole cloud: PE(L) ~ KE(L), (i.e. vir ~ 1)

Linewidth-size relation: = cs (l/s)1/2

In average region, PE(l) l5, KE(l) l4 most regions have KE(l) » PE(l)

In average region, PE(l) l5, KE(l) l4 most regions have KE(l) » PE(l)

This also implies that This also implies that

The Turbulent SFRThe Turbulent SFR Turbulent gas has lognormal PDF of densities that depends on M

J ≈ s gives instability condition on density

Gas above critical density collapses on time scale tff

Result: an estimate

Turbulent gas has lognormal PDF of densities that depends on M

J ≈ s gives instability condition on density

Gas above critical density collapses on time scale tff

Result: an estimate

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

SFRff ~ 1-5% for any turbulent, virialized objectSFRff ~ 1-5% for any turbulent, virialized object

Comparison to Milky Way

Comparison to Milky Way

In MW, properties of GMCs observable Integrate over GMC distribution to get SFR:

Observed SFR ~ 3 M / yr: good agreement! Also reproduce radial distribution (Luna et al.

2006)

Direct test: repeat calculation once a comparable catalog is available for M33, M64, LMC.

In MW, properties of GMCs observable Integrate over GMC distribution to get SFR:

Observed SFR ~ 3 M / yr: good agreement! Also reproduce radial distribution (Luna et al.

2006)

Direct test: repeat calculation once a comparable catalog is available for M33, M64, LMC.

SF Law in Other Galaxies

SF Law in Other Galaxies

For other galaxies, GMCs not directly observable

Estimate GMC properties based on (1) pressure balance with ISM, (2) virial balance in GMCs

Result is SF law in terms of observables:

For other galaxies, GMCs not directly observable

Estimate GMC properties based on (1) pressure balance with ISM, (2) virial balance in GMCs

Result is SF law in terms of observables:

Theory (solid line, KM05), empirical fit (dashed line, Kennicutt 1998), and data (K1998) on galactic SFRs

Theory (solid line, KM05), empirical fit (dashed line, Kennicutt 1998), and data (K1998) on galactic SFRs

SF Laws in Different Tracers

(Krumholz & Thompson, 2007, ApJ, in press, arXiv:0704.0792)

SF Laws in Different Tracers

(Krumholz & Thompson, 2007, ApJ, in press, arXiv:0704.0792) SF law depends on

tracer: SFR LCO1.5,

or SFR LHCN1.0

Depends on ncrit: CO: low ncrit all gas, varying n

HCN: high ncrit dense gas, fixed n

SFR ~ 0.01 M/tff; tff fixed for HCN, not CO, so CO gets extra power of 0.5

SF law depends on tracer: SFR LCO

1.5, or SFR LHCN

1.0

Depends on ncrit: CO: low ncrit all gas, varying n

HCN: high ncrit dense gas, fixed n

SFR ~ 0.01 M/tff; tff fixed for HCN, not CO, so CO gets extra power of 0.5

L IRL CO

1.5

L IRL CO

1.5

L IRL HCN

L IRL HCN

Star Formation- Regulated Turbulence(Krumholz, Matzner, & McKee 2006, ApJ, 653, 361)

Star Formation- Regulated Turbulence(Krumholz, Matzner, & McKee 2006, ApJ, 653, 361)

Observed GMCs are turbulent, virialized, all have about same NH

Turbulence decays in ~1 crossing time (Stone, Ostriker, & Gammie 1998; Mac Low et al. 1998)

Large GMCs live 20–30 Myr, ~3–4 tcr, ~6–8 tff

Need to explain cloud lifetimes and invariance of cloud properties

Observed GMCs are turbulent, virialized, all have about same NH

Turbulence decays in ~1 crossing time (Stone, Ostriker, & Gammie 1998; Mac Low et al. 1998)

Large GMCs live 20–30 Myr, ~3–4 tcr, ~6–8 tff

Need to explain cloud lifetimes and invariance of cloud properties

HII region in 30 Doradus, MCELS teamHII region in 30 Doradus, MCELS team

A Semi-Analytic GMC Model

A Semi-Analytic GMC Model

Goal: model GMC energy and momentum budget, including decay of turbulence, turbulent driving and mass loss due to HII regions

Goal: model GMC energy and momentum budget, including decay of turbulence, turbulent driving and mass loss due to HII regions

Mg, M*, R, dR/dt, Mg, M*,

R, dR/dt,

Evolution eqns: non-equilibrium virial theorem and energy conservation

Evolution eqns: non-equilibrium virial theorem and energy conservation

N H,22=1

N H,22=1

Clouds Stay Near Observed Values of vir,

NH, tdep

Clouds Stay Near Observed Values of vir,

NH, tdep

vir=3vir=3

M=104 M

M=104 M

No clouds

GMCs

Massive clouds all have vir = 0.5 – 3(Heyer et al. 2001)

Massive clouds all have vir = 0.5 – 3(Heyer et al. 2001)

GMCs in MW and LG all have NH ~ 1022 cm–2 (Blitz et al. 2006)

GMCs in MW and LG all have NH ~ 1022 cm–2 (Blitz et al. 2006)

Depletion time constant, ~1 Gyr (Wong & Blitz 2002)

Depletion time constant, ~1 Gyr (Wong & Blitz 2002)

Global ResultsGlobal ResultsMass Lifetime SFE Destroyed By?

2 105 M 9.9 Myr

(1.6 tcr, 3.2 tff)5.3%

Unbinding, dissociation by interstellar UV

1 106 M20 Myr

(2.2 tcr, 4.4 tff)5.4%

Unbinding by HII regions

5 106 M43 Myr

(3.2 tcr, 6.4 tff)8.2%

Unbinding by HII regions

Large clouds quasi-stable, live 20-40 Myr: agrees with observed ~30 Myr lifetime of LMC GMCs (Fukui et al. 2007)!

Small clouds live ~1 crossing time, consistent with small, local clouds

Large clouds quasi-stable, live 20-40 Myr: agrees with observed ~30 Myr lifetime of LMC GMCs (Fukui et al. 2007)!

Small clouds live ~1 crossing time, consistent with small, local clouds

Next Step: Ionization MHD Simulations

(Krumholz, Stone, & Gardiner 2007, ApJ, in press, astro-ph/0606539)

Next Step: Ionization MHD Simulations

(Krumholz, Stone, & Gardiner 2007, ApJ, in press, astro-ph/0606539)

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

ConclusionsConclusions

Star formation is SLOW on all scales, in all environments

Feedback-driven turbulence can explain this observation

This model explains / predicts:Low SFR even in very dense gasGMC lifetimes and propertiesRate of star formation in MWKennicutt Law and IR-HCN correlation

Star formation is SLOW on all scales, in all environments

Feedback-driven turbulence can explain this observation

This model explains / predicts:Low SFR even in very dense gasGMC lifetimes and propertiesRate of star formation in MWKennicutt Law and IR-HCN correlation

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

this is all from someone dumb enough to agree to give a talk in Garching today and a talk in Heidelberg tomorrow morning…

this is all from someone dumb enough to agree to give a talk in Garching today and a talk in Heidelberg tomorrow morning…

3.5 hours!3.5 hours!

Final caveat:Final caveat: