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Maximal Starbursts: Theory & Reality. Todd Thompson The Ohio State University Department of Astronomy Center for Cosmology & Astro-Particle Physics. Principal collaborators: Norm Murray & Eliot Quataert. HST: IRAS 19297-0406. Maximal Starbursts: Motivation. Star Formation is Slow.

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Maximal starbursts theory reality

Maximal Starbursts: Theory & Reality

Todd Thompson

The Ohio State University

Department of Astronomy

Center for Cosmology & Astro-Particle Physics

Principal collaborators: Norm Murray & Eliot Quataert

HST: IRAS 19297-0406



Star formation is slow
Star Formation is Slow

  • Galaxies (from normal spirals to the densest starbursts) are observed to be globally inefficient at forming stars; ~few% of the available gas supply is converted to stars per dynamical timescale (e.g., Kennicutt 1998).

  • Star formation is similarly “slow” on sub-galactic scales down to sub-GMC scales (e.g., Wong & Blitz ‘02; Bigiel et al. ‘09; Leroy et al. ‘09; Krumholz & Tan ‘07).

  • Galaxies are observed to be marginally stable to their own self-gravity (Toomre’s Q ~ 1), and in approximate hydrostatic equilibrium (Martin & Kennicutt ‘01).

    Why? How?


Turbulence feedback
Turbulence & Feedback

  • Answer: Turbulence (e.g., Krumholz’s talk).

    The gas in galaxies is stirred, mixed, & shocked, slowing star formation on all scales & in galaxies ranging from normal spirals to the densest & most luminous starbursts.

    What drives the turbulence?

    Answer(?): “Feedback:” the injection of energy and momentum into the ISM by stellar processes (Supernovae, expanding HII regions, stellar winds, etc.). Suggests that the ISM is self-regulated, like a star.


Systematics of star formation
Systematics of Star Formation

  • Schmidt Law:

  • Wide range in

  • Wide range in density.

  • Star formation is inefficient.

  • Why? How?

  • Suggests a mechanism for self-regulation (feedback!) acting over a huge range in density.

  • Maximal star formation?

Starbursts

Star-forming galaxies

Kennicutt (1998)


Systematics of star formation1
Systematics of Star Formation

  • Schmidt Law:

  • Wide range in

  • Wide range in density.

Arp 220 (LFIR ~ 21012 L)

  • Two counter-rotating cores.

  • Circumbinary disk R~300pc.

Starbursts

GHz Continuum

50pc

Star-forming galaxies

Beswick 2006; Mundell et al; Lonsdale et al

Kennicutt (1998)



Sources of feedback regulation processes
Sources of Feedback, Regulation Processes

  • Energy injection by supernova explosions.

  • Expanding HII regions.

  • Proto-stellar jets/outflows.

  • Stellar winds.

  • Radiation pressure on dust grains.

  • Others:

    • Magnetic fields (B potentially not high enough in starbursts; constraint from FIR-radio correlation; Thompson+’06,’07,’09; Lacki+’09)

    • Cosmic rays (inelastic CR proton collisions (pion losses) make CR energy density likely too small in starbursts; Thompson+’07; Lacki+’09)


The failure of energy injection
The Failure of Energy Injection

  • The standard lore: Energy injection by supernovae, etc. (e.g., McKee & Ostriker ‘77). However, in a dense ISM, radiative losses are large. A simple extrapolation of McKee & Ostriker to Arp 220 is a failure

    SFR needed for hydrostatic equilibrium is off by ~ 300 - 4000.

  • Need a mechanism that gets stronger at high density, not weaker. (Note: There is an “irreducible” minimum amount of momentum from SNe.)

  • Recent numerical studies show that the maximum v attained in SN-driven turbulence is just ~10 km/s (Joung, Mac Low, & Bryan ‘08).


Radiation pressure on dust
Radiation Pressure on Dust

  • Starburst photons absorbed & scattered by dust: UV ~ 1000 cm2/g.

  • Dust is collisionally coupled to gas:  ~ 0.01 pc a0.1 n3-1.

  • Starbursts: optically thick to re-radiated IR : IR ~ gasIR > 1-100.

  • Typical opacity IR ~ few - 10 cm2 g-1, depending on gas-to-dust ratio, metallicity.

  • Radiative diffusion: efficient coupling to cold, dusty gas, most of the mass.

O’Dell+(‘67) Thompson, Quataert, & Murray (‘05)

Chiao & Wickramasinghe (‘73) Murray, Quataert, & Thompson (‘05)

Elmegreen & Chiang (‘82) Murray, Quataert, & Thompson (‘09)

Ferrara+(’90), (‘91)

Scoville+(‘01), Scoville (‘03)

Krumholz & Matzner (‘09)




Flux mean opacity three regimes
Flux Mean Opacity: Three Regimes

Optically

thin to UV

Optically

thick to FIR

Optically

thick to UV


The rosseland mean opacity
The Rosseland Mean Opacity

  • Dust dominates T < 1000 K.

  • Sublimation: Tsub ~ 1000 - 2000 K.

  • T < 200 K:  = 0T2(Rayleigh limit).

  • 200K < T < 1000K:  = const.

  • Overall normalization is dependent on metallicity and the dust-to-gas ratio.

Semenov et al. (2003)


Some predictions
Some Predictions

  • The “Schmidt”-law:

  • When = 0T2 (T < 200K):

    If T < 200 K

    & FIR-thick:

    no dependence on anything, but 0.


Hypotheses
Hypotheses

  • The maximal flux from a galaxy at all wavelengths is bounded by the dust Eddington limit; this is a non-grey criterion.

  • If this limit is exceeded, the remaining gas is ejected, and star formation is quelled.

  • If radiation pressure is the dominant feedback mechanism, the minimum flux is the maximum flux:

  • In this limit, the galaxy is analogous to a single radiation pressure supported massive star.

Thompson, Quataert, & Murray ‘05


Stability
Stability

  • Radiation pressure supported starbursts are dynamically (Jeans) unstable.

    • Radiation rapidly diffuses. No restoring force against gravity even though radiation pressure is sufficient for hydrostatic equilibrium.

  • It (probably) maintains hydrostatic equilibrium in a statistical sense, coupling to the generation of supersonic turbulence. (Like convection).

Thompson (2008)


Stars form in clusters tan krumholz bigiel koda leroy talks
Stars Form in Clusters(Tan, Krumholz, Bigiel, Koda, Leroy talks)

  • Gas in Q ~ 1 disk collapses to form GMC, proto-stellar cluster. Gas from volume ~ h3participates.

  • GMC forms some stars, and is then disrupted by stellar feedback.

  • Assess all stellar feedback processes to determine which dominates. In massive clusters, radiation pressure on dust.

  • Predictions: overall, similar to galaxy-wide work; the criterion to balance the self-gravity of the disk is the same as the criterion to disrupt a self-gravitating GMC composed of gas from one ~ h3in a Q ~ 1 disk.

Murray, Quataert, & Thompson (‘09)

see also Krumholz & Matzner (‘09)




Evidence for a Characteristic Flux

  • The Case of Arp 220:

  • On “large” scales of ~100pc (T < 200K):

Beswick 2006; Mundell+; Lonsdale+

Also Scoville+’98; Sakamoto+99; Downes & Solomon ’98;

Sakamoto+08;

Downes & Eckart 08;

Matsushita+09


Eddington limited starbursts
Eddington-Limited Starbursts

ULIRGs are compact. Intrinsic size?

Appeal to radio size, hoping that the radio reliably traces the star formation.

Data from Condon et al. (1991)



Evidence for Eddington-Limited Star Formation

  • The Case of M82

  • Starburst modeling gives

    (Rieke et al. 1993; McLeod et al. 1993; Forster-Schreiber et al. 2003ab).

  • Potential evidence for the dominance of radiation pressure on dust in the very recent past.

Forster-Schreiber+03

Similarly, observed clusters in M82 can disrupt themselves

with radiation pressure on dust (Murray+09; Krumholz & Matzner 09).


Yet more extreme eddington limited star formation
Yet More Extreme Eddington-Limited Star Formation

Where is star formation sufficiently intense that it approaches this limit?

Semenov et al. (2003)


Evidence for Eddington-Limited Star Formation

  • T > 200K: Most extreme: dense stellar cluster scale:

    • 0.1pc young stellar disk at the Galactic Center: F ~ 5 x 1014Lsun/kpc2 ~ Fedd. Vertical structure of star-forming disk could have been maintained by radiation pressure.

    • Inner ~35-70 pc of West nucleus of Arp 220 has flux of ~ 1-5 x 1014Lsun/kpc2 (Downes & Eckart 07; Sakamoto+08).

    • Dense star clusters & ellipticals reach a stellar surface density indicating the potential dominance of radiation pressure during formation (Hopkins+09).

  • T < 200K: Galaxy scale:

    • Riechers+08, 09a,b; Walter+09; Younger+08: Intensely star-forming galaxies at high-z meet, but do not exceed, the dust Eddington limit.

  • Optically-thin to FIR: star-forming galaxies:

    • Andrews & Thompson+09, in prep: star-forming galaxies meet, but do not exceed, the dust Eddington limit.

    • Pelligrini+07,09: Rad P comparable to other forces in M17 & Orion.


Summary
Summary

  • Observations suggest that starbursts radiate close to the Eddington limit for dust.

  • Radiation pressure may thus be the dominant feedback mechanism in starbursts; it may define a “maximal starburst.”

  • In this picture, starburst galaxies can be thought of in analogy with individual radiation pressure supported massive stars.

  • The maximum Eddington flux changes as a function of wavelength because of the flux-mean opacity (e.g., compare with Meurer+97).

  • This picture can be generalized from the galaxy scale to the scale of individual GMCs & super-star clusters.(Murray, Quataert, & Thompson ‘09; but see also Scoville+’01; Krumholz & Matzner ‘09).


Some questions
Some Questions

  • Starbursts are already super-maximal, in the sense that they are observed to drive large-scale outflows (e.g., Heckman, Armus, & Miley ‘90; Strickland & Heckman ‘09).

    Is this a prediction of Eddington-limited star formation, or not? (Thompson ‘09, in prep; Murray, Quataert, & Thompson ‘05)

  • Can we show that the maximum flux/luminosity predicted appears empirically? Does this theory predict the Schmidt Law? (Andrews & Thompson+ ‘09, in prep)

  • How does this picture change as a function of metallicity/gas-to-dust ratio? Compare with observations (e.g., Bolatto’s talk).


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