Maximal Starbursts: Theory & Reality

1. 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

2. Maximal Starbursts:Motivation

3. 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?

4. 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.

5. 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)

6. 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)

7. Maximal Starbursts:Theory

8. 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)

9. 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).

10. 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)

11. Radiation Pressure Supported Starbursts

12. Radiation Pressure Supported Starbursts A bit complicated

13. Flux Mean Opacity: Three Regimes Optically thin to UV Optically thick to FIR Optically thick to UV

14. 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)

15. Some Predictions • The “Schmidt”-law: • When = 0T2 (T < 200K): If T < 200 K & FIR-thick: no dependence on anything, but 0.

16. 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

17. 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)

18. 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)

19. Maximal Starbursts:Reality

20. Radiation Pressure Supported Starbursts

21. 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

22. 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)

23. Evidence for Eddington-Limited Star Formation Davies et al. (2006)

24. 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).

25. Yet More Extreme Eddington-Limited Star Formation Where is star formation sufficiently intense that it approaches this limit? Semenov et al. (2003)

26. 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.

27. 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).

28. 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).