Feedback in Starburst Galaxies

Todd ThompsonPrinceton University

with Eliot Quataert, Norm Murray, & Eli Waxman

Outline

• Goal: A model for the global structure of starbursts.

• Why starbursts? The physical conditions.

• Radiation pressure feedback.

• Magnetic fields, cosmic rays, & -rays.

Kennicutt (1998)

Systematics of Star Formation

• Schmidt Law:

• ``Star-forming” galaxies:– Extended, few-kpc scales.– ~ billion year timescales.

• ``Starburst” galaxies:– Compact, 100’s pc scales.– 1-100 million year

timescales.

• Pressure: P ~ G g2

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˙ Σ *∝ Σg7 / 5

Starbursts

Star-forming galaxies

Kennicutt (1998)

Regulation & Feedback in Galaxies

• Low star formation efficiency:

Suggests feedback and/or regulation over a broad range of conditions.

• Q~1 observed in disks.(Martin & Kennicutt 2001)

• Stellar processes (?): Stellar winds, radiation, supernovae, HII regions, etc.

• Non-stellar processes (?): MRI. (Sellwood & Balbus 99;Piontek & Ostriker 04)

Starbursts

Star-forming galaxies

Why Starbursts?

NOAO

M82

M51

Arp 220

IRAS 19297-0406

NGC 253

Backgrounds & Starbursts

Dole et al. (2006)

Why Starbursts?• Starbursts & U/LIRGs

– lie on the same scaling relations with normal galaxies.

– constitute a large fraction of the IR background, the star-formation rate density at high z (also, -ray & MeV/TeV backgrounds).

– may be a key phase in the growth of super-massive black holes & spheroids.

– are connected physically to super-star clusters, starburst cores.

– have turbulent velocities v > 10 km/s.

• What do we want to know?– Constituents: radiation, gas/dust, magnetic fields, and cosmic rays.

– The origin and systematics of the scaling relations of galaxies.

The Physical Conditions

Arp 220 (d ~ 80 Mpc):

• Two counter-rotating cores, ~100pc.• Circumbinary disk R~300pc.

gas ~ 5 g cm-2

n ~ 103-104 cm-3

• Mgas ~ 109 -1010 M

v ~ 100 km s-1

• LFIR ~ 21012 L

• LX ~ 3109 L

• tdyn ~ 106 n4-1/2 yr

Solomon, Sakamoto

300 pc

Beswick 2006; Mundell et al; Lonsdale et al

Pressures• Accounting:

What processes regulate Star Formation in ULIRGs?

• The standard lore: Energy injection by supernovae, stellar winds, HII regions (e.g., McKee & Ostriker ‘77). However, in a dense ISM, radiative losses are large: E n-1/4.

• Another Option: Radiation Pressure:

– Starburst photons absorbed & scattered by dust: UV ~ 100’s cm2/g.

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

– Starbursts: optically thick to re-radiated IR : IR ~ gasIR > 1.

– Radiative diffusion: efficient coupling to cold, dusty component, most of the mass.

Scoville (2003) Thompson, Quataert, & Murray (2005)

Radiation Pressure Supported Starbursts

• Radiative flux:

• Radiative diffusion:

• Radiation pressure:

• Obtain Eddington-limited starbursts:

Some Predictions

• The “Schmidt”-law for optically-thick starbursts:

Higher implies more pressure support, which implies a lower star formation rate & efficiency.

Kennicutt (1998)€

˙ Σ *∝Σg

κ

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˙ Σ *∝ Σg7 / 5

The Rosseland Mean Opacity

• Sublimation: Tsub ~ 1000 K.

• Dust dominates T < 1000 K.

• At T < 200 K — in the Rayleigh limit — = 0T2.

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

Semenov et al. (2003)

Some Predictions• The “Schmidt”-law:

• When = 0T2:

no dependence on anything, but 0.

Data from Condon et al. (1991)

ULIRGs are compact. Intrinsic size?

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

A Characteristic Flux?

Evidence for a Characteristic Flux?

Davies et al. (2006)

Why Radiation Always Wins

• Schmidt law:• Flux:• Radiation pressure:

• Hydrostatic pressure:

• Critical surface density:

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˙ Σ *∝ Σg7 / 5

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˙ Σ *∝ Σg7 / 5

Magnetic Fields & Cosmic Rays

How do CR electrons cool?

Radio synchrotron from CR e-’s accelerated by SNe.

FIR traces star formation, massive stars, SNe.

“Calorimeter” theory: synchrotron cooling timescale shorter than the escape time:

tsynch < < tescape

(Völk‘89; generally unaccepted)

galaxy = CR beam dump

The FIR-Radio Correlation

Yun et al. (2001)

Starbursts

Star-Forming Galaxies

Magnetic Fields & Cosmic Rays

• In the Milky Way, B~5-10G and

• In starburst galaxies, how do we estimate B?– “Minimum energy” (UB~UCR; Burbidge 1956): (~5-10G in MW).

Depends on the ratio [p/e] and on the injected CR spectral index.

– Magnetic energy density in equipartition with total hydrostatic pressure: (~5-10G in MW)

Magnetic Fields

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Bmin∝ (Lνrad /V )2 / 7

Conclusion:

Magnetic fields in star-forming galaxies are both minimum energy & equipartition.

and

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B2 /8π ~ UCR

Magnetic Fields

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Bmin∝ (Lνrad /V )2 / 7

Conclusion:

Either

the minimum energy estimate is wrong,

or

magnetic fields are dynamically weak in starburst galaxies.

Thompson et al. (2006)

UBmin/Uph measures the importance of synchrotron relative to IC cooling.

If Bmin is correct, IC dominates for starbursts.

This contradicts the linearity of the FIR-radio correlation.

Bmin Must Underestimate the True Field

UBmin /Uph

Magnetic Fields & FIR-Radio Correlation

• In the limit of very strong cooling (the “calorimeter” limit):

• The observed Schmidt Law says that

• Therefore, in the limit of strong cooling:

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˙ Σ *∝ Σg7 / 5

Magnetic Fields

€

Bmin∝ (Lνrad /V )2 / 7

Conclusion:

If a fraction ~1% of 1051 ergs per SN goes to CR electrons, and they cool rapidly, the observed trend is reproduced.

Implies that B is in fact larger than Bmin.

Thompson et al. (2006)

Magnetic Fields in Starbursts

• Observations thus imply rapid electron cooling.– Strong evidence for the calorimeter theory for the FIR-

radio correlation: tcool< < tescape.

• So, how big is B? – Well, B is big enough that the synchrotron cooling

timescale is << tesc. But, what is tesc?

Very uncertain:

Diffusion in MW tesc ~107.5 yrs. Maybe advection (winds!) in starbursts tesc ~105.5 yrs (?).

Magnetic Fields in Starbursts• Argument/Problem:

– The strongest objection to the calorimeter theory for FIR-radio correlation: if synchrotron dominates cooling and tcool< < tesc, the radio spectral indices of starbursts at GHz should be steep “cooled” : F ~ - , with ~ 1-1.2.

– This is not observed. Spectral indices at GHz are ~constant & not steep: F ~ - , with ~ 0.7.

• Solution: – If CRs interact with matter at mean density & B~Beq, then

Ionization losses dominate for low-energy CRs, not high. This effect changes the expected slope of the radio spectrum at a characteristic frequency ~GHz.

Magnetic Fields in Starbursts

p=2.0

p=2.5

• Ionization losses flatten the radio spectra

• Ionization is important only if CRs interact with ISM of ~mean density.

• Prediction: spectral break ubiquitous at GHz ’s for all galaxies obeying FIR-radio.

• Because this only works if B~Beq, this is the best argument for B >> Bmin in starbursts.

Steeper

Flatter

Summary• Observations indicate

– feedback is important, SF is inefficient, starbursts are dusty, disks have Q~1.

• Radiation pressure – can dominate feedback in the optically thick regions of starbursts.– yields qualitative change to Schmidt Law.– couples to the cold dusty component, most of the mass.– predict starburst structure: T, Teff, F, , , v, SFR/area, efficiency– are in good agreement with observations (local & high-z ULIRGs).

• Magnetic Fields in Starbursts– are larger than Bmin and probably ~ Beq.– are large enough that the “calorimeter” theory for FIR-radio is preferred.– are consistent with starburst radio spectral indices only if CRs interact with

ISM of mean density so that ionization/bremsstrahlung losses are important. -Ray Observations of Starbursts

– will constrain the ISM density seen by CR protons.– will constrain the energetics of CR acceleration. - Lastly, (CRp/CRe) ~ 10.

Thompson et al. (2005), (2006ab)

The Present & The Future • Radiation pressure feedback:

– Embedded sources, porosity, transport, multi-phase ISM.

– The gravitational instability in radiation pressure dominated backgrounds.

– Starburst winds, scaling relations: Faber-Jackson, M-.

• Other mechanisms for feedback:– HII regions, stellar winds, supernovae, gravity.

• The starburst-AGN fueling connection.

• The FIR-radio correlation:– Test prediction of spectral breaks at GHz.

– Electron calorimetry in normal star-forming galaxies (?).

• Starbursts: what is the role of the secondary electron/positrons?

• Backgrounds: neutrino (MeV to >TeV), -ray, FIR, & radio.

• What is the energy density of cosmic rays in starburst galaxies?

The End

Constraining the Average Density “Seen” by Cosmic Rays

-Rays from Starbursts• Assume SNe accelerate both CR protons & electrons.• The GeV protons collide with ambient gas:

• Proton-proton collisions produce

• If pp<< esc, then the starburst is a “proton calorimeter,” and all of the proton energy goes into ’s (1/3), e+,-’s (1/6), and ’s (1/2).

• What is esc? As for CR electrons, very uncertain.

Thompson et al. (2006)

-Rays from Starbursts• Massive star formation IR emission Supernovae:

where is the fraction of 1051 ergs per supernova to CRp’s. This is a FIR--Ray correlation analogous to FIR-radio.

• How do we constrain ? Assume the e+,-’s from p-p cool via only synchrotron in the starburst:

• Observed FIR-radio correlation:Thompson et al. (2006)

-Rays from Starbursts

€

Lν (GeV) ~ 10-5η 0.05LTIR

Arp 220

NGC 253

-Rays from Starbursts• If GLAST sees a larger flux from NGC 253:

– Then > 0.05 more energy per SN to CR protons.

– Because from secondary electrons/positrons, another process (not synchrotron) must dominate CR electron cooling.

• If GLAST sees a smaller flux from NGC 253:– Either the CRs interact ISM below mean density, rapid escape,

– or, < 0.05 less energy per SN to CR protons.

– These options can in principle be distinguished by modeling the IC

and relativistic bremsstrahlung emission at -ray energies since the latter also depends on density.

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Lν (GHz) ~ 3×10-6η 0.05LTIR

The Diffuse -Ray Background• Massive star formation IR emission Supernovae.

+ star formation rate history of the universe.

+ the fraction of all star formation at high-z that occurs in “proton calorimeters” (high density).

• For an individual galaxy:

• For the history of star formation:

Thompson et al. (2006)

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Lν (GeV) ~ 10-5η 0.05LTIR

The -Ray Background