H 3 cooling in primordial gas
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H 3 + cooling in primordial gas. S. Glover (A.I.P.) D. W. Savin (Columbia). A brief introduction to galaxy formation. The first protogalaxies form at z ~ 20 - 30 Typical masses ~ 10 6 M  , sizes ~ 100 pc Composed of primordial gas: no enrichment by heavy elements:

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H 3 + cooling in primordial gas

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H 3 cooling in primordial gas

H3+ cooling in primordial gas

S. Glover (A.I.P.)

D. W. Savin (Columbia)


H 3 cooling in primordial gas

A brief introduction to galaxy formation

  • The first protogalaxies form at z ~ 20 - 30

  • Typical masses ~ 106 M, sizes ~ 100 pc

  • Composed of primordial gas: no enrichment by

  • heavy elements:

  • xHe = 0.083

  • xD = 2.6  10-5

  • xLi = 4.3  10-10

  • (Cyburt, 2004)


H 3 cooling in primordial gas

  • At low density, cooling dominated by H2

H + e- H- + h

H- + H  H2 + e-

  • Gas-phase reactions produce only a small H2 fraction.

  • Typically, no more than 0.1% of the gas is molecular.

  • BUT: this is enough to cool the gas to ~ 200K.

  • This allows runaway gravitational collapse to occur,

  • which leads to star formation.


H 3 cooling in primordial gas

  • Detailed numerical simulations follow collapse

  • down to scales ~ 10,000 AU (Abel et al. 2000, 2002)

  • These simulations suggest that each protogalaxy

  • forms only one single massive star (at least initially)

  • Models assume H2 cooling dominates at ALL densities.

  • But is this true?


H 3 cooling in primordial gas

  • H2 is not a very effective coolant, particularly at high n.

  • In LTE, cooling rate per H2 molecule at 1000K is:

  •  = 1.4  10-21 erg s-1 molecule-1

  • For comparison, the LTE value for H3+ is:

  •  = 2.8  10-12 erg s-1 molecule-1

  • (Neale, Miller & Tennyson, 1996)

  • So cooling from H3+ is potentially very important.


H 3 cooling in primordial gas

Modeling protogalactic H3+

  • Very simple dynamical model: single zone, free-fall

  • Detailed chemical model: 162 reactions between 23

  • species.

  • Rates primarily taken from two existing compilations:

  • Galli & Palla (1998), Stancil, Lepp & Dalgarno (1998)

  • Include some additional three-body reactions, e.g:

  • H2 + H+ + H2 H3+ + H2

  • (Gerlich & Horning, 1992)


H 3 cooling in primordial gas

  • Radiative cooling from: H2, HD, LiH, H2+, H3+, HeH+…

  • Also include cooling due to Compton scattering of

  • CMB photons

  • At n > 104 cm-3, include H2 formation heating

  • At n > 1010 cm-3, include effects of H2 line opacity

  • following prescription of Ripamonti & Abel (2004)


H 3 cooling in primordial gas

The H3+ cooling function

  • In LTE limit, use values from Neale et al (1996)

  • But what do we do at low density?

  • No complete set of vibrational excitation rates for

  • H3+ - H or H3+ - H2 collisions exists, so we can’t

  • treat the low density regime accurately.

  • Approximate the cooling rate per H3+ ion as:

  •  = LTE n > ncr

  •  = LTE (n / ncr) n < ncr


H 3 cooling in primordial gas

  • What’s an appropriate value for ncr?

  • Assume that at 1000 K, there is a total excitation rate

  • coefficient kex ~ 10-9 cm3 s-1

  • Assume that each collision leads, on average, to the loss

  • of approximately kT of energy

  • Then the low density cooling rate per H3+ ion is:

     ~ 10-9 nkT ~ 10-22 n erg s-1 ion-1

  • Comparison with the LTE rate then implies ncr ~ 1010 cm-3


H 3 cooling in primordial gas

Summary

  • H3+ cooling can be important in the density range

  • n = 107 - 1011 cm-3 if:

  • The H3+ critical density ncr < 1010 cm-3

  • OR:

  • The ionization rate  > 10-19 s-1 at n > 107 cm-3

  • H3+ cooling is unimportant at n > 1011 cm-3, as too

  • little H3+ survives at these densities


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