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Astron. Astrophys. 333, 399-410 (1998)

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6. Simulations with an ionizing field and metal enrichment

The cooling rate of interstellar gas depends sensitively on the composition assumed. Metals can increase the cooling rate by an order or two of magnitude in the temperature range relevant here. This increase is mostly due to collisional line cooling by oxygen and carbon. We employ a very simplistic scheme to estimate the metallicity of the gas, as described earlier in this paper. We have included the effects of this modulation of the gas cooling rate, in addition to the inclusion of a background radiation field. To some extent these two processes are expected to counteract each other, since a background field limits the ability of the gas to cool, and a high metal content increases the cooling rate. However, the effects of a background radiation field vanishes at low redshifts, whereas the metal content of the gas is only significant at lower redshifts, after significant star formation has taken place.

Through the amount of collapsed gas it is possible to get a very rough estimate of the star formation rate, and, with the further assumption of instant mixing, consequently an estimate of the metal enrichment. Metallicity as a function of redshift is displayed in Fig. 10, for the different simulations. The gas in the [FORMULA] simulation does not reach metallicities high enough to significantly affect the gas cooling rate, and the result for this mass is therefore practically identical to that of the corresponding simulation without metal enrichment.

[FIGURE] Fig. 10. Metallicity as a function of redshift. The values are the logarithm of the metal content normalized to the solar value. Notation as in Fig. 2.

The early time evolution, for [FORMULA], is almost unchanged for all simulated proto-galaxies, when comparing with the simulations that include a background field, but no metal enrichment. At later times, the metallicity of the gas leads to more efficient cooling, as can be seen when comparing Fig. 11 with Fig. 6. A hot halo of gas, at [FORMULA], is only present in the [FORMULA] and [FORMULA] simulations, and, in fact, all galactic halos contain less gas at virial temperatures than in the corresponding simulations without a background field and metal enrichment. The gas mass of the most massive progenitor is displayed in Fig. 12.


[FIGURE] Fig. 11. The mass fraction of gas inside the virial radius that has a temperature exceeding half the virial temperature, for the simulations with a background UV field and metal enrichment. These curves are for the simulations including a background radiation field, and metal enriched gas. Notation as in Fig. 2.

[FIGURE] Fig. 12. The mass of the most massive progenitor, as a function of redshift, for the simulations with a background UV field and metal enrichment. Notation as in Fig. 2.

[FIGURE] Fig. 13. Total angular momentum of the gas component, normalized to the initial value, as a function of redshift, for the simulations with a background UV field and metal enrichment. Notation as in Fig. 2.

[FIGURE] Fig. 14. Circular velocity as a function of radius, for the simulations with a background UV field and metal enrichment. Notation as in Fig. 2.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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