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

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5. Simulations with an ionizing field

A background UV field affects the gas in two ways: (i) the gas is heated by photo-ionization; and (ii) it ionizes the gas and thereby reduces its ability to cool by collisional excitation of neutral atoms. In these simulations the background radiation field is non-zero in the range [FORMULA].

The rate of photo-ionization heating by the background field is proportional to the local gas density, and the rate of cooling by collisional line radiation, the dominant cooling mechanism, is proportional to the square of the local gas density. Heating by photo-ionization is therefore most important in low density regions, where the equilibrium temperature can rise to [FORMULA] K. This temperature is comparable to the virial temperature of the galactic halos that form in the [FORMULA] and [FORMULA] simulations, but much less than the corresponding temperature for the largest simulations. The dynamical effects of heating should therefore be small for the largest galaxies.

Fig. 6 shows the mass fraction of gas within the virial radius that has a temperature exceeding half the virial temperature as a function of redshift. The difference is modest for the more massive galaxies, when comparing with the corresponding curves for the simulations without a background field. In the [FORMULA] simulation the fraction of hot gas at [FORMULA] increases from around 10% to 20%, when the background radiation field is included. On the other hand, the smallest galaxies show dramatic changes. In the [FORMULA] simulation, most of the gas is heated to temperatures above the virial temperature by photo-ionization. This prevents the gas from falling into the galactic halo potential well. When the background field falls in strength, after [FORMULA], about [FORMULA] of the gas is able to cool and condense into a galactic object.

[FIGURE] Fig. 6. 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. The curves represent simulations including a background radiation field. Notation as in Fig. 2.

The reduced cooling rate in this series of simulations, with a background UV field, also reduces the gas mass of the most massive objects that form. As can be seen when comparing Fig. 7 with Fig. 2, the reduction in object masses is most pronounced in the smallest simulations, with a total mass of [FORMULA] and [FORMULA].

[FIGURE] Fig. 7. The mass of the most massive progenitor, as a function of redshift, for the simulations with a background UV field. Notation as in Fig. 2.

There is less mass in the clumpy and cold gas component when a background UV field is incorporated. This lowers the angular momentum transfer to the dark matter component, as can be seen in Fig. 8, to be compared with Fig. 5.

[FIGURE] Fig. 8. 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. Notation as in Fig. 2.

In all cases, the cold collapsed gas is concentrated in a compact object with an extent of less than a few smoothing lengths. Around this object is a second gas component in the form of a hot pressure supported halo. In the simulations where most of the gas can cool and condense into an object, most of the angular momentum is transferred to the dark matter component. In the cases where cooling is less efficient, more angular momentum is retained in the gas component, but instead the gas angular momentum is contained in the hot gas halo. The resulting gas cores are still very compact, as can be seen in Fig. 9.

[FIGURE] Fig. 9. Circular velocity as a function of radius, for the simulations with a background UV field. Notation as in Fig. 2.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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