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Astron. Astrophys. 333, 399-410 (1998) 5. Simulations with an ionizing fieldA 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 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 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
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
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.
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.
![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: April 20, 1998 ![]() |