![]() | ![]() |
Astron. Astrophys. 333, 399-410 (1998) 7. ConclusionsThe mass of the most massive progenitor as a function of redshift, for all the simulations presented here, is shown in Figs. 15, 16 17, 18, and 19. These simulations clarify some points of previous arguments about
galaxy formation that are based on simple analytic models, and
estimates of the efficiency of gas cooling. It is clear that the
inclusion of a background radiation field, consistent with the
observed Gunn-Petersson effect, can strongly suppress the formation of
galaxies with total mass less than Hierarchical models of galaxy formation tend to over-produce
galaxies with circular velocities less than 100 km/s. Our results
indicate that photo-ionization alone is not sufficient to suppress the
formation of these galaxies, since the effects on galaxies with
circular velocities larger than The galactic objects that form in three-dimensional hydrodynamical
simulations, are too compact when compared with observed disk
galaxies. The reason for this is that most of the angular momentum in
the gas component is transferred to the dark matter. Navarro &
Steinmetz (1997) find that collapsed objects acquire even less angular
momentum, when the effects of a UV field is included. Comparing
Fig. 5 and Fig. 8, these simulations show that the angular
momentum transfer, from the gas to the dark matter, decreases in
magnitude when a UV field is included. This is not a contradiction.
When a UV field is included, most of the gas angular momentum at
Metal enrichment of the interstellar gas increases the gas cooling
rate at late times, and may have significant effects on the amount of
gas that may cool and sink to the center of a galactic halo in a
Hubble time. The inclusion of a background radiation field leads to
more massive hot halos in large galaxies, If inhomogeneities in the gas are smoothed out by the limited resolution used, the average cooling rate in the region will change. This is a problem common to all hydrodynamical simulations of galaxy formation. Some implicit assumption must be made, e.g., that the density field is smooth on unresolved scales due to physical processes not incorporated into the simulation or, that the density in regions where this could have an effect is already so high that the cooling is extremely efficient both with and without unresolved density fluctuations. Previous simulations without UV background ionization and heating, did not suffer from resolution effects, as badly as one might (naively) expect from the squared density dependence of the cooling function. The reason being that the cooling function is divided by the density, i.e. the gas cooling rate, per unit mass, is (roughly) proportional to the density, when thermal energy is integrated. For example, Navarro & White (1994) varied the gas mass- resolution by a factor of two, and Hultman & Källander (1997), by a factor of ten, both showing comparatively small effects. Then, when a UV background field is included, there is a
competition between density vs. density squared dependencies, being
explicitly sensitive to resolution. Indeed, this was observed by
Weinberg et al. (1997). Varying the mass-resolution by a factor of
eight, had severe effects on the outcome of their results. Navarro
& Steinmetz (1997) performed similar simulations varying the
mass-resolution of identical runs with a factor of six. They found
much smaller effects, that in addition decreased with redshift.
However, the mass- resolution was much higher than in Weinberg
et al. (In fact, even the lower resolution runs of Navarro &
Steinmetz had slightly higher resolution as compared to the "high
resolution" runs of Weinberg et al.) For the simulations presented
here, the absolute resolution varies, since it is proportional to the
total mass, but for comparison the ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: April 20, 1998 ![]() |