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

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4. Primordial gas simulations

A first series of simulations does not include a UV field, nor metal enrichment of gas. The gas has a primordial composition, with a 24% mass fraction of helium, and the rest of the mass in hydrogen.

All the simulations in this series result in the formation of a single dominant collapsed gas object at [FORMULA], which contain more than 90% of the collapsed gas mass. This object is built up through hierarchical merging of smaller objects. The mass of the most massive progenitor as a function of redshift can be seen in Fig. 2. The redshift, at which the mass of the largest progenitor has acquired half of the final mass, is an increasing function of total proto-galactic mass. For the [FORMULA] simulation this redshift is [FORMULA], and for the [FORMULA] simulation it is [FORMULA].

[FIGURE] Fig. 2. The mass of the most massive progenitor, as a function of redshift, for the primordial gas simulations. The different curves represent values for simulations with different total mass: [FORMULA] (solid), [FORMULA] (dashed), [FORMULA] (dotted), [FORMULA] (dot-dashed), [FORMULA] (fat solid).

During the collapse of the proto-galaxy the gas acquires kinetic energy, which is then converted into thermal energy by shocks and radiated away. In the low mass galaxies the radiative gas cooling is efficient and keeps the gas cool, as can be seen in Fig. 3. The fraction of gas that can cool depends on the redshift of formation. At high redshifts the density of the Universe is higher, and the cooling therefore stronger.

[FIGURE] Fig. 3. The mass fraction of gas inside the virial radius that has a temperature exceeding half the virial temperature, for the primordial gas simulations. Notation as in Fig. 2.

The shape of the cooling curve also gives a strong implicit redshift dependence of the efficiency of gas cooling. Radiative energy losses are highest in the temperature range [FORMULA] K. Most gas that is shock heated to temperatures above [FORMULA] K stays hot for more than a Hubble time, whereas gas that is heated to less than [FORMULA] K experiences an order of magnitude stronger cooling and cools well within a Hubble time.

It can be seen in Fig. 3 and Table A1 that the galactic halos that contain gas at virial temperatures at [FORMULA], are those with a virial temperature exceeding [FORMULA] K. These galactic halos form late, and have accumulated less than half the final mass at [FORMULA].

The gas core in the [FORMULA] simulation forms late, at [FORMULA], see Fig. 2, in a series of merging events. However, the hot halo remaining at [FORMULA], and containing [FORMULA] of the gas mass, forms much earlier, at [FORMULA]. Between [FORMULA] and [FORMULA], several collapsed gas clumps, none of which has a mass in excess of 20% of the mass of the final gas core, share a common hot pressure supported gas halo. The gas cooling is relatively inefficient at the high temperatures of this hot halo, and there is no significant amount of cooling inflow of gas until the present epoch.

The central gas core that forms in all simulations is very compact, and is not resolved. This over-concentration of mass can be seen from the rotation curves in Fig. 4, which rise very rapidly in the innermost regions.

[FIGURE] Fig. 4. Circular velocity as a function of radius, for the primordial gas simulations. Notation as in Fig. 2.

The galactic objects that form are very compact because the collapsed gas has lost much of the angular momentum that was injected at the start of the simulations. This effect is well known, and have been previously investigated by Navarro & Benz (1991), Katz & Gunn (1991), Vedel, Hellsten, & Sommer-Larsen (1994), Navarro, Frenk, & White (1995), Navarro & Steinmetz (1997), but was first found by Lake & Carlberg (1988), (using a "sticky particle" method).

The total angular momentum is in all cases conserved to well within 1%, and if the collapsed gas has lost angular momentum that angular momentum must have been transferred to the dark matter component. Fig. 5 shows the total angular momentum of the gas, normalized to the initial value. It is clear that between 10 and 90% of the gas angular momentum has been transferred to the dark matter component. In the [FORMULA] and [FORMULA] simulations this angular momentum transfer is, however, not very pronounced. Instead, most of the gas angular momentum is contained in the hot halo of pressure supported gas that surrounds the central collapsed object. (Note that above mentioned authors considered the angular momentum of the disk, and did not include the halo.)

[FIGURE] Fig. 5. Total angular momentum of the gas component, normalized to the initial value, as a function of redshift, for the primordial gas simulations. Notation as in Fig. 2.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998