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After this efficient formation, most of the gas is not in favorable conditions for star formation. Indeed, larger potential wells have not formed yet, and the density threshold is not reached. It is therefore likely that the gas will slowly accumulate in bigger potentials to form galaxies, as sketched in fig. 3. In the inner parts of these early galaxies, star formation can then begin. Then galaxy interactions will stirr and heat the gas through shocks and gravitational perturbations.

Figure 3

Figure 3. Schematic view of the evolution of the baryonic dark matter, under the form of cold H2 gas: formation of clumpuscules at a redshift around 100, which are progressively involved in the formation of larger structures and proto-galaxies. A typical early galaxy is shown at z = 6, with the cold gas settling into a flaring disk, and the star formation beginning in the center, where the surface density is above threshold. Later on, when groups and clusters virialise, the gas is stripped and heated to contribute to the hot X-ray gas.

Besides, interactions accelerate the angular momentum transfer: part of the HI gas is dragged outwards in tails. Most of the gas is driven inwards, giving rise to huge nuclear starbursts (and may be AGN). Galaxy evolution is highly accelerated. The cold gas that was settled around each galaxy is heated and virialised in the new common potential and might be visible through X-rays.

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