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Abstract: We study the degree to which non-radiative gas dynamics affects the mergerhistories of haloes along with subsequent predictions from a semi-analyticmodel SAM of galaxy formation. To this aim, we use a sample of dark matteronly and non-radiative SPH simulations of four massive clusters. The presenceof gas-dynamical processes e.g. ram-pressure from the hot ICM makes haloesmore fragile in the runs which include gas. This results in a 25 per centdecrease in the total number of subhaloes at z = 0. The impact on the galaxypopulation predicted by SAMs is complicated by the presence of -orphan-galaxies, i.e. galaxies whose parent substructures are reduced below theresolution limit of the simulation. In the model employed in our study, thesegalaxies survive for a residual merging time that is computed using a variationof the Chandrasekhar formula. Due to ram-pressure stripping, haloes in gassimulations tend to be less massive than their counterparts in the dark mattersimulations. The resulting merging times for satellite galaxies are then longerin these simulations. On the other hand, the presence of gas influences theorbits of haloes making them on average more circular and therefore reducingthe estimated merging times with respect to the dark matter only simulation.This effect is particularly significant for the most massive satellites and isat least in part responsible for the fact that brightest cluster galaxies inruns with gas have stellar masses which are about 25 per cent larger than thoseobtained from dark matter only simulations. Our results show that gas-dynamicshas only a marginal impact on the statistical properties of the galaxypopulation, but that its impact on the orbits and merging times of haloesstrongly influences the assembly of the most massive galaxies.

Autor: A. Saro, G. De Lucia, K. Dolag, S. Borgani


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