1,721,212 research outputs found
The two regimes of the cosmic sSFR evolution are due to spheroids and discs
No full textThis paper aims at explaining the two phases in the observed specific star formation rate (sSFR), namely the high (>3/Gyr) values at z > 2 and the smooth decrease since z = 2. In order to do this, we compare to observations the sSFR evolution predicted by well-calibrated models of chemical evolution for elliptical and spiral galaxies, using the additional constraints on the mean stellar ages of these galaxies (at a given mass). We can conclude that the two phases of the sSFR evolution across cosmic time are due to different populations of galaxies. At z > 2, the contribution comes from spheroids: the progenitors of present-day massive ellipticals (which feature the highest sSFR) as well as haloes and bulges in spirals (which contribute with average and lower-than-average sSFR). In each single galaxy, the sSFR decreases rapidly and the star formation stops in <1 Gyr. However, the combination of different generations of ellipticals in formation might result in an apparent lack of strong evolution of the sSFR (averaged over a population) at high redshift. The z < 2 decrease is due to the slow evolution of the gas fraction in discs, modulated by the gas accretion history and regulated by the Schmidt law. The Milky Way makes no exception to this behaviour
A fast and accurate method to compute the mass return from multiple stellar populations
No full textThe mass returned to the ambient medium by aging stellar populations over cosmological times sums up to a significant fraction (20-30 per cent or more) of their initial mass. This continuous mass injection plays a fundamental role in phenomena, such as galaxy formation and evolution, fuelling of supermassive black holes in galaxies and the consequent (negative and positive) feedback phenomena, and the origin of multiple stellar populations in globular clusters. In numerical simulations, the calculation of the mass return can be time consuming, since it requires at each time step the evaluation of a convolution integral over the whole star formation history, so the computational time increases quadratically with the number of time steps. The situation can be especially critical in hydrodynamical simulations, where different grid points are characterized by different star formation histories, and the gas cooling and heating times are shorter by orders of magnitude than the characteristic stellar lifetimes. In this paper, we present a fast and accurate method to compute the mass return from stellar populations undergoing arbitrarily complicated star formation histories. At each time step the mass return is calculated from its value at the previous time, and the star formation rate over the last time step only. Therefore, in the new scheme there is no need to store the whole star formation history, and the computational time increases linearly with the number of time steps
Effective N-body models of composite collisionless stellar systems
Full text linkGas-poor galaxies can be modelled as composite collisionless stellar systems, with a dark matter halo and one or more stellar components, representing different stellar populations. The dynamical evolution of such composite systems is often studied with numerical N-body simulations, whose initial conditions typically require realizations with particles of stationary galaxy models. We present a novel method to conceive these N-body realizations, which allows one to exploit at best a collisionless N-body simulation that follows their evolution. The method is based on the use of an effective N-body model of a composite system, which is in fact realized as a one-component system of particles that is interpreted a posteriori as a multicomponent system, by assigning in post-processing fractions of each particle's mass to different components. Examples of astrophysical applications are N-body simulations that aim to reproduce the observed properties of interacting galaxies, satellite galaxies, and stellar streams. As a case study we apply our method to an N-body simulation of tidal stripping of a two-component (dark matter and stars) satellite dwarf galaxy orbiting in the gravitational potential of the Milky Way
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