Simulations of the formation and evolution of isolated dwarf galaxies

Abstract

We present new fully self-consistent models of the formation and evolution of isolated dwarf galaxies (DGs). We have used the publicly available N-body/smoothed particle hydrodynamics (SPH) code HYDRA, to which we have added a set of star formation criteria, and prescriptions for chemical enrichment [taking into account contributions from both Type Ia supernova (SN Ia) and Type II supernova (SN II)], supernova feedback, and gas cooling. We extensively tested the soundness of these prescriptions and the numerical convergence of the models. The models follow the evolution of an initially homogeneous gas cloud collapsing in a pre-existing dark matter (DM) halo. These simplified initial conditions are supported by the merger trees of isolated DGs extracted from the milli-Millennium Simulation. The star formation histories (SFHs) of the model galaxies exhibit burst-like behaviour. These bursts are a consequence of the blow-out and subsequent in-fall of gas. The amount of gas that leaves the galaxy for good is found to be small, in absolute numbers, ranging between 3 x 10(7) and 6 x 10(7) M(circle dot). For the least massive models, however, this is over 80 per cent of their initial gas mass. The local fluctuations in gas density are strong enough to trigger starbursts in the massive models, or to inhibit anything more than small residual star formation (SF) for the less massive models. Between these starbursts there can be time intervals of several gigayears. The models' surface brightness profiles are well fitted by Sersic profiles and the correlations between the models' Sersic parameters and luminosity agree with the observations. We have also compared model predictions for the half-light radius R(e), central velocity dispersion sigma(c), broad-band colour B - v, metallicity [Z/Z(circle dot)] versus luminosity relations and for the location relative to the fundamental plane with the available data. The properties of the model DGs agree quite well with those of observed DGs. However, the properties of the most massive models deviate from those of observed galaxies. This most likely signals that galaxy mergers are starting to affect the galaxies' SFHs in this mass regime (M greater than or similar to 10(9) M(circle dot)). We found that a good way to assess the soundness of models is provided by the combination of R(e) and sigma(c). The demand that these are reproduced simultaneously places a stringent constraint on the spatial distribution of SF and on the shape and extent of the DM halo relative to that of the stars.