A fundamental problem in the theory of low mass galaxy evolution?

Astronomy and Astrophysics – Astrophysics – Cosmology and Extragalactic Astrophysics

Scientific paper

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17 pages, 7 figures, submitted to MNRAS. Comments welcome

Scientific paper

Recent studies have found a dramatic difference between the observed number density evolution of low mass galaxies and that predicted by semi-analytic models. Whilst models accurately predict the z=0 number density, they require that the evolution occurs rapidly at early times, which is incompatible with the strong late evolution found in observational results. We report here the same discrepancy in two state-of-the-art cosmological hydrodynamical simulations, which is evidence that the problem is fundamental. We search for the underlying cause of this problem using two complementary methods. Firstly, we try to find evidence for the evolutionary history of today's low mass galaxies being different in models and observations. We find that the exclusion of satellite galaxies from the analysis brings the median ages and star formation rates of galaxies into good agreement. However, the models yield too few young, strongly star-forming galaxies. Secondly, we construct a toy model to link the observed evolution of specific star formation rates and the galaxy stellar mass function. We infer from this model that a key cause of the discrepancy is the presence of a positive correlation between specific star formation rate and stellar mass in both semi-analytical and hydrodynamical models. A similar positive correlation is found between the specific dark matter accretion rate and the halo mass, indicating that model galaxies are growing in a way that follows the growth of their host haloes too closely. It therefore appears necessary to find a mechanism that decouples the growth of low mass galaxies, which occurs at late times, from the growth of their host haloes, which occurs at early times. We argue that the current form of star-formation driven feedback implemented in most galaxy formation models is unlikely to achieve this goal, owing to its fundamental dependence on host halo mass and time.

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