The growth of galaxies and their gaseous haloes

Freeke van de Voort

November 29, 2011

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Abstract: Simulations predict that the spatial distribution of the intergalactic medium, the cosmic web, has a profound impact on the evolution of galaxies. Gas accretion provides the fuel for star formation, which is inhibited by outflows powered by supernova explosions and active galactic nuclei (AGN). I will discuss the accretion of gas onto haloes and galaxies, its impact on star formation, and its observability. Using cosmological simulations we found that galaxy accretion rates are much lower than halo accretion rates and much more dependent on processes such as metal-line cooling and feedback from star formation and AGN. For massive haloes, the maximum past temperature distribution is bimodal. Cold-mode gas does not experience a virial shock near the virial radius, whereas hot-mode gas heats up to the virial temperature. Cold-mode gas accretes along the filaments of the cosmic web, has a higher density and larger inward velocities, whereas hot-mode gas is more vulnerable to galactic winds and pushed out to several times the virial radius. Cold-mode gas is therefore much more important for the fuelling of galaxies, and thus for the star formation that follows, than it is for the growth of haloes. Unfortunately there is little direct observational evidence for the predicted cold accretion flows inside haloes. I will show that they are, however, crucial for matching the neutral hydrogen column density distribution, allowing us to conclude that they do not only exist, but have already been detected as Lyman-limit and damped Lyman-$ alpha$ systems.