Balancing the baryon budget: the fraction of the IGM due to galaxy mergers

Observations indicate that roughly 60 per cent of the baryons may exist in a warm-hot intergalactic medium (WHIM) at low redshifts. Following up on previous results showing that gas is released through galaxy mergers, we use a semi-analytic technique to estimate the fraction of gas mass lost from haloes solely due to mergers. We find that up to similar to 25 per cent of the gas in a halo can unbind over the course of galaxy assembly. This process does not act preferentially on smaller mass haloes; bigger haloes always release larger amounts of gas in a given volume of the Universe. However, if we include multiphase gas accretion on to haloes, we find that only a few per cent is unbound. We conclude that either non-gravitational processes may be in play to heat up the gas in the galaxies prior to unbinding by mergers or most of the baryons in the WHIM have never fallen into virialized dark matter haloes. We present a budget for stocking the WHIM compiled from recent work.