Are the Missing Baryons from Galaxies in Hot Extended Haloes?

Astronomy and Astrophysics – Astronomy

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Scientific paper

Galaxies are missing most of their baryons, and many ΛCDM models predict these baryons lie in a hot halo around galaxies. We examine whether these haloes are consistent with extant observations, and whether the missing baryons were expelled from galaxies before or after disk formation. We present three independent lines of argument. First, if the baryons collapse onto galaxies and are subsequently ejected by supernovae and AGN heating, the available energy seems marginally insufficient for the Milky Way, and insufficient by an order of magnitude for some galaxies. Moreover, if supernovae or AGNs drove gas from galaxies, departures from the baryonic Tully-Fisher relationship might be related to the star-to-baryon ratio and the central black hole mass. We search for this effect and find no relation, suggesting these are not the primary mechanisms of baryon expulsion. Second, we place constraints on the column density of haloes of missing baryons using nondetections of OVII absorption along AGN sightlines: haloes must contain less than 70% of the missing baryons or extend to no more than 40 kpc. Third, we compute the X-ray surface brightness signature if the missing baryons are at the virial temperature in an NFW potential, and find the emission measure would be 2-3 orders of magnitude higher than the estimated detection threshhold, implying a hot halo can only contain at most 10% of the missing baryons. We conclude that most of missing baryons from galaxies do not lie in hot haloes around the galaxies, and that the missing baryons must have been expelled from the galaxies as part of the process of galaxy formation.

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