A high baryon fraction in massive haloes at z ∼ 3

We investigate the baryon content of the circumgalactic medium (CGM) within the virial radius of M_h ∼ 10¹² M_⊙ haloes at z ∼ 3, by modelling the surface brightness profile of the giant Lyα nebulae recently discovered by the Multi Unit Spectroscopic Explorer (MUSE) around bright QSOs at this redshift. We initially assume fluorescent emission from cold photo-ionized gas confined by the pressure of a hot halo. Acceptable CGM baryon fractions (equal to or smaller than the cosmological value) require that the cold gas occupies {≲} 1 per cent of the volume, but is about as massive as the hot gas. CGM baryon fractions as low as 30 per cent of the cosmic value, as predicted by some strongly ejective feedback models at this redshift, are not easy to reconcile with observations, under our assumptions, unless both the QSO-hosting haloes at z ∼ 3 are more massive than recent estimates from the Baryon Oscillation Spectroscopic Survey (BOSS) based on clustering and the photo-ionized gas is colder than expected in a standard QSO ionizing radiation field. We also consider the option that the emission is dominated by photons scattered from the QSO broad-line region. In this scenario, a very stringent lower limit to the baryon fraction can be obtained under the extreme assumption of optically thin scattering. We infer in this case a baryon fraction of at least 70 per cent of the cosmic value, for fiducial parameters. Lower values require halo masses or gas temperatures different than expected, or that some mechanism keeps the cold gas systematically overpressured with respect to the ambient medium.