The host dark matter haloes of the first quasars

Abstract

If z> 6 quasars reside in rare, massive haloes, ΛCDM cosmology predicts they should be surrounded by an anomalously high number of bright companion galaxies. Here, I show that these companion galaxies should also move unusually fast. Using a new suite of cosmological, 'zoom-in' hydrodynamic simulations, I present predictions for the velocity distribution of quasar companion galaxies and its variation with quasar host halo mass at Z=6⁠. Satellites accelerate as they approach the quasar host galaxy, producing a line-of-sight velocity profile that broadens with decreasing distance to the quasar host galaxy. This increase in velocity dispersion is particularly pronounced if the host halo mass is ≳5×1012M. Typical line-of-sight speeds rise to ≈500kms−1 at projected radii ∼10kpc⁠. For about 10percent of satellites, they should exceed 800kms−1⁠, with ≈5 per cent of companions reaching line-of-sight speeds ∼1000kms−1⁠. For lower host halo masses ≈5×1011−1012M, the velocity profile of companion galaxies is significantly flatter. In this case, typical line-of-sight velocities are ≈250kms−1 and do not exceed ≈500kms−1⁠. A comparison with existing Atacama Large Millimeter/submillimeter Array (ALMA), JWST and Multi-Unit Spectroscopic Explorer (MUSE) line-of-sight velocity measurements reveals that observed z > 6 quasar companions closely follow the velocity distribution expected for a host halo with mass ≳5×1012M, ruling out a light host halo. Finally, through an estimate of ultraviolet and [OIII]luminosity functions, I show that the velocity distribution more reliably discriminates between halo mass than companion number counts, which are strongly affected by cosmic variance..