The host dark matter haloes of the first quasars

Abstract

If $z>6$ quasars reside in rare, massive haloes, $\Lambda$CDM 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 $\gtrsim 5\times 10^{12}{\mathrm{M}}_{\odot }$. Typical line-of-sight speeds rise to $\approx 500\mathrm{km}{\mathrm{s}}^{-1}$ at projected radii $\sim 10\mathrm{kpc}$. For about $10\%$ of satellites, they should exceed $800\mathrm{km}{\mathrm{s}}^{-1}$, with $\approx 5\%$ of companions reaching line-of-sight speeds $\sim 1000\mathrm{km}{\mathrm{s}}^{-1}$. For lower host halo masses $\approx 5\times 10^{11}-10^{12}{\mathrm{M}}_{\odot }$, the velocity profile of companion galaxies is significantly flatter. In this case, typical line-of-sight velocities are $\approx 250\mathrm{km}{\mathrm{s}}^{-1}$ and do not exceed $\approx 500\mathrm{km}{\mathrm{s}}^{-1}$. A comparison with existing ALMA, JWST and 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 $\gtrsim 5\times 10^{12}{\mathrm{M}}_{\odot }$, ruling out a light host halo. Finally, through an estimate of UV 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.

14 pages, accepted in MNRAS (April 2024)

Cited by in corpus (2)

• A unified model for the clustering of quasars and galaxies at $z\approx 6$
• Revisiting the Extreme Clustering of $z\approx 4$ Quasars with Large Volume Cosmological Simulations