Dynamical signature of a stellar bulge in a quasar host galaxy at $z\simeq 6$

Roberta Tripodi, Federico Lelli, Chiara Feruglio, Fabrizio Fiore, Fabio Fontanot, Manuela Bischetti, Roberto Maiolino

Published 2023-01-17Version 1

We present a dynamical analysis of a quasar-host galaxy at $z\simeq 6$ (SDSS J2310+1855) using a high-resolution ALMA observation of the [CII] emission line. The observed rotation curve is fitted with mass models that consider the gravitational contribution of a thick gas disc, a thick star-forming stellar disc, and a central mass concentration, likely due to a combination of a spheroidal component (i.e. a stellar bulge) and a supermassive black hole (SMBH). The SMBH mass of $5\times 10^9\ \rm M_{\odot}$, previously measured using the CIV and MgII emission lines, is not sufficient to explain the high velocities in the central regions. Our dynamical model suggests the presence of a stellar bulge with a mass of $\rm M_{bulge}\sim 10^{10}\ \rm M_{\odot}$ in this object, when the Universe was less than 1 Gyr old. To end up on the local $M_{\rm SMBH}-M_{\rm bulge}$ relation, the bulge mass should increase by a factor of $\sim$40 from $z=6$ to 0, while the SMBH mass should grow at most by a factor of 4, pointing to asynchronous galaxy-BH co-evolution. Imaging with JWST will allow us to validate this scenario.