Dynamical signature of a stellar bulge in a quasar-host galaxy at z ≃ 6

We present a dynamical analysis of a quasar-host galaxy at z≃6 (SDSS J2310+1855) using a high-resolution ALMA observation of the [CII] emission line. The observed rotation curve was fitted with mass models that considered the gravitational contribution of a thick gas disc, a thick star-forming stellar disc, and a central mass concentration, which is likely due to a combination of a spheroidal component (i.e. a stellar bulge) and a supermassive black hole (SMBH). The SMBH mass of 5×109M·, 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 Mbulge∼1010 M· in this object, when the Universe was younger than 1 Gyr. To finally be located on the local MSMBHMbulge relation, the bulge mass should increase by a factor of ∼40 from z= 6 to 0, while the SMBH mass should grow by a factor of 4 at most. This points towards asynchronous galaxy-BH co-evolution. Imaging with the JWST will allow us to validate this scenario.