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

Abstract

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 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 $5\times 10^9{\mathrm{M}}_{\odot }$, 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 ${\mathrm{M}}_{\mathrm{bulge}}\sim 10^{10}{\mathrm{M}}_{\odot }$ in this object, when the Universe was younger than 1 Gyr. To finally be located on the local $M_{\mathrm{SMBH}}-M_{\mathrm{bulge}}$ relation, the bulge mass should increase by a factor of $\sim$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.

8 pages, 4 figures, 3 tables. Accepted by A&A