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

TL;DR

This study uses high-resolution ALMA observations to analyze the dynamics of a quasar host galaxy at redshift 6, revealing a massive stellar bulge and suggesting asynchronous growth of the galaxy and its supermassive black hole.

Contribution

First dynamical analysis of a z~6 quasar host galaxy showing evidence of a stellar bulge and its implications for galaxy-BH co-evolution.

Findings

Detection of a stellar bulge with mass ~10^10 solar masses.

SMBH mass of 5x10^9 solar masses insufficient to explain central velocities.

Bulge mass needs to grow by factor of ~40 to match local relations.

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 $5\times 10^9\ \rm M_{\odot}$, 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 younger than 1 Gyr. To finally be located 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 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.