ALMA detection of a disc-dominated [C II] emission line at z=4.6 in the luminous QSO J1554+1937

TL;DR

This study reports the detection of a broad, disc-like [C II] emission line in a luminous quasar at z=4.6, indicating large-scale rotation and a massive galaxy disc, with implications for understanding black hole-galaxy relations.

Contribution

It provides evidence that [C II] emission at high redshift can originate from rotating discs, affecting interpretations of galaxy dynamics and black hole mass estimates.

Findings

[C II] line shows large-scale rotation in a high-redshift quasar.

The galaxy's dynamical mass suggests consistency with the local M--sigma relation.

Assuming a bulge origin would imply a black hole much more massive than expected.

Abstract

We present observations and analysis of an unusual [C II] emission line in the very luminous QSO SDSS J155426.16+193703.0 at z~4.6. The line is extremely broad (FWHM 735 km/s) and seems to have a flat-topped or double-peaked line profile. A velocity map of the line shows a gradient across the source that indicates large-scale rotation of star-forming gas. Together, the velocity map and line profile suggest the presence of a massive rotating disc with a dynamical mass M_dyn > 5x10^10 M_sun. Using the assumption of a rotating disc origin, we employ an empirical relation between galaxy disc circular velocity and bulge velocity dispersion (sigma) to estimate that sigma > 310 km/s, subject to a correction for the unknown disc inclination. This result implies that this source is consistent with the local M--sigma relation, or offset at most by an order of magnitude in black hole mass. In contrast, the assumption of a bulge origin for the [C II] emission line would lead to a conclusion that the black hole is nearly two orders of magnitude more massive than predicted by the M--sigma relation, similar to previous findings for other high-redshift QSOs. As disc rotation may be a common origin for [C II] emission at high redshifts, these results stress that careful consideration of dynamical origins is required when using observations of this line to derive properties of high-redshift galaxies.