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

Abstract

We present observations and analysis of an unusual [C II] emission line in the very luminous quasi-stellar object (QSO) SDSS J155426.16+193703.0 at z ~ 4.6. The line is extremely broad (full width at half-maximum 735 km s-1) and seems to have a flat-topped or doublepeaked 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 Mdyn ≳ 5 × 1010M⊙. Using the assumption of a rotating disc origin, we employ an empirical relation between galaxy disc circular velocity and bulge velocity dispersion (σ) to estimate that σ > 310 km s-1, subject to a correction for the unknown disc inclination. This result implies that this source is consistent with the local M-σ 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-σ 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.