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

摘要

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