2021ApJ...913..111Z

We present a detailed analysis of the single-slit optical spectrum of the flat-spectrum radio quasar (FSRQ) B2 0003+38A, taken by the Echellette Spectrograph and Imager (ESI) on the Keck II telescope. This classical low-redshift FSRQ (z = 0.22911, as measured from the stellar absorption lines) remains underexplored in its emission lines, though its broadband continuum properties from radio to X-ray are well studied. After removing the unresolved quasar nucleus and the starlight from the host galaxy, we obtain a spatially resolved 2D spectrum, which clearly shows three components, indicating a rotating disk, an extended emission-line region (EELR), and an outflow. The bulk of the EELR, with a characteristic mass M_EELR ∼ 10⁷ M_☉, and redshifted by v_EELR ≃ 120 km s^–1 with respect to the quasar systemic velocity, shows a one-sided structure stretching to a projected distance of r_EELR ∼ 20 kpc from the nucleus. The rotation curve of the rotating disk is consistent with that of a typical galactic disk, suggesting that the FSRQ is hosted by a disk galaxy. This conclusion is in accordance with the facts that strong absorption in the H I 21 cm line was previously observed, and that Na I λλ5891, 5897 and Ca II λλ3934, 3969 doublets are detected in the optical ESI spectrum. B2 0003+38A will become the first FSRQ discovered to be hosted by a gas-rich disk galaxy, if this is confirmed by follow-up deep imaging and/or integral field unit mapping with a high spatial resolution. These observations will also help unravel the origin of the EELR.