Black hole mass measurement using ALMA observations of [CI] and CO emissions in the Seyfert 1 galaxy NGC 7469

Abstract

We present a supermassive black hole (SMBH) mass measurement in the Seyfert 1 galaxy NGC 7469 using Atacama Large Millimeter/submillimeter Array (ALMA) observations of the atomic-[CI](1-0) and molecular-¹²CO(1-0) emission lines at the spatial resolution of ≈0${_{.}^{\prime\prime}}$3 (or ≈100 pc). These emissions reveal that NGC 7469 hosts a circumnuclear gas disc (CND) with a ring-like structure and a two-arm/bi-symmetric spiral pattern within it, surrounded by a starbursting ring. The CND has a relatively low σ_gas/V ≈ 0.35 (r ≲ 0${_{.}^{\prime\prime}}$5) and ≈0.19 (r > 0${_{.}^{\prime\prime}}$5), suggesting that the gas is dynamically settled and suitable for dynamically deriving the mass of its central source. As is expected from X-ray dominated region (XDR) effects that dramatically increase an atomic carbon abundance by dissociating CO molecules, we suggest that the atomic [CI](1-0) emission is a better probe of SMBH masses than CO emission in active galactic nuclei (AGNs). Our dynamical model using the [CI](1-0) kinematics yields a $M_{\rm BH}=1.78^{+2.69}_{-1.10}\times 10^7$ M_⊙ and $M/L_{\rm F547M}=2.25^{+0.40}_{-0.43}$ (M_⊙/L_⊙). The model using the ¹²CO(1-0) kinematics also gives a consistent M_BH with a larger uncertainty, up to an order of magnitude, i.e. $M_{\rm BH}=1.60^{+11.52}_{-1.45}\times 10^7$ M_⊙. This newly dynamical M_BH is ≈2 times higher than the mass determined from the reverberation mapped (RM) method using emissions arising in the unresolved broad-line region (BLR). Given this new M_BH, we are able to constrain the specific RM dimensionless scaling factor of $f=7.2^{+4.2}_{-3.4}$ for the AGN BLR in NGC 7469. The gas within the unresolved BLR thus has a Keplerian virial velocity component and the inclination of $i\approx {11.0^\circ }_{-2.5}^{+2.2}$, confirming its face-on orientation in a Seyfert 1 AGN by assuming a geometrically thin BLR model.