WISDOM Project -- XXVIII. Molecular gas measurement of the supermassive black hole mass of the galaxy NGC 1387

Abstract

Supermassive black hole (SMBH) masses can be measured using molecular gas kinematics. Here we present high angular resolution ($0.12$ arcsec or $\approx11$ pc) Atacama Large Millimeter/submillimeter Array observations of the $^{12}$CO(2-1) line emission of the early-type galaxy NGC 1387. The observations reveal a face-on, regularly-rotating central molecular gas disc with a diameter of $\approx18$ arcsec ($\approx1.7$ kpc) and a central depression slightly larger than the SMBH sphere of influence. We forward model the CO data cube in a Bayesian framework with the \textsc{Kinematic Molecular Simulation} code, and use \textit{Hubble Space Telescope} data to constrain the stellar gravitational potential contribution to the molecular gas kinematics. We infer a SMBH mass of $1.10^{+1.71}_{-0.95}[\text{stat},3σ]^{+2.45}_{-1.09}[\text{sys}]\times10^8$ M$_\odot$ and a F160W-filter stellar mass-to-light ratio of $0.90^{+0.44}_{-0.35}[\text{stat}, 3σ]^{+0.46}_{-0.36}[\text{sys}]$ M$_\odot$/L$_{\odot,\text{F160W}}$. This SMBH mass is consistent with the SMBH mass -- stellar velocity dispersion relation.

Figures (10)

• Figure 1: Unsharp-masked HST Wide Field Camera 3 F160W-filter ($H$-band) image of NGC 1387 (left), overlaid with the $^{12}$CO(2–1) integrated intensity contours (cyan) from our high-resolution ALMA data cube (right). Note that the unsharp masking has removed the smooth outer galaxy light and instead highlights the face-on nuclear disc and bar of the galaxy.
• Figure 2: Central region ($0.18$ arcsec $\times$$0.18$ arcsec) of the NGC 1387 $1.25$ mm continuum image. Contours are evenly spaced between the peak intensity of $1.20\pm0.03$ mJy beam$^{-1}$ and 3 times the RMS noise. The synthesised beam is shown in the bottom-left corner as a black open ellipse.
• Figure 3: $^{12}$CO(2–1) data products of NGC 1387 derived from our ALMA high-resolution data cube. Top-left: Zeroth-moment (integrated-intensity) map. Top-right: First-moment (intensity-weighted mean line-of-sight velocity) map. The dashed black line shows the kinematic major axis. Bottom-left: Second-moment (intensity-weighted line-of-sight velocity dispersion) map. The synthesised beam is shown in the bottom-left corner of each map as a black filled ellipse. Bottom-right: Kinematic major-axis position-velocity diagram. An error bar is shown in the bottom-left corner, showing the size of the synthesised beam along the kinematic major axis and the channel width.
• Figure 4: HST WFC3 F160W-filter ($H$-band) image of NGC 1387 (black contours), overlaid with our best-fitting MGE model (red contours). Left: Full extent of the galaxy, showing the galaxy bulge, bar and extended stellar disc. Right: Central region only.
• Figure 5: $^{12}$CO(2–1) zeroth-moment (integrated-intensity) map of NGC 1387 derived from our ALMA low-resolution data cube. The synthesised beam is shown in the bottom-left corner as a black open ellipse.