The MBHBM* Project -- I: Measurement of the Central Black Hole Mass in Dwarf Galaxy NGC 3504 Using Molecular Gas Kinematics

Abstract

We present the first measurement of the mass of a supermassive black hole (SMBH) in the nearby double-barred spiral galaxy NGC 3504 as part of the Measuring Black Holes Below the Milky Way ($M_{\star }$) mass galaxies (MBHBM${}_{\star }$) Project. Our analysis is based on Atacama Large Millimeter/submillimeter Array (ALMA) Cycle-5 observations of the ${}^{12}\mathrm{CO}(2-1)$ emission line. NGC 3504 has a circumnuclear gas disk (CND), which has a relatively high-velocity dispersion of 30 \kms. Our dynamical models of the CND yield a \Mbh~of $M_{\mathrm{BH}}=1.02_{-0.15}^{+0.18}\times 10^7$\Msun and a mass-to-light ratio in $H$-band of \ml${}_{\mathrm{H}}=0.66_{-0.65}^{+1.44}$ (\Msun/\Lsun). This black hole (BH) mass is consistent with BH--galaxy scaling relations. We also detect a central deficiency in the ${}^{12}\mathrm{CO}(2-1)$ integrated intensity map with a diameter of 2.7 pc at the putative position of the SMBH. However, this hole is filled by a dense gas tracer $\mathrm{CS}(5-4)$ that peaks at the galaxy center found in one of the three low-velocity-resolution continuum spectral correlators. The $\mathrm{CS}(5-4)$ line has the same kinematics with the ${}^{12}\mathrm{CO}(2-1)$ line within the CND, suggesting that it is also an alternative transition for measuring the central \Mbh~in NGC 3504 probably more accurately than the current commonly used of ${}^{12}\mathrm{CO}(2-1)$ due to its centralization.

25 pages, 15 Figures, 6 Tables, Submitted to ApJ