The MBHBM_⋆ Project. I. Measurement of the Central Black Hole Mass in Spiral Galaxy NGC 3504 Using Molecular Gas Kinematics

<jats:title>Abstract</jats:title> <jats:p>We present a dynamical mass measurement of the supermassive black hole (SMBH) in the nearby double-barred spiral galaxy NGC 3504 as part of the Measuring Black Holes in below Milky Way (<jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub>) Mass Galaxies Project. Our analysis is based on Atacama Large Millimeter/submillimeter Array cycle 5 observations of the <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab77aaieqn1.gif" xlink:type="simple"/> </jats:inline-formula> emission line. These observations probe NGC 3504's circumnuclear gas disk (CND). Our dynamical model of the CND simultaneously constrains a black hole (BH) mass of <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab77aaieqn2.gif" xlink:type="simple"/> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, which is consistent with the empirical BH–galaxy scaling relations and a mass-to-light ratio in the <jats:italic>H </jats:italic>band of 0.44 ± 0.12 (<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>/<jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab77aaieqn3.gif" xlink:type="simple"/> </jats:inline-formula>). This measurement also relies on our new estimation of the distance to the galaxy of 32.4 ± 2.1 Mpc using the surface brightness fluctuation method, which is much further than the existing distance estimates. Additionally, our observations detect a central deficit in the <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab77aaieqn4.gif" xlink:type="simple"/> </jats:inline-formula> integrated intensity map with a diameter of 6.3 pc at the putative position of the SMBH. However, we find that a dense gas tracer CS(5 − 4) peaks at the galaxy center, filling in the <jats:sup>12</jats:sup>CO(2 − 1)-attenuated hole. Holes like this one are observed in other galaxies, and our observations suggest these may be caused by changing excitation conditions rather than a true absence of molecular gas around the nucleus.</jats:p>