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We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 5 andCycle 6 observations of CO(2$-$1) and CO(3$-$2) emission at 0.2''$-$0.3''resolution in two radio-bright, brightest group/cluster early-type galaxies,NGC 315 and NGC 4261. The data resolve CO emission that extends within theirblack hole (BH) spheres of influence ($r_\mathrm{g}$), tracing regularKeplerian rotation down to just tens of parsecs from the BHs. The projectedmolecular gas speeds in the highly inclined ($i>60^\circ$) disks rises at least500 km s$^{-1}$ near their galaxy centers. We fit dynamical models of thin-diskrotation directly to the ALMA data cubes, and account for the extended stellarmass distributions by constructing galaxy surface brightness profiles correctedfor a range of plausible dust extinction values. The best-fit models yield$(M_\mathrm{BH}/10^9\,M_\odot)=2.08\pm0.01(\mathrm{stat})^{+0.32}_{-0.14}(\mathrm{sys})$for NGC 315 and$(M_\mathrm{BH}/10^9\,M_\odot)=1.67\pm0.10(\mathrm{stat})^{+0.39}_{-0.24}(\mathrm{sys})$for NGC 4261, the latter of which is larger than previous estimates by a factorof $\sim$3. The BH masses are broadly consistent with the relations between BHmasses and host galaxy properties. These are among the first ALMA observationsto map dynamically cold gas kinematics well within the BH-dominated regions ofradio galaxies, resolving the respective $r_\mathrm{g}$ by factors of$\sim$5$-$10. The observations demonstrate ALMA's ability to precisely measureBH masses in active galaxies, which will enable more confident probes ofaccretion physics for the most massive galaxies.

Black Hole Mass Measurements of Radio Galaxies NGC 315 and NGC 4261 Using ALMA CO Observations