2008ApJ...689..775T

Supermassive black holes in giant elliptical galaxies are remarkably faint given their expected accretion rates. This motivates models of radiatively inefficient accretion due to either ion-electron thermal decoupling, generation of outflows that inhibit accretion, or settling of gas to a gravitationally unstable disk that forms stars in preference to feeding the black hole. The latter model predicts the presence of cold molecular gas in a thin disk around the black hole. Here we report Submillimeter Array observations of the nucleus of the giant elliptical galaxy M87 that probe 230 GHz continuum and CO (J=2-1) line emission. Continuum emission is detected from the nucleus and several knots in the jet, including one that has been undergoing flaring behavior. We estimate a conservative upper limit on the mass of molecular gas within ∼100 pc and ±400 km/s line-of-sight velocity of the central black hole of ∼8x10⁶ M_☉, which includes an allowance for possible systematic errors associated with subtraction of the continuum. Ignoring such errors, we have a 3 σ sensitivity to ∼3x10⁶ M_☉. In fact, the continuum-subtracted spectrum shows weak emission features extending up to 4 σ above the rms dispersion of the line-free channels. These may be artifacts of the continuum subtraction process. Alternatively, if they are interpreted as CO emission, then the implied molecular gas mass is ∼5x10⁶ M_☉ spread out over a velocity range of 700 km/s. These constraints on molecular gas mass are close to the predictions of the model of self-gravitating, star-forming accretion disks fed by Bondi accretion (Tan & Blackman (ref???)2005).