A Search for Molecular Gas in the Nucleus of M87 and Implications for the Fueling of Supermassive Black Holes

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 an upper limit on the mass of molecular gas within ∼ 100pc and ±400km s line of sight velocity of the central black hole of∼ 8×10M⊙. The upper limit for molecular gas mass within any 300 km s velocity interval within this range is ∼ 3 × 10M⊙. Our sensitivity is limited by the subtraction of the central continuum source. The continuum-subtracted spectrum shows weak emission features extending up to 4σ 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 from molecular gas, then the implied gas mass is 5± 4× 10M⊙ spread out over a velocity range of about 700 km s. These constraints on molecular gas mass are close to the predictions of the model of self-gravitating, star-forming accretion disks (Tan & Blackman 2005). Future, higher sensitivity observations are needed to constrain the central molecular gas reservoir further. Subject headings: accretion — stars: formation — galaxies: active, individual (M87), jets