M94 As A Unique Testbed for Black Hole Mass Estimates and AGN Activity At Low Luminosities

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We discuss the peculiar nature of the nucleus of M94 (NGC 4736) in the context of new measurements of the broad Hₐlpha emission from HST-STIS observations. We show that this component is unambiguously associated with the high-resolution X-ray, radio, and variable UV sources detected at the optical nucleus of this galaxy. These multi-wavelength observations suggest that NGC 4736 is one of the least luminous broad-line (type 1) LINERs, with Lbol = 2.5 × 10⁴0 erg/s. This LINER galaxy has also possibly the least luminous broad line region known (LHₐlpha =2.2×10³7 erg/s). We compare black hole mass estimates of this system to the recently measured 7 × 10⁶ Mₛun dynamical black hole mass measurement. The fundamental plane and M-sigma relationship roughly agree with the measured black hole mass, while other accretion based estimates (the M-FWHM(Hₐlpha) relation, empirical correlation of BH mass with high-ionization mid IR emission lines, and the X-ray excess variance) provide much lower estimates (10⁵ Mₛun). An energy budget test shows that the AGN in this system may be deficient in ionizing radiation relative to the observed emission-line activity. This deficiency may result from source variability or the superposition of multiple sources including supernovae.