M94 as a Unique Testbed for Black Hole Mass Estimates and AGN Activity at Low Luminosities

• Main Authors: Anca Constantin, Anil C. Seth
• Format: Article
• Language: English
• Published: Hindawi Limited 2012-01-01
• Series: Advances in Astronomy
• Online Access: http://dx.doi.org/10.1155/2012/178060

We discuss the peculiar nature of the nucleus of M94 (NGC 4736) in the context of new measurements of the broad Hα 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 multiwavelength observations suggest that NGC 4736 is one of the least luminous broad-line (type 1) LINERs, with Lbol=2.5×1040 erg s-1. This LINER galaxy has also possibly the least luminous broad-line region known (LHα=2.2×1037 erg s-1). We compare black hole mass estimates of this system to the recently measured ∼7×106 M⨀ dynamical black hole mass measurement. The fundamental plane and M-σ* relationship roughly agree with the measured black hole mass, while other accretion-based estimates (the M-FWHM(Hα) relation, empirical correlation of BH mass with high-ionization mid-IR emission lines, and the X-ray excess variance) provide much lower estimates (∼105 M⨀). 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.