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Advances in Astronomy
Volume 2012, Article ID 178060, 14 pages
Research Article

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

1Department of Physics and Astronomy, James Madison University, Harrisonburg, VA 22807, USA
2Department of Physics and Astronomy,University of Utah, Salt Lake City, UT 84112, USA

Received 28 September 2011; Accepted 12 December 2011

Academic Editor: Antonis Georgakakis

Copyright © 2012 Anca Constantin and Anil C. Seth. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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 7×106M 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 (105M). 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.