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

TL;DR

This paper investigates the nucleus of galaxy M94, analyzing its low-luminosity active galactic nucleus and black hole mass estimates using multi-wavelength data, highlighting the galaxy's unique properties and challenges in mass determination.

Contribution

It provides the first detailed analysis of M94's nucleus at multiple wavelengths, comparing various black hole mass estimates and discussing the galaxy's low luminosity AGN characteristics.

Findings

M94's nucleus hosts one of the least luminous broad-line LINERs.

Different black hole mass estimation methods yield significantly varying results.

The AGN may be deficient in ionizing radiation, possibly due to variability or multiple sources.

Abstract

We discuss the peculiar nature of the nucleus of M94 (NGC 4736) in the context of new measurements of the broad H_alpha 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 \times 10^40 erg/s. This LINER galaxy has also possibly the least luminous broad line region known (LH_alpha =2.2\times10^37 erg/s). We compare black hole mass estimates of this system to the recently measured ~7 \times 10^6 M_sun 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(H_alpha) relation, empirical correlation of BH mass with high-ionization mid IR emission lines, and the X-ray excess variance) provide much lower estimates (~10^5 M_sun). 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.