A Deep Chandra, Very Large Array, and Spitzer Infrared Array Camera Study of the Very Low Luminosity Nucleus of the Elliptical NGC 821

The relatively nearby (distance = 24.1 Mpc) elliptical galaxy NGC 821 hosts an extreme example of a quiescent central massive black hole, for which deep Chandra observations revealed a nuclear source for the first time, with L2-10 keV/LEdd ~ 10-8. We present here a multiwavelength study of this nucleus, including Very Large Array (VLA) observations that detect a radio counterpart to the Chandra nuclear source at 1.4 GHz, with a flux density of 127 μJy and possibly a flat spectral shape; we also consider new Spitzer Infrared Array Camera (IRAC) observations and archival Hubble Space Telescope (HST) images. With these data we discuss possible scenarios for the accretion modalities of the sole material that seems available for fueling, i.e., the stellar mass losses steadily replenishing the circumnuclear region. The final stages of accretion could be radiatively inefficient and coupled to a compact nuclear jet/outfow. The stellar mass losses could instead end up in a standard disk only if a Compton-thick active galactic nucleus (AGN) is present. Two extended sources detected by Chandra close to the nucleus could be due to several unresolved knots in a jet. If a jet is present, however, its kinetic energy would be only a very small fraction of the energy associated with the rest mass of the material being accreted. Star formation close to the nucleus is not shown by the available data. Deeper Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) radio and far-IR observations are expected to further constrain the accretion process in this intriguing nucleus.