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We report the discovery of an Active Galactic Nucleus (AGN) in the nearby SAdgalaxy NGC 3621 using Spitzer high spectral resolution observations. Theseobservations reveal the presence of [NeV] 14 um and 24 um emission which iscentrally concentrated and peaks at the position of the near-infrared nucleus.Using the [NeV] line luminosity, we estimate that the nuclear bolometricluminosity of the AGN is ~ 5 X 10^41 ergs s^-1, which corresponds based on theEddington limit to a lower mass limit of the black hole of ~ 4 X 10^3 Msun.Using an order of magnitude estimate for the bulge mass based on the Hubbletype of the galaxy, we find that this lower mass limit does not put a strain onthe well-known relationship between the black hole mass and the host galaxy'sstellar velocity dispersion established in predominantly early-type galaxies.Mutli-wavelength follow-up observations of NGC 3621 are required to obtain moreprecise estimates of the bulge mass, black hole mass, accretion rate, andnuclear bolometric luminosity. The discovery reported here adds to the growingevidence that a black hole can form and grow in a galaxy with no or minimalbulge.

The Discovery of an Active Galactic Nucleus in the Late-type Galaxy NGC 3621: Spitzer Spectroscopic Observations