  We present multi-wavelength observations that trace more than 40 years in the
life of the active galactic nucleus (AGN) in Mrk 590, traditionally known as a
classic Seyfert 1 galaxy. From spectra recently obtained from HST, Chandra, and
the Large Binocular Telescope, we find that the activity in the nucleus of Mrk
590 has diminished so significantly that the continuum luminosity is a factor
of 100 lower than the peak luminosity probed by our long baseline observations.
Furthermore, the broad emission lines, once prominent in the UV/optical
spectrum, have all but disappeared. Since AGN type is defined by the presence
of broad emission lines in the optical spectrum, our observations demonstrate
that Mrk 590 has now become a "changing look" AGN. If classified by recent
optical spectra, Mrk 590 would be a Seyfert ~1.9-2, where the only broad
emission line still visible in the optical spectrum is a weak component of
Halpha. As an additional consequence of this change, we have definitively
detected UV narrow-line components in a Type 1 AGN, allowing an analysis of
these emission-line components with high-resolution COS spectra. These
observations challenge the historical paradigm that AGN type is only a
consequence of the line of sight viewing angle toward the nucleus in the
presence of a geometrically-flattened, obscuring medium (i.e., the torus). Our
data instead suggest that the current state of Mrk 590 is a consequence of the
change in luminosity, which implies the black hole accretion rate has
significantly decreased.
