The [O III] Nebula of the Merger Remnant NGC 7252: A Likely Faint
  Ionization Echo
  We present images and spectra of a ~10 kpc-sized emission-line nebulosity
discovered in the prototypical merger remnant NGC 7252 and dubbed the `[O III]
nebula' because of its dominant [O III]_5007 line. This nebula seems to yield
the first sign of episodic AGN activity still occurring in the remnant, ~220
Myr after the coalescence of two gas-rich galaxies. Its location and kinematics
suggest it belongs to a stream of tidal-tail gas falling back into the remnant.
Its integrated [O III]_5007 luminosity is 1.4x10^40 erg/s, and its spectrum
features some high-excitation lines, including He II_4686. In diagnostic line-
ratio diagrams, the nebula lies in the domain of Seyfert galaxies, suggesting
that it is photoionized by a source with a power-law spectrum. Yet, a search
for AGN activity in NGC 7252 from X-rays to radio wavelengths yields no
detection, with the most stringent upper limit set by X-ray observations. The
upper luminosity limit of L_{2-10 keV,0} < 5x10^39 erg/s estimated for the
nucleus is ~10^3 times lower than the minimum ionizing luminosity of >5x10^42
erg/s necessary to excite the nebula. This large discrepancy suggests that the
nebula is a faint ionization echo excited by a mildly active nucleus that has
declined by ~3 orders of magnitude over the past 20,000-200,000 years. In many
ways this nebula resembles the prototypical `Hanny's Voorwerp' near IC 2497,
but its size is 3x smaller and its [O III] luminosity ~100x lower. We propose
that it be classified as an extended emission-line region (EELR). The [O III]
nebula is then the lowest-luminosity ionization echo and EELR discovered so
far, indicative of recent, probably sputtering AGN activity of Seyfert-like
intensity in NGC 7252.
