2013ApJ...773..148S

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] λ5007 line. This nebula seems to yield the first sign of episodic active galactic nucleus (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] λ5007 luminosity is 1.4x10⁴⁰ erg/s, and its spectrum features some high-excitation lines, including He II λ4686. 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,O_ < 5 x 10³⁹erg/s estimated for the nucleus is ∼10³ times lower than the minimum ionizing luminosity of ≳ 5x10⁴² 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 yr. 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.