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 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] ?5007 luminosity is 1.4 ? 1040?erg?s?1, 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 ?erg?s?1 estimated for the nucleus is ~103?times lower than the minimum ionizing luminosity of 5 ? 1042?erg?s?1 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 3? smaller and its [O III] luminosity ~100? 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.