The [O III] Nebula of the Merger Remnant NGC 7252: A Likely Faint Ionization Echo

August 2013 • 2013ApJ...773..148S

Authors • Schweizer, François • Seitzer, Patrick • Kelson, Daniel D. • Villanueva, Edward V. • Walth, Gregory L.

Abstract • 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 1.4 × 1040 erg s-1, 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,\,0<5\times 10^{39} 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.

Based in part on data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.


IPAC Authors


Greg Walth

Assistant Scientist