The [O III] Nebula of the Merger Remnant NGC 7252: A Likely Faint Ionization Echo
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.- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2013
- DOI:
- 10.1088/0004-637X/773/2/148
- arXiv:
- arXiv:1307.2233
- Bibcode:
- 2013ApJ...773..148S
- Keywords:
-
- galaxies: active;
- galaxies: evolution;
- galaxies: formation;
- galaxies: individual: NGC 7252;
- galaxies: interactions;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 21 pages, 7 figures, 5 tables (emulateapj). Accepted for publication in The Astrophysical Journal, Vol. 773. Fig. 5 degraded to reduce file size