Constraining the outburst properties of the SMBH in fornax a through X-ray, infrared, and radio observations

Abstract

Combined Spitzer, Chandra, XMM-Newton, and VLA observations of the giant radio galaxy NGC 1316 (Fornax A) show a radio jet and X-ray cavities from active galactic nucleus (AGN) outbursts most likely triggered by a merger with a late-type galaxy at least 0.4 Gyr ago. We detect a weak nucleus with a spectral energy distribution typical of a low-luminosity AGN with a bolometric luminosity of 2.4 × 1042 erg s-1. We examine the Spitzer IRAC and MIPS images of NGC 1316. We find that the dust emission is strongest in regions with little or no radio emission and that the particularly large infrared luminosity relative to the galaxy's K-band luminosity implies an external origin for the dust. The inferred dust mass implies that the merger spiral galaxy had a stellar mass of (1-6)×1010 M and a gas mass of (2-4)×109 M. X-ray cavities in the Chandra and XMM-Newton images likely result from the expansion of relativistic plasma ejected by the AGN. The soft (0.5-2.0 keV) Chandra images show a small ̃15′′ (1.6 kpc) cavity coincident with the radio jet, while the XMM-Newton image shows two large X-ray cavities lying 320′′ (34.8 kpc) east and west of the nucleus, each approximately 230′′ (25 kpc) in radius. Current radio observations do not show emission within these cavities. The radio lobes lie at radii of 14.3 (93.3 kpc) and 15.6 (101 kpc), more distant from the nucleus than the detected X-ray cavities. The relative morphology of the large scale 1.4 GHz and X-ray emission suggests they were products of two distinct outbursts, an earlier one creating the radio lobes and a later one producing the X-ray cavities. Alternatively, if a single outburst created both the X-ray cavities and the radio lobes, this would require that the radio morphology is not fully defined by the 1.4 GHz emission. For the more likely two outbursts scenarios, we use the buoyancy rise times to estimate an age for the more recent outburst that created the X-ray cavities of 0.1 Gyr and the PV work done by the expanding plasma that created the X-ray cavities to estimate the outburst's energy to be 1058 erg. The present size and location of the radio lobes imply that the outburst that created them happened ̃0.4 Gyr ago and released ̃5 × 1058 erg. © 2010. The American Astronomical Society. All rights reserved.