Mid-infrared dust in two nearby radio galaxies, NGC 1316 (Fornax A) and NGC 612 (PKS 0131-36)^⋆

¹ Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, Johannesburg, South Africa
² Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: cathy.horellou@chalmers.se
³ Current address: Ghana Space Science and Technology Institute, Ghana Atomic Energy Commission, PO Box LG 80, Legon, Accra, Ghana
⁴ Astronomy Department, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa

Received: 27 December 2015
Accepted: 24 May 2016

Abstract

Context. Most radio galaxies are hosted by giant gas-poor ellipticals, but some contain significant amounts of dust, which is likely to be of external origin.

Aims. In order to characterize the mid-IR properties of two of the most nearby and brightest merger-remnant radio galaxies of the Southern hemisphere, NGC 1316 (Fornax A) and NGC 612 (PKS 0131−36), we used observations with the Wide-field Infrared Survey Explorer (WISE) at wavelengths of 3.4, 4.6, 12 and 22 μm and Spitzer mid-infrared spectra.

Methods. By applying a resolution-enhancement technique, new WISE images were produced at angular resolutions ranging from 2.̋6 to 5.̋5. Global measurements were performed in the four WISE bands, and stellar masses and star-formation rates were estimated using published scaling relations. Two methods were used to uncover the distribution of dust, one relying on two-dimensional fits to the 3.4 μm images to model the starlight, and the other one using a simple scaling and subtraction of the 3.4 μm images to estimate the stellar continuum contribution to the emission in the 12 and 22 μm bands.

Results. The two galaxies differ markedly in their mid-IR properties. The 3.4 μm brightness distribution can be well represented by the superposition of two Sérsic models in NGC 1316 and by a Sérsic model and an exponential disk in NGC 612. The WISE colors of NGC 1316 are typical of those of early-type galaxies; those of NGC 612 are in the range found for star-forming galaxies. From the 22 μm luminosity, we infer a star-formation rate of ~0.7 M_⊙ yr^-1 in NGC 1316 and ~7 M_⊙ yr^-1 in NGC 612. Spitzer spectroscopy shows that the 7.7-to-11.3 μm PAH line ratio is significantly lower in NGC 1316 than in NGC 612. The WISE images reveal resolved emission from dust in the central 1′−2′ of the galaxies. In NGC 1316, the extra-nuclear emission coincides with two dusty regions NW and SE of the nucleus seen in extinction in optical images and where molecular gas is known to reside. In NGC 612 it comes from a warped disk. This suggests a recent infall onto NGC 1316 and disruption of one or several smaller gas-rich galaxies, but a smoother accretion in NGC 612. While the nucleus of NGC 1316 is currently dormant and the galaxy is likely to evolve into a passive elliptical, NGC 612 has the potential of growing a larger disk and sustaining an active nucleus.

Conclusions. NGC 1316 and NGC 612 represent interesting challenges to models of formation and evolution of galaxies and AGNs.