Modeling the Infrared Emission in Cygnus A

TL;DR

This paper models the infrared emission of Cygnus A using a comprehensive SED that includes AGN, starburst, and synchrotron components, revealing insights into its luminosity, dust structure, and star formation activity.

Contribution

It presents a detailed physical model of Cygnus A's infrared emission combining multiple components, which was not previously done with such spectral coverage.

Findings

The AGN bolometric luminosity is approximately 10^12 L_0.

A clumpy dust medium extends to about 130 pc from the black hole.

Star formation contributes at least 6 7 10^10 L_0 to the total luminosity.

Abstract

We present new Spitzer IRS spectroscopy of Cygnus A, one of the most luminous radio sources in the local universe. Data on the inner 20" are combined with new reductions of MIPS and IRAC photometry as well as data from the literature to form a radio through mid-infrared spectral energy distribution (SED). This SED is then modeled as a combination of torus reprocessed active galactic nucleus (AGN) radiation, dust enshrouded starburst, and a synchrotron jet. This combination of physically motivated components successfully reproduces the observed emission over almost 5 dex in frequency. The bolometric AGN luminosity is found to be 10^12 L_\odot (90% of LIR), with a clumpy AGN-heated dust medium extending to \sim130 pc from the supermassive black hole. Evidence is seen for a break or cutoff in the core synchrotron emission. The associated population of relativistic electrons could in principle be responsible for some of the observed X-ray emission though the synchrotron self-Compton mechanism. The SED requires a cool dust component, consistent with dust-reprocessed radiation from ongoing star formation. Star formation contributes at least 6 \times 10^10 L_\odot to the bolometric output of Cygnus A, corresponding to a star formation rate of \sim10 M_\odot yr-1.