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Separating infrared spectral energy distributions into their AGN and host-galaxy components.

Both active galactic nuclei (hereafter, AGNs) and their host-galaxies emit strongly at infrared wavelengths (i.e., 1-1000 um). Separating the emission from these components can provide insights into the levels of AGN and star-forming activity taking place within a galaxy.  

DecompIR is an IDL script that takes infrared photometry and/or spectra as inputs and calculates the approximate levels of AGN and host-galaxy contribution to the total infrared output of a composite galaxy. It does this by fitting (via chi2 minimisation) the input data with a combination of a host-galaxy component and an AGN component (see the Templates page for a description of the AGN and host-galaxy components).  

The routine will fit either infrared photometry or spectra alone, or a combination of both, and is able to distinguish between these two types of data. When considering photometry, it will recognise most IRAS, Spitzer and Herschel wavebands and properly account for the filter+instrument response functions.

In its default mode, the routine will only allow the normalisation of the host-galaxy and AGN components to vary during the fitting process. However, the complexity (or, flexibility) of the fitted components can be adjusted depending on the number independent input data bins. For example, if fitting an infrared spectrum obtained with the IRS spectrograph on-board Spitzer (i.e., a large number of data-bins), the routine can be instructed to be highly flexible by allowing parameters such as the AGN continuum slope and silicate emission/absorption features to be fit.  See the Manual page for more details.

The following image provides an example of the kind of spectral energy decompositions that can be performed by DecompIR. 

Main Plot: Full IR SED (6-200um); Inset: IRS spectrum,

Grey shading: IRS spectra +/- 5-sigma

Solid line: Fit; Dashed line: AGN component; Dotted line: Host galaxy component

Error bars: IRAS photometry

Image taken from Mullaney et al (2011)

Further descriptions of DecompIR can be found on the Manual page attached to this site and in Mullaney et al (2011). The latter also presents a full description of how the AGN and host-galaxy components were derived and compares the results derived from DecompIR against other measures of AGN/host-galaxy contribution.  

The full IDL code can be downloaded from the Downloads page.

Also, read the Caveats regarding the interpretation of results produced by DecompIR.

If you use DecompIR in your research, please cite Mullaney et al (2011) (Bibcode: 2011MNRAS.414.1082M [links to ADS page]).

Website last updated: 28 Aug 2015 by J. R. Mullaney