Mid-Infrared Spectral Indicators of Star-Formation and AGN Activity in Normal Galaxies

TL;DR

This study uses mid-infrared spectral features from Spitzer data to distinguish star formation from AGN activity in normal galaxies, improving diagnostic methods and linking MIR components to galaxy properties.

Contribution

It introduces a refined spectral decomposition method and demonstrates its effectiveness in diagnosing galaxy activity and estimating dust attenuation.

Findings

PAH equivalent widths are significantly larger with PAHFIT than spline methods.

The PAH and emission line diagnostics better distinguish AGN from star-forming galaxies.

MIR component luminosities correlate strongly with total IR luminosity.

Abstract

We investigate the use of mid-infrared PAH bands, continuum and emission lines as probes of star-formation and AGN activity in a sample of 100 `normal' and local (z~0.1) galaxies. The MIR spectra were obtained with the Spitzer IRS as part of the Spitzer-SDSS-GALEX Spectroscopic Survey (SSGSS) which includes multi-wavelength photometry from the UV to the FIR and optical spectroscopy. The spectra were decomposed using PAHFIT (Smith et al. 2007), which we find to yield PAH equivalent widths (EW) up to ~30 times larger than the commonly used spline methods. Based on correlations between PAH, continuum and emission line properties and optically derived physical properties (gas phase metallicity, radiation field hardness), we revisit the diagnostic diagram relating PAH EWs and [NeII]/[OIV] and find it more efficient as distinguishing weak AGNs from star-forming galaxies than when spline decompositions are used. The luminosity of individual MIR component (PAH, continuum, Ne and molecular hydrogen lines) are found to be tightly correlated to the total IR luminosity and can be used to estimate dust attenuation in the UV and in Ha lines based on energy balance arguments.