Far-infrared Spectral Energy Distribution Fitting for Galaxies Near and Far

TL;DR

This paper introduces a simple, effective FIR SED fitting method combining a greybody and powerlaw to accurately measure galaxy properties with minimal parameters, outperforming traditional templates.

Contribution

The novel FIR SED fitting technique uses only 2-4 free parameters and provides better fits than existing templates, improving measurements of galaxy infrared properties.

Findings

Better fit residuals than literature templates (~2x lower)

Measured dust emissivity index beta=1.60+-0.38

Dust temperatures 25-45K for local (U)LIRGs, colder than previous estimates

Abstract

Spectral Energy Distribution (SED) fitting in the far-infrared (FIR) is greatly limited by a dearth of data and an excess of free parameters - from galaxies' dust composition, temperature, mass, orientation, opacity, to heating from AGN. This paper presents a simple FIR SED fitting technique joining a modified, single dust temperature greybody, representing the reprocessed starburst emission in the whole galaxy, to a mid-infrared powerlaw, which approximates hot-dust emission from AGN heating or clumpy, hot starbursting regions. This FIR SED can be used to measure infrared luminosities, dust temperatures and dust masses for both local and high-z galaxies with 3 to 10+ FIR photometric measurements. This fitting method is compared to infrared template SEDs in the literature using photometric data on 65 local luminous and ultraluminous infrared galaxies, (U)LIRGs. Despite relying only on 2-4 free parameters, the coupled greybody/powerlaw SED fitting described here produces better fits to photometric measurements than best-fit literature template SEDs (with residuals a factor of ~2 lower). A mean emissivity index of beta=1.60+-0.38 and mid-infrared powerlaw slope of alpha=2.0+-0.5 is measured; the former agrees with the widely presumed emissivity index of beta=1.5 and the latter is indicative of an optically-thin dust medium with a shallow radial density profile, ~r^-0.5. Adopting characteristic dust temperature as the inverse wavelength where the SED peaks, dust temperatures ~25-45K are measured for local (U)LIRGs, ~5-15K colder than previous estimates using only simple greybodies. This comparative study highlights the impact of SED fitting assumptions on the measurement of physical properties such as infrared luminosity (and thereby infrared-based star formation rate), dust temperature and dust mass, for both local and high-redshift galaxies. [abridged]