Far-infrared spectral energy distribution fitting for galaxies near and far: Far-infrared SED fitting

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 active galactic nuclei (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 (MIR) power law, which approximates hot-dust emission from AGN heating or clumpy, hot starbursting regions. This FIR SED can be used to measure IR luminosities, dust temperatures and dust masses for both local and high-z galaxies with three to 10+ FIR photometric measurements. While the fitting technique does not model emission from polycyclic aromatic hydrocarbons (PAHs) in the MIR, the impact of PAH features on integrated FIR properties is negligible when compared to the bulk emission at longer wavelengths. This fitting method is compared to IR 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/power-law SED fitting described here produces better fits to photometric measurements than best-fitting literature template SEDs (with residuals a factor of ∼2 lower). A mean emissivity index of β = 1.60 ± 0.38 and MIR power-law slope of α = 2.0 ± 0.5 is measured; the former agrees with the widely presumed emissivity index of β = 1.5 and the latter is indicative of an optically thin dust medium with a shallow radial density profile, ≈r−1/2. Adopting characteristic dust temperature as the inverse wavelength where the SED peaks, dust temperatures ∼25–45 K are measured for local (U)LIRGs, ∼5–15 K 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 IR luminosity (and thereby IR-based star formation rate), dust temperature and dust mass, for both local and high-redshift galaxies.