Constraining the Active Galactic Nucleus and Starburst Properties of the IR-luminous Quasar Host Galaxy APM08279+5255 at Redshift 4 with SOFIA

Abstract

We present far-IR photometry and the infrared spectrum of the z = 3.9114 quasar/starburst composite system APM 08279+5255, obtained using the Stratospheric Observatory for Infrared Astronomy (SOFIA)/High-resolution Airborne Wideband Camera+ (HAWC+) and the Spitzer Space Telescope Infrared Spectrograph. We decompose the IR-to-radio spectral energy distribution (SED), sampled in 51 bands, using (i) a model comprised of two-temperature modified blackbodies and radio power laws and (ii) a semi-analytic model, which also accounts for emission from a clumpy torus. The latter is more realistic but requires a well-sampled SED, which is possible here. In the former model, we find temperatures of {T}_d^warm} = 296{}_-15⁺¹⁷ K and {T}_d^cold} = 110{}_-3⁺³ K for the warm and cold dust components, respectively. This model suggests that the cold dust component dominates the far-infrared (FIR) energy budget (66%) but contributes only 17% to the total IR luminosity. Based on the torus models, we infer an inclination angle of i = 15{}_-8⁺⁸° and the presence of silicate emission, in accordance with the Type-1 active galactic nucleus nature of APM 08279+5255. Accounting for the torus’ contribution to the FIR luminosity, we find a lensing-corrected star formation rate of SFR = 3075 × (4/μ _L ) M _⊙ yr^-1. We find that the central quasar contributes 30% to the FIR luminosity but dominates the total IR luminosity (93%). The 30% correction is in contrast to the 90% reported in previous work. In addition, the IR luminosity inferred from the torus model is a factor of two higher. These differences highlight the importance of adopting physically motivated models to properly account for IR emission in high-z quasars, which is now possible with SOFIA/HAWC+.