2004ApJS..154..151S

The Infrared Spectrograph (IRS) on board the Spitzer Space Telescope has been used to obtain low- and moderate-resolution spectra of the dust- and gas-rich quasar APM 08279+5255 (z=3.91). Broad Paα and Paβ recombination lines of hydrogen were detected at wavelengths of 9.235 and 6.315 µm, respectively, as well as a strong, red continuum that is a smooth power law over the observed (rest-frame) wavelength range 5.3-35 µm (1.08-7.1 µm). The observed Paα/Paβ line flux ratio of 1.05±0.2 is far from the case B value of ∼2 and simple models of high-density, high optical depth ionized-gas regions (∼1.8). This deviation is opposite in sense to the expected effect of reddening. No evidence is found in the spectrum for either the 3.3 or the 6.2 µm emission features usually attributed to aromatic hydrocarbons in gas-rich galaxies in the local universe. This is consistent with the high-luminosity active galactic nucleus (AGN) nature of APM 08279+5255.