Spitzer Infrared Spectrograph (IRS) observations of the redshift 3.91 quasar APM 08279+5255

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 Paalpha and Pabeta recombination lines of hydrogen were detected at wavelengths of 9.235 and 6.315 mum, respectively, as well as a strong, red continuum that is a smooth power law over the observed (rest-frame) wavelength range 5.3-35 mum (1.08-7.1 mum). The observed Paalpha/Pabeta line flux ratio of 1>05 +/- 0>2 is far from the case B value of similar to2 and simple models of high-density, high optical depth ionized-gas regions (similar to1.8). 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 mum 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.