2004ApJ...604..565D

The Infrared Space Observatory (ISO) was used to search for a tracer of the warm and dense neutral interstellar medium, the [O I] 63.18 µm line, in four ultraluminous IRAS sources lying at redshifts between 0.6 and 1.4. While these sources are quasars, their infrared continuum emission suggests a substantial interstellar medium. No [O I] flux was detected after probing down to a 3 σ sensitivity level sufficient for detecting line emission in starbursts with similar continuum emission. However, if the detection threshold is slightly relaxed, one target is detected with 2.7 σ significance. For this radio-quiet quasar, there is likely a substantial dense and warm interstellar medium; the upper limits for the three radio-loud sources do not preclude the same conclusion. Using a new, uniformly processed database of the ISO extragalactic far-infrared spectroscopy observations, we show that nearby Seyfert galaxies typically have higher [O I]-to-far-infrared ratios than do normal star-forming galaxies, so the lack of strong [O I] 63 µm emission from these high-redshift ultraluminous sources cannot be attributed to their active cores.