Measuring the Interstellar Medium Content of Nearby, Luminous, Type 1 and Type 2 QSOs through CO and [C <scp>ii</scp>]

Abstract We present observations of CO(1–0) and CO(2–1) lines from the Institut de radioastronomie millimétrique 30 m telescope toward 20 nearby, optically luminous type 2 quasars (QSO2s) and observations of the [C ii ] 158 μ m line from the Stratospheric Observatory for Infrared Astronomy for five QSO2s in the CO sample and five type 1 quasars (QSO1s). In the traditional evolutionary scenario explaining different types of QSOs, obscured QSO2s emerge from gas-rich mergers observed as luminous infrared galaxies (LIRGs) and then turn into unobscured QSO1s as the black holes clear out the obscuring material in a blowout phase. We test the validity of this theoretical prediction by comparing the gas fractions and star formation efficiencies among LIRGs and QSOs. We find that CO luminosity, CO-derived gas masses, and gas fractions in QSO1s are consistent with those estimated for QSO2s, while LIRGs exhibit a closer resemblance to QSO2s in terms of CO-derived gas masses and gas fractions. Comparisons between [C ii ] luminosity and star formation tracers such as the CO and infrared luminosity imply additional sources of [C ii ] emission in QSO1s likely tracing neutral atomic or ionized gas with the caveat of a small sample size. All three types of galaxies have statistically indistinguishable distributions of star formation efficiency. Our results are consistent with part of the evolutionary scenario where nearby QSO2s could emerge from LIRGs, but they may not be the precursors of nearby QSO1s.