Dario Fadda
Space Telescope Science Institute
An analysis of the interstellar medium of nearby luminous QSOs via their CO and [CII] emission
We present the analysis of a sample of QSOs observed with SOFIA and IRAM. These are the most distant extragalactic objects successfully detected with FIFI-LS spectroscopy. We examined if and how the ISM content traced by CO and [CII] differs across luminous infrared galaxies (LIRGs) and QSOs of type 1 and 2. In particular, we tested the scenario where obscured type 2 QSOs emerge from gas rich mergers observed as LIRGs to later become type 1 QSOs once the central black hole clears out the obscuring material in a blow-out phase. Specifically, we estimate how our sample's gas fraction and star-formation efficiency estimates compare to those of LIRGs and QSO1s. We find out that QSO of type 1 and 2 are indistiguishable in CO luminosities and CO derived gas mass and gas fraction. However, SOFIA [CII] measurements show lower CO/[CII] luminosity ratios for QSO1s than QSO2s This suggests that QSO1s could contain extra sources of [CII] emission likely tracing neutral atomic or ionized gas. LIRGs display a closer resemblance to QSO2s in CO- and [C II]-properties than to QSO1s. All three types of galaxies have statistically indistinguishable distributions of star formation efficiency. We suggest that while nearby QSO2s could emerge from LIRGs, they are unlikely the precursors of nearby QSO1s. The evolutionary scenario might not be correct in the nearby universe, where black holes do not act once in a violent way but are more likely to affect their host galaxies continuously by altering the spatial distribution and phase of the gas.
