Measuring the ISM Content of Nearby, Luminous, Type 1 and Type 2 QSOs through CO and [C II]

TL;DR

This study investigates the gas content and star formation in nearby luminous quasars and LIRGs, testing evolutionary models by comparing CO and [C II] line emissions across different quasar types.

Contribution

It provides the first direct comparison of gas fractions and star formation efficiencies between QSO1s, QSO2s, and LIRGs using CO and [C II] observations.

Findings

QSO1s and QSO2s have similar gas fractions and CO luminosities.

LIRGs resemble QSO2s more than QSO1s in gas properties.

Star formation efficiencies are statistically similar across all galaxy types.

Abstract

We present observations of CO(1--0) and CO(2--1) lines from the Institut de radioastronomie millim\'etrique (IRAM) 30m telescope toward 20 nearby, optically luminous type 2 quasars (QSO2s) and observations of [C II] 158$\mu$m line from the Stratospheric Observatory For Infrared Astronomy (SOFIA) for 5 QSO2s in the CO sample and 5 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 blow-out 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.