The evolutionary connection between QSOs and SMGs: molecular gas in far-infrared luminous QSOs at z ~ 2.5

TL;DR

This study investigates the molecular gas properties of far-infrared luminous QSOs at z ~ 2.5, revealing they have less gas than SMGs and are likely in a transitional evolutionary phase from starburst galaxies to unobscured QSOs.

Contribution

It provides new observational evidence linking QSOs and SMGs, showing their different gas contents and supporting a short-lived transitional phase in galaxy evolution.

Findings

Far-infrared bright QSOs have ~50% less warm/dense gas than SMGs.

QSOs lack the extended cool gas reservoirs found in SMGs.

Results support a short (~1 Myr) transitional phase from SMGs to QSOs.

Abstract

We present IRAM Plateau de Bure Interferometer observations of the 12CO(3-2) emission from two far-infrared luminous QSOs at z ~ 2.5 selected from the Herschel-ATLAS survey. These far-infrared bright QSOs were selected to have supermassive black holes (SMBHs) with masses similar to those thought to reside in sub-millimetre galaxies (SMGs) at z ~ 2.5; making them ideal candidates as systems in transition from an ultraluminous infrared galaxy phase to a sub-mm faint, unobscured, QSO. We detect 12CO(3-2) emission from both QSOs and we compare their baryonic, dynamical and SMBH masses to those of SMGs at the same epoch. We find that these far-infrared bright QSOs have similar dynamical but lower gas masses than SMGs. In particular we find that far-infrared bright QSOs have ~50+-23% less warm/dense gas than SMGs, which combined with previous results showing the QSOs lack the extended, cool reservoir of gas seen in SMGs, suggests that they are at a different evolutionary stage. This is consistent with the hypothesis that far-infrared bright QSOs represent a short (~1Myr) but ubiquitous phase in the transformation of dust obscured, gas-rich, starburst-dominated SMGs into unobscured, gas-poor, QSOs.