An ALMA Band 7 survey of SDSS/Herschel quasars in Stripe 82: I. The properties of the 870 micron counterparts

TL;DR

This study uses high-resolution ALMA observations to identify the true sources of far-infrared emission in a large sample of quasars, revealing the prevalence of single versus multiple counterparts and exploring their implications for galaxy mergers and star formation.

Contribution

First high-resolution ALMA Band 7 survey of SDSS quasars clarifies FIR emission origins and assesses merger roles in SMBH growth and star formation.

Findings

~60% of FIR emission from single counterparts within SPIRE beam

Multiplicity rate increases from 1 to 2.5 between redshifts 1 and 2.5

Detection of CO and potential H2O/HCN lines in some quasars

Abstract

(Abridged) Quasar studies with Herschel/SPIRE often report host luminosities ranging from 10^{12} to 10^{14} Lsun, suggestive of star formation rates (SFRs) of up to several thousand Msun/yr. Due to the limited spatial resolution of SPIRE, it is uncertain whether the far-infrared (FIR) emission originates from the quasar itself, nearby sources, or unrelated sources within the SPIRE beam. High-resolution observations at wavelengths close to the SPIRE coverage are needed to pinpoint the true source of the FIR emission. In this work we unambiguously identify the ALMA Band 7 counterparts of a statistical sample of 152 FIR-bright SDSS quasars and estimate the multiplicity rates among these systems. Based on the multiplicities, we assess the importance of mergers as triggers for concomitant accretion onto supermassive black holes (SMBHs) and extreme star formation. In ~60% of cases, the submm emission originates from a single counterpart within the SPIRE beam, centred on the optical coordinates of the quasar. The multiplicity rate increases by a factor of ~2.5 between redshifts 1 and 2.5. The incidence of multiplicities is consistent among broad absorption line (BAL) quasars and non-BAL quasars. The multiplicities observed in a fraction of the sample indicate that, while mergers enhance gas inflow efficiency, there must be viable alternatives for driving synchronous SMBH growth and intense star formation in isolated systems. We report the serendipitous detection of two CO(6-5) and three CO(7-6) transitions out of the eight such transitions expected based on the spectral setup and the redshifts of the objects in the sample. Higher transitions are not detected, indicating that the quasars are not exciting sufficiently the gas in their hosts. Finally, we also detect a potential emission of H2O, HCN (10-9) or a combination of both in the spectrum of a quasar at redshift 1.67.