Etching glass in the early Universe: Luminous HF and H₂O emission in a QSO-SMG pair at z = 4.7^⋆

¹ Sorbonne Université, CNRS UMR 7095, Institut d’Astrophysique de Paris, 98bis bvd Arago, 75014 Paris, France
e-mail: lehnert@iap.fr
² European Southern Observatory, Ave. Alonso de Córdova 3107, Vitacura, Santiago, Chile
³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁴ Laboratoire de Physique de l’ENS, École Normale Supérieure, Université PSL, CNRS, Sorbonne Université, Université Paris-Diderot, Paris, France

Received: 23 April 2019
Accepted: 30 March 2020

Abstract

We present ALMA observations of hydrogen fluoride, HF J = 1–0, water, H₂O (2₂₀–2₁₁), and the 1.2 THz rest-frame continuum emission from the z = 4.7 system BR 1202-0725. System BR 1202-0725 is a galaxy group consisting of a quasi-stellar object (QSO), a sub-millimeter galaxy (SMG), and a pair of Lyα emitters. We detected HF in emission in the QSO and possibly in absorption in the SMG, while water was detected in emission in both the QSO and the SMG. The QSO is the most luminous HF J = 1–0 emitter that has yet been found and has the same ratio of HF emission-line to infrared luminosity, L_HF/L_IR, as a small sample of local active galactic nuclei and the Orion Bar. This consistency covers about ten orders of magnitude in L_IR. Based on the conclusions of a study of HF emission in the Orion Bar and simple radiative transfer modeling, the HF emission in the QSO is excited either by collisions with electrons (and H₂) in molecular plasmas irradiated by the AGN and intense star formation, or predominately by collisions with H₂, with a modest contribution from electrons, in a relatively high temperature (∼120 K), dense (∼10⁵ cm⁻³) medium. The high density of electrons necessary to collisionally excite the HF J = 1–0 line can be supplied in sufficient quantities by the estimated column density of C⁺. Although HF should be an excellent tracer of molecular outflows, we found no strong kinematic evidence for outflows in HF in either the QSO or the SMG. From a putative absorption feature in HF observed against the continuum emission from the SMG, we conducted a bootstrap analysis to estimate an upper limit on the outflow rate, Ṁ_outflow ≲ 45 M_⊙ yr⁻¹. This result implies that the ratio of the molecular outflow rate to the star formation rate is Ṁ_outflow/SFR ≲ 5% for the SMG. Both the QSO and the SMG are among the most luminous H₂O (2₂₀–2₁₁) emitters currently known and are found to lie along the same relationship between L_{H2O (220 − 211)}/L_IR and L_IR as a large sample of local and high-redshift star-forming galaxies. The kinematics of the H₂O (2₂₀–2₁₁) line in the SMG is consistent with a rotating disk as found previously but the line profile appears broader than other molecular lines, with a full width at half maximum of ∼1020 km s⁻¹. The broadness of the line, which is similar to the width of a much lower resolution observation of CO(2-1), may suggest that either the gas on large scales (≳4 kpc) is significantly more disturbed and turbulent due either to interactions and mass exchange with the other members of the group, or to the dissipation of the energy of the intense star formation, or both. Overall however, the lack of significant molecular outflows in either source may imply that much of the energy from the intense star formation and active galactic nucleus in this pair is being dissipated in their interstellar media.