Discovery of Water Vapor in the High-Redshift Quasar APM 08279+5255 at Z=3.91

TL;DR

This paper reports the first detection of interstellar water vapor at high redshift in a quasar, revealing insights into water excitation and fluorine abundance in the early universe.

Contribution

It presents the highest-redshift detection of interstellar water vapor and analyzes its excitation, also providing new limits on fluorine abundance at z=3.91.

Findings

Detection of water vapor at z=3.91

Water excitation is mainly radiatively pumped

No HF absorption detected, suggesting lower fluorine abundance

Abstract

We report a detection of the excited 220-211 rotational transition of para-H2O in APM 08279+5255 using the IRAM Plateau de Bure interferometer. At z = 3.91, this is the highest-redshift detection of interstellar water to date. From LVG modeling, we conclude that this transition is predominantly radiatively pumped and on its own does not provide a good estimate of the water abundance. However, additional water transitions are predicted to be detectable in this source, which would lead to an improved excitation model. We also present a sensitive upper limit for the HF J = 1 - 0 absorption toward APM 08279+5255. While the face-on geometry of this source is not favorable for absorption studies, the lack of HF absorption is still puzzling and may be indicative of a lower fluorine abundance at z = 3.91 compared with the Galactic ISM.