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

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, LHF/LIR, as a small sample of local active galactic nuclei and the Orion Bar. This consistency covers about ten orders of magnitude in LIR. 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, Ṁoutflow ≲ 45 M⊙ yr−1. This result implies that the ratio of the molecular outflow rate to the star formation rate is Ṁoutflow/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 LH2O (220 − 211)/LIR and LIR 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.

Black hole and host galaxy growth in an isolated z ∼ 6 QSO observed with ALMA

“…The emission line is detected with a 10σ statistical significance, and the velocity gradient and PA are consistent with those derived for [CII]. Water vapour emission traces the molecular warm dense phase of the interstellar medium (Liu et al 2017), and is detected in only a few QSOs at z>6 (Pensabene et al 2021;Lehnert et al 2020;Yang et al 2019). This is the first time that the emission is spatially resolved and consistent with a rotating water vapour disk.…”

The ramp-up of interstellar medium enrichment at z>4

“…If we fit the line with a double Gaussian, the centre of the strongest component is close to zero velocity, −23 ± 30 km s −1 , and a weaker component is blue-shifted by −277 ± 35 km s −1 . HF(1-0) is a potential tracer of molecular outflows 13,14 , and therefore we could be seeing tentative evidence of a molecular outflow, as has been detected in a high fraction of similarly luminous objects at high redshift 15 . However, the reduced χ 2 of the double Gaussian fit is not significantly better than the single Gaussian fit and could be affected by differential lensing.…”

“…HF has also been detected mostly in absorption in a handful of other local galaxies (see Figure 4-left panel). At higher redshifts, observations are scarce: the molecule has only been detected quasars; in absorption in PKS 1830−211 [22] at z = 0.9, in the Cloverleaf 23 at z = 2.6 and in emission in BR 1202−0725 at z = 4.7 [14] . This makes NGP-190387, in addition to being the most distant detection observed in absorption, the only detection outside of the local Universe in a non-quasar host.…”

Photodissociation and X-Ray Dominated Regions

“…Nevertheless, the unprecedented sensitivity and resolution of ALMA enable observations approaching sub-kpc scales, thus providing a first glimpse on the overall ISM conditions of distant sources. The [C II] 158 µm line is by far the most widely targeted PDR tracer in normal (SFR= 1 − 100 M yr −1 ) sources at high-z, while [O I] 63 and 145 µm (e.g., Rybak et al 2020b, Lee et al 2021), [C I] 609 and 370 µm (e.g., Strandet et al 2017, Valentino et al 2020, CO (e.g., D'Odorico et al 2018, Pavesi et al 2019, Apostolovski et al 2019, and H2O, HF, and OH + lines (e.g., Casey et al 2019, Lehnert et al 2020, Riechers et al 2021, Pensabene et al 2021) are mostly detected in rare/massive and highly star forming galaxies and quasars.…”