A study of submillimeter methanol absorption toward PKS 1830−211:

Abstract

Context. Methanol is an important tracer to probe physical and chemical conditions in the interstellar medium of galaxies. Methanol is also the most sensitive target molecule for probing potential space-time variations of the proton-electron mass ratio, μ, a dimensionless constant of nature. Aims. We present an extensive study of the strongest submillimeter absorption lines of methanol (with rest frequencies between 300 and 520 GHz) in the z = 0.89 molecular absorber toward PKS 1830−211, the only high-redshift object in which methanol has been detected. Our goals are to constrain the excitation of the methanol lines and to investigate the cosmological invariance of μ based on their relative kinematics. Methods. We observed 14 transitions of methanol, five of the A-form and nine of the E-form, and three transitions of A-13CH3OH, with ALMA. We analyzed the line profiles with a Gaussian fitting and constructed a global line profile that is able to match all observations after allowing for variations of the source covering factor, line opacity scaling, and relative bulk velocity offsets. We explore methanol excitation by running the non local thermal equilibrium radiative transfer code RADEX on a grid of kinetic temperatures and H2 volume densities. Results. Methanol absorption is detected in only one of the two lines of sight (the southwest) to PKS 1830−211. There, the excitation analysis points to a cool (∼10 − 20 K) and dense (∼104 − 5 cm−3) methanol gas. Under these conditions, several methanol transitions become anti-inverted, with excitation temperatures below the temperature of the cosmic microwave background. In addition, we measure an abundance ratio A/E = 1.0 ± 0.1, an abundance ratio CH3OH/H2 ∼ 2 × 10−8, and a 12CH3OH/13CH3OH ratio 62 ± 3. Our analysis shows that the bulk velocities of the different transitions are primarily correlated with the observing epoch due to morphological changes in the background quasar’s emission. There is a weaker correlation between bulk velocities and the lower level energies of the transitions, which could be a signature of temperature-velocity gradients in the absorbing gas. As a result, we do not find evidence for variations of μ, and we estimate Δμ/μ=(−1.8 ± 1.2) × 10−7 at 1-σ from our multivariate linear regression. Conclusions. We set a robust upper limit |Δμ/μ| < 3.6 × 10−7 (3σ) for the invariance of μ at a look-back time of half the present age of the Universe. Our analysis highlights that systematics need to be carefully taken into account in future radio molecular absorption studies aimed at testing Δμ/μ below the 10−7 horizon.

Discovery of Hydrogen Radio Recombination Lines at z=0.89 towards PKS 1830-211

“…We assume the continuum flux is equally split between the two core components following Koopmans & de Bruyn (2005) et al ( 2021) which show this to be the case and that the core components dominate the emission at least down to 1.4 GHz. Furthermore, ALMA observations measure a NE/SW flux density ratio close to one in July 2019 (Muller et al 2021). Fig.…”

Protonated acetylene in the z = 0.89 molecular absorber toward PKS 1830-211

“…The spectra of the ortho and para lines are not exactly aligned in velocity centroids but clearly overlap in velocity. However, the system is known to have significant time variations of the absorption profiles which are due to structural changes in the background quasar (Muller et al 2006(Muller et al , 2020(Muller et al , 2021(Muller et al , 2023. Since the spectra have been taken eight years apart, it is therefore possible that the quasar illumination changed between the two epochs.…”

“…More complex molecules like CH 3 OH, HC 3 N, CH 3 NH 2 have their line barycenter offset compared to the bulk of other species (Muller et al 2011), suggesting further chemical segregation. Finally, the time variability of the absorption profiles, caused by structural changes in the quasar's morphology (Muller & Guélin 2008), has highlighted at least one velocity component that shows similarities with Galactic dark clouds, both in physical conditions and chemical composition (Muller et al 2021(Muller et al , 2023.…”

Absorption of Millimeter-band CO and CN in the Early Universe: Molecular Clouds in the Radio Galaxy B2 0902+34 at Redshift 3.4

“…Studies of molecular absorbers at low and intermediate redshifts provided insight into the raw materials that fuel early star formation and active galactic nucleus (AGN) activity (e.g., Gerin et al 1997;Wiklind & Combes 1999;Maccagni et al 2018;Wiklind et al 2018;Allison et al 2019;Combes et al 2019;Rose et al 2019;Morganti et al 2023;Rose et al 2023). In addition, they have been used to study extragalactic chemistry (e.g., Muller et al 2011Muller et al , 2014, time variability in the background radio source (e.g., Muller et al 2023), the temperature of the cosmic microwave background (CMB; e.g., Muller et al 2013;Riechers et al 2022), and potential spacetime variations of fundamental constants (e.g., Carilli et al 2000;Curran et al 2011a;Muller et al 2021). When extended to higher redshifts, such studies could provide critical new insights into physical processes that govern cosmology and galaxy evolution in the early Universe.…”