The inhomogeneous ISM toward PKS 1830–211 SW: A detailed view of molecular gas at a look-back time of 7.5 Gyr

Abstract

Based on measurements with the Effelsberg 100-m telescope, a multi-line study of molecular species is presented toward the south-western source of the gravitational lens system PKS 1830-211, which is by far the best known target to study molecular gas in absorption at intermediate redshift. Determining line parameters and optical depths and performing Large Velocity Gradient radiative transfer calculations, the aims of this study are (1) to evaluate physical parameters of the absorbing foreground gas at z ∼ 0.89, in particular its homogeneity, and (2) to monitor the spectroscopic time variability caused by fluctuations of the z ∼ 2.5 background continuum source. We find, that the gas is quite inhomogeneous with n(H2) ∼ 2 × 10 3 cm −3 for most molecular species but with higher values for H2CO and lower ones for SO. Measuring the CS J=1←0 transition during a time interval of more than a decade, from 2001 to 2012, the peak absorption depth of the line remains approximately constant, while the line shape undergoes notable variations. Covering the time between 1996 and 2013, CS, HCO + , and CH3OH data indicate maximal integrated optical depths in ∼2001 and 2011/2012. This is compatible with a ∼10 yr periodicity, which, however, needs confirmation by substantially longer time monitoring. Comparing molecular abundances with those of different types of Galactic and nearby extragalactic clouds we find that the observed cloud complex does not correspond to one particular type but to a variety of cloud types with more diffuse and denser components as can be expected for an observed region with a transverse linear scale of several parsec and a likely larger depth along the line-of-sight. A tentative detection of Galactic absorption in the c-C3H2 110 − 101 line at 18.343 GHz is also reported.

An ALMA Early Science survey of molecular absorption lines toward PKS 1830−211

“…The continuum changes are potentially significantly magnified close to the lens caustics. Past observations have shown significant variations of the absorption intensity of the different velocity components (Muller & Guélin 2008;Muller et al 2011Muller et al , 2013Schultz et al 2014) but no shifts in velocity, or only small ones. The small variations of ∼1 km s −1 in the centroid of methanol lines reported by Bagdonaite et al (2013b) might be due, as they note, to the simplifying assumption of only one Gaussian component in the fit of the profile.…”

“…In short, the absorption depths toward both the NE and SW images can change by a factor of at least two within a timescale of a year and may not be as predictible as suggested by the long-term trends in monitoring so far (Muller & Guélin 2008;Schultz et al 2014). Clearly, a regular monitoring with a timescale on the order of one month would be desirable and could trigger dedicated studies (e.g., search of new species, high signal-to-noise-ratio observations) when the absorption depths increases significantly.…”

“…Recently, variations for other species such as CH 3 OH and CS have also been reported (Bagdonaite et al 2013b;Schultz et al 2014). The origin of these long-term variations is thought to be due to changes in the continuum emission (Muller & Guélin 2008) possibly associated with a precessing jet (Nair et al 2005), although evidence of a periodicity of the absorption changes remains to be seen.…”

“…So far, the variability has only been monitored in the lines of HCO + (and to a lesser extend of HCN, Muller & Guélin 2008) and CS (Schultz et al 2014). The time variations have the potential to reveal sub-parsec scale structures in the absorbing gas and the chemical correlation between different molecular species.…”

Detection of extragalactic CF+toward PKS 1830−211

“…As we already mentioned, the line crowding toward PKS 1830−211 is low and we could not find any other possible alternative identification, except 13 CN N = 1−0 at z = 0, but we would expect the line to be: 1) very narrow (a few km s −1 ; see the Galactic absorption of c-C 3 H 2 toward PKS 1830−211 by Schulz et al 2015) and decomposed into its characteristic spin-rotation and hyperfine pattern; and 2) very weak, as even the HCO + /HCN/C 2 H J = 1−0 Galactic absorptions toward PKS 1830−211 are no more than a few percent deep (Muller et al, in prep).…”