Abstract
The Large Survey Project (LSP) “MeerKAT Absorption Line Survey” (MALS) is a blind H I 21 cm and OH 18 cm absorption line survey in the L- and UHF-bands, primarily designed to better determine the occurrence of atomic and molecular gas in the circumgalactic and intergalactic medium, and its redshift evolution. Here we present the first results using the UHF band obtained towards the strongly lensed radio source PKS 1830−211, revealing the detection of absorption produced by the lensing galaxy. With merely 90 min of data acquired on-source for science verification and processed using the Automated Radio Telescope Imaging Pipeline (ARTIP), we detect in absorption the known H I 21 cm and OH 18 cm main lines at z = 0.89 at an unprecedented signal-to-noise ratio (4000 in the continuum, in each 6 km s−1 wide channel). For the first time we report the detection of OH satellite lines at z = 0.89, which until now have not been detected at z > 0.25. We decompose the OH lines into a thermal and a stimulated contribution, where the 1612 and 1720 MHz lines are conjugate. The total OH 1720 MHz emission line luminosity is 6100 L⊙. This is the most luminous known 1720 MHz maser line and is also among the most luminous of the OH main line megamasers. The absorption components of the different images of the background source sample different light paths in the lensing galaxy, and their weights in the total absorption spectrum are expected to vary in time on daily and monthly time scales. We compare our normalized spectra with those obtained more than 20 years ago, and find no variation. We interpret the absorption spectra with the help of a lens galaxy model derived from an N-body hydrodynamical simulation, with a morphology similar to its optical HST image. The resulting absorption lines depend mainly on the background continuum and the radial distribution of the gas surface density for each atomic and molecular species. We show that it is possible to reproduce the observations assuming a realistic spiral galaxy disk without invoking any central gas outflows. However, there are distinct and faint high-velocity features in the ALMA millimeter absorption spectra that most likely originate from high-velocity clouds or tidal features. These clouds may contribute to broaden the H I and OH spectra.
Highlights
PKS 1830−211 is a highly reddened radio quasar at z = 2.51 (Lidman et al 1999), and the brightest known radio lens in the sky
We zoom in on these in the top two panels of Fig. 2. These lines were detected in the overlapping L-band (900−1670 MHz) spectrum obtained with MeerKAT on December 19, 2019 (Gupta et al 2021)
The MeerKAT Absorption Line Survey (MALS) Large Survey Project (LSP) project on the MeerKAT array has carried out its first science verification observations in the UHF-band, covering the 580 to 1015 MHz frequency range
Summary
Throughout the present paper, the velocity scale is defined with respect to z = 0.88582 (heliocentric reference frame), which corresponds to the main molecular (e.g., CO, HCO+, and HCN) absorption components detected at millimeter (mm) wavelengths (Wiklind & Combes 1998). Compared to the mm absorptions, the broader line widths of ∼280 km s−1 and ∼150 km s−1 (FWHM) for H i and OH cm-absorption, respectively, at z = 0.89 are explained by the greater thickness of the H i or OH plane, and to some extent by the more extended background radio continuum at lower frequencies Such a strong lens system should be useful to determine the Hubble constant through the cosmography method or time-delay between the images. At the distance of the PKS 1830−211 absorber, 1 arcsec corresponds to 7.8 kpc in physical units
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