PKS 1413+135: OH and H I at z = 0.247 with MeerKAT^⋆

¹ Observatoire de Paris, LERMA, Collège de France, CNRS, PSL University, Sorbonne University, 75014 Paris, France
e-mail: francoise.combes@obspm.fr
² Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India
³ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁴ Ioffe Institute, Polyteknicheskaya 26, 194021 Saint-Petersburg, and HSE University, Saint-Petersburg, Russia
⁵ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Astrophysics Research Centre, University of KwaZulu-Natal, Durban 4041, South Africa
⁸ School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal, Durban 4041, South Africa
⁹ National Radio Astronomy Observatory, Socorro, NM 87801, USA
¹⁰ Institut d’Astrophysique de Paris, CNRS-SU, UMR 7095, 98bis bd Arago, 75014 Paris, France
¹¹ French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and U. de Chile, Casilla 36-D, Santiago, Chile
¹² Green Bank Observatory, PO Box 2 Green Bank, WV 24944, USA

Received: 16 November 2022
Accepted: 8 December 2022

Abstract

The BL Lac object PKS 1413+135 was observed by the Large Survey Project MeerKAT Absorption Line Survey (MALS) in the L-band, at 1139 MHz and 1293–1379 MHz, targeting the H I and OH lines in absorption at z = 0.24671. The radio continuum might come from the nucleus of the absorbing galaxy or from a background object at redshift lower than 0.5, as suggested by the absence of gravitational images. The H I absorption line is detected at a high signal-to-noise ratio, with a narrow central component, and with a red wing, confirming previous results. The OH 1720 MHz line is clearly detected in (maser) emission, peaking at a velocity shifted by −10 to −15 km s⁻¹ with respect to the H I peak. The 1612 MHz line is lost due to radio frequency interference. The OH 1667 MHz main line is tentatively detected in absorption, but not the 1665 MHz line. Over 30 years a high variability is observed in optical depths, due to the rapid changes of the line of sight caused by the superluminal motions of the radio knots. The H I line has varied by 20% in depth, while the OH-1720 MHz depth has varied by a factor of ∼3. The position of the central velocity and the widths also varied. The absorbing galaxy is an early-type spiral (maybe S0) seen edge-on, with a prominent dust lane, covering the whole disk. Given the measured mass concentration and the radio continuum size at centimeter wavelengths (100 mas corresponding to 400 pc at z = 0.25), the width of the absorption lines from the nuclear regions are expected up to 250 km s⁻¹. The narrowness of the observed lines (< 15 km s⁻¹) suggests that the absorption comes from an outer gas ring, as frequently observed in S0 galaxies. The millimetric lines are even narrower (< 1 km s⁻¹), which corresponds to the continuum size restricted to the core. The radio core is covered by individual 1 pc molecular clouds, whose column density is a few 10²² cm⁻², which is compatible with the gas screen detected in X-rays.