Spectral survey of the diffuse gas toward BL Lac in the Q band

¹ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 75014 Paris, France
² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
³ Observatorio Astronómico Nacional (OAN-IGN), Calle Alfonso XII, 3, 28014 Madrid, Spain
⁴ Observatorio de Yebes (OY-IGN), Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain
⁵ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain

^★ Corresponding author: maryvonne.gerin@obspm.fr.

Received: 25 June 2025
Accepted: 16 September 2025

Abstract

Context. The chemical composition of diffuse interstellar clouds is not fully established. They host an active chemistry despite their relatively low density and the ubiquitous presence of far-UV radiation.

Aims. To further explore the chemical composition of diffuse clouds, we performed a spectral scan toward the bright radio source BL Lac in the Q band (from 32 to 50 GHz) using the Yebes 40 m telescope.

Methods. Yebes observations were performed interleaving frequency switching and position switching integrations toward BL Lac, using a spectral resolution of 38 kHz. The data were reduced with the CLASS software.

Results. We achieved an unprecedented sensitivity on the continuum of 0.02–0.07%, allowing for the detection of very faint absorption features. We confirm previous detections of HCS⁺, C₃H, C₃H⁺, CH₃CN, and HC₃N in diffuse clouds and report new detections of CCS, C₄H, CH₃CHO, H₂CCO, HNCO, and H₂CS along the line of sight to BL Lac, with abundances relative to H₂ from a few 10⁻¹¹ to a few 10⁻¹⁰. We compiled molecular detections toward diffuse clouds to obtain the chemical inventory of a typical diffuse interstellar cloud.

Conclusions. The chemical inventory of diffuse interstellar clouds includes complex organic species with up to four heavy atoms. These species are efficiently formed in the diffuse interstellar gas and reach abundances similar to those measured in dense photodissociation regions, pointing to similar gas-phase chemical processes.