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

TL;DR

This study presents a deep Q-band spectral survey (32–50 GHz) of diffuse interstellar gas toward BL Lac using the Yebes 40 m telescope, achieving unprecedented continuum sensitivity and enabling detection of very faint absorption features. It confirms previous detections of several small molecules and reports new detections of CCS, C_4H, CH_3CHO, H_2CCO, H_2CS, expanding the chemical inventory of diffuse clouds to include complex organics with up to four heavy atoms. Column densities and H_2–relative abundances (≈10^{-11}–10^{-10}) show ketene and acetaldehyde at levels comparable to UV-illuminated dense PDRs, implying similar gas-phase chemical processes in diffuse gas. The results highlight a rich, diverse chemistry in diffuse lines of sight and suggest connections between small gas-phase species, PAHs, and fullerenes, motivating further Q-band surveys and potential K-band extensions to detect longer carbon chains. Overall, the work demonstrates the potential of deep Q-band observations to reveal complex organic chemistry in the diffuse ISM and to constrain chemical networks under low-density, UV-illuminated conditions.

Abstract

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. 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 40m telescope. Yebes observations were performed interleaving Frequency Switching and Position Switching integrations toward BL Lac, using a spectral resolution of 38 kHz. The data have been reduced with the CLASS software. We achieved an unprecedented sensitivity on the continuum of 0.02 - 0.07 %, allowing 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. 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.

Figures (5)

• Figure 1: Absorption lines detected toward BL Lac in the Q band. All spectra are normalized by the continuum and shifted vertically for clarity. The fainter lines are scaled as indicated. For comparison, we also show the absorption lines of HCO$^+$ ($J=1-0$) Lucas:1996, HCN ($J=1-0$) Liszt:2001, and c-C$_3$H$_2$ ($J_{K_1K_2} = 1_{1,0}-1_{0,1}$) Liszt:2012 in gray with their scaling factor.
• Figure 2: Summary of abundances of detected species in diffuse and translucent clouds as listed in Table \ref{['tab:abun']}.
• Figure 3: Variation of the continuum flux in H (red) and V (blue) polarization with time in 2024 for each spectral window. The central frequency of each spectral window in MHz is indicated in each panel. The gain varies from 3.7 Jy/K (at 32 GHz) to 4.7 Jy/K (at 48 GHz) (see https://rt40m.oan.es/).
• Figure 4: Variation of the polarization fraction $p$ with observed antenna temperature. The polarization fraction is defined as $p = \frac{T_V-T_H}{T_V+T_H}$ where $T_V$ and $T_H$ are the continuum temperatures in the V and H polarizations, respectively.
• Figure 5: Variation of the spectral index of the continuum antenna temperature with the antenna temperature measured in the spectral window at the low end of the spectral range, 33.2 GHz.