3 mm spectral line survey of two lines of sight toward two typical cloud complexes in the Galactic Center

TL;DR

This study presents detailed spectral line surveys of two lines of sight in the Galactic Center, detecting numerous molecules and isotopologues, and compares their chemical conditions to other galactic environments, revealing similar excitation conditions.

Contribution

First detection of HCO and HOC+ in LOS+0.693, and a comprehensive comparison of molecular abundances with PDRs and starburst galaxies, highlighting potential chemical tracers.

Findings

Isotopic ratios are consistent with canonical values, indicating negligible fractionation.

Molecular abundances and temperatures are similar across the two lines of sight.

The CS/HOC+ ratio may serve as a tracer for PDR components in galactic nuclei.

Abstract

We present the results of two Mopra 3-mm spectral line surveys of the Lines of Sight (LOS) toward the Galactic Center (GC) molecular complexes Sgr B2 (LOS+0.693) and Sgr A (LOS-0.11). The spectra covered the frequency ranges of ~77-93 GHz and ~105-113 GHz. We have detected 38 molecular species and 25 isotopologues. The isotopic ratios derived from column density ratios are consistent with the canonical values, indicating that chemical isotopic fractionation and/or selective photodissociation can be considered negligible (<10%) for the GC physical conditions. The derived abundance and rotational temperatures are very similar for both LOSs, indicating very similar chemical and excitation conditions for the molecular gas in the GC. The excitation conditions are also very similar to those found for the nucleus of the starburst galaxy NGC 253. We report for the first time the detection of HCO and HOC+ emission in LOS+0.693. Our comparison of the abundance ratios between CS, HCO, HOC+ and HCO+ found in the two LOSs with those in typical Galactic photodissociation regions (PDRs) and starbursts galaxies does not show any clear trend to distinguish between UV and X-ray induced chemistries. We propose that the CS/HOC+ ratio could be used as a tracer of the PDR components in the molecular clouds in the nuclei of galaxies.