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Surveying Radio Line Spectra of Nearby Galaxies

Abstract

Centimeter wavelengths hold promise for astrochemistry, but remain understudied. Spectra of astronomical objects at millimeter and submillimeter wavelengths are approaching the limit of line saturation, decreasing the yield of any future instruments at these higher frequencies. In addition, the rotational transitions of complex molecules tend to be located in the centimeter wavelengths, and represent better reservoirs of information than those of simpler molecules due to having more degrees of freedom. The upcoming Next-Generation Very Large Array (ngVLA) will represent a 10x increase from the sensitivity of current instruments, potentially allowing extragalactic observations of these complex molecules. We sample the 4cm- or 15mm-band of nine nearby star-forming galaxies (NGC 253, NGC 1068, NGC 1266, NGC 1365, NGC 1808, NGC 3256, NGC 4945, M83, and Circinus) using the Australia Telescope Compact Array in the H75 (4cm beam ≈140″, 15mm beam ≈40″) configuration. Our velocity resolution ranges from ≈13 km/s at the low-frequency end to ≈2.5 km/s at the high-frequency end (resulting 4cm RMS ≈ 1 mJy/bm and 15mm RMS ≈4 mJy/bm). The galaxies in our sample display transitions from radio recombination lines (RRLs) and five different molecular species to thresholds of ≈1 mJy: H2CO, H2O, OH, NH3, and c-C3H2. RRLs dominate the 4cm-band, while the 15mm-band has a higher density of molecular lines. Notably, 2Π3/2 OH is detected in the highly-excited J=9/2 state in NGC 4945 and Circinus, only the third and fourth extragalactic detections of this state, while a potential NH3 (3,3) maser is detected in NGC 4945. The ngVLA will be able to probe a 150 times as many galaxies at the same S/N ratio as our sample. A larger variety of galaxies will similarly be available for study, and complex molecules should be observable in the closest galaxies.

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