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19th of September 2019
Excited extragalactic OH
by Eisner et al.
Centimeter-wave transitions are important counterparts to the rotational mm-wave transitions usually observed to study gas in star-forming regions. However, given their relative weakness, these transitions have historically been neglected. Eisner et al. present Australia Telescope Compact Array 4cm- and 15mm-band spectral line observations of nine nearby star-forming galaxies. Thirteen different molecular lines are detected across the sample from OH, NH3, H2O, H2CO, and c-C3H2, as well as 18 radio recombination lines (RRLs) in NGC 253. Excited OH absorption is detected at rest-frame frequencies of 23.81762 and 23.82662 GHz towards NGC 4945 and Circinus; only the third and fourth extragalactic detections of this (J=9/2) OH transition. Circinus’s OH lines are blueshifted from the systemic velocity by ~35 km s−1 , while NGC 4945’s are redshifted by ~100 km/s, with the latter likely indicating infall onto the nucleus.

The images above show the profiles of the detected OH lines in NGC 4945 and Circinus. The blue vertical lines show the systemic velocities of the galaxies, while the shaded regions represent the approximate extents of the lines. Both hyperfine components are seen in each figure. More details are give in the paper, published in the Astrophysical Journal.

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