2025ApJ...990...66K

We analyzed the archival Atacama Large Millimeter/submillimeter Array data of the nuclear region of M87 and evaluated the molecular gas content from the CO(2-1) absorption line. We found an enigmatic variability in the absorption line depth between two epochs separated by only two months. We reexamined the data set used in the analysis and found that the bandpass calibration source within the same data set also revealed a similar absorption line structure. Furthermore, we observed a rise in the system noise temperature spectrum. We concluded that the absorption line structure identified in a previous study, and attributed to CO(2-1), does not originate from M87 but instead results from telluric contamination, and that we still have only the upper limit on the molecular gas around the nucleus of M87.