CO Emission and Absorption-line Survey of the M87 Nucleus Using Archival ALMA Imaging

Abstract We present an M87 molecular line search from archival Atacama Large Millimeter/submillimeter Array imaging, covering the circumnuclear disk (CND) as well as ionized gas filaments and dusty cloud regions. We find no evidence for CO emission in the central ∼kiloparsec and place an upper limit of M H 2 2.3 × 1 0 5 M ⊙ in the atomic gas CND region, a factor of 20× lower than previous surveys. During this search, we discovered extragalactic CO absorption lines in the J = 1−0, 2−1, and 3−2 transitions against the bright (jansky-scale) active nucleus. These CO lines are narrow (∼5 km s −1 ) and blueshifted with respect to the galaxy’s systemic velocity by −75 to −84 km s −1 . This CO absorber appears to be kinematically distinct from outflowing atomic gas seen in absorption. Low integrated opacities ranging from τ CO ∼ 0.02−0.06 km s −1 and a column density N CO ≈ (1.2 ± 0.2) × 10 15 cm −2 translate to N H 2 ∼ ( 1 − 2 ) × 1 0 20 cm −2 . CO excitation temperatures spanning T ex ∼ 8–30 K do not follow local thermodynamic equilibrium (LTE) expectations, and non-LTE radex radiative transfer modeling of the CO absorber is consistent with a number density n H 2 ∼ 5000 cm −3 embedded in a ∼60 K environment. Taken together, the observed CO absorption lines are most consistent with a thin, pressure-confined filament seen slightly off-center from the M87 nucleus. We also explore the impact of residual telluric lines and atmospheric variability on narrow extragalactic line identification and demonstrate how bandpass calibration limitations may introduce broad but very low signal-to-noise ratio and spurious absorption and emission signatures.