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/sub-millimeter Array (ALMA) 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 $\sim$kpc and place an upper limit of $M_\mathrm{H_2} < 2.3\times 10^5$ $M_\odot$ in the atomic gas CND region, a factor of 20$\times$ lower than previous surveys. During this search, we discovered extragalactic CO absorption lines in the $J$ = 1$-$0, 2$-$1, and 3$-$2 transitions against the bright (Jy-scale) active nucleus. These CO lines are narrow ($\sim$5 km s$^{-1}$) and blueshifted with respect to the galaxy's systemic velocity by $-$75 to $-$84 \kms. This CO absorber appears to be kinematically distinct from outflowing atomic gas seen in absorption. Low integrated opacities ranging from $\tau_\mathrm{CO} \sim 0.02-0.06$ \kms\ and a column density $N_\mathrm{CO} \approx (1.2\pm0.2)\times 10^{15}$ cm$^{-2}$ translate to $N_\mathrm{H_2} \sim (1-2) \times 10^{20}$ cm$^{-2}$. CO excitation temperatures spanning $T_\mathrm{ex} \sim 8$ K to $\sim$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 $n_\mathrm{H_2} \sim 5000$ cm$^{-3}$ embedded in a $\sim$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 S/N and spurious absorption and emission signatures.