Physical origin of the western HCO-plus-to-CO enhancement

Determine which physical process is responsible for the localized HCO$^{+}$(3--2)/CO(2--1) enhancement at the western boundary of the circumnuclear disc, particularly whether it is caused by shock processing at the interface between infalling molecular filaments and the rotating disc.

Background

The paper identifies a secondary peak in the HCO+^{+}/CO ratio in the western sector of the circumnuclear disc, coinciding spatially with the interface where molecular filaments accrete onto the rotating disc. The authors discuss photodissociation-region chemistry and shock processing as possible explanations, with shocks potentially enhancing HCO+^{+} production and excitation while suppressing CO emission.

The shock interpretation remains unresolved because the CO emission is faint in the western region and the available 72 pc-resolution data do not resolve a definitive kinematic shock signature. Higher-resolution observations and targeted shock tracers are therefore needed to distinguish among the proposed mechanisms.

References

The localised nature of this enhancement, confined to the western boundary and absent in the other sectors, raises the question of what physical process is responsible.

— Spatially resolved molecular gas conditions in the circumnuclear disc of 3C 84  (2608.24678 - Jia et al., 25 Aug 2026) in Section 4.2, “Spatial variations of the HCO$^+/CO$ ratio and possible physical origins”