2024A&A...690L..18S

We present the first observations of HCO⁺(1-0) and HCN(1-0) emission in the northern filaments of Centaurus A with ALMA. HCO⁺(1-0) is detected in nine clumps of the Horseshoe complex, with similar velocities as the CO(1-0) emission. Conversely, HCN(1-0) is not detected, and we derive upper limits on the flux. At a resolution of ∼40 pc, the line ratio of the velocity-integrated intensities I_HCO+/I_CO varies between 0.03 and 0.08, while I_HCO+/I_HCN is higher than unity, with an average lower limit of 1.51. These ratios are significantly higher than what is observed in nearby star-forming galaxies. Moreover, the ratio I_HCO+/I_CO decreases with increasing CO-integrated intensity, contrary to what is observed in the star-forming galaxies. This indicates that the HCO⁺ emission is enhanced and may not arise from dense gas within the Horseshoe complex. This hypothesis is strengthened by the average line ratio I_HCN/I_CO < 0.03, which suggests that the gas density is rather low. Using non-local thermal equilibrium, large velocity gradient modelling with RADEX, we explored two possible phases of the gas, which we call 'diffuse' and 'dense' and are characterised by a significant difference in the HCO⁺ abundance relative to CO, respectively N_HCO+/N_CO = 10^–3 and N_HCO+/N_CO = 3 × 10^–5. The average CO(1-0) and HCO⁺(1-0) integrated intensities and the upper limit on HCN(1-0) are compatible with both diffuse (n_H = 10³ cm^–3, T_kin = 15 - 165 K) and dense gas (n_H = 10⁴ cm^–3, T_kin > 65 K). The spectral setup of the present observations also covers SiO(2-1). While undetected, the upper limit on SiO(2-1) is not compatible with the RADEX predictions for the dense gas. We conclude that the nine molecular clouds detected in HCO⁺(1-0) are likely dominated by diffuse molecular gas. While the exact origin of the HCO⁺(1-0) emission remains to be investigated, it is likely related to the energy injection within the molecular gas that prevents gravitational collapse and star formation.