2019A&A...627A.147A

High-resolution (0.03"-0.09" (9-26pc)) ALMA (100-350GHz (λ3 to 0.8mm)) and (0.04" (11pc)) VLA 45 GHz measurements have been used to image continuum and spectral line emission from the inner (100pc) region of the nearby infrared luminous galaxy IC 860. We detect compact (r∼10pc), luminous, 3 to 0.8mm continuum emission in the core of IC 860, with brightness temperatures T_B>160K. The 45GHz continuum is equally compact but significantly fainter in flux. We suggest that the 3 to 0.8mm continuum emerges from hot dust with radius r∼8pc and temperature T_d∼280K, and that it is opaque at millimetre wavelengths, implying a very large H₂ column density N(H₂)≥10²⁶cm^–2. Vibrationally excited lines of HCN ν₂=1f J=4-3 and 3-2 (HCN-VIB) are seen in emission and spatially resolved on scales of 40-50pc. The line-to-continuum ratio drops towards the inner r = 4pc, resulting in a ring-like morphology. This may be due to high opacities and matching HCN-VIB excitation- and continuum temperatures. The HCN-VIB emission reveals a north-south nuclear velocity gradient with projected rotation velocities of ν=100km/s at r=10pc. The brightest emission is oriented perpendicular to the velocity gradient, with a peak HCN-VIB 3-2 T_B of 115K (above the continuum). Vibrational ground-state lines of HCN 3-2 and 4-3, HC¹⁵N 4-3, HCO⁺ 3-2 and 4-3, and CS 7-6 show complex line absorption and emission features towards the dusty nucleus. Redshifted, reversed P-Cygni profiles are seen for HCN and HCO⁺ consistent with gas inflow with ν_in≤50km/s. Foreground absorption structures outline the flow, and can be traced from the north-east into the nucleus. In contrast, CS 7-6 has blueshifted line profiles with line wings extending out to -180km/s. We suggest that a dense and slow outflow is hidden behind a foreground layer of obscuring, inflowing gas. The centre of IC 860 is in a phase of rapid evolution where an inflow is building up a massive nuclear column density of gas and dust that feeds star formation and/or AGN activity. The slow, dense outflow may be signaling the onset of feedback. The inner, r=10pc, IR luminosity may be powered by an AGN or a compact starburst, which then would likely require a top-heavy initial mass function.