2018A&A...617A..20A

Aiming to characterise the properties of the molecular gas in the ultra-luminous infrared galaxy Mrk 273 and its outflow, we used the NOEMA interferometer to image the dense-gas molecular tracers HCN, HCO⁺, HNC, HOC⁺ and HC₃N at ∼86GHz and ∼256GHz with angular resolutions of 4.9"x4.5" (∼3.7x3.4kpc) and 0.61"x0.55" (∼460x420pc). We also modelled the flux of several H₂O lines observed with Herschel using a radiative transfer code that includes excitation by collisions and far-infrared photons. The disc of the Mrk 273 north nucleus has two components with decoupled kinematics. The gas in the outer parts (R∼1.5kpc) rotates with a south-east to north-west direction, while in the inner disc (R∼300pc) follows a north-east to south-west rotation. The central 300pc, which hosts a compact starburst region, is filled with dense and warm gas, and contains a dynamical mass of (4-5)x10⁹M_☉, a luminosity of L'_HCN=(3-4)x10⁸K.(km/s).pc², and a dust temperature of 55K. At the very centre, a compact core with R∼50pc has a luminosity of L_IR=4x10¹¹L_☉ (30% of the total infrared luminosity), and a dust temperature of 95K. The core is expanding at low velocities ∼50-100km/s, probably affected by the outflowing gas. We detect the blue-shifted component of the outflow, while the red-shifted counterpart remains undetected in our data. Its cold and dense phase reaches fast velocities up to ∼1000km/s, while the warm outflowing gas has more moderate maximum velocities of ∼600km/s. The outflow is compact, being detected as far as 460pc from the centre in the northern direction, and has a mass of dense gas ≥8x10⁸M_☉. The difference between the position angles of the inner disc (∼70°) and the outflow (∼10°) indicates that the outflow is likely powered by the AGN, and not by the starburst. Regarding the chemistry in Mrk 273, we measure an extremely low HCO⁺/HOC⁺ ratio of 10±5 in the inner disc of Mrk 273.