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 HC _ { 3 } N at \sim 86 GHz and \sim 256 GHz with angular resolutions of 4 . ^ { \prime \prime } 9 \times 4 . ^ { \prime \prime } 5 ( \sim 3.7 \times 3.4 kpc ) and 0 . ^ { \prime \prime } 61 \times 0 . ^ { \prime \prime } 55 ( \sim 460 \times 420 pc ) .
We also modelled the flux of several H _ { 2 } O 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 \sim 1.5 kpc ) rotates with a south-east to north-west direction , while in the inner disc ( R \sim 300 pc ) follows a north-east to south-west rotation .
The central 300 pc , which hosts a compact starburst region , is filled with dense and warm gas , and contains a dynamical mass of ( 4 - 5 ) \times 10 ^ { 9 } M _ { \odot } , a luminosity of L ^ { \prime } _ { HCN } = ( 3 - 4 ) \times 10 ^ { 8 } K km s ^ { -1 } pc ^ { 2 } , and a dust temperature of 55 K. At the very centre , a compact core with R \sim 50 pc has a luminosity of L _ { IR } = 4 \times 10 ^ { 11 } L _ { \odot } ( 30 % of the total infrared luminosity ) , and a dust temperature of 95 K .
The core is expanding at low velocities \sim 50-100 km s ^ { -1 } , 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 \sim 1000 km s ^ { -1 } , while the warm outflowing gas has more moderate maximum velocities of \sim 600 km s ^ { -1 } .
The outflow is compact , being detected as far as 460 pc from the centre in the northern direction , and has a mass of dense gas \leq 8 \times 10 ^ { 8 } M _ { \odot } .
The difference between the position angles of the inner disc ( \sim 70 \degree ) and the outflow ( \sim 10 \degree ) 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 \pm 5 in the inner disc of Mrk 273 .