Molecular gas in the northern nucleus of Mrk 273: Physical and chemical properties of the disc and its outflow

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: aladro@mpifr-bonn.mpg.de
² Chalmers University of Technology, Department of Space, Earth and Environment, Onsala Space Observatory, 43992 Onsala, Sweden
³ Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura 763 0355 Santiago, Chile
⁵ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura 763 0355 Santiago, Chile
⁶ Observatorio de Madrid, OAN-IGN, Alfonso XII, 3, 28014 Madrid, Spain
⁷ Astron. Dept., King Abdulaziz University, PO Box 80203 21589 Jeddah, Saudi Arabia
⁸ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁹ San Jose State University, 1 Washington Square, San Jose, CA, 95192 USA
¹⁰ George Mason University, Department of Physics & Astronomy, MS 3F3, 4400 University Drive, Fairfax, VA, 22030 USA
¹¹ Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, Dom. Univ., 38406 St. Martin d’Hères, France

Received: 2 May 2018
Accepted: 25 May 2018

Abstract

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 ∼86 GHz and ∼256 GHz with angular resolutions of 4ʺ̣9 × 4ʺ̣5 (∼3.7 × 3.4 kpc) and 0ʺ̣61 × 0ʺ̣55 (∼460 × 420 pc). 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 ∼ 1.5 kpc) rotates with a south-east to north-west direction, while in the inner disc (R ∼ 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) × 10⁹ M_⊙, a luminosity of L′_HCN = (3–4) × 10⁸ K km s⁻¹ pc², and a dust temperature of 55 K. At the very centre, a compact core with R ∼ 50 pc has a luminosity of L_IR = 4 × 10¹¹ L_⊙ (30% of the total infrared luminosity), and a dust temperature of 95 K. The core is expanding at low velocities ∼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 ∼1000 km s⁻¹, while the warm outflowing gas has more moderate maximum velocities of ∼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 ≤8 × 10⁸ M_⊙. 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.