Molecular line emission in NGC 4945, imaged with ALMA

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: chenkel@mpifr-bonn.mpg.de
² Astronomy Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia
³ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
⁴ Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁵ Jodrell Bank Centre for Astrophysics, University of Manchester, Oxford Road, Manchester M13, 9PL, UK
⁶ UK ALMA Regional Centre Node, University of Manchester, Oxford Road, Manchester M13, 9PL, UK
⁷ SARAO, SKA South Africa, The Park, Park Road, Pinelands 7405, South Africa
⁸ Rhodes University, RARG, RATT, PO Box 94, Grahamstown 6140, South Africa
⁹ Purple Mountain Observatory & Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, 210008 Nanjing, PR China
¹⁰ Department of Physics and Astronomy, UCL, Gower St., London, WC1E 6BT, UK
¹¹ Dept. of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 43992 Onsala, Sweden
¹² LERMA, Observatoire de Paris, College de France, CNRS, PSL Univ., UPMC, Sorbonne Univ., Paris, France
¹³ Observatorio de Madrid, OAN-IGN, Alfonso XII, 3, 28014 Madrid, Spain
¹⁴ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
¹⁵ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹⁶ European Southern Observatory, Alonso de Córdova 3107, Vitacura Casilla 763 0355, Santiago, Chile
¹⁷ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura Casilla 763 0355, Santiago, Chile
¹⁸ National Radio Astronomy Observatory, PO Box O, 1003, Lopezville Road, Socorro, NM 87801-0387, USA
¹⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁰ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
²¹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
²² Netherlands Institue for Radioastronomy ASTRON, 7991 PD Dwingeloo, The Netherlands
²³ Dept. of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
²⁴ Universität Heidelberg, Zentrum für Astronomie, Inst. für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
²⁵ Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 830011 Urumqi, PR China
²⁶ Center for Astrophysics, Guangzhou University, Guangzhou 510006, PR China
²⁷ Universität Heidelberg, Interdisziplinäres Zentrum für Wissens. Rechnen, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany

Received: 25 October 2017
Accepted: 20 February 2018

Abstract

NGC 4945 is one of the nearest (D ≈ 3.8 Mpc; 1″ ≈ 19 pc) starburst galaxies. To investigate the structure, dynamics, and composition of the dense nuclear gas of this galaxy, ALMA band 3 (λ ≈ 3−4 mm) observations were carried out with ≈2″ resolution. Three HCN and two HCO⁺ isotopologues, CS, C₃H₂, SiO, HCO, and CH₃C₂H were measured. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 10″ × 2″ reaching out to a galactocentric radius of r ≈ 100 pc with position angle PA = 45° ± 2°, inclination i = 75° ± 2° and an unresolved bright central core of size ≲2″. The continuum source, representing mostly free-free radiation from star forming regions, is more compact than the nuclear disk by a linear factor of two but shows the same position angle and is centered 0.′′39 ± 0.′′14 northeast of the nuclear accretion disk defined by H₂O maser emission. Near the systemic velocity but outside the nuclear disk, both HCN J = 1 → 0 and CS J = 2 → 1 delineate molecular arms of length ≳15″ (≳285 pc) on opposite sides of the dynamical center. These are connected by a (deprojected) ≈ 0.6 kpc sized molecular bridge, likely a dense gaseous bar seen almost ends-on, shifting gas from the front and back side into the nuclear disk. Modeling this nuclear disk located farther inside (r ≲100 pc) with tilted rings provides a good fit by inferring a coplanar outflow reaching a characteristic deprojected velocity of ≈50 km s⁻¹. All our molecular lines, with the notable exception of CH₃ C₂H, show significant absorption near the systemic velocity (≈571 km s⁻¹), within the range ≈500–660 km s⁻¹. Apparently, only molecular transitions with low critical H₂ density (n_crit ≲ 10⁴ cm⁻³) do not show absorption. The velocity field of the nuclear disk, derived from CH₃ C₂H, provides evidence for rigid rotation in the inner few arcseconds and a dynamical mass of M_tot = (2.1 ± 0.2) × 10⁸ M_⊙ inside a galactocentric radius of 2.′′45 (≈45 pc), with a significantly flattened rotation curve farther out. Velocity integrated line intensity maps with most pronounced absorption show molecular peak positions up to ≈1.′′5 (≈30 pc) southwest of the continuum peak, presumably due to absorption, which appears to be most severe slightly northeast of the nuclear maser disk. A nitrogen isotope ratio of ¹⁴N/¹⁵N ≈ 200–450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, because ¹⁵N is less abundant than expected, the question for strong ¹⁵N enrichment by massive star ejecta in starbursts still remains to be settled.