Molecular line emission in NGC 4945, imaged with ALMA
Journal article, 2018

NGC 4945 is one of the nearest (D ≈ 3.8 Mpc; 1 00 ≈ 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 00 resolution. Three HCN and two HC + isotopologues, CS, C 3 H 2 , SiO, HCO, and CH 3 C 2 H were measured. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 10 00 × 2 00 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 00 . 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 00 . 39 ± 0 00 . 14 northeast of the nuclear accretion disk defined by H 2 O 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 00 ( >∼ 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 CH 3 C 2 H, 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 H 2 density (n crit<∼ 10 4 cm −3 ) do not show absorption. The velocity field of the nuclear disk, derived from CH 3 C 2 H, provides evidence for rigid rotation in the inner few arcseconds and a dynamical mass of M tot = (2.1 ± 0.2) × 10 8 M inside a galactocentric radius of 2 00 . 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 00 . 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 14 N/ 15 N ≈ 200-450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, because 15 N is less abundant than expected, the question for strong 15 N enrichment by massive star ejecta in starbursts still remains to be settled.

Galaxies: individual: NGC4945

Nuclear reactions

Galaxies: starburst

Nucleosynthesis

Galaxies: structure

Radio lines: ISM

Galaxies: ISM

Abundances

Author

C. Henkel

Chinese Academy of Sciences

King Abdulaziz University

Max Planck Society

S. Muhle

University of Bonn

G. J. Bendo

University of Manchester

G. I.G. Józsa

Rhodes University

University of Bonn

Square Kilometre Array, South Africa

Y. Gong

Max Planck Society

Chinese Academy of Sciences

Serena Viti

University College London (UCL)

Susanne Aalto

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics

F. Combes

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

S. G. Burillo

Spanish National Observatory (OAN)

L. K. Hunt

Arcetri Astrophysical Observatory

J. G. Mangum

National Radio Astronomy Observatory

S. Martin

Atacama Large Millimeter-submillimeter Array (ALMA)

European Southern Observatory Santiago

S. Muller

Chalmers, Space, Earth and Environment, Onsala Space Observatory

J. Ott

National Radio Astronomy Observatory Socorro

P. van der Werf

National Radio Astronomy Observatory Socorro

A. A. Malawi

King Abdulaziz University

H. Ismail

King Abdulaziz University

E. Alkhuja

King Abdulaziz University

H. Asiri

King Abdulaziz University

Rebeca Aladro

Max Planck Society

F. O. Alves

University of Bonn

Max Planck Society

Y. Ao

National Astronomical Observatory of Japan

Willem A. Baan

Netherlands Institute for Radio Astronomy (ASTRON)

Francesco Costagliola

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Extragalactic Astrophysics

G. A. Fuller

University of Manchester

J. Greene

Princeton University

C. M. V. Impellizzeri

European Southern Observatory Santiago

F. Kamali

Max Planck Society

R. S. Klessen

Heidelberg University

R. Mauersberger

Max Planck Society

X. D. Tang

Max Planck Society

Chinese Academy of Sciences

K. Tristram

European Southern Observatory Santiago

M. Wang

Chinese Academy of Sciences

J. S. Zhang

Guangzhou University

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 615 A155

Subject Categories

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics

Theoretical Chemistry

DOI

10.1051/0004-6361/201732174

More information

Latest update

9/10/2019