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.
Radio lines: ISM
Galaxies: structure
Galaxies: individual: NGC4945
Nuclear reactions
Abundances
Galaxies: ISM
Nucleosynthesis
Galaxies: starburst
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
Square Kilometre Array, South Africa
Rhodes University
University of Bonn
Y. Gong
Chinese Academy of Sciences
Max Planck Society
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
European Southern Observatory Santiago
Atacama Large Millimeter-submillimeter Array (ALMA)
S. Muller
Chalmers, Space, Earth and Environment, Onsala Space Observatory
J. Ott
National Radio Astronomy Observatory
P. van der Werf
National Radio Astronomy Observatory
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
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
Chinese Academy of Sciences
Max Planck Society
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 A155Subject Categories (SSIF 2011)
Astronomy, Astrophysics and Cosmology
Atom and Molecular Physics and Optics
Theoretical Chemistry
DOI
10.1051/0004-6361/201732174