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Article Dans Une Revue Astronomy and Astrophysics - A&A Année : 2018

Molecular line emission in NGC 4945, imaged with ALMA

C. Henkel
  • Fonction : Auteur
S. Mühle
  • Fonction : Auteur
Y. Gong
  • Fonction : Auteur
S. Viti
  • Fonction : Auteur
L. Hunt
  • Fonction : Auteur
S. Muller
  • Fonction : Auteur
J. Ott
  • Fonction : Auteur
A. Malawi
  • Fonction : Auteur
H. Ismail
  • Fonction : Auteur
E. Alkhuja
  • Fonction : Auteur
H. Asiri
  • Fonction : Auteur
F. Alves
  • Fonction : Auteur
Y. Ao
  • Fonction : Auteur
W. Baan
  • Fonction : Auteur
J. Greene
  • Fonction : Auteur
C. Impellizzeri
  • Fonction : Auteur
F. Kamali
  • Fonction : Auteur
R. Klessen
R. Mauersberger
  • Fonction : Auteur
X. Tang
  • Fonction : Auteur
K. Tristram
  • Fonction : Auteur
M. Wang
  • Fonction : Auteur
J. Zhang
  • Fonction : Auteur

Résumé

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 (ncrit ≲ 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 Mtot = (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.
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Dates et versions

hal-02320489 , version 1 (26-04-2023)

Identifiants

Citer

C. Henkel, S. Mühle, G. Bendo, G. I G Jozsa, Y. Gong, et al.. Molecular line emission in NGC 4945, imaged with ALMA. Astronomy and Astrophysics - A&A, 2018, 615, pp.A155. ⟨10.1051/0004-6361/201732174⟩. ⟨hal-02320489⟩
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