Issue |
A&A
Volume 615, July 2018
|
|
---|---|---|
Article Number | A155 | |
Number of page(s) | 29 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/201732174 | |
Published online | 31 July 2018 |
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
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.
Key words: galaxies: starburst / galaxies: structure / galaxies: ISM / nuclear reactions, nucleosynthesis, abundances / galaxies: individual: NGC4945 / radio lines: ISM
© ESO 2018
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