TY - JOUR
T1 - Molecular line emission in NGC 4945, imaged with ALMA
AU - Henkel, C.
AU - Mühle, S.
AU - Bendo, G.
AU - Józsa, G. I.G.
AU - Gong, Y.
AU - Viti, S.
AU - Aalto, S.
AU - Combes, F.
AU - García-Burillo, S.
AU - Hunt, L. K.
AU - Mangum, J.
AU - Martín, S.
AU - Muller, S.
AU - Ott, J.
AU - Van Der Werf, P.
AU - Malawi, A. A.
AU - Ismail, H.
AU - Alkhuja, E.
AU - Asiri, H. M.
AU - Aladro, R.
AU - Alves, F.
AU - Ao, Y.
AU - Baan, W. A.
AU - Costagliola, F.
AU - Fuller, G.
AU - Greene, J.
AU - Impellizzeri, C. M.V.
AU - Kamali, F.
AU - Klessen, R. S.
AU - Mauersberger, R.
AU - Tang, X. D.
AU - Tristram, K.
AU - Wang, M.
AU - Zhang, J. S.
N1 - Funding Information:
Acnowledgements. We wish to thank an anonymous referee for helpful comments. This work was partially carried out within the Collaborative Research Council 956, subproject A6, funded by the Deutsche Forschungsgemeinschaft (DFG). G.I.G. Józsa was supported by the National Research Foundation of South Africa (grant number 109048). Y. Gong was supported by the National key research and development program under grant 2017YFA0402702 and the National Natural Science Foundation of China (NSFC grant nos. 11127903). S. García-Burillo appreciates economic support from Spanish grants ESP2015-68964-P and AYA2016-76682-C3-2-P. R. S. Klessen acknowledges support from the DFG via SFB 881 (subprojects B1, B2, and B8) and in SPP 1573 (grant numbers KL 1358/18.1, KL 1358/19.2). RSK furthermore thanks the European Research Council for funding in the ERC Advanced Grant STARLIGHT (project number 339177). The paper is based on observations carried out with the Atacama Large Millimeter/Submillimeter Array (ALMA). Project: ADS/JAO.ALMA#2012.1.00912.S (Cycle 1). ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. We wish to thank the members of the German ALMA Regional Center (ARC) node at the Argelander-Institut für Astronomie of the University of Bonn for their support and help with the data reduction. This research has made use of NASA’s Astrophysical Data System. The NASA/IPAC Extragalactic Database (NED), also used, is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Finally, we made use of HyperLeda (http://leda.univ-lyon1.fr) from the University of Lyon.
Publisher Copyright:
© ESO 2018
PY - 2018
Y1 - 2018
N2 - NGC 4945 is one of the nearest (D ≈ 3.8 Mpc; 100 ≈ 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 ≈200 resolution. Three HCN and two HC+ isotopologues, CS, C3H2, SiO, HCO, and CH3C2H were measured. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 1000 × 200 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<∼200. 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 000 . 39 ± 000 . 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>∼1500 (>∼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 CH3C2H, 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 CH3C2H, 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 200 . 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 ≈100 . 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 of14N/15N ≈ 200-450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, because15N is less abundant than expected, the question for strong15N enrichment by massive star ejecta in starbursts still remains to be settled.
AB - NGC 4945 is one of the nearest (D ≈ 3.8 Mpc; 100 ≈ 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 ≈200 resolution. Three HCN and two HC+ isotopologues, CS, C3H2, SiO, HCO, and CH3C2H were measured. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 1000 × 200 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<∼200. 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 000 . 39 ± 000 . 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>∼1500 (>∼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 CH3C2H, 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 CH3C2H, 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 200 . 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 ≈100 . 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 of14N/15N ≈ 200-450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, because15N is less abundant than expected, the question for strong15N enrichment by massive star ejecta in starbursts still remains to be settled.
KW - Abundances
KW - Galaxies: ISM
KW - Galaxies: individual: NGC4945
KW - Galaxies: starburst
KW - Galaxies: structure
KW - Nuclear reactions
KW - Nucleosynthesis
KW - Radio lines: ISM
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U2 - 10.1051/0004-6361/201732174
DO - 10.1051/0004-6361/201732174
M3 - Article
AN - SCOPUS:85054732559
SN - 0004-6361
VL - 615
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
M1 - A155
ER -