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|Title:||Molecular line emission in NGC 4945, imaged with ALMA||Authors:||Henkel, C.
Józsa, G. I. G.
HUNT, Leslie Kipp
van der Werf, P.
Malawi, A. A.
Asiri, H. M.
Baan, W. A.
Impellizzeri, C. M. V.
Klessen, R. S.
Tang, X. D.
Zhang, J. S.
|Issue Date:||2018||Journal:||ASTRONOMY & ASTROPHYSICS||Number:||615||First Page:||A155||Abstract:||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<SUP>+</SUP> isotopologues, CS, C<SUB>3</SUB>H<SUB>2</SUB>, SiO, HCO, and CH<SUB>3</SUB>C<SUB>2</SUB>H 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 H<SUB>2</SUB>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″ (≳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<SUP>-1</SUP>. All our molecular lines, with the notable exception of CH<SUB>3</SUB> C<SUB>2</SUB>H, show significant absorption near the systemic velocity (≈571 km s<SUP>-1</SUP>), within the range ≈500-660 km s<SUP>-1</SUP>. Apparently, only molecular transitions with low critical H<SUB>2</SUB> density (n<SUB>crit</SUB> ≲ 10<SUP>4</SUP> cm<SUP>-3</SUP>) do not show absorption. The velocity field of the nuclear disk, derived from CH<SUB>3</SUB> C<SUB>2</SUB>H, provides evidence for rigid rotation in the inner few arcseconds and a dynamical mass of M<SUB>tot</SUB> = (2.1 ± 0.2) × 10<SUP>8</SUP> M<SUB>☉</SUB> 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 <SUP>14</SUP>N/<SUP>15</SUP>N ≈ 200-450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, because <SUP>15</SUP>N is less abundant than expected, the question for strong <SUP>15</SUP>N enrichment by massive star ejecta in starbursts still remains to be settled.||URI:||http://hdl.handle.net/20.500.12386/28329||URL:||https://www.aanda.org/articles/aa/abs/2018/07/aa32174-17/aa32174-17.html||ISSN:||0004-6361||DOI:||10.1051/0004-6361/201732174||Bibcode ADS:||2018A&A...615A.155H||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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checked on Jun 22, 2021
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