NGC 4945 is one of the nearest ( D \approx 3.8 Mpc ; 1 ^ { \prime \prime } \approx 19 pc ) starburst galaxies .
To investigate structure , dynamics , and composition of its dense nuclear gas , ALMA band 3 ( \lambda \approx 3–4 mm ) observations were carried out with \approx 2 ^ { \prime \prime } resolution .
Measured were three HCN and two HCO ^ { + } isotopologues , CS , C _ { 3 } H _ { 2 } , SiO , HCO , and CH _ { 3 } C _ { 2 } H. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 10 ^ { \prime \prime } \times 2 ^ { \prime \prime } reaching out to a galactocentric radius of r \approx 100 pc with position angle PA = 45 ^ { \circ } \pm 2 ^ { \circ } , inclination i = 75 ^ { \circ } \pm 2 ^ { \circ } and an unresolved bright central core of size { { { { \mathrel { \mathchoice { \vbox { \offinterlineskip \halign { \cr } $ \displaystyle < $ \cr$% \displaystyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \textstyle < $ \cr$% \textstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptstyle < $ \cr$% \scriptstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptscriptstyle < $% \cr$ \scriptscriptstyle \sim$ } } } } 2 ^ { \prime \prime } .
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 . ^ { \prime \prime } 39 \pm 0 . ^ { \prime \prime } 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 \rightarrow 0 and CS J =2 \rightarrow 1 delineate molecular arms of length { { { { \mathrel { \mathchoice { \vbox { \offinterlineskip \halign { \cr } $ \displaystyle > $ \cr$% \displaystyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \textstyle > $ \cr$% \textstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptstyle > $ \cr$% \scriptstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptscriptstyle > $% \cr$ \scriptscriptstyle \sim$ } } } } 15 ^ { \prime \prime } ( { { { { \mathrel { \mathchoice { \vbox { \offinterlineskip \halign { \cr } $ \displaystyle > $ \cr$% \displaystyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \textstyle > $ \cr$% \textstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptstyle > $ \cr$% \scriptstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptscriptstyle > $% \cr$ \scriptscriptstyle \sim$ } } } } 285 pc ) on opposite sides of the dynamical center .
These are connected by a ( deprojected ) \approx 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 { { { { \mathrel { \mathchoice { \vbox { \offinterlineskip \halign { \cr } $ \displaystyle < $ \cr$% \displaystyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \textstyle < $ \cr$% \textstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptstyle < $ \cr$% \scriptstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptscriptstyle < $% \cr$ \scriptscriptstyle \sim$ } } } } 100 pc ) with tilted rings provides a good fit by inferring a coplanar outflow reaching a characteristic deprojectd velocity of \approx 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 ( \approx 571 km s ^ { -1 } ) , within a range of \approx 500 – 660 km s ^ { -1 } .
Apparently , only molecular transitions with low critical H _ { 2 } -density ( n _ { crit } { { { { \mathrel { \mathchoice { \vbox { \offinterlineskip \halign { \cr } $ \displaystyle < $ \cr$% \displaystyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \textstyle < $ \cr$% \textstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptstyle < $ \cr$% \scriptstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptscriptstyle < $% \cr$ \scriptscriptstyle \sim$ } } } } 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 \pm 0.2 ) \times 10 ^ { 8 } M _ { \odot } inside a galactocentric radius of 2 . ^ { \prime \prime } 45 ( \approx 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 \approx 1 . ^ { \prime \prime } 5 ( \approx 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 \approx 200–450 is estimated .
This range of values is much higher then previously reported on a tentative basis .
Therefore , with ^ { 15 } N being less abundant than expected , the question for strong ^ { 15 } N enrichment by massive star ejecta in starbursts still remains to be settled .