Fire in the Heart: A Characterization of the High Kinetic Temperatures
  and Heating Sources in the Nucleus of NGC253
  The nuclear starburst within the central $\sim 15^{\prime\prime}$ ($\sim 250$
pc; $1^{\prime\prime} \simeq 17$ pc) of NGC253 has been extensively studied as
a prototype for the starburst phase in galactic evolution. Atacama Large
Millimeter/submillimeter Array (ALMA) imaging within receiver Bands 6 and 7
have been used to investigate the dense gas structure, kinetic temperature, and
heating processes which drive the NGC253 starburst. Twenty-nine transitions
from fifteen molecular species/isotopologues have been identified and imaged at
$1.^{\prime\prime}5$ to $0.^{\prime\prime}4$ resolution, allowing for the
identification of five of the previously-studied giant molecular clouds (GMCs)
within the central molecular zone (CMZ) of NGC253. Ten transitions from the
formaldehyde (H$_2$CO) molecule have been used to derive the kinetic
temperature within the $\sim 0.^{\prime\prime}5$ to $5^{\prime\prime}$
dense-gas structures imaged. On $\sim 5^{\prime\prime}$ scales we measure $T_K
\gtrsim 50$ K, while on size scales $\lesssim 1^{\prime\prime}$ we measure $T_K
\gtrsim 300$ K. These kinetic temperature measurements further delineate the
association between potential sources of dense gas heating. We have
investigated potential heating sources by comparing our measurements to models
which predict the physical conditions associated with dense molecular clouds
that possess a variety of heating mechanisms. This comparison has been
supplemented with tracers of recently-formed massive stars (Br$\gamma$) and
shocks ([FeII]). Derived molecular column densities point to a
radially-decreasing abundance of molecules with sensitivity to cosmic ray and
mechanical heating within the NGC253 CMZ. These measurements are consistent
with radio spectral index calculations which suggest a higher concentration of
cosmic ray producing supernova remnants within the central 10 pc of NGC253.
