We present interferometric maps of ammonia ( NH _ { 3 } ) of the nearby starburst galaxy NGC 253 .
The observations have been taken with the Australia Telescope Compact Array and include the para-NH _ { 3 } ( 1,1 ) , ( 2,2 ) , and the ortho-NH _ { 3 } ( 3,3 ) and ( 6,6 ) inversion lines .
Six major complexes of dense ammonia are identified , three of them on either side of the starburst center , out to projected galactocentric radii of \sim 250 pc .
Rotational temperatures are derived toward selected individual positions as well as for the entire south-eastern and north-western molecular complexes .
The application of radiative transfer large velocity gradient models reveals that the bulk of the ammonia molecules is embedded in a one-temperature gas phase .
Kinetic temperatures of this gas are \sim 200 and 140 K toward the south-west and north-east , respectively .
The temperatures under which ammonia was formed in the past are with \gtrsim 30 K also warmer toward the south-west than toward the north-east ( \sim 15 - 20 K ) .
This is indicated by the ortho–to–para ammonia ratio which is \sim 1 and 1.5 - 2.5 toward the south-west and north-east , respectively .
Ammonia column densities in the brightest complexes are in the range of 6 - 11 \times 10 ^ { 14 } cm ^ { -2 } , which adds up to a total ammonia mass of \sim 20 M _ { \sun } , about evenly distributed toward both sides of the nucleus .
Ammonia abundances relative to H _ { 2 } are \sim 3 \times 10 ^ { -8 } .
In the south–western complex , the ammonia abundances increase from the starburst center to larger galactocentric radii .
Toward the center of NGC 253 , NH _ { 3 } ( 1,1 ) , ( 2,2 ) , and ( 6,6 ) is detected in absorption against an unresolved continuum source .
At the same location , however , ammonia ( 3,3 ) is found in emission which indicates maser activity .
This would be the first detected extragalactic NH _ { 3 } maser .
Evidence for an expanding shell in the south-western complex is provided .
The shell , with a dynamical age of \sim 1.3 Myr , is centered on an X-ray point source which must be located within the dense gas of NGC 253 .
The shell and X-ray properties can be reproduced by the energy input of a highly obscured young stellar cluster with a mass of \sim 10 ^ { 5 } M _ { \sun } which also heats the dense gas .
A current star formation rate of \sim 2.8 M _ { \sun } yr ^ { -1 } is derived for the nuclear starburst in NGC 253 based on its 1.2 cm continuum emission .