Context :

Aims : We aim at deriving the excitation conditions of the interstellar gas as well as the local FUV intensities in the molecular cloud surrounding NGC 3603 to get a coherent picture of how the gas is energized by the central stars .

Methods : The NANTEN2-4m submillimeter antenna is used to map the [ CI ] 1-0 , 2-1 and CO 4-3 , 7-6 lines in a 2 ^ { \prime } \times 2 ^ { \prime } region around the young OB cluster NGC 3603 YC .
These data are combined with C ^ { 18 } O 2–1 data , HIRES-processed IRAS 60 \mu m and 100 \mu m maps of the FIR continuum , and Spitzer/IRAC maps .

Results : The NANTEN2 observations show the presence of two molecular clumps located south-east and south-west of the cluster and confirm the overall structure already found by previous CS and C ^ { 18 } O observations .
We find a slight position offset of the peak intensity of CO and [ CI ] , and the atomic carbon appears to be further extended compared to the molecular material .
We used the HIRES far-infrared dust data to derive a map of the FUV field heating the dust .
We constrain the FUV field to values of \chi = 3 - 6 \times 10 ^ { 3 } in units of the Draine field across the clouds .
Approximately 0.2 to 0.3 % of the total FUV energy is re-emitted in the [ CII ] 158 \mu m cooling line observed by ISO .
Applying LTE and escape probability calculations , we derive temperatures ( T _ { \mathrm { MM 1 } } = 43 K , T _ { \mathrm { MM 2 } } = 47 K ) , column densities ( N _ { \mathrm { MM 1 } } = 0.9 \times 10 ^ { 22 } cm ^ { -2 } , N _ { \mathrm { MM 2 } } = 2.5 \times 10 ^ { 22 } cm ^ { -2 } ) and densities ( n _ { \mathrm { MM 1 } } = 3 \times 10 ^ { 3 } cm ^ { -3 } , n _ { \mathrm { MM 2 } } = 10 ^ { 3 } -10 ^ { 4 } cm ^ { -3 } ) for the two observed molecular clumps MM1 and MM2 .

Conclusions : The cluster is strongly interacting with the ambient molecular cloud , governing its structure and physical conditions .
A stability analysis shows the existence of gravitationally collapsing gas clumps which should lead to star formation .
Embedded IR sources have already been observed in the outskirts of the molecular cloud and seem to support our conclusions .