We present large scale 9 ^ { \prime } \times 27 ^ { \prime } ( \sim 25 pc \times 70 pc ) far–IR observations around Sgr B2 using the Long–wavelength spectrometer ( LWS ) on board the Infrared Space Observatory ( ISO ) .
The spectra are dominated by the strong continuum emission of dust , the widespread molecular absorption of light hydrides ( OH , CH and H _ { 2 } O ) and the fine structure lines of [ N ii ] , [ N iii ] , [ O iii ] , [ C ii ] and [ O i ] .
The widespread dust emission is reproduced by a cold component ( T _ { d } \simeq 13–22 K ) together with a warm component ( T _ { d } \simeq 24–38 K ) representing \lesssim 10 % of the dust opacity .
The fine structure line emission reveals a very extended component of ionized gas .
The [ O iii ] 52 \mu m / 88 \mu m and [ N iii ] 57 \mu m / [ N ii ] 122 \mu m line intensity ratios show that ionized gas has an averaged electron density of \sim 240 cm ^ { -3 } .
The ionizing radiation can be characterized by a hard but diluted continuum , with effective temperatures of \sim 36000 K and a Lyman continuum photon flux of \sim 10 ^ { 50.4 } s ^ { -1 } .
The spatial distribution of the ionizing sources respect to the extended cloud and the clumpyness of the medium determine the large scale effects of the radiation .
Photo–Dissociation Regions ( PDRs ) can be numerous at the interface of the ionized and neutral gas .
The analysis of the [ C ii ] 158 \mu m and [ O i ] 63 and 145 \mu m lines indicates a far–UV radiation field of G _ { 0 } \simeq 10 ^ { 3 - 4 } 
and a density of n _ { H } =10 ^ { 3 - 4 } cm ^ { -3 } in these PDRs .
The widespread OH lines are reproduced by nonlocal radiative transfer models for clouds of moderate volume density ( n _ { H _ { 2 } } \simeq 10 ^ { 3 - 4 } cm ^ { -3 } ) at T _ { k } \gtrsim 40–100 K. PDR models can explain the enhanced column density of species such as H _ { 2 } O , OH and O ^ { 0 } .
However , they fail to reproduce the observed NH _ { 3 } /NH _ { 2 } /NH \simeq 100/10/1 abundance ratios .
For N–bearing species it seems that shock chemistry has to be invoked .
The molecular richness in the outer layers of Sgr B2 is probed by the ISO-LWS Fabry–Perot ( \sim 35 km s ^ { -1 } ) detections towards Sgr B2 ( M ) , where more that 70 lines from 15 molecular and atomic species are observed at high signal to noise ratio .