  Context: The north-west photo-dissociation region (PDR) in the reflection
nebula NGC 7023 displays a complex structure. Filament-like condensations at
the edge of the cloud can be traced via the emission of the main cooling lines,
offering a great opportunity to study the link between the morphology and
energetics of these regions. Aims: We study the spatial variation of the
far-infrared fine-structure lines of [C II] (158 um) and [O I] (63 and 145 um).
These lines trace the local gas conditions across the PDR. Methods: We used
observations from the Herschel/PACS instrument to map the spatial distribution
of these fine-structure lines. The observed region covers a square area of
about 110" x 110" with an angular resolution that varies from 4" to 11". We
compared this emission with ground-based and Spitzer observations of H2 lines,
Herschel/SPIRE observations of CO lines, and Spitzer/IRAC 3.6 um images that
trace the emission of polycyclic aromatic hydrocarbons. Results: The [C II]
(158 um) and [O I] (63 and 145 um) lines arise from the warm cloud surface
where the PDR is located and the gas is warm, cooling the region. We find that
although the relative contribution to the cooling budget over the observed
region is dominated by [O I]63 um (>30%), H2 contributes significantly in the
PDR (35%), as does [C II]158 um outside the PDR (30%). Other species contribute
little to the cooling ([O I]145 um 9%, and CO 4%). The [O I] maps resolve these
condensations into two structures and show that the peak of [O I] is slightly
displaced from the molecular H2 emission. The size of these structures is about
8" (0.015 pc) and in surface cover about 9% of the PDR emission. Finally, we
did not detect emission from [N II]122 um, suggesting that the cavity is mostly
filled with non-ionised gas.
