An imaging line survey of OMC-1 to OMC-3
  Aims: Our aim is to identify the dominant molecular cooling lines and
characteristic emission features in the 1.3 mm window of distinct regions in
the northern part of the Orion A molecular cloud. By defining and analysing
template regions, we also intend to help with the interpretation of
observations from more distant sources which cannot be easily spatially
resolved.
  Methods: We analyse an imaging line survey covering the area of OMC-1 to
OMC-3 from 200.2 to 281.8 GHz obtained with the PI230 receiver at the APEX
telescope. Masks are used to define regions with distinct properties (e.g.
column density or temperature ranges) from which we obtain averaged spectra.
Lines of 29 molecular species (55 isotopologues) are fitted for each region to
obtain the respective total intensity.
  Results: We find that strong sources like Orion KL have a clear impact on the
emission on larger scales. Although not spatially extended, their line emission
contributes substantially to spectra averaged over large regions. Conversely,
the emission signatures of dense, cold regions like OMC-2 and OMC-3 (e.g.
enhanced N$_{2}$H$^{+}$ emission and low HCN/HNC ratio) seem to be difficult to
pick up on larger scales, where they are eclipsed by signatures of stronger
sources. In all regions, HCO$^{+}$ appears to contribute between 3% and 6% to
the total intensity, the most stable value for all bright species.
N$_{2}$H$^{+}$ shows the strongest correlation with column density, but not
with typical high-density tracers like HCN, HCO$^{+}$, H$_{2}$CO, or HNC.
Common line ratios associated with UV illumination, CN/HNC and CN/HCO$^{+}$,
show ambiguous results on larger scales, suggesting that the identification of
UV illuminated material may be more challenging. The HCN/HNC ratio may be
related to temperature over varying scales.
