APEX-SEPIA660 Early Science: Gas at densities above $10^7$ cm$^{-3}$
  towards OMC-1
  Context. The star formation rates and stellar densities found in young
massive clusters suggest that these stellar systems originate from gas at
densities n(H$_2$) $>10^7$ cm$^{-3}$. Until today, however, the physical
characterization of this ultra high density material remains largely
unconstrained in observations. Aims. We investigated the density properties of
the star-forming gas in the OMC-1 region located in the vicinity of the Orion
Nebula Cluster (ONC). Methods. We mapped the molecular emission at 652 GHz in
OMC-1 as part of the APEX-SEPIA660 Early Science. Results. We detect bright and
extended N$_2$H$^+$ (J=7-6) line emission along the entire OMC-1 region.
Comparisons with previous ALMA data of the (J=1-0) transition and radiative
transfer models indicate that the line intensities observed in this N$_2$H$^+$
(7-6) line are produced by large mass reservoirs of gas at densities n(H$_2$)
$>10^7$ cm$^{-3}$. Conclusions. The first detection of this N$_2$H$^+$ (7-6)
line at parsec-scales demonstrates the extreme density conditions of the
star-forming gas in young massive clusters such as the ONC. Our results
highlight the unique combination of sensitivity and mapping capabilities of the
new SEPIA660 receiver for the study of the ISM properties at high frequencies.
