Spectral imaging of the Central Molecular Zone in multiple 7-mm
  molecular lines
  We have imaged 24 spectral lines in the Central Molecular Zone (CMZ) around
the Galactic Centre, in the range 42 to 50 GHz. The lines include emission from
the CS, CH3OH, HC3N, SiO, HNCO, HOCO+, NH2CHO, OCS, HCS+, CCS, C34S, 13CS,
29SiO, H13CCCN, HCC13CN and HC5}N molecules, and three hydrogen recombination
lines. The area covered is Galactic longitude -0.7 to 1.8 deg. and latitude
-0.3 to 0.2 deg., including the bright cores around Sgr A, SgrB2, SgrC and
G1.6-0.025. This work used the 22-m Mopra radio telescope in Australia,
obtaining ~ 1.8 km/s spectral and ~ 65 arcsec spatial resolution. We present
peak images from this study and conduct a principal component analysis on the
integrated emission from the brightest 10 lines, to study similarities and
differences in the line distribution. We examine the integrated line
intensities and line ratios in selected apertures around the bright cores, as
well as for the complete mapped region of the CMZ. We compare these 7-mm lines
to the corresponding lines in the 3-mm band, for five molecules, to study the
excitation. There is a variation in 3-mm to 7-mm line ratio across the CMZ,
with relatively higher ratio in the centre around Sgr B2 and Sgr A. We find
that the lines are sub-thermally excited, and from modelling with RADEX find
that non-LTE conditions apply, with densities of order 10^4 cm^{-3}.
