Molecules as tracers of galaxy evolution: an EMIR survey. I.
  Presentation of the data and first results
  We investigate the molecular gas properties of a sample of 23 galaxies in
order to find and test chemical signatures of galaxy evolution and to compare
them to IR evolutionary tracers. Observation at 3 mm wavelengths were obtained
with the EMIR broadband receiver, mounted on the IRAM 30 m telescope on Pico
Veleta, Spain. We compare the emission of the main molecular species with
existing models of chemical evolution by means of line intensity ratios
diagrams and principal component analysis. We detect molecular emission in 19
galaxies in two 8 GHz-wide bands centred at 88 and 112 GHz. The main detected
transitions are the J=1-0 lines of CO, 13CO, HCN, HNC, HCO+, CN, and C2H. We
also detect HC3N J=10-9 in the galaxies IRAS 17208, IC 860, NGC 4418, NGC 7771,
and NGC 1068. The only HC3N detections are in objects with HCO+/HCN<1 and warm
IRAS colours. Galaxies with the highest HC3N/HCN ratios have warm IRAS colours
(60/100 {\mu}m>0.8). The brightest HC3N emission is found in IC 860, where we
also detect the molecule in its vibrationally excited state.We find low HNC/HCN
line ratios (<0.5), that cannot be explained by existing PDR or XDR chemical
models. Bright HC3N emission in HCO+-faint objects may imply that these are not
dominated by X-ray chemistry. Thus the HCN/HCO+ line ratio is not, by itself, a
reliable tracer of XDRs. Bright HC3N and faint HCO+ could be signatures of
embedded starformation, instead of AGN activity.
