The Redshift Search Receiver 3 mm Wavelength Spectra of 10 Galaxies
  The 3 mm wavelength spectra of 10 galaxies have been obtained at the Five
College Radio Astronomy Observatory using a new, very broadband receiver and
spectrometer, called the Redshift Search Receiver (RSR). The RSR has an
instantaneous bandwidth of 37 GHz covering frequencies from 74 to 111 GHz, and
has a spectral resolution of 31 MHz (~100 km/s). During tests of the RSR on the
FCRAO 14 m telescope the complete 3 mm spectra of the central regions of NGC
253, Maffei 2, NGC1068, IC 342, M82, NGC 3079, NGC 3690, NGC 4258, Arp 220 and
NGC 6240 were obtained. Within the wavelength band covered by the RSR, 20
spectral lines from 14 different atomic and molecular species were detected.
Based on simultaneous fits to the spectrum of each galaxy, a number of key
molecular line ratios are derived. A simple model which assumes the emission
arises from an ensemble of Milky Way-like Giant Nolecular Cloud cores can
adequately fit the observed line ratios using molecular abundances based on
Galactic molecular cloud cores. Variations seen in some line ratios, such as
13CO/HCN and HCO+/HCN, can be explained if the mean density of the molecular
gas varies from galaxy to galaxy. However, NGC 3690, NGC 4258 and NGC 6240 show
very large HCO+/HCN ratios and require significant abundance enhancement of
HCO+ over HCN, possible due to the proximity to active galactic nucleus
activity. Finally, the mass of dense molecular gas is estimated and we infer
that 25-85 % of the total molecular gas in the central regions of these
galaxies must have densities greater than 10^4 cm^-3.
