  The molecular phase of the interstellar medium (ISM) in galaxies offers
fundamental insight for understanding star-formation processes and how stellar
feedback affects the nuclear activity of certain galaxies. We present here
Large Millimeter Telescope spectra obtained with the Redshift Search Receiver,
a spectrograph that cover simultaneously the 3 mm band from 74 to 111 GHz with
a spectral resolution of around 100 km/s. The observed galaxies that have been
detected previously in HCN, have different degrees of nuclear activity, one
normal galaxy (NGC 6946), the starburst prototype (M 82) and two ultraluminous
infrared galaxies (ULIRGs, IRAS 17208-0014 and Mrk 231). We plotted our data in
the HCO+/HCN vs. HCN/13CO diagnostic diagram finding that NGC 6946 and M 82 are
located close to other normal galaxies; and that both IRAS 17208-0014 and Mrk
231 are close to the position of the well known ULIRG Arp 220 reported by Snell
et al. (2011). We found that in Mrk 231 -- a galaxy with a well known active
galactic nucleus -- the HCO+/HCN ratio is similar to the ratio observed in
other normal galaxies.
