Millimeter line observations toward four local galaxies
  We present results of millimeter line observations toward four local gas-rich
galaxies (NGC 3079, NGC 4258, NGC 6240 and VII Zw 31) with the IRAM 30 meter
millimeter telescope. More than 33 lines in these four sources were detected,
including normal dense gas tracers (HCN 1-0, HCO$^+$ 1-0, and C$_2$H 1-0, etc)
and their isotopic species. H$^{13}$CN (1-0) and H$^{13}$CO$^+$ (1-0) are
detected for the first time in NGC 4258. Optical depths of HCN 1-0 and
HCO$^{+}$ 1-0 were estimated with detected isotopic lines in NGC 4258, which
were 4.1 and 2.6, respectively. HC$_3$N $J=29-28$, which requires high volume
density and high temperature to excite, was detected in NGC 6240. High ratios
of HCO$^+$/HCN in NGC 4258 and NGC 6240 imply that this ratio might not be a
perfect diagnostic tool between AGN and starburst environments, due to
contamination/combination of both processes. The low HC$_3$N/HCN line ratios
with less than 0.15 in NGC 4258, NGC 6240 and the non-detection of HC$_3$N line
in NGC 3079 and VII Zw 31 indicates that these four galaxies are HC$_3$N-poor
galaxies. The variation of fractional abundance of CN in different types of
galaxies is large.
