Dense gas in low-metallicity galaxies
  Stars form out of the densest parts of molecular clouds. Far-IR emission can
be used to estimate the Star Formation Rate (SFR) and high dipole moment
molecules, typically HCN, trace the dense gas. A strong correlation exists
between HCN and Far-IR emission, with the ratio being nearly constant, over a
large range of physical scales. A few recent observations have found HCN to be
weak with respect to the Far-IR and CO in subsolar metallicity (low-Z) objects.
We present observations of the Local Group galaxies M33, IC10, and NGC6822 with
the IRAM 30meter and NRO 45m telescopes, greatly improving the sample of low-Z
galaxies observed. HCN, HCO$^+$, CS, C$_2$H, and HNC have been detected.
Compared to solar metallicity galaxies, the Nitrogen-bearing species are weak
(HCN, HNC) or not detected (CN, HNCO, N$_2$H$^+$) relative to Far-IR or CO
emission. HCO$^+$ and C$_2$H emission is normal with respect to CO and Far-IR.
While $^{13}$CO is the usual factor 10 weaker than $^{12}$CO, C$^{18}$O
emission was not detected down to very low levels. Including earlier data, we
find that the HCN/HCO$^+$ ratio varies with metallicity (O/H) and attribute
this to the sharply decreasing Nitrogen abundance. The dense gas fraction,
traced by the HCN/CO and HCO$^+$/CO ratios, follows the SFR but in the low-Z
objects the HCO$^+$ is much easier to measure. Combined with larger and smaller
scale measurements, the HCO$^+$ line appears to be an excellent tracer of dense
gas and varies linearly with the SFR for both low and high metallicities.
