Dense Molecular Gas in the Nearby Low Metallicity Dwarf Starburst Galaxy
  IC 10
  Dense molecular gas and star formation are correlated in galaxies. The effect
of low metallicity on this relationship is crucial for interpreting
observations of high redshift galaxies, which have lower metallicities than
galaxies today. However, it remains relatively unexplored because dense
molecular gas tracers like HCN and HCO+ are faint in low metallicity systems.
We present Green Bank Telescope observations of HCN(1-0) and HCO+(1-0) on giant
molecular cloud (34pc) scales in the nearby low metallicity ($12+\log({\rm
O/H})=8.2$) starburst IC 10 and compare them to those in other galaxies. We
detect HCN and HCO+ in one and three of five pointings, respectively. The
$I_{\rm HCN}/I_{\rm HCO+}$ values are within the range seen in other galaxies,
but are most similar to those seen in other low metallicity sources and in
starbursts. The detections follow the fiducial $L_{\rm IR}$-$L_{\rm HCN}$ and
$L_{\rm IR}$-$L_{\rm HCO+}$ relationships. These trends suggest that HCN and
HCO+ can be used to trace dense molecular gas at metallicities of 1/4
$Z_\odot$, to first order. The dense gas fraction is similar to that in spiral
galaxies, but lower than that in U/LIRGs. The dense molecular gas star
formation efficiency, however, is on the upper end of those in normal galaxies
and consistent with those in U/LIRGs. These results suggest that the CO and
HCN/HCO+ emission occupy the same relative volumes as at higher metallicity,
but that the entire emitting structure is reduced in size. Dense gas mass
estimates for high redshift galaxies may need to be corrected for this effect.
