HCO+ Observations toward Compact Radio Continuum Sources Using the KVN 21-m Telescopes to Trace Dark Molecular Gas

It has been known that there is "dark gas" invisible either in 21-cm HI or 2.6-mm CO emission which are general tracers of atomic and molecular gas, respectively. Many researchers consider that the dark gas is "Dark Molecular Gas (DMG)" composed of CO-free $H_2$ in the intermediate zone between atomic and full-fledged molecular gas and that HCO+ and OH molecules are good tracers of the DMG since they can form in much lower $H_2$ column densities where CO does not. We have carried out HCO+ J=1-0 absorption observations toward nine bright extragalactic radio continuum sources using the KVN 21-m telescopes as single dishes. We detected HCO+ absorption lines toward two sources. We derive HCO+ and $H_2$ column densities or their limits, and discuss the implications of our results.