We present observations of HCO+ 1–0 absorption lines toward two extragalactic compact radio sources, NRAO 150 and BL Lac with the Korean VLBI Network in order to investigate their time variation over 20 years by Galactic foreground clouds. It is found that the line shape of –17 km s−1 component changed marginally during 1993–1998 period and has remained unaltered thereafter for NRAO 150. Its behavior is different from that of H2CO 110–111, suggesting chemical differentiation on ∼ 20 AU scale, the smallest ever seen. On the other hand, BL Lac exhibits little temporal variation for the HCO+ and H2CO lines. Our observation also suggests that Korea VLBI Network performs reliably in the spectrum mode in that the shapes of the new HCO+ 1–0 spectra are in good agreement with the previous ones to an accuracy of a few percent except the time varying component toward NRAO 150.

In order to reveal physical conditions of molecular gas in active galaxies (active galaxies mean both starbursts and AGNs in this paper), we carried out systematic observations (R = 19 ~ 120) of CO funda- mental band at 4.7 m in absorption with AKARI. We also made follow-up CO absorption observations at higher spectral resolution (R = 5000 ~ 1000) with Subaru. Recently, Herschel made extensive ob- servations of highly-excited CO lines in emission in the far-infrared. The two data sets (absorption and emission) sometimes provide us with apparently inconsistent results. One case is starburst galaxies: Sub- aru observations showed low temperature of molecular gas toward the starburst NGC 253, while Herschel detected highly excited CO lines in the starburst. This suggests that warm molecular clouds are more deeply embedded than newly formed star clusters. The other case is obscured AGNs; Herschel detected highly excited CO lines in emission in nearby AGNs, while AKARI and Subaru observations showed CO absorption only in some of the obscured AGNs. This could re ect the dierence of nature of molecular tori in these AGNs. We propose the combination of the absorption and emission observations as an eective tool to reveal geometry of warm molecular clouds in active galaxies.