In the Milky Way, the 12CO J = 1 → 0 line traces the molecular gas content. The conversion factor, X
, between the H2 column density, N(H
2), and the velocity integrated intensity of CO, I(CO), has been measured via the virial theorem or via γ-ray emission (Solomon et al. 1987; Bloemen et al. 1986; Digel et al. 1997; and references therein).
How can we apply XGAL
to other galaxies where the metallicities are different from that of our Galaxy? The metallicity dependence of the conversion factor has been an issue. Cohen et al. (1988), Wilson (1995), and Arimoto, Sofue, & Tsujimoto (1996) argued that the value of X increases as the metallicity of the individual galaxy decreases. In contrast, Taylor, Kobulnicky, & Skillman (1996) showed that some low abundance galaxies have lower X.