The derivation of the factor 'X' used to convert the line integral of the 12 CO emission over velocity (W CO ) to the column density of molecular hydrogen (N(H 2 )) is a topic that has exercised the astronomical community for over a decade. Early work gave 'high' values whereas we ourselves have consistently advocated a 'low' value, specifically, for the Inner Galaxy, X ∼ 1.0 × [10 20 cm 2 (K km s −1 ) −1 ] (see Bhat et al 1985 Nature 314 511, and later papers). Very recently, a new γ -ray value has come from the analysis of the EGRET data from the Compton Gamma Ray Observatory (CGRO) (Hunter et al 1996 Astrophys. J. in press). This new value is X ∼ 1.5 units but we here advocate its reduction by ∼ 40% to allow for unresolved discrete sources.

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