Unresolved Galactic gamma ray sources and the determination of the CO/H2 conversion factor

The derivation of the factor 'X' used to convert the line integral of the 12CO emission over velocity (WCO) to the column density of molecular hydrogen (N(H2)) 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 ∼ 1.0x[1020 cm2 (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. (author)