GR 8: CO-to-H 2 Conversion Factor at Extremely Low Metallicity

We have obtained CO(2-1) upper limits of 6 and 7 mK toward two positions in the extremely metal poor dwarf irregular galaxy GR 8. The Hα emission of GR 8 shows active star formation which requires that the galaxy contain at least 1.5 × 104 Msun of molecular gas. Our inability to detect CO emission implies that the CO-to-H₂conversion factor in the vicinity of the dominant star-formation region is at least 80 × 1020 molecules cm-2 (K km s-1)-1. We compare GR 8 to all other irregular galaxies for which N(H2)/WCO has been measured; we confirm that the CO-to-H2 conversion factor increases with decreasing metallicity or with increasing linear scale of the observations.