[C-II] 158 MICRON OBSERVATIONS OF IC-10 - EVIDENCE FOR HIDDEN MOLECULAR-HYDROGEN IN IRREGULAR GALAXIES

We have mapped the [C II] 158 mu m line over 8.5' x 6.5' in the Magell anic irregular galaxy IC 10, thus presenting the first complete [C II] map of an entire low-metallicity galaxy. The total luminosity in the [C II] line in IC 10 is 1.5 x 10(6) L-.. We discuss the origin of the [C II] emission toward different regions in the galaxy. Overall, about 10% of the [C II] emission can originate in standard H I clouds (n si milar to 80, T similar to 100 K), while up to about 10% of the emissio n can originate in ionized gas, either the low-density warm gas or the denser H II regions. For the two brightest regions, most of the [C II ] emission is associated with dense photodissociation regions (PDRs). For several regions, however, the [C II] emission may not be explained by standard PDR models. For these regions, emission solely from the a tomic medium can also be precluded because the cooling rate per hydrog en atom would be much greater than the heating rate provided by photoe lectric UV heating. We speculate that in these regions the presence of an additional column density of H-2, 5 times that observed in H I, is required to explain the [C II] emission. The ambient UV fields presen t in these regions, combined with the low metallicity, create a situat ion where small CO cores exist surrounded by a relatively large [C II] -emitting envelope where molecular hydrogen is self-shielded. This add itional molecular mass is equivalent to at least 100 times the mass in the CO core that one would derive from the CO integrated intensity al one using the standard CO-to-H-2 conversion factor. These [C II] obser vations may, therefore, make a more reliable inventory of the gas rese rvoir in dwarf irregular galaxies where use of CO alone may significan tly underestimate the molecular mass.