[Cii] 158 Micron Observations of IC 10: Evidence for Hidden Molecular Hydrogen in Irregular Galaxies

Abstract

We have mapped the [C II] 158 km line over in the Magellanic irregular galaxy IC 10, thus 8@ .5 ] 6@ .5 presenting the Ðrst complete [C II] map of an entire low-metallicity galaxy. The total luminosity in the [C II] line in IC 10 is 1.5 ] 106We discuss the origin of the [C II] emission toward di †erent regions L _ . in the galaxy. Overall, about 10% of the [C II] emission can originate in standard H I clouds (n D 80, T D 100 K), while up to about 10% of the emission 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 atomic medium can also be precluded because the cooling rate per hydrogen atom would be much greater than the heating rate provided by photoelectric UV heating. We speculate that in these regions the presence of an additional column density of 5 times that observed in H I, is required to explain the [C II] emission. H 2 , The ambient UV Ðelds present in these regions, combined with the low metallicity, create a situation where small CO cores exist surrounded by a relatively large [C II]-emitting envelope where molecular hydrogen is self-shielded. This additional molecular mass is equivalent to at least 100 times the mass in the CO core that one would derive from the CO integrated intensity alone using the standard CO-to-H 2 conversion factor. These [C II] observations may, therefore, make a more reliable inventory of the gas reservoir in dwarf irregular galaxies where use of CO alone may signiÐcantly underestimate the molecular mass.

Photodissociation regions in the interstellar medium of galaxies

“…Since H 2 may exist in the C ϩ zones because of self-shielding, the ratio X of H 2 mass to CO Jϭ1 -0 luminosity may be significantly higher in the Magellanic Clouds than in the Galaxy (Israel, 1997). Madden et al (1997) arrive at a similar conclusion from [CII] observations of IC10, an irregular galaxy with low metallicity.…”

Probing the dust properties of galaxies up to submillimetre wavelengths

“…Thus, there could be a non negligeable amount of molecular gas not traced by CO (e.g. Poglitsch et al 1995;Madden et al 1997). In order to reach the D/G value expected by chemical models, a large molecular gas mass on the order of 10 10 M would be required to account for the "missing" gas mass, that is to say an order of magnitude more molecular gas than that deduced from current CO upper limits.…”

“…In order to reach the D/G value expected by chemical models, a large molecular gas mass on the order of 10 10 M would be required to account for the "missing" gas mass, that is to say an order of magnitude more molecular gas than that deduced from current CO upper limits. Molecular hydrogen could be embedded in [CII] emiting envelopes as described in Madden et al (1997) for the low-metallicity galaxy IC 10. The high L CII /L CO value ( 4 × 10 5 ) deduced by Bergvall et al (2000) and also seen in other low metallicity galaxies (Madden 2000) is coherent with this theory.…”

Star Forming Dwarf Galaxies

“…The emission is strongly correlated with H 2 [17] and can be used to estimate the H 2 mass. As an example we can have a look at IC 10 where Madden et al [99] speculate that, as a consequence of the strong UV field and low metallicity, small CO cores are formed, surrounded by extensive [C II] emitting regions containing H 2 . H 2 masses based on the CO flux would therefore be severely underestimated.…”