Submillimeter observations of IC 10: the dust properties and neutral carbon content of a low metallicity starburst

Abstract

We present submillimeter observations of the Local Group, metal-poor, irregular dwarf galaxy IC 10, directly relevant to the interaction between interstellar medium and star formation activity in primeval galaxies. Using the JCMT, we have observed the fine structure neutral carbon transition at 492 GHz and the rotational J=3-2 transition of 12CO and 13CO in the most massive giant molecular cloud complex in this galaxy, IC10-SE. We find that, although the I([CII])/I(CO) ratio for this object is a factor of 4 larger than the typical Milky Way value, its [C~I] to CO intensity ratio is I([CI])/I(CO)~18 is similar (only about 50% larger) to that of the Milky Way. Modelling of the [CII]/CO and [CI]/CO intensity ratios with metallicity indicates that, if C+ and C are chiefly produced by UV photodissociation, both ratios should increase sharply with decreasing metallicity (and consequently diminished UV shielding; Bolatto, Jackson, and Ingalls 1999). These data then suggest a different origin for an important fraction of C in these clouds, unrelated to photodissociation. We have also mapped the 850 um continuum in this region using SCUBA. Employing these data in conjunction with KAO and IRAM measurements we find an extremely low emissivity exponent, b~0.5. We conclude that this low exponent is most likely due to the destruction of small dust grains, brought about by the increased penetration of UV radiation in the low metallicity ISM. If a low b in the submillimeter is a general property of metal-poor systems then the interpretation of millimeter and submillimeter surveys of high-z galaxies should be revised.