Photodissociation Regions in the Large Magellanic Cloud and IC 10

Extract

Both theory and observation suggest that metallicity profoundly alters the properties and structure of the Photodissociation Regions (PDRs; e.g., Madden et al. 1997). Several factors contribute to the differences between high and low metallicity systems: altered gas phase and grain surface chemistry due to the low Si, C and S elemental abundances, and diminished dust shielding because of the low dust-to-gas ratio. Since there is less dust shielding, UV photons penetrate more deeply into the molecular clouds leaving H₂ unaffected but photodissociating most other molecules everywhere except in the most opaque clumps. Thus, a low-metallicity system contains large regions where hydrogen remains molecular but the usual tracers of molecular gas like CO are photodissociated.