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Very Large Array Observations of Ammonia in Infrared-Dark Clouds I: Column Density and Temperature Structure

We present Very Large Array observations of NH3 (1,1) and (2,2) in a sample of six infrared-dark clouds (IRDCs) with distances from 2 to 5 kpc. We find that ammonia serves as an excellent tracer of dense gas in IRDCs, showing no evidence of depletion, and the average abundance in these clouds is 8.1 x 10^-7 relative to H2. Our sample consists of four IRDCs with 24 micron embedded protostars and two that appear starless. We calculate the kinetic temperature of the gas in IRDCs and find no significant difference between starless and star-forming IRDCs. We find that the bulk of the gas is between 8 and 13 K, indicating that any embedded or nearby stars or clusters do not affect the gas temperature dramatically. Though IRDCs have temperatures and volume densities on par with local star formation regions of lower mass, they consist of much more mass which induces very high internal pressures. In order for IRDCs to survive as coherent structures, the internal pressure must be balanced by a confining pressure provided by the high concentration of molecular clouds in the spiral arm in which they reside. The high molecular concentration and pressure is roughly consistent with gas dynamics of a bar galaxy.