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We present Very Large Array observations of NH3 (1,1) and (2,2) in a sampleof six infrared-dark clouds (IRDCs) with distances from 2 to 5 kpc. We findthat ammonia serves as an excellent tracer of dense gas in IRDCs, showing noevidence of depletion, and the average abundance in these clouds is 8.1 x 10^-7relative to H2. Our sample consists of four IRDCs with 24 micron embeddedprotostars and two that appear starless. We calculate the kinetic temperatureof the gas in IRDCs and find no significant difference between starless andstar-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 gastemperature dramatically. Though IRDCs have temperatures and volume densitieson par with local star formation regions of lower mass, they consist of muchmore mass which induces very high internal pressures. In order for IRDCs tosurvive as coherent structures, the internal pressure must be balanced by aconfining pressure provided by the high concentration of molecular clouds inthe spiral arm in which they reside. The high molecular concentration andpressure is roughly consistent with gas dynamics of a bar galaxy.

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