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Infrared Dark Clouds (IRDCs) are cold, dense regions that are usually foundwithin Giant Molecular Clouds (GMCs). Ongoing star formation within IRDCs istypically still deeply embedded within the surrounding molecular gas.Characterising the properties of relatively quiescent IRDCs may therefore helpus to understand the earliest phases of the star formation process. Studies oflocal molecular clouds have revealed that deuterated species are enhanced inthe earliest phases of star formation. In this paper we test this towards IRDCG035.39-00.33. We present an 80 arcsec by 140 arcsec map of the J=2-1transition of N2D+, obtained with the IRAM-30m telescope. We find that N2D+ iswidespread throughout G035.39-00.33. Complementary observations of N2H+(1-0)are used to estimate the deuterium fraction, N(N2D+)/N(N2H+). We report a meandeuterium fraction of 0.04+-0.01, with a maximum of 0.09+-0.02. The meandeuterium fraction is ~3 orders of magnitude greater than the interstellar[D]/[H] ratio. High angular resolution observations are required to excludebeam dilution effects of compact deuterated cores. Using chemical modelling, wefind that the average observed values of the deuterium fraction are inagreement with an equilibrium deuterium fraction, given the general propertiesof the cloud. This implies that the IRDC is at least ~3Myr old, which is ~8times longer than the mean free-fall time of the observed deuterated region.

Widespread deuteration across the IRDC G035.39-00.33