12 mm line survey of the dense molecular gas towards the W28 field TeV gamma-ray sources

Abstract

We present 12mm Mopra observations of dense molecular gas towards the W28 supernovaremnant (SNR) field. The focus is on the dense molecular gas towards the TeV gamma-raysources detected by the HESS telescopes, which likely trace the cosmic rays from W28 andpossibly other sources in the region. Using the NH3 inversion transitions we reveal severaldense cores inside the molecular clouds, the majority of which coincide with high-mass starformation and HII regions, including the energetic ultracompact HII region G5.89-0.39. Akey exception to this is the cloud north-east of W28, which is well known to be disruptedas evidenced by clusters of 1720MHz OH masers and broad CO line emission. Here wedetect broad NH3, up to the (9,9) transition, with linewidths up to 16 kms-1. This broad NH3emission spatially matches well with the TeV source HESS J1801-233 and CO emission, andits velocity dispersion distribution suggests external disruption from the W28 SNR direction.Other lines are detected, such as HC3Nand HC5N,H2Omasers, and many radio recombinationlines, all of which are primarily found towards the southern high-mass star formation regions.These observations provide a new view on to the internal structures and dynamics of thedense molecular gas towards the W28 SNR field and, in tandem with future higher-resolutionTeV gamma-ray observations, will offer the chance to probe the transport of cosmic rays intomolecular clouds.