A 7 mm line survey of the shocked and disrupted molecular gas towards the W28 field TeV gamma-ray sources

Abstract

We present 7 mm Mopra observations of the dense molecular gas towards the W28 supernova remnant (SNR) field, following a previous 12 mm line survey of this region. These observations take advantage of the 7 mm beam size to probe the dense and disrupted gas in the region at ∼1 arcmin scales. Our observations are focused towards the north-eastern (NE) HESS J1801−233 and southern HESS J1800−240 B TeV gamma-ray sources, with slightly less observations towards HESS J1800−240 A and C. Using the CS (1–0) transition we reveal multiple regions of dense gas, cm−3. We report the discovery of dense gas towards HESS J1800−240 C, at the site of a 1720 MHz OH maser. The NE molecular cloud is known to be disrupted; many 1720 MHz OH masers and broad CO line emission are detected at the rim of W28. Here, we reveal this shock interaction region contains generally extended clumpy CS, as well as clumpy SiO and CH3OH emission with broad line profiles. The full width at half maximum (FWHM) of the molecular lines extend up to 18 km s−1 on the W28 side of the NE cloud. The detection of SiO towards maser clumps OH C, D, E and F provides further evidence of the shocked conditions in the NE cloud. Several other lines associated with star formation are also detected towards the southern source, notably the energetic H ii complex G5.89−0.39. The spatial match of dense gas with the TeV emission further supports the cosmic ray (CR) origin for the gamma-rays. We estimate the mass of several extended dense clouds within the field and predict the TeV flux from the dense cloud components. The predicted fluxes are of the order of 10−14 to 10−13 photons cm−2 s−1, which should be detectable and possibly resolved by a future TeV instrument, such as the Cherenkov Telescope Array.