Probing the interstellar medium in space
Herschel space observatory, European Space Agency's cutting-edge observatory, carries the largest, most powerful infrared telescope ever flown in space. It is the first space observatory to cover the entire range from far-infrared to sub-millimetre wavelengths. With Herschel's observations, scientists working on the 'The dust-gas synergy in the ISM' (SYNISM) project were able to deeply explore the interstellar medium. About 99% of the interstellar medium is atomic and molecular gas and only 1% consists of tiny pieces of solid particles of dust. This dust plays a key role in the processes that regulate the formation of stars, and in the synthesis of complex molecules present in space. However, dust is an evolving component of the interstellar medium, and its evolution is intimately related to the gas's physical conditions. It is this circular dependence that has previously made dust-gas studies so complex. Dust grains absorb visible and ultraviolet light, thus heating up and radiating in infrared and submiilimetre wavelengths, now accessible with Herschel. By tapping these unexploited wavelengths, the SYNISM scientists were able to see phenomena beyond the reach of other observatories. Specifically, they were able to study photodissociation regions (PDRs) at an unprecedented level of detail. These regions of the interstellar medium are at the edge of molecular clouds, the dense regions where stars form, and are the source of most non-stellar infrared emission. Structures such as filaments that are in nearby PDRs are easily observable due to the spatial resolution of the Herschel telescope. Using the Herschel detectors, the SYNISM scientists were able to unveil this symbiotic system. The physical conditions prevailing around PDRs' fine structure were determined based on observations of emission lines which were the fingerprints of atoms and molecules. Theoretical models were also used by SYNISM scientists to interpret emission lines observe, and compared with the dust emission. The SYNISM scientists linked variations in gas physical properties with the changes in dust in the dense interstellar medium where stars form. The results of the SYNISM study have improved our understanding of processes that govern galaxy evolution.
