Observations of complex organic molecules in the gas phase of the interstellar medium
Thanks to the advent of sensitive and broad bandwidth instrumentation, complex organic molecules (COMs) have been found in a wide variety of interstellar environments, not only in our Galaxy but also in external galaxies up to a redshift of 0.89. The detection of COMs in cold environments such as starless or prestellar cores has challenged our understanding of COM formation and new ideas are being implemented in chemical models and explored in laboratory experiments. At the protostellar stage, the advent of new interferometers such as the Atacama Large Millimeter/submillimeter Array (ALMA) has allowed the mapping of the weak emission of COMs in the protostellar envelopes and protoplanetary disks around both low-mass and high-mass protostars, pinpointing their location and revealing differentiation between the different families of molecules. In this way, thermal and non-thermal desorption mechanisms can be probed, constraining the efficiency of formation of COMs in the gas phase versus on grain surfaces. Some degree of continuity in the COM composition is found from the early to late stages of star formation, suggesting that a significant fraction of COMs are formed at the initial conditions of star formation. For extreme environments such as the Galactic Center, cosmic rays and low-velocity shocks seem to influence the COM composition of low and high-density gas components. The spectral confusion limit will be a major challenge for the detection of new COMs in future spectroscopic surveys. However, low-frequency interferometers targeting sources with low-excitation temperatures may help to overcome this limit.