Detection of the Aromatic Molecule Benzonitrile ($c$-C$_6$H$_5$CN) in
  the Interstellar Medium
  Polycyclic aromatic hydrocarbons and polycyclic aromatic nitrogen
heterocycles are thought to be widespread throughout the Universe, because
these classes of molecules are probably responsible for the unidentified
infrared bands, a set of emission features seen in numerous Galactic and
extragalactic sources. Despite their expected ubiquity, astronomical
identification of specific aromatic molecules has proven elusive. We present
the discovery of benzonitrile ($c$-C$_6$H$_5$CN), one of the simplest
nitrogen-bearing aromatic molecules, in the interstellar medium. We observed
hyperfine-resolved transitions of benzonitrile in emission from the molecular
cloud TMC-1. Simple aromatic molecules such as benzonitrile may be precursors
for polycyclic aromatic hydrocarbon formation, providing a chemical link to the
carriers of the unidentified infrared bands.
