  Long-duration gamma-ray bursts are associated with the explosions of massive
stars and are accordingly expected to reside in star-forming regions with
molecular gas (the fuel for star formation). Previous searches for carbon
monoxide (CO), a tracer of molecular gas, in burst host galaxies did not detect
any emission. Molecules have been detected as absorption in the spectra of
gamma-ray burst afterglows, and the molecular gas is similar to the translucent
or diffuse molecular clouds of the Milky Way. Absorption lines probe the
interstellar medium only along the line of sight, so it is not clear whether
the molecular gas represents the general properties of the regions where the
bursts occur. Here we report spatially resolved observations of CO line
emission and millimetre-wavelength continuum emission in two galaxies hosting
gamma-ray bursts. The bursts happened in regions rich in dust, but not
particularly rich in molecular gas. The ratio of molecular gas to dust (<9-14)
is significantly lower than in star-forming regions of the Milky Way and nearby
star-forming galaxies, suggesting that much of the dense gas where stars form
has been dissipated by other massive stars.
