  We present a detection of correlated clustering between MeerKAT radio
intensity maps and galaxies from the WiggleZ Dark Energy Survey. We find a
$7.7\sigma$ detection of the cross-correlation power spectrum, the amplitude of
which is proportional to the product of the HI density fraction ($\Omega_{\rm
HI}$), HI bias ($b_{\rm HI}$) and the cross-correlation coefficient ($r$). We
therefore obtain the constraint $\Omega_{\rm HI} b_{\rm HI}
r\,{=}\,[0.86\,{\pm}\,0.10\,({\rm stat})\,{\pm}\,0.12\,({\rm
sys})]\,{\times}\,10^{-3}$, at an effective scale of $k_{\rm
eff}\,{\sim}\,0.13\,h\,{\rm Mpc}^{-1}$. The intensity maps were obtained from a
pilot survey with the MeerKAT telescope, a 64-dish pathfinder array to the SKA
Observatory (SKAO). The data were collected from 10.5 hours of observations
using MeerKAT's L-band receivers over six nights covering the 11hr field of
WiggleZ, in the frequency range $1015-973\,{\rm MHz}$
(0.400$\,{<}\,z\,{<}\,$0.459 in redshift). This detection is the first
practical demonstration of the multi-dish auto-correlation intensity mapping
technique for cosmology. This marks an important milestone in the roadmap for
the cosmology science case with the full SKAO.
