CO(1--0) imaging reveals 10-kiloparsec molecular gas reservoirs around star-forming galaxies at high redshift

Massive, intensely star-forming galaxies at high redshift require a supply of molecular gas from their gas reservoirs, replenished by infall from the surrounding circumgalactic medium, to sustain their immense star-formation rates. However, our knowledge of the extent and morphology of their cold-gas reservoirs is still in its infancy. We present the results of stacking 80 hours of JVLA observations of CO(1--0) emission -- which traces the cold molecular gas -- in nineteen $z=2.0-4.5$ dusty, star-forming galaxies from the AS2VLA survey. The visibility-plane stack reveals extended emission with a half-light radius of $3.8\pm0.5$~kpc, 2--3$\times$ more extended than the dust-obscured star formation and $1.4\pm0.2\times$ more extended than the stellar emission revealed by JWST. Stacking the [CI](1--0) observations for ten galaxies from our parent sample yields a half-light radius $\leq$2.6~kpc, marginally smaller than CO(1--0). The CO(1--0) size is also comparable to the [CII] halos detected around high-redshift star-forming galaxies, suggesting these arise from molecular gas. Photo-dissociation region modelling indicates that the extended CO(1--0) emission arises from clumpy, dense clouds rather than smooth, diffuse gas. Our results show that the bulk (up to 80\%) of molecular gas resides outside the star-forming region; with only a small part directly contributing to their current star formation.