  The goal of this science case is to address the use of a ngVLA as a CO
redshift machine for dust-obscured high-redshift galaxies which lack of clear
counterparts at other wavelengths. Thanks to its unprecedentedly large
simultaneous bandwidth and sensitivity, the ngVLA will be able to detect low--J
CO transitions at virtually any $z>1$. In particular, at $z>4.76$ two CO
transitions will be covered in a single frequency setting, thus ensuring
unambiguous line identification. The ngVLA capabilities fill in a redshift
range where other approaches (e.g., photometric redshifts, search for
optical/radio counterparts, etc) typically fail due to the combination of
intrinsically faint emission and increasing luminosity distance. This will
allow us to explore the formation of massive galaxies in the early cosmic
times.
