A census of the dense gas stored in galaxies and available to form new stars as a function of the cosmic time is a key ingredient to understand the star-formation history of the Universe. A direct estimate of the gas mass can be derived from observations of the carbon monoxide 12CO, the second most abundant molecule in the universe after H2 . Building the CO luminosity function at different redshift represents a promising approach to evaluate how the molecular gas content evolves and its relation with the star formation activity. Thanks to the sensitivity level reached in deep interferometric observations it is possible to study this molecule up to high redshift. 

In this talk I will present a blind search of CO emitters that was performed using ALMA deep observations from the ALMA archive. I will describe the line extractor algorithm, the fidelity of the line candidates and the identification procedure. I will also show some preliminary results of ALPINE (ALMA Large Program to INvestigate [C II] at Early times), a project which is focused on the characterization of the interstellar medium and the measurement of the star formation rate density in 122 galaxies at 4 < z < 6 using [C II] emission.