At least half of the local galaxies reside in galaxy groups, which indicates that the group is the common environment where galaxies evolve. Therefore, it is important to probe how significantly galaxies are affected by group environmental processes, in order to obtain a better understanding of galaxy evolution. We carried out a new CO imaging survey for 31 galaxies in the IC 1459 and NGC 4636 groups, using the Atacama Compact Array, to study the effect of the group environment on the molecular gas properties and the star formation activity. With our resolved CO data, combined with high-resolution H i images, we find asymmetric CO and H i distributions in the group galaxies. Compared to isolated galaxies, group members have relatively low molecular gas fraction and low star formation rate. These results suggest that the group environment can change the properties of cold gas components and star formation in group galaxies.

Green valley galaxies (by selection) exhibit lower specific star formation rates and are thought to be in the transition from the active star-forming phase to the quiescent state. Physical mechanisms responsible for the depleted star formation in green valley galaxies, however, are still under debate. Using the ALMA-MaNGA Quenching and STar formation (ALMaQUEST) CO observations, we study the so-called ‘resolved star formation scaling relations’, which describe relationships among surface densities of star formation rate, stellar mass, and molecular gas mass. By comparing the kpc-scale scaling relations between the main sequence and green valley galaxies, we are able to quantify if the deficit of star formation in green valley galaxies is driven by depleted molecular gas or inefficient star formation. And finally, we present our recent ALMA dense gas (HCN and HCO+) observations for a set of selected ALMaQUEST galaxies to discuss whether the green valley galaxies lack dense molecular gas or not.

The cosmic star formation rate density first increases with time towards a pronounced peak 10 Gyrs ago (or z=1-2) and then slows down, dropping by more than a factor 10 since z=1. The processes at the origin of the star formation quenching are not yet well identified: either the gas is expelled by supernovae and AGN feedback, or prevented to inflow. Morphological transformation or environment effects are also invoked. Recent IRAM/NOEMA and ALMA results are reviewed about the molecular content of galaxies and its dynamics, as a function of redshift. Along the main sequence of massive star forming galaxies, the gas fraction was higher in the past (up to 80%), and galaxy disks were more unstable and more turbulent. The star formation efficiency increases with redshift, or equivalently the depletion time decreases, whatever the position of galaxies, either on the main sequence or above. Attempts have been made to determine the cosmic evolution of the H2 density, but deeper ALMA observations are needed to effectively compare with models.