Deciphering Galaxy Disk Assembly through Stellar Counter-Rotation

Understanding how galaxies assemble their baryonic components requires disentangling the relative contributions of in-situ formation and external accretion. One striking example of externally driven assembly is the presence of stellar counter-rotation (CR) disks, where a substantial fraction of stars in a galaxy rotate opposite to the older stellar disk. These CR disks were formed by the accretion of gas (wet mergers or cosmic filaments) with opposite angular momentum to the preexisting stellar component, providing a unique observational window into how external gas contributes to the buildup of galactic disks. Using data from the MaNGA SDSS survey, we have identified a diverse sample of 120 galaxies with CR disks (~2% of all early-type galaxies), spanning a wide range of masses, environments, and star formation histories. These systems offer a unique opportunity to study how external accretion, angular momentum redistribution, and mergers shape galaxy evolution. Some of the most intriguing examples are massive galaxies in dense environments, where the gas accretion is difficult or impossible. This talk will present the insights into how accreted gas contributes to the growth of new disks, shedding light on the interplay between external gas accretion, counter-rotation, and the baryon cycle in galaxy evolution.