The difference in age of the two counter-rotating stellar disks of the spiral galaxy NGC 4138

Context. Galaxies accrete material from the environment through acquisitions and mergers. These processes contribute to the galaxy assembly and leave their fingerprints on the galactic morphology, internal kinematics of gas and stars, and stellar populations. Aims. The Sa spiral NGC 4138 is known to host two counter-rotating stellar disks, with the ionized gas co-rotating with one of them. We measured the kinematics and properties of the two counter-rotating stellar populations to constrain their formation scenario. Methods. A spectroscopic decomposition of the observed major-axis spectrum was performed to disentangle the relative contribution of the two counter-rotating stellar and one ionized-gas components. The line-strength indices of the two counter-rotating stellar components were measured and modeled with single stellar population models that account for the alpha/Fe overabundance. Results. The counter-rotating stellar population is younger, marginally more metal poor, and more a-enhanced than the main stellar component. The younger stellar component is also associated with a star-forming ring. Conclusions. The different properties of the counter-rotating stellar components of NGC 4138 rule out the idea that they formed because of bar dissolution. Our findings support the results of numerical simulations in which the counter-rotating component assembled from gas accreted on retrograde orbits from the environment or from the retrograde merging with a gas-rich dwarf galaxy.