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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 kinematicsand properties of the two counter-rotating stellar populations to constraintheir formation scenario. A spectroscopic decomposition of the observedmajor-axis spectrum was performed to disentangle the relative contribution ofthe two counter-rotating stellar and one ionized-gas components. Theline-strength indices of the two counter-rotating stellar components weremeasured and modeled with single stellar population models that account for thealpha/Fe overabundance. The counter-rotating stellar population is younger,marginally more metal poor, and more alpha-enhanced than the main stellarcomponent. The younger stellar component is also associated with a star-formingring. The different properties of the counter-rotating stellar components ofNGC~4138 rule out the idea that they formed due to bar dissolution. Ourfindings support the results of numerical simulations in which thecounter-rotating component assembled from gas accreted on retrograde orbitsfrom the environment or from the retrograde merging with a gas-rich dwarfgalaxy.

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