Spectroscopic evidence of distinct stellar populations in the counter-rotating stellar disks of NGC 3593 and NGC 4550

Aims. We present the results of integral-field spectroscopic observations of the two disk galaxies NGC 3593 and NGC 4550 obtained with the Visible Multi Object Spectrograph at the Very Large Telescope. Both galaxies are known to host two counter-rotating stellar disks, with the ionized gas co-rotating with one of them. We measured in each galaxy the surface brightness, kinematics, mass surface density, and the stellar populations of the two stellar components, as well as the distribution, kinematics, and metallicity of the ionized-gas component to constrain the formation scenario of these peculiar galaxies. Methods. We applied a novel spectroscopic decomposition technique to both galaxies, to disentangle at each position in the field of view the relative contribution of the two counter-rotating stellar and one ionized-gas components to the observed spectrum. We measured the kinematics and the line strengths of the Lick indices of the two counter-rotating stellar components. We modeled the data of each stellar component with single stellar population models that account for the α/Fe overabundance. Results. In both galaxies we successfully separated the main from the secondary stellar component that is less massive and rotates in the same direction as the ionized-gas component. The two stellar components have exponential surface-brightness profiles. In NGC 3593 they have different scale lengths, with the secondary one dominating the innermost 500 pc. In NGC 4550 they have the same scale lengths, but slightly different scale heights. In both galaxies, the two counter-rotating stellar components have different stellar populations. The secondary stellar disk is younger, more metal poor, and more α-enhanced than the main galaxy stellar disk. Such a difference is stronger in NGC 3593 than in NGC 4550. Conclusions. Our findings rule out an internal origin of the secondary stellar component and favor a scenario where it formed from gas accreted on retrograde orbits from the environment fueling an in situ outside-in rapid star formation. The event occurred about 2 Gyr ago in NGC 3593 (1.6 ± 0.8 Gyr after the formation of the main galaxy disk), and about 7 Gyr ago in NGC 4550 (less than 1 Gyr after the formation of the main galaxy disk). The formation through a binary galaxy merger cannot be ruled out, and a larger sample is required to statistically determine which mechanism is the most efficient to build counter-rotating stellar disks.