The different formation mechanisms of counter-rotating disk galaxies are expected to leave different signatures in the properties of the prograde and retrograde stellar populations. In particular, their age difference may be used to discriminate between competing scenarios for the origin of counter-rotation.
The gas accretion followed by star formation always predicts a younger age for the counter-rotating component, whereas the counter-rotating component formed by the retrograde capture of stars through minor or major mergers may be either younger or older with respect to the pre-existing stellar disk. The external origin also allows that the two counter-rotating components have different metallicities and α-enhancements. In contrast, the formation of large-scale counter-rotating stellar disks due to bar dissolution predicts the same mass, chemical composition, and age for both the prograde and retrograde components.
A spectroscopic decomposition that separates the relative contribution of the counter-rotating stellar components to the observed galaxy spectrum is therefore needed to disentangle their stellar populations. This has been recently done for the counter-rotating stellar disks of NGC 3593 (Coccato et al. 2013), NGC 4550 (Coccato et al. 2013, Johnston et al. 2013), and NGC 5719 (Coccato et al. 2011). In all of them, the counter-rotating stellar disk rotates in the same direction as the ionized gas, and it is less massive, younger, more metal poor, and more α-enhanced than the main stellar disk. These 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 Sab spiral NGC 5179 shows a spectacular on-going interaction with its face-on Sbc companion NGC 5713 (Fig. 2, top panel). The interaction is traced by a tidal bridge of neutral hydrogen, which feeds the counter-rotating gaseous and stellar components (Vergani et al. 2007). NGC 5719 is the first interacting disk galaxy in which counter-rotation has been detected (Fig. 2, bottom panels). The age of counter-rotating stellar population ranges from 0.7 to 2.0 Gyr and metallicity changes from subsolar ([Z/H] ≃ -1.0 dex) in the outskirts to supersolar ([Z/H] ≃ 0.3 dex) in the center, whereas the main stellar component has ages ranging from 2 to 13.5 Gyr and nearly solar metallicity. The youngest ages and highest metallicities are found in correspondence of the star forming regions. The α-enhancement of the counter-rotating component indicates a star formation history with a time-scale of 2 Gyr (Fig. 2, middle panels). On the contrary, the formation through a major galaxy merger cannot be completely ruled out for NGC 3593 and NGC 4550, which are both quite isolated and undisturbed galaxies (Coccato et al. 2011, Coccato et al. 2013). A larger sample is required to understand by statistical arguments whether it was accretion or merger the most efficient mechanism to assembly counter-rotating spirals.
Figure 2. The counter-rotating stellar disks of the interacting Sab spiral galaxy NGC 5179. Top panels: Contour map of the HI column density distribution of NGC 5719 superimposed on an optical image from the Digitized Sky Survey. The lowest contour level is at 7.0 × 1019 atoms cm-2 and the increment is 2.4 × 1020 atoms cm-2. From Vergani et al. (2007). Middle panels: Field of view observed with integral-field spectroscopy (left panel) and measured equivalent width of the line-strength indices Hβ vs. Mg b (central panel) and ‹Fe› vs. [MgFe]' (right panel) with the predictions for age, metallicity, and α-enhancement from single stellar population models. Each spatial bin returns the indices of both the main stellar (red diamonds) and secondary counter-rotating stellar component (blue circles). From Coccato et al. (2011). Bottom panels: Two-dimensional velocity fields of the main stellar component (left panel), secondary stellar component (central panel), and ionized-gas component (right panel). From Coccato et al. (2011).