Evolution of dark gaps in barred galaxies

Aguerri, J. A. L.; D'Onghia, E.; Cuomo, V.; Morelli, L.
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Astronomy and Astrophysics

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Context. Recent studies show that barred galaxies have a light deficit, called a dark gap (DG), in the direction of the bar minor axis with respect to the major axis. The properties of these DGs might be related to the properties of the bars and to the location of some resonances of the galaxies.
Aims: We have analyzed the evolution of the DGs properties in the barred galaxy of the GALAKOS simulation to compare them with those from real galaxies and infer conclusions about the evolutionary status of the bars.
Methods: The DGs were measured by using the bar and the interbar surface brightness profiles from the Fourier decomposition of the light of the galaxies. We characterized them by two parameters: the prominence of the DG (Δμmax), and its location in the galaxy (RDG).
Results: In the GALAKOS simulation, both Δμmax and RDG evolve with time. Thus, the DGs are more prominent and are located at larger radii as the bar evolves. In addition, RDG is smaller than the bar radius (Rb) at all time steps of the simulation, being always Rb/RDG > 1.2. About 90% of the real galaxies show Rb/RDG > 1.2 similar to the GALAKOS simulation. For these objects, the ratio of the corrotation radius (RCR) and the DG radius is RCR/RDG ≈ 1.8. This is similar to the expected ratio of the corrotation resonance (CR) and the ultraharmonic resonance (UHR) radius. This indicates a link between the DGs and the UHR of these galaxies. The remaining 10% of the galaxies show Rb/RDG < 1.2 and RCR/RDG ≈ 1. In these cases, the DG would be linked with the CR of the galaxy. We have found that the bar in GALAKOS, as well as real bars in galaxies, can be located in distinct places on the RCR/h − Rb/h plane based on the prominence of their DGs. In particular, galaxies with high values of Δμmax are located at RCR/h and Rb/h larger than 1.5. The simulated bar turned to be a slow rotator when Δμmax was larger than 0.8. When this occurs, RCR/h and Rb/h are larger than 2.0 and 1.5, respectively, for the GALAKOS bar.
Conclusions: The location of the DG seems to be a robust signature of the location of the galaxy resonances. In addition, the prominence of the DG could be used as indicators of the amount of angular momentum exchange by the bar and other galactic components. Galaxies with Δμmax > 0.8 would be candidates for slow bar rotators. In addition, flat bars would be more evolved or would have exchanged more angular momentum than exponential bars.
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