Dark Halo Shapes and the Fate of Stellar Bars

Abstract

We investigate the stability properties of trajectories in barred galaxies with mildly triaxial halos by means of Liapunov exponents. This method is perfectly suitable for time-dependent three-dimensional potentials where surfaces of sections and other simple diagnostics are not applicable. We find that when halos are centrally concentrated most trajectories starting near the plane containing the bar become chaotic. The spatial density distribution of these orbits does not match that of the bar, being overextended in and out of the plane compared to the latter. Moreover, the shape of many of the remaining regular trajectories does not match the bar density distribution, being too round. Therefore, time-independent self-consistent solutions are highly unlikely to be found. When the nonrotating nonaxisymmetric perturbation in the potential reaches 10%, almost all trajectories integrated are chaotic and have large Liapunov exponents. No regular trajectories aligned with the bar have been found. Hence, if the evolution of the density figure is directly related to the characteristic timescale of orbital instability, bar dissolution would take place on a timescale of a few dynamical times. The slowly rotating nonaxisymmetric contribution to the potential required for the onset of widespread chaotic behavior is remarkably small. Even a potential axis ratio of 0.99 results in large connected chaotic regions dominating the space of initial conditions. Systems consisting of centrally concentrated axisymmetric halos and stellar bars thus appear to be structurally unstable, and small ($1%) deviations from perfect axisymmetry should result in a bar dissolution on a timescale significantly smaller than the Hubble time. Since halos found in cold dark matter simulations of large-scale structure are both centrally concentrated and triaxial, it is unlikely that stellar bars embedded in such halos would form and survive unless the halos are modified during the formation of the baryonic component.

Structure Formation inside Triaxial Dark Matter Halos: Galactic Disks, Bulges, and Bars

“…On the other hand, halo shapes inferred from observations in the local universe are axisymmetric, although individual objects may exhibit some mild degree of prolateness (e.g., Rix & Zaritsky 1995;Merrifield 2002). Theoretical arguments supplemented by numerical simulations have shown that bars are incompatible with prolate halos (El-Zant & Shlosman 2002;. When taken in tandem with the plethora of stellar bars observed locally (e.g., Marinova & Jogee 2007;Menendez-Delmestre et al 2007;Knapen et al 2000) and at the intermediate redshifts of $1, corresponding to the lookback time of 8 Gyr (Jogee et al 2004), these results indicate strongly that the halo shapes have evolved only mildly during this time period.…”

“…While prolate DM halos appear to be incompatible with large-scale stellar bars ( El-Zant & Shlosman 2002;, the growth of disks acts to wash out the equatorial prolateness within, at least, the inner halos . The present work confirms this trend; we observe a decrease in the halo flatness even well outside the disk, elimination of its prolateness within the inner (<20 kpc) halo, and reduction in the outer halo.…”

Long-lived Double-barred Galaxies: Critical Mass and Length Scales

“…Indeed, in a somewhat analogous situation of a bar embedded in a nonrotating triaxial halo, we have shown that the bar is unsustainable unless its contribution significantly exceeds that of the halo, in which case it is able to trap its supporting orbits and stabilize (El-Zant & Shlosman 2002). In nested bar systems a similar situation can be constructed by invoking separation of mass and length scales, with each bar dominating its own domain.…”

Evolution of Stellar Bars in Live Axisymmetric Halos: Recurrent Buckling and Secular Growth

“…Recent efforts include but are not limited to the issues related to the bar lifetime cycles, gas-star interactions, bar amplitudes and sizes, and bar slowdown (e.g., Bournaud & Combes 2002;Valenzuela & Klypin 2003;Shen & Sellwood 2004;Weinberg 1985;Hernquist & Weinberg 1992;Debattista & Sellwood 1998. The bigger issue, of course, is how the observational and theoretical aspects of bar evolution fit within the emerging understanding of cosmological galaxy evolution (e.g., Jogee et al 2004;Elmegreen et al 2004), specifically, the bar evolution in triaxial halos ( El-Zant & Shlosman 2002;Berentzen et al 2006). …”