Evolution of galaxies in triaxial halos with figure rotation

Firstly, we demonstrate that unusually large outer HI spiral arms observed in NGC 2915 can form in an extended gas disk embedded in a massive triaxial dark matter halo with slow figure rotation, through the strong gravitational torque of the rotating halo. Secondly, we show that the figure rotation of a triaxial dark matter halo can influence dynamical evolution of disk galaxies by using fully self-consistent numerical simulations. We particularly describe the formation processes of “halotriggered” bars in thin galactic disks dominated by dark matter halos with figure rotation and discuss the origin of stellar bars in low luminosity, low surface brightness (LSB) disk galaxies. Thirdly, we provide several implications of the present numerical results in terms of triggering mechanism of starbursts in galaxies and stellar bar formation in high redshifts. 1. Kinematics of dark matter halos and galaxy evolution Although several attempts have been so far made to reveal the shapes (e.g., the degree of oblateness or triaxiality) of dark matter halos in galaxies (e.g., Sackett & Sparke 1990; Franx, van Gorkom, & de Zeeuw 1994; Olling 1995; Salucci & Persic 1997; Sackett 2003 in this conference), rotational properties of dark matter halos have been less discussed. Based on the detailed analysis of structure and kinematics of the very extended HI disk around NGC 2915, Bureau et al (1999) first suggested that the observed spiral-like structures in the HI disk can be formed by a triaxial halo with figure rotation. However, it is unclear whether a triaxial dark halo with figure rotation is really responsible for the observed extended spiral structures in NGC 2915, because of the lack of numerical studies of gas dynamics in the gravitational potentials of triaxial halos with figure rotation. Also it is an important problem to understand how figure rotation of a dark matter halo in a disk galaxy influences dynamical evolution, star formation history, and chemical evolution of the disk, because previous theoretical/numerical studies so far have not extensively investigated such influence of figure rotation