Long-lived triaxiality in the dynamically old elliptical galaxy NGC 4365: a limit on chaos and black hole mass

ABSTRACT

Supermassive black holes in the centres of giant elliptical galaxies are thought to be capable of inducing chaos and eliminating or preventing triaxiality in their hosts if they are sufficiently massive. We address whether this process operates in real systems, by modelling the stellar kinematics of the old elliptical NGC 4365. This galaxy has a mean stellar population age >12 Gyr and is known for its kinematically decoupled core and skew rotation at larger radii. We fit the two-dimensional mean velocity field obtained by the SAURON integral-field spectrograph, and the isophotal ellipticity and position-angle profiles, using the velocity field (VF) fitting approach. The models constrain the intrinsic shape of the system between 0.03 and 0.5 effective radii, as well as its orientation in space. We find NGC 4365 to be strongly triaxial  (〈T〉≈ 0.45)  and somewhat flatter than it appears  (〈c/a〉≈ 0.6) . Axisymmetry or near axisymmetry  (T < 0.1)  is ruled out at >95 per cent confidence for  1.6  arcsec < R < 3.2 arcsec (0.03 < R/r_e < 0.06) , and at >99 per cent confidence at larger radii. There is an indication of an outward triaxiality gradient. The line of sight is constrained to two narrow bands on the viewing hemisphere. In the most probable orientation, the long axis points roughly toward the observer, extending to the south-west in projection. The stellar population age implies that strong triaxiality has persisted for hundreds of dynamical times. This rules out black holes  >3 × 10⁹ M_⊙ , which numerical simulations indicate would either have globally axisymmetrized the galaxy or made the inner several arcsec spherical. The  M_BH–σ  relation predicts  M_BH≈ 4 × 10⁸ M_⊙ , which would probably not preclude long-lived triaxiality and is consistent with the observations. There must also be an unequal population of direct and retrograde long-axis tube orbits outside the kinematically decoupled core. This, combined with the small isophotal twist, limits the rate of figure rotation (tumbling) about the short axis, and places corotation at >8 effective radii. NGC 4365 lends support to a picture in which supermassive black holes, though omnipresent in luminous giant elliptical galaxies, are not massive enough to alter their global structure through chaos.