SAURON'S CHALLENGE FOR THE MAJOR MERGER SCENARIO OF ELLIPTICAL GALAXY FORMATION

The intrinsic anisotropyand flatteningof simulated merger remnants is compared with elliptical galaxies that have been observed by the SAURON collaboration, and that were analyzed using axisymmetric Schwarzschild models. Collisionless binary mergers of stellar disks and disk mergers with an additional isothermal gas component, neglecting star formation, cannot reproduce the observed trend � ¼ 0:55� (SAURON relationship). An excellent fit of the SAURON relationship for flattened ellipticals with � � 0:25 is, however, found for merger simulations of disks with gas fractions � 20%, including star formation and stellar energy feedback. Massive black hole feedback does not strongly affect this result. Subsequent dry merging of these merger remnants does not generate the slowly rotatingSAURON ellipticals,which are characterized bylow ellipticities �< 0:25 and low anisotropies.These objects therefore might not havebeenshapedbyafinalmajor merger of eitherearly-typegalaxiesordisks.We show,however, that stellar spheroids resulting from multiple, hierarchical mergers of star-bursting subunits in a cosmological context are in excellent agreement with the low ellipticities and anisotropies of the slowly rotating SAURON ellipticals and their observed trend ofwith � . The numerical simulations indicate that the SAURON relation might be a result of strong violent relaxation and phase mixing of multiple, kinematically cold stellar subunits, with the angular mo- mentum of the system determining its location on the relation. Subject headingg galaxies: elliptical and lenticular, cD — galaxies: evolution — galaxies: formation — galaxies: kinematics and dynamics — galaxies: structure — methods: n-body simulations