Galaxy mapping with the SAURON integral-field spectrograph: The star formation history of NGC 4365

We report the first wide-field mapping of the kinematics and stellar popula tions in the E3 galaxy NGC 4365. The velocity maps extend previous long-sli t work. They show two independent kinematic subsystems: the central 300 pc x 700 pc rotates about the projected minor axis, and the main body of the g alaxy, 3 kpc x 4 kpc, rotates almost at right angles to this. The line stre ngth maps show that the metallicity of the stellar population decreases fro m a central value greater than solar to one-half solar at a radius of 2 kpc . The decoupled core and main body of the galaxy have the same luminosity- weighted age, approximate to 14 Gyr, and the same elevated magnesium- to-ir on ratio. The two kinematically distinct components have thus shared a comm on star formation history. We infer that the galaxy underwent a sequence of mergers associated with dissipative star formation that ended greater than or similar to 12 Gyr ago. The misalignment between the photometric and kin ematic axes of the main body is unambiguous evidence of triaxiality. The si milarity of the stellar populations in the two components suggests that the observed kinematic structure has not changed substantially in 12 Gyr.