The extended Planetary Nebula Spectrograph (ePN.S) early-type galaxy survey. The kinematic diversity of stellar halos and the relation between halo transition scale and stellar mass

摘要

In the hierarchical two-phase formation scenario, the extended halos of earlytype galaxies (ETGs) are expected to have different physical properties fromthose of the galaxies' central regions. This work aims at characterizing thekinematic properties of ETG halos using planetary nebulae (PNe) as tracers,which allow us to overcome the limitations of absorption line spectroscopy ofcontinuum at low surface brightness. We present two-dimensional velocity andvelocity dispersion fields for 33 ETGs, including both fast and slow rotators,making this the largest kinematic survey to-date of extragalactic PNe. Thevelocity fields typically extend out to 6 effective radii (Re), with a range[3Re-13Re] for the PN.S ETGs. We complemented the PN kinematics with absorptionline data from the literature. We find that ETGs typically show a kinematictransition between inner regions and halo. Estimated transition radii in unitsof Re anticorrelate with stellar mass. Slow rotators have increased but stillmodest rotational support at large radii, while most of the fast rotators showa decrease in rotation, due to the fading of the stellar disk in the outer,more slowly rotating spheroid. 30% of the fast rotators are dominated byrotation also at large radii. Most ETGs have flat or slightly falling halovelocity dispersion profiles, but 15% of the sample have steeply fallingprofiles. One third of the fast rotators show kinematic twists, misalignments,or rotation along two axes, indicating that they turn from oblate near thecenter to triaxial in the halo. ETGs have more diverse kinematic properties intheir halos than in the central regions, and a significant fraction showssignatures of triaxial halos in the PNe data. The observed kinematic transitionto the halo and its dependence on stellar mass is consistent with LambdaCDMsimulations and supports a two-phase formation scenario. [abridged]