The extended planetary nebula spectrograph (ePN.S) early type galaxy survey: the kinematic diversity of stellar halos

Pulsoni, C., Gerhard, O., Arnaboldi, M., Coccato, L., Longobardi, A., Napolitano, N. R., et al., 2018, A&A, 618, A94

The current understanding of the formation scenario for early type galaxies (ETGs) is that it proceeds in two phases: an initial fast assembly stage, in which galaxies form in-situ stars, followed by a phase dominated by the hierarchical accretion of smaller galaxies, whose stars mainly deposit in the halos. Because of the different origin of the halos with respect to the central regions, ETGs are expected to have different kinematic properties at large radii than at the centers (typically inside 1 effective radii, Re), where they neatly divide between fast (FRs) and slow rotators (SRs) according to their angular momentum. Measuring the kinematics of ETG halos is challenging, as they are too faint to be studied with absorption line spectroscopy, and they require alternative kinematic tracers such as planetary nebulae (PNe). The extended PN Spectrograph (ePN.S) survey is the largest survey to-date of ETG kinematics with PNe, based on data from the Planetary Nebula Spectrograph (PN.S), counter-dispersed imaging, and high-resolution PN spectroscopy. With the ePN.S data we investigated the kinematics of 33 ETGs out to their faintest outskirts (6 Re, on average). We find that ETGs typically show a kinematic transition between inner regions and halos. SRs have increased  rotational support at large radii. Most of the FRs show a decrease in rotation, due to the fading of the stellar disk in the outer, more slowly rotating spheroid. 30% of these fast rotators are dominated by rotation also at large radii, 40% show kinematic twists or misalignments, indicating a transition from oblate to triaxial in the halo. Despite this variety of kinematic behaviors, most of the ePN.S halos have similar rotational support, independently of fast/slow rotation of the central regions. The location where the observed variation of the kinematics from the central regions to the halos occurs defines a transition radius. Estimated transition radii in units of Re are ~1-3 Re and anti-correlate with stellar mass, meaning that these kinematically distinct halos are more prominent in the more massive galaxies. These results are consistent with cosmological simulations and support the two-phase formation scenario for ETGs.

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