The study of kinematic properties, angular momentum and amount of dark matter in the halos of earlytype galaxies (ETGs) is limited by the rapid fall-off of the stellar surface brightness. This difficulty can be overcome by using radial velocities of Planetary Nebulae (PNe), which can be obtained much further out than traditional absorption-line kinematics, given their bright [OIII] emission lines. The use of PNe allowed us to measure for the very first time two-dimensional velocity and velocity dispersion fields out to ~6-9 effective radii in nearby ETGs.

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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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V/σ(1Re) from absorption line data compared with V/σ(halo) from PN data. Different symbols show SRs, FRs, FRs with triaxial halos, and FRs with small numbers of PNe; for the latter V/σ(halo) is a lower limit estimate.

Transition radius RT in units of Re as a function of stellar mass M*. The shaded area shows the corresponding quantities from the simulations of Cooper et al. (2013).


Planetary nebula kinematics in NGC 1316: a young Sombrero

McNeil-Moylan E. K., Freeman K. C., Arnaboldi M., Gerhard O. E., 2012, A&A, 539, A11

Jeans model with no dark matter compared with the velocity dispersion profile of NGC 1316. The disagreement between the model and the observations in the outer part underlines the importance of dark matter in the dynamics of NGC 1316

An exceptionally large sample of PN velocities makes it possible to explore in detail the kinematics of the Fornax Brightest Cluster Galaxy, NGC 1316, a merger remnant. The two-dimensional velocity field shows dynamically-important rotation that rises in the outer parts, possibly due to the outward transfer of angular momentum during the merger. We use spherical, non-rotating, constant-anisotropy Jeans models were to analyze the kinematic data. These indicate a high dark matter content, particularly in the outer parts. NGC 1316 represents, then, a transition phase from a major-merger event to a bulge-dominated galaxy.

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Kinematic properties of early-type galaxy haloes using planetary nebulae

Coccato, L., Gerhard, O., Arnaboldi, M., Das, P., Douglas, N. G., Kuijken, K., et al. 2009, MNRAS, 394, 1249

DSS image of NGC 4474 with PNe data superimposed. Crosses represent the position of each PNe; colors indicate the mean value of the two-dimensional velocity field reconstructed from the PNe data. Iso-velocity contours are also shown. The field of view is 11'x11', North is up, East is left.

Over the past years we conducted an observational campain with the Planetary Nebulae Spectrograph (Douglas et al. 2002) aimed to measure the radial velocities of PNe in the halos of ETGs (see Figure 1). In our first official data release (Coccato et al. 2009) we combined absorption line data and PNe radial velocity measurements in 16 ETGs. Our analysis showed that: i) PNe are good tracers of the mean stellar population kinematics, as their kinematics and number density agrees with the stellar absorption line kinematics and surface brightness; ii) outer halos have more complex radial profiles of the lR parameter (a proxy for the angular momentum, Emsellem et al. 2007) than observed within 1 Re. Interestingly, in the halo, some fast rotators have declining lR radial profiles, almost reaching the slow rotator regime, while some slow rotators have slowly increasing lR profiles, which reach the fast rotator regime (see Figure 2); iii) the velocity dispersion profiles fall into two groups, with part of the galaxies characterized by slowly decreasing profiles and the remainder having steeply falling profiles; iv) the halo kinematics are correlated with other galaxy properties, such as luminosity, shape, total stellar mass, V/s, and number of PNe per unit luminosity, with a clear distinction between fast and slow rotators.

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