Probing the Outer Halo Kinematics of Early-Type Galaxies using Planetary Nebulae

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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.


1. Planetary nebula kinematics in NGC 1316: a young Sombrero

N4374 velocity field Lambda radial profiles
Figure 1. Spatial distribution of the PNs tha were found in the NGC 1316 observations. The north-east is rotating away from the observer while the south-west is moving towards us relative to the mean. North is up and East is left. Figure 2. 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.

2. Kinematics and Angulare Momentum in the Outer Halos of Early Type Galaxies
N4374 velocity field Lambda radial profiles
Figure 1. 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. Figure 2. Radial profile of the angular momentum proxy LR. Red and blue profiles are taken from the SAURON sample (Emsellem et al. 2007) and represent slow rotators (galaxies with LR < 0.1) and fast rotators (LR > 0.1), respectively. Black profiles are computed using PNe radial velocity measurements.

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.



Coccato, L., Gerhard, O., Arnaboldi, M., Das, P., Douglas, N. G., Kuijken, K., et al. 2009, MNRAS, 394, 1249.
Kinematic properties of early-type galaxy haloes using planetary nebulae. 2009MNRAS.394.1249C

Coccato, L., Gerhard, O., Arnaboldi, M., Das, P., Douglas, N. G., Kuijken, K., et al. 2008, AN, 329, 912.
Probing the kinematics of early-type galaxy halos using planetary nebulae. 2008AN....329..912C

Noordermeer, E., Merrifield, M. R., Coccato, L., Arnaboldi, M., Capaccioli, M., Douglas, N. G., et al. 2008, MNRAS, 384, 943.
Testing the nature of S0 galaxies using planetary nebula kinematics in NGC 1023. 2008MNRAS.384..943N

Douglas, N. G., Napolitano, N. R., Romanowsky, A. J., Coccato, L., Kuijken, K., Merrifield, M. R., et al. 2007, ApJ, 664, 257.
The PN.S Elliptical Galaxy Survey: Data Reduction, Planetary Nebula Catalog, and Basic Dynamics for NGC 3379. 2007ApJ...664..257D

Merrett, H. R., Merrifield, M. R., Douglas, N. G., Kuijken, K., Romanowsky, A. J., Napolitano, N. R., et al. 2006, MNRAS, 369, 120.
A deep kinematic survey of planetary nebulae in the Andromeda galaxy using the Planetary Nebula Spectrograph. 2006MNRAS.369..120M