DYNAMICAL MAPPING OF ELLIPTICAL GALAXIES

We need very high accuracy long-slit spectra at multiple position angles to get good results from dynamical model fitting. Such observations typically require 1 or more nights per galaxy on a 4-meter-class telescope. In addition to containing clues to the shape of the galaxy, the data can also be used to derive absorption line indices as a function of distance from the the galactic center, revealing much about the stellar populations and dynamical history.

The well known E1 elliptical NGC 3379 (M105, the galaxy on the right) is now strongly suspected of harboring a central massive black hole. Central mass concentrations, in turn, are implicated in the destruction of triaxiality through the creation of orbital chaos. We want, therefore, to measure the triaxiality of this system.

From our spectrocopic data on NGC 3379 (Statler & Smecker-Hane 1999) we can compute a Fourier reconstruction of the mean velocity and dispersion fields out to large radii. The circular maps extend 56" from the center. (The velocity scales are 0 to 60 km/s and 100 to 205 km/s in the left and right maps, respectively.) Note that the velocity field is slightly twisted relative to the outer isophotes, signaling a weak triaxiality.

Dynamical modeling (Statler 2001) is able to recover, as a probability amplitude, the shape of the system in each of 9 radial zones, which can then be turned into an estimate of the intrinsic shape profile. The figure at lower right shows the most likely profiles of triaxiality T and flattening cL (the "L" refers to the luminous mass), with 1- and 2-sigma error regions. The galaxy is most probably axisymmetric in the center, and weakly triaxial in the outer regions. This is consistent with a scenario in which triaxiality is gradually destroyed from the center of the galaxy outward. However, there are models that are strongly triaxial, even in the center, which also fit the data.


This mean velocity field for NGC 1700 was similarly reconstructed from MMT spectra (Statler et al. 1996). The reflection symmetry and the lack of minor-axis rotation show that the galaxy is nearly oblate inside 2.5 effective radii. The diagram below shows the probability distribution for the shape of the galaxy in a zone about 4 kpc from the center, in terms of the triaxiality T and the overall flattening (Statler et al. 1999). Other details of the kinematic and photometric structure suggest that NGC 1700 owes its present form to a merger of 3 or more stellar systems 4 to 8 billion years ago (for H0 = 50).

Our Cross-Correlation software is available to interested researchers.

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Last updated 31 July 01.