DYNAMICAL THEORY OF ELLIPTICAL GALAXIES
DYNAMICAL THEORY OF ELLIPTICAL GALAXIES 
The 3D shapes of elliptical galaxies are still not known; knowing them
would tell us much about the processes operating during the epoch of galaxy
formation. The obstacle to our undertanding is the bewildering array of
orbits on which the stars in the galaxy can move. Otherwise identical
systems can present entirely different dynamical faces, depending on
which orbits are populated. Disentangling the effects of orbit populations
from the influence of the shape of the gravitational potential -- especially
when we can observe only the line-of-sight component of the motion -- is
a challenge.
Theoretical models for the average flow of the stellar "fluid" through the galaxy suggest that the triaxiality (the degree of "hamburgerness" vs. "hotdogness") of ellipticals can be distinguished by the symmetry, or lack of symmetry, of their projected velocity fields. Although small compared to the random motions, the mean velocity field in effect shows the locations of critical points associated with orbit family boundaries, which reveal the shape of the potential; this is why the velocity field can be a shape diagnostic.
We use Bayesian statistical methods to estimate the true shapes of
elliptical galaxies from observations of their projected light
distributions and stellar motions. Results from this work and the
efforts of others present a puzzle: from the distribution of apparent
shapes, we know with better than 95% confidence that not all
ellipticals are axisymmetric -- either oblate (hamburger) or prolate
(hot dog). At the same time, dynamical modeling suggests that many, if
not most, ellipticals are nearly oblate. And yet, there are some
objects -- NGC 5128 and NGC 4589 among them -- that are unambiguously
triaxial. Does this suggest that there are two or more sub-families of
ellipticals, formed by different processes?
SPECTROSCOPIC OBSERVATIONS 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 (taken at the MMT in February 1995)
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. Dynamical models are being run to
work out the consequences of this twist for the shape of the galaxy.
This mean velocity field for NGC 1700 was similarly reconstructed from MMT
spectra 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. 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 H_0 = 50).
Our Cross-Correlation
software is available to interested researchers.
ELLIPTICAL GALAXIES IN THE HUBBLE DEEP FIELD
Undergraduate students have catalogued all the galaxies in the Hubble Deep
Field. By applying numerical filters we have identified objects that
Edwin Hubble would have classified as ellipticals, if he had had access to
the HDF in 1926. Out of some 2100 objects (most of which are galaxies), we
find only 63 clear ellipticals, some of which are shown in
the illustration at right.
The object at upper left has a redshift of 0.764, corresponding to
a distance of about 4 billion light-years; in other words we see
it as it was 4 billion years ago. The other galaxies are probably
more distant, and appear as they were at even earlier times.
Faculty Postdocs