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TwitterA&A 424, 779-791 (2004)
DOI: 10.1051/0004-6361:20041306
The ESO Nearby Abell Cluster Survey
XIII. The orbits of the different types of galaxies in rich clusters
A. Biviano1 and P. Katgert21 INAF - Osservatorio Astronomico di Trieste, Italy
e-mail: biviano@ts.astro.it
2 Sterrewacht Leiden, The Netherlands
(Received 17 May 2004 / Accepted 3 June 2004 )
Abstract
We study the orbits of the various types of galaxies
observed in the ESO Nearby Abell Cluster Survey. We combine the
observed kinematics and projected distributions of galaxies of various
types with an estimate of the mass density profile of the ensemble
cluster to derive velocity-anisotropy profiles. Galaxies within and
outside substructures are considered separately. Among the galaxies
outside substructures we distinguish four classes, on the basis of
their projected phase-space distributions. These classes are: the
brightest ellipticals (with
), the other
ellipticals together with the S0's, the early-type spirals (Sa-Sb),
and the late-type spirals and irregulars (Sbc-Irr) together with the
emission-line galaxies (except those of early morphology). The mass
profile was determined from the distribution and kinematics of the
early-type (i.e. elliptical and S0) galaxies outside substructures;
the latter were assumed to be on isotropic orbits, which is supported
by the shape of their velocity distribution. The projected
distribution and kinematics of the galaxies of other types are used to
search for equilibrium solutions in the gravitational potential
derived from the early-type galaxies. We apply the method described by
Binney & Mamon as implemented by Solanes & Salvador-Solé to
derive, to our knowledge for the first time, the velocity
anisotropy profiles of all galaxy classes individually (except, of
course, the early-type class). We check the validity of the solutions
for
![$\beta'(r) \equiv [{{\langle} v_{\rm r}^2{\rangle}(r)}/{{\langle} v_{\rm t}^2
{\rangle}(r)}]^{1/2} $](/articles/aa/abs/2004/36/aa1306/img2.gif){kind=small}
, where
and
are the mean squared components of the radial
and tangential velocity, respectively, by comparing the observed and
predicted velocity-dispersion profiles. For the brightest ellipticals
we are not able to construct equilibrium solutions. This is most
likely the result of the formation history and the special location of
these galaxies at the centres of their clusters. For both the early
and the late spirals, as well as for the galaxies in substructures,
the data allow equilibrium solutions. The data for the early spirals
are consistent with isotropic orbits (
), although
there is an apparent radial anisotropy at
. For the late spirals an equilibrium solution with isotropic
orbits is rejected by the data at the
>99% confidence level. While
within
,
increases
linearly with radius to a value
1.8 at
.
Taken at face value, the data for the galaxies in substructures
indicate that isotropic solutions are not acceptable, and tangential
orbits are indicated. Even though the details of the tangential
anisotropy remain to be determined, the general conclusion appears
robust. We briefly discuss the possible implications of these
velocity-anisotropy profiles for current ideas of the evolution and
transformation of galaxies in clusters.
Key words: galaxies: clusters: general -- galaxies: kinematics and dynamics -- cosmology: observations
© ESO 2004