Publications (2)0 Total impact
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ABSTRACT: Mass modelling of spherical systems through internal motions is hampered by
the mass/velocity anisotropy (VA) degeneracy inherent in the Jeans equation, as
well as the lack of techniques that are both fast and adaptable to realistic
systems. A new fast method, called MAMPOSSt, which performs a maximum
likelihood fit of the distribution of observed tracers in projected phase
space, is developed and thoroughly tested. MAMPOSSt assumes a shape for the
gravitational potential, but instead of postulating a shape for the
distribution function in terms of energy and angular momentum, or supposing
Gaussian line-of-sight velocity distributions, MAMPOSSt assumes a VA profile
and a shape for the 3D velocity distribution, here Gaussian. MAMPOSSt requires
no binning, differentiation, nor extrapolation of the observables. Tests on
cluster-mass haloes from LambdaCDM cosmological simulations show that, with 500
tracers, MAMPOSSt is able to jointly recover the virial radius, tracer scale
radius, dark matter scale radius and outer or constant VA with small bias (<10%
on scale radii and <2% on the two other quantities) and inefficiencies of 10%,
27%, 48% and 20%, respectively. MAMPOSSt does not perform better when some
parameters are frozen, and even worse when the virial radius is set to its true
value, which appears to be the consequence of halo triaxiality. The accuracy of
MAMPOSSt depends weakly on the adopted interloper removal scheme, including an
efficient iterative Bayesian scheme that we introduce here, which can directly
obtain the virial radius with as good precision as MAMPOSSt. Our tests show
that MAMPOSSt with Gaussian 3D velocities is very competitive with, and up to
1000x faster than other methods. Hence, MAMPOSSt is a very powerful and rapid
tool for the mass and anisotropy modeling of systems such as clusters and
groups of galaxies, elliptical and dwarf spheroidal galaxies.
12/2012;
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ABSTRACT: When clusters of galaxies are viewed in projection, one cannot avoid picking up foreground/background interlopers (FBIs), that lie within the virial cone (VC), but outside the virial sphere. Structural & kinematic deprojection equations are not known for an expanding Universe, where the Hubble flow (HF) stretches the line-of-sight (LOS) distribution of velocities. We analyze 93 mock relaxed clusters, built from a cosmological simulation. The stacked mock cluster is well fit by an m=5 Einasto DM density profile (but only out to 1.5 virial radii [r_v]), with velocity anisotropy (VA) close to the Mamon-Lokas model with VA radius equal to that of density slope -2. The surface density of FBIs is nearly flat out to r_v, while their LOS velocity distribution shows a dominant gaussian cluster-outskirts component and a flat field component. This distribution of FBIs in projected phase space is nearly universal in mass. A local k=2.7 sigma velocity cut returns the LOS velocity dispersion profile (LOSVDP) expected from the NFW density and VA profiles measured in 3D. The HF causes a shallower outer LOSVDP that cannot be well matched by the Einasto model for any k. After this velocity cut, FBIs still account for 23% of DM particles within the VC (close to the observed fraction of cluster galaxies lying off the Red Sequence). The best-fit projected NFW/Einasto models underestimate the 3D concentration by 6+/-6% (16+/-7%) after (before) the velocity cut, unless a constant background is included in the fit. Assuming the correct mass profile, the VA profile is well recovered from the measured LOSVDP, with a slight bias towards more radial orbits in the outer regions. These small biases are overshadowed by large cluster-cluster variations caused by cosmic variance. An appendix provides an analytical approximation to the surface density, projected mass and tangential shear profiles of the Einasto model. Comment: Version published in A&A, 23 pages, 22 figures
02/2010;
