31 research outputs found

### Angular Momentum Transfer in Dark Matter Halos: Erasing the Cusp

We propose that angular momentum transfer from the baryons to the Dark Matter
(DM) during the early stages of galaxy formation can flatten the halo inner
density profile and modify the halo dynamics. We compute the phase-space
distribution function of DM halos, that corresponds to the density and
anisotropy profiles obtained from N-body simulations in the concordance
cosmology. We then describe an injection of angular momentum into the halo by
modifying the distribution function, and show that the system evolves into a
new equilibrium configuration; the latter features a constant central density
and a tangentially-dominated anisotropy profile in the inner regions, while the
structure is nearly unchanged beyond 10% of the virial radius. Then we propose
a toy model to account for such a halo evolution, based on the angular momentum
exchange due to dynamical friction; at the epoch of galaxy formation this is
efficiently exerted by the DM onto the gas clouds spiralling down the potential
well. The comparison between the angular momentum profile gained by the halo
through dynamical friction and that provided by the perturbed distribution
function reveals a surprising similarity, hinting at the reliability of the
process.Comment: 10 pages, 6 figures. Minor changes, ApJ accepte

### The Hubble Constant from the Gravitational Lens B1608+656

We present a refined gravitational lens model of the four-image lens system
B1608+656 based on new and improved observational constraints: (i) the three
independent time-delays and flux-ratios from VLA observations, (ii) the
radio-image positions from VLBA observations, (iii) the shape of the
deconvolved Einstein Ring from optical and infrared HST images, (iv) the
extinction-corrected lens-galaxy centroids and structural parameters, and (v) a
stellar velocity dispersion, sigma_ap=247+-35 km/s, of the primary lens galaxy
(G1), obtained from an echelle spectrum taken with the Keck--II telescope. The
lens mass model consists of two elliptical mass distributions with power-law
density profiles and an external shear, totaling 22 free parameters, including
the density slopes which are the key parameters to determine the value of H_0
from lens time delays. This has required the development of a new lens code
that is highly optimized for speed. The minimum-chi^2 model reproduces all
observations very well, including the stellar velocity dispersion and the shape
of the Einstein Ring. A combined gravitational-lens and stellar dynamical
analysis leads to a value of the Hubble Constant of H_0=75(+7/-6) km/s/Mpc (68
percent CL; Omega_m=0.3, Omega_Lambda=0.7. The non-linear error analysis
includes correlations between all free parameters, in particular the density
slopes of G1 and G2, yielding an accurate determination of the random error on
H_0. The lens galaxy G1 is ~5 times more massive than the secondary lens galaxy
(G2), and has a mass density slope of gamma_G1=2.03(+0.14/-0.14) +- 0.03 (68
percent CL) for rho~r^-gamma', very close to isothermal (gamma'=2). (Abridged)Comment: 17 pages, 6 figures, 5 tables; revised version with correct fig.6 and
clarified text based on referee report; conclusions unchange

### Do current WIMP direct measurements constrain light relic neutralinos?

New upper bounds on direct detection rates have recently been presented by a
number of experimental collaborations working on searches for WIMPs. In this
paper we analyze how the constraints on relic neutralinos which can be derived
from these results is affected by the uncertainties in the distribution
function of WIMPs in the halo. Various different categories of velocity
distribution functions are considered, and the ensuing implications for
supersymmetric configurations derived. We conservatively conclude that current
experimental data do not constrain neutralinos of small mass (below 50 GeV).Comment: 9 pages, 7 figures, typeset with ReVTeX4. The paper may also be found
at http://www.to.infn.it/~fornengo/papers/constraints05.ps.gz or through
http://www.astroparticle.to.infn.it/index.htm

### Distribution function of the dark matter

There is good evidence from N-body simulations that the velocity distribution
in the outer parts of halos is radially anisotropic, with the kinetic energy in
the radial direction roughly equal to the sum of that in the two tangential
directions. We provide a simple algorithm to generate such cosmologically
important distribution functions. Introducing r_E(E), the radius of the largest
orbit of a particle with energy E, we show how to write down almost trivially a
distribution function of the form f(E,L)=g(r_E)/L for any spherical model --
including the NFW profile. We in addition give the generic form of the
distribution function for any model with a local density power-law index and
anisotropy parameter, and provide limiting forms appropriate for the central
parts and envelopes of dark matter halos. From those, we argue that, regardless
of the anisotropy, the density fall-off at large radii must evolve to 1/r^4 or
steeper ultimately.Comment: to appear in PRD, including 3 figures, typo correcte

### Partial suppression of the radial orbit instability in stellar systems

It is well known that the simple criterion proposed originally by Polyachenko
and Shukhman (1981) for the onset of the radial orbit instability, although
being generally a useful tool, faces significant exceptions both on the side of
mildly anisotropic systems (with some that can be proved to be unstable) and on
the side of strongly anisotropic models (with some that can be shown to be
stable). In this paper we address two issues: Are there processes of
collisionless collapse that can lead to equilibria of the exceptional type?
What is the intrinsic structural property that is responsible for the sometimes
noted exceptional stability behavior? To clarify these issues, we have
performed a series of simulations of collisionless collapse that start from
homogeneous, highly symmetrized, cold initial conditions and, because of such
special conditions, are characterized by very little mixing. For these runs,
the end-states can be associated with large values of the global pressure
anisotropy parameter up to 2K_r/K_T \approx 2.75. The highly anisotropic
equilibrium states thus constructed show no significant traces of radial
anisotropy in their central region, with a very sharp transition to a radially
anisotropic envelope occurring well inside the half-mass radius (around 0.2
r_M). To check whether the existence of such almost perfectly isotropic
"nucleus" might be responsible for the apparent suppression of the radial orbit
instability, we could not resort to equilibrium models with the above
characteristics and with analytically available distribution function; instead,
we studied and confirmed the stability of configurations with those
characteristics by initializing N-body approximate equilibria (with given
density and pressure anisotropy profiles) with the help of the Jeans equations.Comment: 26 pages, 9 figures, accepted for publication in The Astrophysical
Journa

### Modeling the dynamical evolution of the M87 globular cluster system

We study the dynamical evolution of the M87 globular cluster system (GCS)
with a number of numerical simulations. We explore a range of different initial
conditions for the GCS mass function (GCMF), for the GCS spatial distribution
and for the GCS velocity distribution. We confirm that an initial power-law
GCMF like that observed in young cluster systems can be readily transformed
through dynamical processes into a bell-shaped GCMF. However,only models with
initial velocity distributions characterized by a strong radial anisotropy
increasing with the galactocentric distance are able to reproduce the observed
constancy of the GCMF at all radii.We show that such strongly radial orbital
distributions are inconsistent with the observed kinematics of the M87 GCS. The
evolution of models with a bell-shaped GCMF with a turnover similar to that
currently observed in old GCS is also investigated. We show that models with
this initial GCMF can satisfy all the observational constraints currently
available on the GCS spatial distribution,the GCS velocity distribution and on
the GCMF properties.In particular these models successfully reproduce both the
lack of a radial gradient of the GCS mean mass recently found in an analysis of
HST images of M87 at multiple locations, and the observed kinematics of the M87
GCS.Our simulations also show that evolutionary processes significantly affect
the initial GCS properties by leading to the disruption of many clusters and
changing the masses of those which survive.The preferential disruption of inner
clusters flattens the initial GCS number density profile and it can explain the
rising specific frequency with radius; we show that the inner flattening
observed in the M87 GCS spatial distribution can be the result of the effects
of dynamical evolution on an initially steep density profile. (abridged)Comment: 15 pages,14 figures;accepted for publication in The Astrophysical
Journa

### Galactic cannibalism in the galaxy cluster C0337-2522 at z=0.59

According to the galactic cannibalism model, cD galaxies are formed in the
center of galaxy clusters by merging of massive galaxies and accretion of
smaller stellar systems: however, observational examples of the initial phases
of this process are lacking. We have identified a strong candidate for this
early stage of cD galaxy formation: a group of five elliptical galaxies in the
core of the X-ray cluster C0337-2522 at redshift z=0.59. With the aid of
numerical simulations, in which the galaxies are represented by N-body systems,
we study their dynamical evolution up to z=0; the cluster dark matter
distribution is also described as a N-body system. We find that a multiple
merging event in the considered group of galaxies will take place before z=0
and that the merger remnant preserves the Fundamental Plane and the
Faber-Jackson relations, while its behavior with respect to the Mbh-sigma
relation is quite sensitive to the details of black hole merging [abridged].Comment: 30 pages, 7 figures, MNRAS (accepted

### Numerical stability of a family of Osipkov-Merrit models

We have investigated the stability of a set of non-rotating anisotropic
spherical models with a phase-space distribution function of the
Osipkov-Merritt type. The velocity distribution in these models is isotropic
near the center and becomes radially anisotropic at large radii. They are
special members of the family studied by Dehnen and Tremaine et al. where the
mass density has a power-law cusp $\rho\propto r^{-\gamma}$ at small radii and
decays as $\rho\propto r^{-4}$ at large radii. The radial-orbit instability of
models with $\gamma$ = 0, 1/2, 1, 3/2, and 2, was studied using an N-body code
written by one of us and based on the `self-consistent field' method developed
by Hernquist and Ostriker. These simulations have allowed us to delineate a
boundary in the $(\gamma,r_{a})$-plane that separates the stable from the
unstable models. This boundary is given by $2T_{r}/T_{t} = 2.31 \pm 0.27$, for
the ratio of the total radial to tangential kinetic energy. We also found that
the stability criterion $df/dQ\le 0$, recently raised by Hjorth, gives lower
values compared with our numerical results.Comment: AASTEX, 22 pages, 11 figures, Figs. 5 available from author. Accepted
for publication in Astrophysical Journa

### Equilibrium Disk-Bulge-Halo Models for the Milky Way and Andromeda Galaxies

We describe a new set of self-consistent, equilibrium disk galaxy models that
incorporate an exponential disk, a Hernquist model bulge, an NFW halo and a
central supermassive black hole. The models are derived from explicit
distribution functions for each component and the large number of parameters
permit detailed modeling of actual galaxies. We present techniques that use
structural and kinematic data such as radial surface brightness profiles,
rotation curves and bulge velocity dispersion profiles to find the best-fit
models for the Milky Way and M31. Through N-body realizations of these models
we explore their stability against the formation of bars. The models permit the
study of a wide range of dynamical phenomenon with a high degree of realism.Comment: 58 pages, 20 figures, submitted to the Astrophysical Journa

### Generating Equilibrium Dark Matter Halos: Inadequacies of the Local Maxwellian Approximation

We describe an algorithm for constructing N-body realizations of equilibrium
spherical systems. A general form for the mass density rho(r) is used, making
it possible to represent most of the popular density profiles found in the
literature, including the cuspy density profiles found in high-resolution
cosmological simulations. We demonstrate explicitly that our models are in
equilibrium. In contrast, many existing N-body realizations of isolated systems
have been constructed under the assumption that the local velocity distribution
is Maxwellian. We show that a Maxwellian halo with an initial r^{-1} central
density cusp immediately develops a constant-density core. Moreover, after just
one crossing time the orbital anisotropy has changed over the entire system,
and the initially isotropic model becomes radially anisotropic. These effects
have important implications for many studies, including the survival of
substructure in cold dark matter (CDM) models. Comparing the evolution and
mass-loss rate of isotropic Maxwellian and self-consistent Navarro, Frenk, &
White (NFW) satellites orbiting inside a static host CDM potential, we find
that the former are unrealistically susceptible to tidal disruption. Thus,
recent studies of the mass-loss rate and disruption timescales of substructure
in CDM models may be compromized by using the Maxwellian approximation. We also
demonstrate that a radially anisotropic, self-consistent NFW satellite loses
mass at a rate several times higher than that of its isotropic counterpart on
the same external tidal field and orbit.Comment: Accepted for publication in ApJ, 10 pages, 6 figures, LaTeX (uses
emulateapj5.sty