21 research outputs found
Cold Dark Matter Halos Must Burn
High-quality optical rotation curves for a sample of low-luminosity spirals
evidence that the dark halos around galaxies are inconsistent with the output
of proper CDM simulations. In fact, dark halos enveloping stellar disks are
structures with approximately a constant density out to the optical edges. This
is in strong disagreement with the characteristic rho(r) ~ r^(-1.5) CDM regime
and severely challenges the "standard" CDM theory, also because the halo
density appears to be heated up, at gross variance with the hierarchical
evolution of collision-free particles.Comment: 2 figures, definitive version to appear in the Proceedings of the
MPA/ESO/MPE/USM Joint Conference: "Lighthouses of the Universe: The Most
Luminous Celestial Objects and their use for Cosmology", August 2001,
Garching, German
The Dark Matter Distribution in Disk Galaxies
We use high-quality optical rotation curves of 9 low-luminosity disk galaxies
to obtain the velocity profile of the surrounding dark matter halos. We find
that they increase linearly with radius at least out to the stellar disk edge,
implying that, over the entire region where the stars reside, the density of
the dark halo is constant. The properties of the halo mass structure found are
similar to that claimed for a number of dwarf and low surface brightness
galaxies, but provide a more substantial evidence of the discrepancy between
the halo mass distribution predicted in standard cold dark matter scenario and
those actually detected around galaxies. We find that the density profile
proposed by Burkert (1995) reproduces the halo rotation curves, with halo
central densities and core radii scaling as .Comment: 8 pages, 6 figures, MNRAS accepted. New section and figures added,
concerning CDM mass models. Minor changes to the rest of the pape
The Distribution of Dark Matter in Galaxies: the Core Radius Issue
I review the up-to-date status on the properties of the Dark Matter density
distribution around Galaxies. The rotation curves of spirals all conform to a
same Universal profile which can be uniquely decomposed as the sum of an
exponential thin stellar disk and a dark halo with a flat density core.
From dwarfs to giants galaxies, the halos embedding the stellar component
feature a constant density region of size and value , which are
inversely correlated. The fine structure of dark halos in the region of the
stellar disk has been derived for a number of low--luminosity disk galaxies:
the halo circular velocity increases almost linearly with radius out to the
edge of the stellar disk, implying, up there, an almost constant dark matter
density. This sets a serious discrepancy between the cuspy density distribution
predicted by N-body simulations of CDM cosmology, and those actually
detected around galaxies. The small scatter around the Fundamental Plane (FP)
of elliptical galaxies constraints the distribution of dark and luminous matter
in these systems. The measured central velocity dispersion in the FP
is linked to both photometric and dynamical properties of luminous and dark
matter. As a consequence, the well-known features of the FP imply that, inside
the effective radius , the stellar spheroid must dominate over the dark
matter, in contrast with CDM predictions.Comment: 14 pages, 10 figures, invited talk given at Beyond the Desert '03,
Ringberg, 11-15 July 200
The Intriguing Distribution of Dark Matter in Galaxies
We review the most recent evidence for the amazing properties of the density
distribution of dark matter around spiral galaxies. Their rotation curves,
coadded according to the galaxy luminosity, conform to an universal profile
which can be represented as the sum of an exponential thin disk plus a
spherical halo with a flat density core. From dwarfs to giants, these halos
feature an inner constant density region. The fine structure of dark matter
halos is obtained from the kinematics of a number of suitable low-luminosity
disk galaxies. The halo circular velocity increases linearly with radius out to
the edge of the stellar disk, implying a constant dark halo density over the
entire disk region. The properties of halos around normal spirals provide
substantial evidence of a discrepancy between the mass distributions predicted
in the Cold Dark Matter scenario and those actually detected around galaxies.Comment: 12 pages, 7 figures. Invited lecture to the 8th Adriatic Meeting:
Particle Physics in the New Millennium, Dubrovnik 4-14 Sep. 2001. To be
published in the series Lecture Notes in Physics, by Springer Verla
Phenomenological covariant approach to gravity
We covariantly modify the Einstein-Hilbert action such that the modified
action perturbatively resolves the flat rotational velocity curve of the spiral
galaxies and gives rise to the Tully-Fisher relation, and dynamically generates
the cosmological constant. This modification requires introducing just a single
new universal parameter.Comment: v6: a mistake in deriving the equation of the cosmological constant
corrected, refs adde
Mirror Matter as Self Interacting Dark Matter
It has been argued that the observed core density profile of galaxies is
inconsistent with having a dark matter particle that is collisionless and
alternative dark matter candidates which are self interacting may explain
observations better. One new class of self interacting dark matter that has
been proposed in the context mirror universe models of particle physics is the
mirror hydrogen atom whose stability is guaranteed by the conservation of
mirror baryon number. We show that the effective transport cross section for
mirror hydrogen atoms, has the right order of magnitude for solving the
``cuspy'' halo problem. Furthermore, the suppression of dissipation effects for
mirror atoms due to higher mirror mass scale prevents the mirror halo matter
from collapsing into a disk strengthening the argument for mirror matter as
galactic dark matter.Comment: 6 pages; some references adde
The radial profiles of the different mass components in galaxy clusters
AIMS. To derive the mass profiles of the different luminous and dark
components in clusters. METHODS. The cluster mass profile is determined by
using the Jeans equation applied to the projected phase-space distribution of
about 3000 galaxies members of 59 nearby clusters from the ESO Nearby Abell
Cluster Survey. The baryonic and subhaloes mass components are determined from
the galaxies' luminosity-density profiles through scaling relations between
luminosities and baryonic and dark halo masses. The baryonic mass component
associated to the intra-cluster gas is determined using X-ray data from ROSAT.
RESULTS. The baryon-to-total mass fraction decreases from a value of 0.12 near
the center, to 0.08 at the distance of 0.15 virial radii, then it increases
again, to reach a value of 0.14 at the virial radius. Diffuse, cluster-scale,
dark matter dominates at all radii, but its contribution to the total mass
content decreases outwards to the virial radius, where the dark matter in
subhaloes may contribute up to 23 %, and the baryons 14 %, of the total mass.
The dark mass, and diffuse dark mass profiles are well fit by both cuspy and
cored models. The subhaloes mass distribution is not fit by either model.Comment: Version accepted for publication in A&A. 10 pages, 7 figures.
Footnote 1, and caption to Fig.3.correcte
Stellar polytropes and Navarro-Frenk-White halo models: comparison with observations
Motivated by the possible conflict between the Navarro-Frenk-White(NFW) model
predictions for the dark matter contents of galactic systems and its
correlation with baryonic surface density, we will explore an alternative
paradigm for the description of dark matter halos. Such an alternative emerges
from Tsallis' non-extensive thermodynamics applied to self-gravitating systems
and leads to the so-called ``stellar polytrope'' (SP) model. We consider that
this could be a better approach to real structures rather than the isothermal
model, given the fact that the first one takes into account the non-extensivity
of energy and entropy present in these type of systems characterized by
long-range interactions. We compare a halo based on the Navarro-Frenk-White
(NFW) and one which follows the SP description. Analyzing the dark matter
contents estimated by means of global physical parameters of galactic disks,
obtained from a sample of actual galaxies, with the ones of the unobserved dark
matter halos, we conclude that the SP model is favored over the NFW model in
such a comparison.Comment: 21 pages, 4 figures. Accepted for publication in the Journal of
Cosmology and Astroparticle Physic