3,600 research outputs found
The fundamental plane of elliptical galaxies with modified Newtonian dynamics
The modified Newtonian dynamics (MOND), suggested by Milgrom as an
alternative to dark matter, implies that isothermal spheres with a fixed
anisotropy parameter should exhibit a near perfect relation between the mass
and the fourth power of the velocity dispersion. This is consistent with the
observed Faber-Jackson relation for elliptical galaxies-- a luminosity-velocity
dispersion relation with large scatter. However, the observable global
properties of elliptical galaxies comprise a three parameter family; they lie
on a ``fundamental plane'' in a logarithmic space consisting of central
velocity dispersion, effective radius, and luminosity. The scatter
perpendicular to this plane is significantly less than that about the
Faber-Jackson relation. I show here that, in order to match the observed global
properties of elliptical galaxies with MOND, models must deviate from being
strictly isothermal and isotropic; such objects can be approximated by
high-order polytropic spheres with a radial orbit anisotropy in the outer
regions. MOND imposes boundary conditions on the inner Newtonian regions which
restrict these models to a dynamical fundamental plane which may differ from
that implied by the traditional virial theorem. Scatter about this plane is
relatively insensitive to the necessary deviations from homology.Comment: 9 pages, 6 eps figures, mn style. Shortened and revised version
includes more recent data on elliptical galaxies. Accepted for publication in
MNRA
Dark matter and non-Newtonian gravity from General Relativity coupled to a fluid of strings
An exact solution of Einstein's field equations for a point mass surrounded
by a static, spherically symmetric fluid of strings is presented. The solution
is singular at the origin. Near the string cloud limit there is a
correction to Newton's force law. It is noted that at large distances and small
accelerations, this law coincides with the phenomenological force law invented
by Milgrom in order to explain the flat rotation curves of galaxies without
introducing dark matter. When interpreted in the context of a cosmological
model with a string fluid, the new solution naturally explains why the critical
acceleration of Milgrom is of the same order of magnitude as the Hubble
parameter.Comment: 12 pages, REVTeX, no figure
MOND and the lensing Fundamental Plane: No need for dark matter on galaxy scales
Bolton et al. (2007) have derived a mass-based fundamental plane using
photometric and spectroscopic observations of 36 strong gravitational lenses.
The lensing allows a direct determination of the mass-surface density and so
avoids the usual dependence on mass-to-light ratio. We consider this same
sample in the context of modified Newtonian dynamics (MOND) and demonstrate
that the observed mass-based fundamental plane coincides with the MOND
fundamental plane determined previously for a set of high-order polytropic
spheres chosen to match the observed range of effective radii and velocity
dispersions in elliptical galaxies. Moreover, the observed projected mass
within one-half an effective radius is consistent with the mass in visible
stars plus a small additional component of ``phantom dark matter'' resulting
from the MOND contribution to photon deflection.Comment: Minor revisions in response to referee. Revised title. Accepted in
MNRA
MOND predictions of "halo" phenomenology in disc galaxies
We examine two corollaries of MOND pertaining to the properties of the
equivalent dark-matter halo. MOND predicts for pure exponential discs a tight
relation involving the halo and disc scale lengths and the mean acceleration in
the disc, which we find to test favorably against the Verheijen sample of Ursa
Major galaxies. A correlation between halo and disc length scales is also
apparent when the "maximum disc" contribution is assumed, but we demonstrate
that this follows from the more general MOND prediction. The second MOND
prediction involves the existence of a maximum halo acceleration, which also
tests favorably against the Ursa Major sample for different assumptions on the
disc contribution.Comment: 5 pages, 4 eps figures, uses mn2e.cls (submitted to MNRAS
Distant star clusters of the Milky Way in MOND
We determine the mean velocity dispersion of six Galactic outer halo globular
clusters, AM 1, Eridanus, Pal 3, Pal 4, Pal 15, and Arp 2 in the weak
acceleration regime to test classical vs. modified Newtonian dynamics (MOND).
Owing to the non-linearity of MOND's Poisson equation, beyond tidal effects,
the internal dynamics of clusters is affected by the external field in which
they are immersed. For the studied clusters, particle accelerations are much
lower than the critical acceleration a_0 of MOND, but the motion of stars is
neither dominated by internal accelerations (a_i >> a_e) nor external
accelerations (a_e >> a_i). We use the N-body code N-MODY in our analysis,
which is a particle-mesh-based code with a numerical MOND potential solver
developed by Ciotti, Londrillo, and Nipoti (2006) to derive the line-of-sight
velocity dispersion by adding the external field effect. We show that Newtonian
dynamics predicts a low-velocity dispersion for each cluster, while in modified
Newtonian dynamics the velocity dispersion is much higher. We calculate the
minimum number of measured stars necessary to distinguish between Newtonian
gravity and MOND with the Kolmogorov-Smirnov test. We also show that for most
clusters it is necessary to measure the velocities of between 30 to 80 stars to
distinguish between both cases. Therefore the observational measurement of the
line-of-sight velocity dispersion of these clusters will provide a test for
MOND.Comment: A&A, accepted, LaTeX, 8 pages, 4 figure
IN VITRO STUDIES ON STRAIN-DEPENDENT PRODUCTION OF THYMUS-SPECIFIC AUTOANTIBODIES
In vitro cultures of spleen cells (S) from normal 8–10-wk-old DBA/2J mice were shown to develop a small number of plaque-forming cells (PFC) that released antibodies lytic to syngenic and autologous thymus cells as well as to syngenic lymphoma L5178Y cells used as the target in the PFC assay. A marked increase in the number of PFC detectable on L5178Y target cells was demonstrated on day 4 in the cultures of S cells to which syngenic or autologous thymus cells had been added (S+T) at time 0, whereas the PFC detectable on thymus cells in such cultures remained at a level similar to that in S cultures. This suggested that two populations of PFC participated in the observed phenomena. No PFC developed in the culture of thymus cells (T). The addition of the cell-free supernatants of 24-h cultures of T or of L5178Y cells to syngenic S cultures also caused a specific increase in the number of the PFC detectable on L5178Y, which suggested that certain immunogenic factors released from the T cells stimulated the response observed in the S+T cultures. Antibodies of IgM nature were detected in the supernatants of S+T cultures by means of cytolysis in agar of L5178Y cells. Although such antibodies did not cause lysis of thymus cells, they could be completely removed by absorption with normal adult or fetal thymus cells of syngenic origin. Still, the absorbing capacity of L5178Y was much higher than that of thymus cells. The absorption was more efficient at 4°C than at 22°C, and hardly any absorption occurred at 37°C. The tissue distribution of the antigen under study seemed to be restricted to thymus cells since no other murine tissue cells tested removed the antibodies. The thymic antigen under study was not restricted to strain DBA/2J and could be demonstrated on thymus cells of all other strains tested. On the other hand, the ability of spleen cells to respond in vitro to this antigen has thus far been observed only in DBA/2J mice. Spleen cells of strains C57BL/6J and NZB/BINJ as well as (DBA/2 x NZB)F1 failed to show any significant increase in the PFC response detectable on the L5178Y target when syngenic thymus cells or DBA/2J thymus cells were added. An intravenous injection of syngenic thymus cells to DBA/2J mice also caused the appearance in their spleens of PFC detectable on the L5178Y target. The described in vitro system may provide a good means of studying the cellular basis of generation of self-tolerance and of its breakdown
The Bright Side of Dark Matter
We show that it is not possible in the absence of dark matter to construct a
four-dimensional metric that explains galactic observations. In particular, by
working with an effective potential it is shown that a metric which is
constructed to fit flat rotation curves in spiral galaxies leads to the wrong
sign for the bending of light i.e. repulsion instead of attraction. Hence,
without dark matter the motion of particles on galactic scales cannot be
explained in terms of geodesic motion on a four- dimensional metric. This
reveals a new bright side to dark matter: it is indispensable if we wish to
retain the cherished equivalence principle.Comment: 7 pages, latex, no figures. Received an honorable mention in the 1999
Gravity research Foundation Essay Competition. Submitted to Phys. Rev. Let
Clusters of galaxies with modified Newtonian dynamics (MOND)
X-ray emitting clusters of galaxies are considered in the context of modified
Newtonian dynamics (MOND). I show that self-gravitating isothermal gas spheres
are not good representations of rich clusters with respect to the radial gas
density distribution as indicated by the X-ray surface brightness. Pure gas
spheres with a density distribution described by a ``beta'' model, as observed,
also fail because, with MOND, these objects are far from isothermal. These
problems may be resolved by adding an additional dark mass component in the
central regions; a constant density sphere contained within two core radii and
having a total mass of one to two times that in the gas. With this additional
component, the observed luminosity-temperature relation for clusters of
galaxies is reproduced. When the observed X-ray surface brightness distribution
in actual clusters is modeled by such a two-component structure, the typical
mass discrepancy is three to four times smaller than with Newtonian dynamics.
Thus while MOND significantly reduces the mass of the dark component in
clusters it does not remove it completely. I speculate on the nature of the
dark component and argue that this is not a fundamental problem for MOND.Comment: 10 pages, 9 figures, MNRAS style (submitted MNRAS
Gravitational anomalies signaling the breakdown of classical gravity
Recent observations for three types of astrophysical systems severely
challenge the GR plus dark matter scenario, showing a phenomenology which is
what modified gravity theories predict. Stellar kinematics in the outskirts of
globular clusters show the appearance of MOND type dynamics on crossing the
threshold. Analysis shows a ``Tully-Fisher'' relation in these systems,
a scaling of dispersion velocities with the fourth root of their masses.
Secondly, an anomaly has been found at the unexpected scales of wide binaries
in the solar neighbourhood. Binary orbital velocities cease to fall along
Keplerian expectations, and settle at a constant value, exactly on crossing the
threshold. Finally, the inferred infall velocity of the bullet cluster
is inconsistent with the standard cosmological scenario, where much smaller
limit encounter velocities appear. This stems from the escape velocity limit
present in standard gravity; the ``bullet'' should not hit the ``target'' at
more than the escape velocity of the joint system, as it very clearly did.
These results are consistent with extended gravity, but would require rather
contrived explanations under GR, each. Thus, observations now put us in a
situation where modifications to gravity at low acceleration scales cease to be
a matter of choice, to now become inevitable.Comment: 10 pages, 5 figures, Astrophysics and Space Science Proceedings 38,
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