29,779 research outputs found
Problems for MOND in Clusters and the Ly-alpha Forest
The observed dynamics of gas and stars on galactic and larger scales cannot
be accounted for by self-gravity, indicating that there are large quantities of
unseen matter, or that gravity is non-Newtonian in these regimes. Milgrom's
MOdified Newtonian Dynamics (MOND) postulates that Newton's laws are modified
at very low acceleration, and can account for the rotation curves of galaxies
and some other astrophysical observations, without dark matter. Here we apply
MOND to two independent physical systems: Ly-alpha absorbers and galaxy
clusters. While physically distinct, both are simple hydrodynamical systems
with characteristic accelerations in the MOND regime. We find that Ly-alpha
absorbers are somewhat smaller than in Newtonian gravity with dark matter, but
the result depends crucially on the (unknown) background acceleration field in
which they are embedded. In clusters MOND appears to explain the observed
(baryonic) mass-temperature relation. However, given observed gas density and
enclosed mass profiles and the assumption of hydrostatic equilibrium, MOND
predicts radial temperature profiles which disagree badly with observations. We
show this explicitly for the Virgo, Abell 2199 and Coma clusters, but the
results are general, and seem very difficult to avoid. If this discrepancy is
to be resolved by positing additional (presumably baryonic) dark matter, then
this dark matter must have ~1-3 times the cluster gas mass within 1 Mpc. This
result strongly disfavors MOND as an alternative to dark matter (Abridged).Comment: Revised version. Important caveat in Ly-alpha calculation discussed;
conclusions weakened. Coma cluster and calculation of dark matter mass
required by MOND added; cluster conclusions strengthened. 11 EmulateApJ pages
with 3 embedded figures. Accepted by Ap
Are Over-massive Haloes of Ultra Diffuse Galaxies Consistent with Extended MOND?
A sample of Coma cluster ultra-diffuse galaxies (UDGs) are modelled in the
context of Extended Modified Newtonian Dynamics (EMOND) with the aim to explain
the large dark matter-like effect observed in these cluster galaxies.
We first build a model of the Coma cluster in the context of EMOND using gas
and galaxy mass profiles from the literature. Then assuming the dynamical mass
of the UDGs satisfies the fundamental manifold of other ellipticals, and that
the UDG stellar mass-to-light matches their colour, we can verify the EMOND
formulation by comparing two predictions of the baryonic mass of UDGs.
We find that EMOND can explain the UDG mass, within the expected modelling
errors, if they lie on the fundamental manifold of ellipsoids, however, given
that measurements show one UDG lying off the fundamental manifold, observations
of more UDGs are needed to confirm this assumption.Comment: 13 figures, 12 pages, Submitted to A&
Morality Grounds Personal Identity
There is a connection between moral facts and personal identity facts: morality grounds personal identity. If, for example, old Sally enters a teletransporter, and new Sally emerges, the fundamental question to ask is: is new Sally morally responsible for actions (and omissions) of old Sally? If the moral facts are such that she is morally responsible, then Sally persisted through the teletransporter event, and if not, Sally ceased to exist
Nonthermal Bremsstrahlung and Hard X-ray Emission from Clusters of Galaxies
We have calculated nonthermal bremsstrahlung (NTB) models for the hard X-ray
(HXR) tails recently observed by BeppoSAX in clusters of galaxies. In these
models, the HXR emission is due to suprathermal electrons with energies of
about 10-200 keV. Under the assumption that the suprathermal electrons form
part of a continuous spectrum of electrons including highly relativistic
particles, we have calculated the inverse Compton (IC) extreme ultraviolet
(EUV), HXR, and radio synchrotron emission by the extensions of the same
populations. For accelerating electron models with power-law momentum spectra
(N[p] propto p^{- mu}) with mu <~ 2.7, which are those expected from strong
shock acceleration, the IC HXR emission exceeds that due to NTB. Thus, these
models are only of interest if the electron population is cut-off at some upper
energy <~1 GeV. Similarly, flat spectrum accelerating electron models produce
more radio synchrotron emission than is observed from clusters if the ICM
magnetic field is B >~ 1 muG. The cooling electron model produces vastly too
much EUV emission as compared to the observations of clusters. We have compared
these NTB models to the observed HXR tails in Coma and Abell 2199. The NTB
models require a nonthermal electron population which contains about 3% of the
number of electrons in the thermal ICM. If the suprathermal electron population
is cut-off at some energy above 100 keV, then the models can easily fit the
observed HXR fluxes and spectral indices in both clusters. For accelerating
electron models without a cutoff, the electron spectrum must be rather steep >~
2.9.Comment: Accepted for publication in the Astrophysical Journal. 10 pages with
5 embedded Postscript figures in emulateapj.sty. An abbreviated abstract
follow
Exploring Cluster Ellipticals as Cosmological Standard Rods
We explore the possibility to calibrate massive cluster ellipticals as
cosmological standard rods using the Fundamental Plane relation combined with a
correction for luminosity evolution. Though cluster ellipticals certainly
formed in a complex way, their passive evolution out to redshifts of about 1
indicates that basically all major merging and accretion events took place at
higher redshifts. Therefore, a calibration of their luminosity evolution can be
attempted. We propose to use the Mg relation for that purpose because
it is independent of distance and cosmology. We discuss a variety of possible
caveats, ranging from dynamical evolution to uncertainties in stellar
population models and evolution corrections to the presence of age spread.
Sources of major random and systematic errors are analysed as well. We apply
the described procedure to nine elliptical galaxies in two clusters at
and derive constraints on the cosmological model. For the best
fitting -free cosmological model we obtain: , with
90% confidence limits being (the lower limit being due to the
presence of matter in the Universe). If the inflationary scenario applies (i.e.
the Universe has flat geometry), then, for the best fitting model, matter and
contribute about equally to the critical cosmic density (i.e.
). With 90% confidence
should be smaller than 0.9.Comment: 21 pages, including 5 eps-figures, Latex, uses aasms4.sty, accepted
by ApJ main journa
Analytical Study of Optical Wavefront Aberrations Using Maple
This paper describes a package for analytical ray tracing of relatively
simple optical systems. AESOP (An Extensible Symbolic Optics Package) enables
analysis of the effects of small optical element misalignments or other
perturbations. (It is possible to include two or more simultaneous independent
perturbations.) Wavefront aberrations and optical path variations can be
studied as functions of the perturbation parameters. The power of this approach
lies in the fact that the results can be manipulated algebraically, allowing
determination of misalignment tolerances as well as developing physical
intuition, especially in the picometer regime of optical path length
variations.Comment: To appear in MapleTech vol. 4, no. 2, pp. 52-62. 11 pages, 5 figures.
PDF may also be accessed at http://aa.usno.navy.mil/AESOP
Influence of Cooling Flow and Galactic Motion on the Iron Distribution in Clusters of Galaxies
Iron abundance distribution is now known for 12 clusters of galaxies. For
some clusters (e.g. Centaurus) the observed abundance increases toward the
cluster center, while for the others (e.g. Coma and Hydra-A) no significant
inhomogeneity was observed. In order to understand this difference, we
investigate the influence of cooling flow and turbulence produced by galactic
motion on the iron abundance distribution by simple spherical models. We show
that the cooling flow has a significant effect to flatten the iron abundance
distribution if the flow velocity is sufficiently large. Further, by applying
our analysis to the above clusters we show that we can give a systematic
account for the observed variety of the iron abundance distribution
qualitatively.Comment: 30 pages, uuencoded compressed postscript with figures, YITP/U-94-2
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