13,305 research outputs found
A new look at microlensing limits on dark matter in the Galactic halo
The motivation for this paper is to review the limits set on the MACHO
content of the Galactic halo by microlensing experiments in the direction of
the Large Magellanic Cloud. This has been prompted by recent measurements of
the Galactic rotation curve, which suggest that the limits have been biassed by
the assumption of an over-massive halo. The paper first discusses the security
of the detection efficiency calculations which are central to deriving the
MACHO content of the Galactic halo. It then sets out to compare the rotation
curves from various halo models with recent observations, with a view to
establishing what limits can be put on an all-MACHO halo. The main thrust of
the paper is to investigate whether lighter halo models which are consistent
with microlensing by an all-MACHO halo are also consistent with recent measures
of the Galactic rotation curve. In this case the population of bodies
discovered by the MACHO collaboration would make up the entire dark matter
content of the Galactic halo. The main result of this paper is that it is easy
to find low mass halo models consistent with the observed Galactic rotation
curve, which also imply an optical depth to microlensing similar to that found
by the MACHO collaboration. This means that all-MACHO halos cannot be ruled out
on the basis of their observations. In conclusion, limits placed on the MACHO
content of the Galactic halo from microlensing surveys in the Magellanic Clouds
are inconsistent and model dependent, and do not provide a secure basis for
rejecting an all-MACHO halo.Comment: 8 pages, 4 figures, accepted for publication in A&
Time Dilation and Quasar Variability
The timescale of quasar variability is widely expected to show the effects of
time dilation. In this paper we analyse the Fourier power spectra of a large
sample of quasar light curves to look for such an effect. We find that the
timescale of quasar variation does not increase with redshift as required by
time dilation. Possible explanations of this result all conflict with widely
held consensus in the scientific community.Comment: 6 pages including 3 figures. Accepted for publication in ApJ Letter
On time dilation in quasar light curves
In this paper we set out to measure time dilation in quasar light curves. In
order to detect the effects of time dilation, sets of light curves from two
monitoring programmes are used to construct Fourier power spectra covering
timescales from 50 days to 28 years. Data from high and low redshift samples
are compared to look for the changes expected from time dilation. The main
result of the paper is that quasar light curves do not show the effects of time
dilation. Several explanations are discussed, including the possibility that
time dilation effects are exactly offset by an increase in timescale of
variation associated with black hole growth, or that the variations are caused
by microlensing in which case time dilation would not be expected.Comment: 8 pages, 5 figures. Accepted for publication in MNRAS. Published
online 9 April 2010
The signature of primordial black holes in the dark matter halos of galaxies
The aim of this paper is to investigate the claim that stars in the lensing
galaxy of a gravitationally lensed quasar system can always account for the
observed microlensing of the individual quasar images. A small sample of
gravitationally lensed quasar systems was chosen where the quasar images appear
to lie on the fringe of the stellar distribution of the lensing galaxy. As with
most quasar systems, all the individual quasar images were observed to be
microlensed. The surface brightness of the lensing galaxy at the positions of
the quasar images was measured from HST frames, and converted to stellar
surface mass density. The surface density of smoothly distributed dark matter
at the image positions was obtained from lensing models of the quasar systems
and applied to the stellar surface mass density to give the optical depth to
microlensing. This was then used to assess the probability that the stars in
the lensing galaxy could be responsible for the observed microlensing. The
results were supported by microlensing simulations of the star fields around
the quasar images combined with values of convergence and shear from the
lensing models. Taken together, the probability that all the observed
microlensing is due to stars was found to be ~0.0003. Errors resulting from
surface brightness measurement, mass-to-light ratio and the contribution of the
dark matter halo do not significantly affect this result. It is argued that the
most plausible candidates for the microlenses are primordial black holes,
either in the dark matter halos of the lensing galaxies, or more generally
distributed along the lines of sight to the quasars.Comment: 15 pages, 8 figures, published in A&
Evidence for microlensing by primordial black holes in quasar broad emission lines
With the detection of black hole mergers by the LIGO gravitational wave
telescope, there has been increasing interest in the possibility that dark
matter may be in the form of solar mass primordial black holes. One of the
predictions implicit in this idea is that compact clouds in the broad emission
line regions of high redshift quasars will be microlensed, leading to changes
in line structure and the appearance of new emission features. In this paper
the effect of microlensing on the broad emission line region is reviewed by
reference to gravitationally lensed quasar systems where microlensing of the
emission lines can be unambiguously identified. It is then shown that although
changes in Seyfert galaxy line profiles occur on timescales of a few years,
they are too nearby for a significant chance that they could be microlensed,
and are plausibly attributed to intrinsic changes in line structure. In
contrast, in a sample of 53 high redshift quasars, 9 quasars show large changes
in line profile at a rate consistent with microlensing. These changes occur on
a timescale an order of magnitude too short for changes associated with the
dynamics of the emission line region. The main conclusion of the paper is that
the observed changes in quasar emission line profiles are consistent with
microlensing by a population of solar mass compact bodies making up the dark
matter, although other explanations like intrinsic variability are possible.
Such bodies are most plausibly identified as primordial black holes.Comment: 10 pages, 7 figure
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