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