Astroparticle Physics

13.2 Motivation for Dark Matter 271Large Magellanic Cloud is sufficiently distant so that thelight of its stars has to pass through a large region of thegalactic halo and therefore has a chance to interact gravitationallywith many brown non-luminous star candidates.Furthermore, the Large Magellanic Cloud is just above thegalactic disk so that the light of its stars really has to passthrough the halo. From considerations of the mass spectrumof brown stars (‘MACHOs’) and the size of the Einstein ringone can conclude that a minimum of 10 6 stars has to be observedto have a fair chance to find some MACHOs.The experiments MACHO, EROS 2 , and OGLE 3 havefound approximately a dozen MACHOs in the halo of ourMilky Way. Figure 13.8 shows the light curve of the firstcandidate found by the MACHO experiment. The observedwidth of the brightness excursion allows to determine themass of the brown objects.If the deflector has a mass corresponding to one solarmass (M ⊙ ), one would expect an average brightness excur-sion of three months while for 10 −6 M ⊙ one would obtainonly two hours. The measured brightness curves all leadto masses of approximately 0.5 M ⊙ . The non-observationof short brightness signals already excludes a large massrange of MACHOs as candidates for dark halo matter. Ifthe few seen MACHOs in a limited solid angle are extrapolatedto the whole galactic halo, one arrives at the conclusionthat possibly 20% of the halo mass, which takes an influenceon the dynamics of the Milky Way, could be hiddenin dark stars. Because of the low number of observed MA-CHOs, this result, however, has a considerable uncertainty((20 +30−12 )%).A remote possibility for additional non-luminous baryonicdark matter could be the existence of massive quarkstars (several hundred solar masses). Because of their anticipatedsubstantial mass the duration of brightness excursionswould be so large that it would have escaped detection.The Andromeda Galaxy with many target stars wouldbe an ideal candidate for microlensing experiments. Thisgalaxy is right above the galactic plane. Unfortunately, it istoo distant that individual stars can be resolved. Still onecould employ the ‘pixel-lensing’ technique by observing theapparent brightness excursions of individual pixels with aCCD camera. In such an experiment one pixel would cover2 EROS – Expérience pour la Recherche d’Objets Sombres3 OGLE – Optical Gravitational Lens Experimentamplification A10864200 50100time [d]Fig. 13.8Light curve of a distant star causedby gravitational lensing. Shown isthe first brown object found by theMACHO experiment in thegalactic halo {30}expected durationsof brightness excursionsestimated mass fractionof dark stars in the halomassive quark starspixel-lensing technique