Magellanic Cloud Gravitational Microlensing Results: What Do They Mean?

Abstract

I review recent results from gravitational microlensing surveys of the Large Magellanic Cloud. The combined microlensing optical depth of the MACHO and EROS-1 surveys is tau_LMC = 2.1{+1.3/-0.8}* 10^{-7} which is substantially larger than the background of tau < 0.5* 10^{-7} from lensing by known stars but is below the expected microlensing optical depth of tau = 4.7*10^{-7} for a halo composed entirely of Machos. The simplest interpretation of this result is that nearly half of the dark halo is composed of Machos with a typical mass of order 0.5 Msun. These Machos could be old white dwarfs, but it is not obvious that the generation of stars that preceded these white dwarfs could have gone undetected. Perhaps, the Machos could be non-baryonic, but there is no compelling model for the formation of non-baryonic Machos. Therefore, some authors have tried to develop alternative models which attempt to explain the LMC microlensing results with non-halo lenses. Many of these models postulate previously unknown dark stellar populations which contribute significantly to the total mass of the Galaxy and are therefore simply variations of the dark matter solution. However, models which postulate an unknown dwarf galaxy along the line of sight to the LMC or a distortion of the LMC which significantly enhances the LMC self-lensing optical depth can potentially explain the LMC lensing results with only a small amount of mass, so these can be regarded as true non-dark matter solutions to the Macho puzzle. All such models that have been proposed so far have serious problems, so there is as yet no compelling alternative to the dark matter interpretation. However, the problem can be solved observationally with a second generation gravitational microlensing survey that is significantly more sensitive than current microlensing surveys.

12 pages, to appear in the Proceedings of the 3rd International Symposium on Sources and Detection of Dark Matter in the Universe (DM98), Feb. 1998, ed. D. Cline

References in corpus (8)

• A Prescription for Building the Milky Way's Halo from Disrupted Satellites
• The Chemical Residue of a White Dwarf-Dominated Galactic Halo
• Microlensing of tidal debris on the Magellanic great circle
• The LMC Microlensing Events: Evidence for a Warped and Flaring Milky Way Disk?
• Is the LMC Microlensing Due to an Intervening Dwarf Galaxy?
• Red Clump Morphology as Evidence Against a New Intervening Stellar Population as the Primary Source of Microlensing Toward the LMC
• No Need for MACHOS in the Halo
• Red Clump Stars as a Tracer of Microlensing Optical Depth

Cited by in corpus (11)

• Astrophysical Applications of Gravitational Microlensing
• Microlensing Searches for Exoplanets
• Is the Large Magellanic Cloud a Large Microlensing Cloud?
• Constraints on Intervening Stellar Populations Toward the Large Magellanic Cloud
• A microlensing search of 700 million VVV light curves
• Are there MACHOs in the Milky Way halo?
• Density Distribution and Shape of Galactic Dark Halo Can be Determined by Low Frequency Gravitational Waves ?
• VVV Survey Microlensing: Catalog of Best and Forsaken Events
• Neutron Stars and Black Holes as MACHOs
• Probing the mass function of halo dark matter via microlensing
• Pixel lensing observations towards globular clusters