SDSS galaxy correlation function: Eisenstein et al 2005.
Dark Matter distribution §Solar neighbourhood (disk): 80-100% visible. §Solar radius, 8 kpc from Galactic centre: 50% - 80% visible. §50 kpc from Galactic centre: 10-20% visible. §DM in our Galaxy less well known than other galaxies ! – unfavourable location. §Universe average: baryons 15%, stars ~ 2%.
Astrophysical DM candidates: §Various candidates excluded: l Neutral HI : 21cm l Hot gas : X-rays l Low mass stars : visible. l Rocks : nucleosynthesis, impacts l Solid H snowballs evaporate §Two main remaining candidates: l Cold molecular hydrogen l MACHOs (also primordial black holes): tested via microlensing.
Microlensing: basic definitions Einstein radius : M dependence optical depth independent of M, event durations ~ M, event rates ~ 1/M Magnification:
MACHO project 5.7-yr LMC summary: §11 million stars, ~ 500 data points each. §13 microlensing candidates in A sample, 17 in B sample. §Predict 2 – 4 events from lensing by known stars (mostly LMC self-lensing). §Event durations too long for substellar lenses.
Summary: Microlensing + wide binaries + disk thinness: Limits on MACHO fraction below 30% over almost entire mass range above 10 -7 M Sun Origin of LMC microlensing events remains unclear: ancient white dwarfs excluded (assuming H atmospheres). τ(EROS) < τ(MACHO), hint of LMC self-lensing ?? New-generation microlensing projects (OGLE-3, SuperMACHO, MEGA, AGAPE, DIME) may resolve. Prospects very good for particle DM searches. Halo substructure probably the dominant astrophysical uncertainty: annual modulation very uncertain, directionality more robust.