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Astronomical Solutions to Galactic Dark Matter Will Sutherland Institute of Astronomy, Cambridge
Overview: • Cosmological parameters • Distribution of DM “locally” • Astrophysical DM candidates: MACHOs etc. • Overview of microlensing results. • Implications for particle DM searches.
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: • Neutral HI : 21cm • Hot gas : X-rays • Low mass stars : visible. • “Rocks” : nucleosynthesis, impacts • Solid H “snowballs” evaporate • Two main remaining candidates: • Cold molecular hydrogen • 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 LMC 5.7 yr event selection
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.
EROS 95% CL limits(preliminary) – Glicenstein, 2004 ML workshop excluded by EROS (95% CL)
CDM small-scale structure “crisis” ? • Several manifestations : • CDM predicts “cuspy” central density profile • CDM predicts numerous low-mass dwarf galaxies • Possible resolutions : • Extrapolation below limits of simulations • Observational issues: resolution, non-circular motions • Baryonic feedback affecting DM distribution.
Recent progress in Galaxy-scale DM distribution: • Galaxy-galaxy lensing: new wide-area imaging surveys – SDSS, COMBO-17, Red-Sequence Cluster Survey (RCS). • Satellite galaxies: large new samples from 2dFGRS + SDSS redshift surveys. • Both probe galaxy DM halos on scales ~ 100 -500 kpc : results broadly consistent with ΛCDM simulations.
2dFGRS satellites Brainerd 2004, astro-ph/0409381
Summary: • Microlensing + wide binaries + disk thinness: • → Limits on MACHO fraction below 30% over almost entire mass range above 10-7 MSun • 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.