Astronomical Solutions to Galactic Dark Matter
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Astronomical Solutions to Galactic Dark Matter
Will Sutherland
Institute of Astronomy, Cambridge
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Overview:
Cosmological parameters
Distribution of DM “locally”
Astrophysical DM candidates: MACHOs etc.
Overview of microlensing results.
Implications for particle DM searches.
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WMAP: CMB power spectrum
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2dFGRS galaxy power spectrum – Cole et al 2005
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SDSS galaxy correlation function:
Eisenstein et al 2005.
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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%.
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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.
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Microlensing: basic definitions
Einstein radius :
√M dependence → optical depth independent of M, event durations ~ √M, event rates ~ 1/√M
Magnification:
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Microlensing lightcurves
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Milky Way + satellites
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LMC with MACHO fields
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Mt Stromlo 50-inch telescope
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MACHO LMC 5.7 yr event selection
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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.
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Event distributions.
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MACHO LMC 5.7-yr : assuming halo lenses.
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Limits on low-mass MACHOs
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EROS 95% CL limits (preliminary) – Glicenstein, 2004 ML workshop
excluded by EROS (95% CL)
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Possible sources of LMC microlensing excess signal:
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Ancient halo white dwarfs: Creze et al 2004
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Wide halo binaries:Yoo, Chaname & Gould, ApJ 2004
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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.
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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.
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RCS: Hoekstra, Yee & Gladders ApJ 2004.
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2dFGRS satellites
Brainerd 2004, astro-ph/0409381
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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.
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GAIA satellite
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