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.

WMAP: CMB power spectrum

2dFGRS galaxy power spectrum – Cole et al 2005

SDSS galaxy correlation function: Eisenstein et al 2005.

Dark Matter distribution §Solar neighbourhood (disk): % 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:

Microlensing lightcurves

Milky Way + satellites

LMC with MACHO fields

Mt Stromlo 50-inch telescope

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.

Event distributions.

MACHO LMC 5.7-yr : assuming halo lenses.

Limits on low-mass MACHOs

EROS 95% CL limits (preliminary) – Glicenstein, 2004 ML workshop excluded by EROS (95% CL)

Possible sources of LMC microlensing excess signal:

Ancient halo white dwarfs: Creze et al 2004

Wide halo binaries: Yoo, Chaname & Gould, ApJ 2004

CDM small-scale structure crisis ? §Several manifestations : l CDM predicts cuspy central density profile l CDM predicts numerous low-mass dwarf galaxies §Possible resolutions : l Extrapolation below limits of simulations l Observational issues: resolution, non-circular motions l 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 ~ kpc : results broadly consistent with ΛCDM simulations.

RCS: Hoekstra, Yee & Gladders ApJ 2004.

2dFGRS satellites Brainerd 2004, astro-ph/

Summary: Microlensing + wide binaries + disk thinness: Limits on MACHO fraction below 30% over almost entire mass range above 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.

GAIA satellite