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): 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:

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 : 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.

RCS: Hoekstra, Yee & Gladders ApJ 2004.

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.

GAIA satellite