Microlensing and Dark Matter

Jan 2005

Kim Griest, UCSD

•Surveys monitor millions of stars for years to find rare lensing events

• Bulge => stars, remnants, planets, etc.

•LMC/SMC/M31 => DM

Microlensing of Dark Matter?

5 collaborations have returned dark matter results• MACHO: strong evidence toward LMC, but

interpretation unclear• EROS: evidence against toward LMC/SMC, but

not inconsistent with MACHO• MEGA: moderate evidence in favor toward M31• POINT/AGAPE: weak evidence against toward

M31, consistent with MACHO• WeCapp, very weak evidence in favor (M31)

MACHO Collaboration (2000)

• Monitored 11.9 million stars for 5.7 years• Found 13-17 events (depending on selection criteria)• Careful efficiency analysis including blending• Removed 8 Supernova behind LMC (contaminants)• Distribution in space, CMD, Amax, consistent with

microlensing interpretation• Likelihood analysis to measure Macho DM, plus

events in disk, LMC, etc.

LMC inneutral Hlooks likea face-ondisk.

Test of systematic error due to contamination, selection bias: compare A B criteria

Criteria A: tighter cuts, with less contamination

Criteria B: looser cuts, with more contamination

•Masses 0.1 - 1.0 Msun preferred•Halo fraction 8% - 40% preferred•Total mass in Machos: 8-10 10^10 Msun

(MW disk=6 10^10 Msun, and MW halo has 4-6 10^11 Msun)

•Optical depth = 1.2+0.4-0.3 10^-7

• Main conclusion: Macho’s as main component of Dark Matter are ruled out

• But found significant extra microlensing

The number of non-Macho events is predicted to be much smaller than the 13-17 events observed (using standard LMC and Milky Way stellar populations.)

But these results need correcting

• Recently EROS (Glicenstein 2004) found that event LMC-23 bumped again after 7 years => variable star, not lensing.

• LMC-23 contributed 8% of optical depth (and halo fraction) (6% for set B), so all our optical depths and halo fractions should be reduced by 8%

• => best f is 18.5%, and tau=1.1 10^-7• More worrying: are there more events like this?

LMC-23

What does extra LMC microlensing mean?

1. If events are in MW halo => - significant portion of DM - problem exists: What are they?

-- stellar mass but can’t be stars (stars shine!)-- stellar remnant (white dwarfs, black holes) would need lots of early stars: no evidence for these (metal enrichment, background light, etc.) WD observed?-- primordial black holes? quark nuggets?

2. If events are LMC self lensing => - current LMC models wrong? - lens stars should be seen?3. Contamination in MACHO dataset?

•MACHO used Gyuk, Dalal, Griest review of LMC models, valid in 2000, to predict 1-2 LMC self-lensing microlensing events. At that time no evidence of other stellar populations to do the self lensing.

•HOW ABOUT RECENT EVIDENCE?

•Zhao, Ibata, Lewis, & Irwin(2003) did 1300 2dF radial velocities: no evidence for any extra population over expected LMC and Galaxy Any new kinematically distinct population less than 1%. (rules out Evans & Kerrins 2000 fluffy stellar halo model)

Much written on LMC self lensing since Sahu/Wu/Gould 1994

• Gallart, Stetson, Hardy, Pont, & Zinn (2004), search for a stellar in a deep surface brightness CMD, and found no evidence for any stellar halo

• However, Minniti, et al (2003), and Alves (2004) found RVs for 43 RR Lyaes and discovered an old and hot stellar halo! But they say it is too small to account for all the extra microlensing

• But the structure of the LMC is being questioned: van der Marel,et al (2002) says the LMC disk is not circular, but Nikolaev, et al. (2004) disagree, saying it is warped. Both say it does not probably affect self lensing much (e.g. Mancinit etal 2003 agree), but it does show the LMC is still not well understood.

• Summary: no clear answer yet

Contamination?• Contamination was studied by MACHO; selection

criteria: A: 13 events, tight cuts, less contamination., lower effs

B: 17 events, loose cuts, more contam., higher effstau(A) =1.1e-7, tau(B)=1.3e-7.

17% difference estimates contamination systematics• But Belokurov, Evans, & LeDu used neural net to

reanalyze MACHO LMC data. Say data set is badly contaminated; find only 6 or 7 microlensing events => tau much smaller => no need for either Machos in dark halo or extra LMC self lensing!

Wrong!• Found events by running only on our selected events, but

calculated efficiencies without including effect of our selection => badly miscalculated efficiencies.

• Analyzed only 22000 lightcurves out of 11.9 million • Also used very weak statistics => much lower eff, and

many false positives (2 out of 22000) => probably would not even work if applied to all 11.9 million lightcurves

• Rejected good microlensing, misidentified SN Conclusion: BEL analysis is meaningless; neural nets may

be useful, but have yet to be applied correctly. Contamination possible, but certainly not shown yet. Results of MACHO LMC5.7 stand after

small correction for LMC-23.

What do to?

Other experiments!

EROS collaboration: 4 events in 50 LMC fields and 4 events in10 SMC fields: Interpreted as limit on Halo dark matter

LMCEvents

Combined MACHO and EROS limits on short duration = smallmass objects

Limits vary according to Milky Way halo model

Limits on Macho Dark Matter

• Objects with 10-7 < m < 10-3 Msun make up less than 25% of DM. Objects with 3.5 10-7 < m < 4.5 10-5 make up less than 10% of DM

MEGA: M31 MicrolensingFound 4 events:

Measure Macho halo fractionf=0.29 +0.30 -0.13 .01< m < 1 Msun => M31 haloDM consistentWith LMC result!

BUT POINT-AGAPE M313 events saysf<.25 (.6) for .0001<m<.1(.1<m<1 Msun)

WeCAPP

• (Wendelstein Calar Alto Pixellensing project)

• Found 2 events toward M31

• Say favor M31 halo lenses, but evidence very weak (in my opinion)

What does it mean?

• Experimentally not clear: need more MEGA/POINT-AGAPE M31 work, Supermacho on LMC. From Space DIME can do parallax and (if approved) can answer question of where lenses are; eventually SIM and do astrometric microlensing. (Measure distance to 2 or 3 LMC lenses as 10 kpc to prove Macho DM. 3 or 4 at 50 kpc proves LMC self-lensing.)

• Theoretically fairly clear: Macho DM consistent with Omega_baryon = 0.04, but causes problems with star and galaxy formation, or requires very exotic objects.

BULGEMicrolensing:threecollaborationsreturnedresults:OGLE,EROS,MACHO

Microlensing towards bulge

• 50 million stars over 7 years

• >450 events, 60 on clump giants (less blended)

• ~40 binary events, parallax, extended source, lensing of variable stars, etc.

• Optical depth = 2.18 +.45-.38 10-6, agrees with models (e.g. Gould and Han 1.63 10-6)

• Also found optical depth as a function of (b,l) and gradient in optical depth

Location of all 500 events.

(b,l)=(0,0) isGalactic center

Many of theseAre blended.

Microlensing should be randomly distributed in Color-Magnitude

Select clump giants from color-magnitude diagram: 62 events

62 Clump giant events.

Circle size is proportional to event duration.

Are events all microlensing?

Microlensing is uniformly distributed in impact parameter, umin ~1/Amax

K-S test shows probability of 2.5% for these 258 events.

Deviation is from blending.

For 60 clump giant events probability is 81%. So these are unblended microlensing

34 candidate events probably from the recently discovered Sagitarious dwarf galaxy

The first planet to be discovered by microlensing: OGLE 2003-BLG-233/MOA 2003-BLG-53; q=.0039. Likely star mass of 0.4 Msun, likelyPlanet mass of 1.5 Mjupiter.

Microlensing PlanetFinder Mission: (Bennett et al.)

4 year mission with 1 mTelescope 290 M pixel focal plane, in 2 bands

Conclusion

• The mystery of LMC microlensing is still unsolved, and more work is needed

• If you want an inventory of all compact objects, independent of luminosity microlensing is the way to go, i.e. Microlensing has a bright future for finding dark objects

Light bending => split and magnify image, move imagesAround, and shear image shape

Are lenses DM in Galaxy or LMC Self lensing?

If events are in MW halo => - significant portion of DM - problem exists: What are they?

-- stellar mass but can’t be stars (stars shine!)-- stellar remnant (white dwarfs, black holes) would need lots of early stars: no evidence for these (metal enrichment, background light, etc.)

If events are LMC self lensing => - current LMC models are wrong - why are the lens stars not seen?

Lots of tests done: none conclusive yet [Other lensing info?]

BULGEMicrolensing:threecollaborationsreturnedresults:OGLE,EROS,MACHO

Microlensing lightcurves have well specified shapes depending on 3 parameters:

Maximum magnification: Amax, event duration t^hat, and time of peak.

Blended lightcurves look very similar, but have different values for Amax and t^hat