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Microlensing and Dark Matter

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. 5 collaborations have returned dark matter results

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Microlensing and Dark Matter

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  1. Microlensing and Dark Matter Jan 2005 Kim Griest, UCSD

  2. Surveys monitor millions of stars for years to find rare lensing events • Bulge => stars, remnants, planets, etc. • LMC/SMC/M31 => DM

  3. 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) Microlensing of Dark Matter?

  4. 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.

  5. LMC in neutral H looks like a face-on disk.

  6. 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

  7. 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

  8. Main conclusion: Macho’s as main component of Dark Matter are ruled out • But found significant extra microlensing

  9. 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.)

  10. 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?

  11. LMC-23

  12. 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?

  13. Much written on LMC self lensing since Sahu/Wu/Gould 1994 • 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)

  14. 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

  15. 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 effs tau(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!

  16. 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.

  17. What do to? Other experiments!

  18. EROS collaboration: 4 events in 50 LMC fields and 4 events in 10 SMC fields: Interpreted as limit on Halo dark matter LMC Events

  19. Combined MACHO and EROS limits on short duration = small mass objects

  20. Limits vary according to Milky Way halo model

  21. 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

  22. MEGA: M31 Microlensing Found 4 events: Measure Macho halo fraction f=0.29 +0.30 -0.13 .01< m < 1 Msun => M31 halo DM consistent With LMC result! BUT POINT- AGAPE M31 3 events says f<.25 (.6) for .0001<m<.1 (.1<m<1 Msun)

  23. WeCAPP • (Wendelstein Calar Alto Pixellensing project) • Found 2 events toward M31 • Say favor M31 halo lenses, but evidence very weak (in my opinion)

  24. 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.

  25. BULGE Microlensing: three collaborations returned results: OGLE, EROS, MACHO

  26. 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

  27. Location of all 500 events. (b,l)=(0,0) is Galactic center Many of these Are blended.

  28. Microlensing should be randomly distributed in Color-Magnitude

  29. Select clump giants from color-magnitude diagram: 62 events

  30. 62 Clump giant events. Circle size is proportional to event duration.

  31. 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.

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

  33. 34 candidate events probably from the recently discovered Sagitarious dwarf galaxy

  34. 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, likely Planet mass of 1.5 Mjupiter.

  35. Microlensing Planet Finder Mission: (Bennett et al.) 4 year mission with 1 m Telescope 290 M pixel focal plane, in 2 bands

  36. 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

  37. Light bending => split and magnify image, move images Around, and shear image shape

  38. 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?]

  39. BULGE Microlensing: three collaborations returned results: OGLE, EROS, MACHO

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