Microlensing and Dark Matter

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Microlensing and Dark Matter

• Jan 2005
• Kim Griest, UCSD

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

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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)

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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.

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LMC in neutral H looks like a face-on disk.
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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
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• Masses 0.1 - 1.0 Msun preferred
• Halo fraction 8 - 40 preferred
• Total mass in Machos 8-10 1010 Msun (MW
  disk6 1010 Msun, and MW halo has 4-6 1011
  Msun)
• Optical depth 1.20.4-0.3 10-7

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• Main conclusion Machos as main component of
  Dark Matter are ruled out
• But found significant extra microlensing

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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.)
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But these results need correcting

• Recently EROS (Glicenstein 2004) found that event
  LMC-23 bumped again after 7 years gt 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
• gt best f is 18.5, and tau1.1 10-7
• More worrying are there more events like this?

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LMC-23
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What does extra LMC microlensing mean?
1. If events are in MW halo gt - significant
portion of DM - problem exists What are
they? -- stellar mass but cant 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 gt -
current LMC models wrong? - lens stars
should be seen? 3. Contamination in MACHO dataset?
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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)

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• 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

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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 gt tau much smaller gt no need for either
  Machos in dark halo or extra LMC self lensing!

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Wrong!

• Found events by running only on our selected
  events, but calculated efficiencies without
  including effect of our selection gt badly
  miscalculated efficiencies.
• Analyzed only 22000 lightcurves out of 11.9
  million
• Also used very weak statistics gt much lower eff,
  and many false positives (2 out of 22000) gt
  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.

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What do to? Other experiments!
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EROS collaboration 4 events in 50 LMC fields
and 4 events in 10 SMC fields Interpreted as
limit on Halo dark matter
LMC Events
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Combined MACHO and EROS limits on short duration
small mass objects
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Limits vary according to Milky Way halo model
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Limits on Macho Dark Matter

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

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MEGA M31 Microlensing Found 4
events Measure Macho halo fraction f0.29
0.30 -0.13 .01lt m lt 1 Msun gt M31 halo DM
consistent With LMC result!
BUT POINT- AGAPE M31 3 events says flt.25 (.6) for
.0001ltmlt.1 (.1ltmlt1 Msun)
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WeCAPP

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

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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.

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BULGE Microlensing three collaborations returned
results OGLE, EROS, MACHO
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Microlensing towards bulge

• 50 million stars over 7 years
• gt450 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

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Location of all 500 events. (b,l)(0,0)
is Galactic center Many of these Are blended.
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Microlensing should be randomly distributed in
Color-Magnitude
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Select clump giants from color-magnitude diagram
62 events
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62 Clump giant events. Circle size is
proportional to event duration.
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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.
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For 60 clump giant events probability is 81. So
these are unblended microlensing
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34 candidate events probably from the recently
discovered Sagitarious dwarf galaxy
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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.
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Microlensing Planet Finder Mission (Bennett et
al.) 4 year mission with 1 m Telescope 290 M
pixel focal plane, in 2 bands
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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

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Light bending gt split and magnify image, move
images Around, and shear image shape
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Are lenses DM in Galaxy or LMC Self lensing?
If events are in MW halo gt - significant
portion of DM - problem exists What are
they? -- stellar mass but cant 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 gt - current LMC models are wrong -
why are the lens stars not seen?
Lots of tests done none conclusive yet
Other lensing info?
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BULGE Microlensing three collaborations returned
results OGLE, EROS, MACHO
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Microlensing lightcurves have well specified
shapes depending on 3 parameters Maximum
magnification Amax, event duration that, and
time of peak. Blended lightcurves look very
similar, but have different values for Amax and
that
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