Kim Griest

Microlensing and Dark Matter
August 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

Executive Summary
• Basic Result: Searches done and show bulk
of DM cannot be in compact objects with
masses between Mars mass and 100 Msun
• Conflicting results on whether ANY halo
DM can exist in this form: 2 collaborations
say yes, 2 say no
• IF measurable amount of Macho halo DM
exists, it would be a major new component
of MW (and M31) => important
consequences for galaxy formation and
evolution

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 in
neutral H
looks like
a face-on
disk.

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 at
least 8%.
• => best f = 18.5%, and tau=1.1 10^-7 (Griest &
Thomas), or f=16%, tau=1 10^-7 (Bennett)
• 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?

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)

• Gallart, Stetson, Hardy, Pont, & Zinn (2004), search for a stellar
population 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 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!

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 in
10 SMC fields: Interpreted as limit on Halo dark matter
LMC
Events

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

Limits vary
according to
Milky Way halo
model

LMC Limits on Macho Dark
Matter
• MACHO plus published EROS: 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
• EROS conference proceedings: (3 events from 33
million stars over 100 sq deg)
Objects with 10 -4 < m < 10 0 Msun make up less
than 12% of DM (best fit Macho halo fraction of
3%). (astro-ph/0501584) (CONFLICTS WITH
MACHO RESULT)

• M31 microlensing is the new frontier
(and site of new controversies!)
• At least 4 groups have returned results
(but situation not clarified!)
• Key idea was to use M31 inclination and
near/far asymmetry: if Macho halo, many
more events should appear on far side of
M31

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

More recently: reversal!
• POINT/AGAPE has 6 events; model M31; say more
than self-lensing: halo fraction: f >20% for .5

Other M31 results
• WeCAPP (Wendelstein Calar Alto
Pixellensing project) found 2 events toward
M31. Say favor M31 halo lenses, but
evidence very weak (in my opinion).
• Nainital group found one M31 event (on far
side), no conclusions.

What does it mean?
• Experimentally not clear: need more M31 work
(MEGA/POINT-AGAPE, etc); Supermacho on LMC.
From space 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.) Need better modeling of
M31.
• Exciting current event: Spitzer IR satellite project
approved to get parallax (Gould et al). Currently following
an SMC event. Very likely to get distance.
• Theoretically: Macho DM consistent with
Omega_baryon = 0.04, but causes problems with star and
galaxy formation, or requires very exotic objects.

BULGE
Microlensing:
three
collaborations
returned
results:
OGLE, EROS, MACHO
Results now very consistent
With modern models of the
Milky Way (e.g. Bissant &
Gerhard; Han & Gould)

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

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, likely
Planet mass of 1.5 M_jupiter.
Second planet found in June (OGLE-2005-BLG-071).
0.05 < m/M_jupiter < 4, at distance of several kpc; very high S/N

Conclusion
• The mystery of LMC/M31 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