P.Tisserand Rencontres du Vietnam 2006 1
Final results on galactic dark matter from the EROS-2 microlensing survey
~ 850 000 images processed - 55 million stars monitored
Large Magellanic Cloud (LMC) Small Magellanic Cloud (SMC)
EROS-2 Expérience de Recherche d’Objets Sombres
Observation : 1996-2003 at La Silla (Chile)CEA/DAPNIA/SPP-Saclay
Patrick TisserandMount Stromlo Obs., Australia
Microlensing formalism History and the EROS-2 experiment Situation before this analysis Microlensing Background Analysis and Candidates status Final Result of EROS-2 Discussions
Astro-ph/0607207
P.Tisserand Rencontres du Vietnam 2006 2
A large amount of dark matter exists at the
galaxy’s scale
Van Albada et al., 1985
Problem : Galaxy
rotation curve
Disc
Su
rfac
e lu
min
osi
ty (
mag
/arc
sec2
)R
ota
tio
n v
elo
city
(km
.sec
-1)
Halo
One hypothesis: A halo full
of machos...
Characteristics:
- spherical isothermal distribution - Radius between 50 and 200 kpc- Mass : M(r) α r- Total Mass ~ 1012 M
- Density : (r) α 1/r2
Machos
« Massive Astronomical Compact Halo Objects »
_ Planets _ Brown dwarfs_ Stellar remnants_ Unknown compact matter
P.Tisserand Rencontres du Vietnam 2006 3
Tool : lensing effect
• Lensing effect : Indirect detection
• For 1 M:
Image Separation ~ 0.2 milli arcsec
Σ
~ milli arcsec ~ arcsecEROS
MACHOOGLE
Exp:
P.Tisserand Rencontres du Vietnam 2006 4
tE ~ 70 ( )½
days
1986ApJ...304....1P, B.Paczyński
Light curve characteristics:
Symmetric
Achromatic
Unique ( ~1 evt / 106)
M M
Microlensing effect :
tE tE (M, Dd, Vt )Degeneracy !
P.Tisserand Rencontres du Vietnam 2006 5
MACHO – LMC#1 Appeared in 1993
tE = 17 days, Amplification ~ 7.5
OGLE2-99-LMC#1
Alert 1999tE ~66 days, Amplification ~ 50
EROS2-LMC#8
Increase by 3.5 magnitudes !
Appeared in 2000 tE ~10 days
Some microlensing events observed :
P.Tisserand Rencontres du Vietnam 2006 6
Event rate predictions from «standard» isothermal halo model
Typical Value(in the case of a
dark halo 100% machos)
LMC
0.45 10-6
SMC
0.65 10-6
Virialised System: ~ ( v / c )2
τ depends mainly on the halo densityIndependent of machos velocity and mass
τ = Probability that, a given time, a source star is inside
one Einstein disk (Amplification > 1.34)
Probability (τ=Optical Depth) :
P.Tisserand Rencontres du Vietnam 2006 7
Full Macho Halo:
LMC
0.45 10-6
SMC
0.65 10-6
Self lensing:
LMC-LMC
0.005 - 0.05 10-6
SMC-SMC
0.04 10-6
Lensing LMC-Galactic stars:
LMC-gal
0.01 10-6
Lensing Galactic-Galactic stars:
gal-gal
2.0 10-6
Events rate comparison :
(MACHO 0.12 10-6)
P.Tisserand Rencontres du Vietnam 2006 8
1986 : B. Paczyński propose microlensing effect to probe the halo.
1990-92 : EROS1/MACHO/OGLE start the adventure.
1993 : First candidates !
1994-95 : First alert system by MACHO & OGLE
Detection of exotic events (binary lenses)
1994-98 : EROS1/MACHO : No short timescale events discovered (10-7M<M<10-3M)
1996 : Start of EROS-2.
jan 2000 : End of the MACHO experiment.
2000 : EROS2/MACHO : First result up to Mass=10M
~ 2002 : Start of the SuperMACHO experiment + 3rd OGLE phase.
feb 2003 : End of the EROS-2 observations.
History :
P.Tisserand Rencontres du Vietnam 2006 9
Collaboration: CEA/DAPNIA, LAL-IN2P3, IAP-INSU, Observatoire de Marseille, Collège de France (PCC), OHPBEros ~ between V and R & REros ~ I
EROS-2 : Expérience de Recherche d’Objets Sombres
Blue filterRed filter
Second Phase : July 1996 - February 2003
Dedicated telescope 1m Ø (Marly), at La Silla (Chile)
2 cameras : test for achromaticity
2×8 CCDs : wide field (~1deg²)
P.Tisserand Rencontres du Vietnam 2006 10
Status before this
analysis
P.Tisserand Rencontres du Vietnam 2006 11
Toward the Galactic center ….
Only Clump giant stars have been used !!
EROS2 : 120MACHO : 62OGLE : 33
Hundreds of microlensing effecthave been observed
Galactic latitude (deg)
P.Tisserand Rencontres du Vietnam 2006 12
Halo constraints in 2003:
Exclusion diagram at 95% C.L.
Excluded at 95% C.L.
Microlensing halo candidates:
EROS1 : 1 LMC
EROS2 : 4 LMC + 3 SMC
MACHO : 13 LMC
P.Tisserand Rencontres du Vietnam 2006 13
Physical Microlensing Background
P.Tisserand Rencontres du Vietnam 2006 14
« BLUE BUMPER »
Bright stars of the upper main sequence
Amplification < 2
+ Chromatic Variation
Easy to reject !
known physical background : (discovered by MACHO)
P.Tisserand Rencontres du Vietnam 2006 15
Candidates follow-up : longer baseline ( + 3 yrs)
EROS 1 – LMC#1 :
MACHO – LMC#23 :
~ 1992
~ 1998
~ 2001
~ 1995
3 candidates show a new bump a few years later !!
Variable Stars = Background Withdrawn !
P.Tisserand Rencontres du Vietnam 2006 16
(Probable) New background: Be type Stars.
EROS1-LMC#1 source star have
emission features.
ZOOM on the 2nd fluctuation:
P.Tisserand Rencontres du Vietnam 2006 17
Supernovae :
~ 590 Supernovae detectable
If : Appeared close to a cataloged star.
or SN cataloged.
26 Supernovae detected at low S/N .(Similar rate for MACHO)
== Serious background !
P.Tisserand Rencontres du Vietnam 2006 18
and / or
Supernovae elimination :• Galaxies seen on reference images
• Fit of an “asymmetric” microlensing light curve :
• Elimination of the 3 remaining EROS-2 LMC candidates (#5, #6 et #7) :Better Photometry!
Elimination if |S| > 0.3
EROS2-LMC#5 : S = 0.5 EROS2-LMC#7 : S = 0.62
P.Tisserand Rencontres du Vietnam 2006 19
Halo microlensing candidates status
EROS MACHO
EROS1-LMC#1 : Variable starEROS2-LMC#3 : Variable star EROS2-LMC#5 : SupernovaeEROS2-LMC#6 : SupernovaeEROS2-LMC#7 : SupernovaeEROS2-SMC#1EROS2-SMC#2 : Long Period VariableEROS2-SMC#3 : Long Period VariableEROS2-SMC#4 : Long Period Variable
MACHO-A-LMC#1 MACHO-A-LMC#4 MACHO-A-LMC#5 : galactic red dwarf lensMACHO-A-LMC#6 MACHO-A-LMC#7 MACHO-A-LMC#8 MACHO-A-LMC#13 MACHO-A-LMC#14 : self-lensingMACHO-A-LMC#15 MACHO-A-LMC#18 MACHO-A-LMC#21 MACHO-A-LMC#23 : Variable starMACHO-A-LMC#25
Only 1 on 9 candidates remain
10 on 13 could be considered as halo candidates
P.Tisserand Rencontres du Vietnam 2006 20
Data Analysis
P.Tisserand Rencontres du Vietnam 2006 21
Principe
of the
analysis:
Detection efficiency controlled by a MONTE-CARLO simulation:
=> False microlensing effects added on real light curve (~ 99% stable)
They passed the same selection cuts!
P.Tisserand Rencontres du Vietnam 2006 22
MACHO : estimate an additional 30% error due to blending
EROS2 : With HST LMC luminosity function weighted with the probability to generate an observable event.
→ ~1%
Blending problem
Star cataloged and surveyed
Fainter star located in the seeing disk
(less than 2”)
Optical depth estimate :
Using bright star, we considerably reduce that problem
under-estimated
P.Tisserand Rencontres du Vietnam 2006 23
Crowded field:
P.Tisserand Rencontres du Vietnam 2006 24
Better resolution → better rejection of variable stars
Statistics still excellent due to a better <efficiency>
Largely reduce the Blending problem
Remember galactic center !
Bright Star Sample
Magnitude cut different for each field: Mag [16-Rmax] with Rmax [18.2-19.7] Homogeneous sample : ~uniform photometric resolution (~7%)
LMC : ~6 Million
SMC : ~0.9 Million
First time in LMC !
On our 33.4 Million stars sample, we retained :
CLUMP
Eff
icie
ncy
LMC
P.Tisserand Rencontres du Vietnam 2006 25
Number of events expected
Macho
mass
Duration tE Number Magellanic events (full halo)
effi. = 100% Real effi.
10-3 M ~ 2.2 days ~ 2500 ~63
10-2 M ~ 7 days ~ 785 ~173
1 M ~ 70 days ~ 78 ~35
10 M ~7.4 months ~ 25 ~9
100 M ~ 2 years ~ 8 ~0.4
in the case of a dark halo
100% machos
τLMC ~ 0.45 10-6
τSMC ~ 0.65 10-6
For 6.9 million bright stars monitored during 6.7 years
× Efficiency
tE ~ 70 ( )½ days
M M
We need ~13 events to confirm the positive
signal of MACHO at 20%
P.Tisserand Rencontres du Vietnam 2006 26
No new microlensing event detected
Known since 1997 (EROS+MACHO) → Probably due to SMC lens
(for a halo lens, earth motion would distort the
light curve visibly)
tE = 120 days
EROS2-SMC#1
Duration expected for SMC self-lensing
1 candidate in the SMC still selected
P.Tisserand Rencontres du Vietnam 2006 27
_3% at 10-2 M
Final EROS combined limit (1990-2003)
_7% at 0.4 M
_10% at 1 M
LMC data set / No event
LMC + SMC data set with 1 SMC halo candidate
Domain excluded from all EROS data
ZOOMZOOM
P.Tisserand Rencontres du Vietnam 2006 28
Comparison : EROS-2/MACHO
2 different strategies : 2 different data sets
EROS2: ~ 7 Million Bright stars in sparse wide field (~84 deg2 LMC + ~10deg2 SMC)
MACHO: ~ 11 Million faint and bright stars in dense field (~13.4 deg2, LMC bar)
~2 Million bright stars in common ! MACHO field
EROS2 field
LMC
Our Measurement is mainly based on a less crowded areaPhotometry easier and result less affected by blending
Remark : A positive result must be seen everywhere, not be concentrated in a special area
P.Tisserand Rencontres du Vietnam 2006 29
Our analysis is conservative : - Use only bright-well measured sub-sample of Magellanic stars (~20% total) - Largely reduce the blending effect
Measurement obtained mainly with stars in the outer part of the LMC (sparse field)
Machos in the mass range 10-7 M< M < 5 M are ruled out as the primary occupants of the Milky Way Halo.
Result compatible with the Optical depth expected from the known star distribution (self-lensing + galactic disk stars)
2 different Monte-Carlo have been computed to estimate our detection efficiency: _ simulated microlensing effect on true light curve _ fake images that pass all the photometric chain with simulated microlensed star ► they are in excellent agreement for the bright star sample.
An all star sample analysis (33.4 millions) has been done with stricter cuts. Only 5 microlensing candidates have been selected : for one, the lens is a galactic red dwarf star located at about 300pc. (result also compatible with self-lensing)
Serious background : Supernovae & Variable stars Many former candidates died for these reasons (ex: EROS2-LMC#1 and MACHO-LMC#23)
Discussion of the EROS2 result
P.Tisserand Rencontres du Vietnam 2006 30
Difference between MACHO/EROS2 2 completely different data sets :
_Most of the MACHO stars are considered too faint for us : ~9 millions. _MACHO observation concentrated in the LMC bar : crowded region
Blending effect : MACHO suggest an additional 30% systematic error on the result.
Our limit is at f<7% for 0.4 M , about 13 events would have been necessary to confirm the MACHO signal.
The higher MACHO optical depth may be due, in part, to self-lensing in central part of the LMC. But this would contradict LMC models (Mancini et al., 2004) which suggest that only 1-2 MACHO candidates should be expected to be due to self-lensing (#9 and #14 are already known to be self-lensing).
5 MACHO candidates are really convincing : #1, #5, #9, #14 and #21. 3 are explained by LMC self-lensing or due to a galactic lens.
Possible confirmation : _ OGLE III and SuperMACHO
_ AGAPE, MEGA and WeCaPP (toward M31)
_ Photometric follow-up of candidates