Final results on galactic dark matter from the EROS-2 microlensing survey

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


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


Tool : lensing effect

• Lensing effect : Indirect detection

• For 1 M:

Image Separation ~ 0.2 milli arcsec

Σ

~ milli arcsec ~ arcsecEROS

MACHOOGLE

Exp:


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 !


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 :


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) :


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)


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 :


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)


Status before this

analysis


Toward the Galactic center ….

Only Clump giant stars have been used !!

EROS2 : 120MACHO : 62OGLE : 33

Hundreds of microlensing effecthave been observed

Galactic latitude (deg)


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


Physical Microlensing Background


« BLUE BUMPER »

Bright stars of the upper main sequence

Amplification < 2

+ Chromatic Variation

Easy to reject !

known physical background : (discovered by MACHO)


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 !


(Probable) New background: Be type Stars.

EROS1-LMC#1 source star have

emission features.

ZOOM on the 2nd fluctuation:


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 !


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


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


Data Analysis


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!


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


Crowded field:


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


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%


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


_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


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


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


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

Final results on galactic dark matter from the EROS-2 microlensing survey

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