bullet cluster talk

8/10/2019 Bullet Cluster Talk

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BULLET CLUSTER

Dhruba Dutta ChowdhuryPresidency University


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Outline of the Talk

Overview of Galaxy Clusters

Bullet Cluster-What it is and Observations

Bullet Cluster and Cosmological Models

Recent Studies on Bullet li!e" clusters

Conclusion


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Galaxy Clusters


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#argest gravitationally bound structures$$

%&& to %&&&s of galaxies bound together$

Galaxies account for only %' of its total mass$

(round )' in the form of intergalactic gas-hightem*erature +,-) !e. X-Ray plasma$

Remaining 90% is invisible$theoreticallyre/uired to ex*lain stability and galaxy rotations*eeds in clusters within the G0R framewor! $


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What is the Bullet Cluster?

1t consists of two colliding clusters ofgalaxies.

More correctly bullet cluster refers to the

smaller subcluster moving away from the largerone$

#ocated at a comoving distance of %$%2% G*c

and redshift &$3 from us$


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Observations


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Observations were carried out with 4S05Magallen5 Chandra5 6SO W71 by the grou*s8$Clowe et al and Mar!evitch et al in (ug 9&&2$

Results were *ublished in 9&&:$

4S0 Credit;


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Magellan=?$(ri@ona=8$Clowe etal$Galaxies in orange and white$

Scale; 7ull field image is ,$A x A$2 arcmin

Optical Ia!e of the collidin! Clusters

0he stars of the galaxieswere not greatly affected

by the collision$0hey *assed right throughgravitionally slowed butnot otherwise altered


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"#$ay and Optical Ia!e superiposed

0his causes the gases toslow downmuch morethan the galaxies+theyinteract only gravitationally

without any drag.$

4ence the galaxies moveahead of the hot gasclouds.

0he o*tical image from Magellan and 4S0 shows galaxies in orange andwhite$ 4ot gas5which contains )&' normal matter in the cluster5 is shown bythe Chandra -ray Observatory image in *in!$


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%ravitational &ensin! studiesof Bullet Cluster

0he images of the bac!ground galaxies are gentlydistorted by the gravitational field of the clusters$

With more than a hundred galaxies5 we notice thatthe images are systematically alignedcharacteristic of *assing through a coherentgravitational lens$

7rom these distortions itDs *ossible to wor!bac!wards and find the mass distributionwhichcould have created such a gravitational lens$


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%ala'ies()ot !as and ore****

#ensing ma* in blue su*erim*osed on#ensing ma* in blue su*erim*osed onthe o*tical imagethe o*tical image#ensing ma* in blue su*erim*osed on#ensing ma* in blue su*erim*osed onthe x-ray imagethe x-ray image

Credit; -ray; O*tical; Magellan=?$(ri@ona=8$Clowe et al$>#ensing Ma*; 6SO W71> Magellan=?$(ri@ona=8$Clowe et al

Scale; 1mages are ,$A x A$2 arcmin


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The invisible ass

!ore bluerregions of the clusters have morelensingeffect-meaning it has more mass$

the total mass peak from the lensing map is

spatially offset from the baryonic masspeak"the hot gas# with a statisticalconfidence of $ sigma$ +8$Clowe et al.

We conclude that most of the mass in the clusteris invisible$


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Properties of the invisible ass

1tinteracts gravitationallywhich is the cause for lensing$

1ts centre coincides with the galaxies-it is alsocollisionless$it has not been slowed down and so doesnot have 6M interaction either with itself or ordinary

matter$ ?**er bound on dark matter self interaction cross-

sectionsigmam & 'cm()gm+Mar!evitch et al 9&&2.

More recent studies-sigma=m E &$, cmF9=gm +Randall et

all.


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%ala'ies+)ot !as+Invisible atter

Credit; -ray; O*tical; Magellan=?$(ri@ona=8$Clowe et al$> #ensing Ma*; 6SO W71>Magellan=?$(ri@ona=8$Clowe et al


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Collision ,niation

(rtists re*resentation of the huge collision in the bullet cluster$4ot gas5 containing most of the normal matter in the cluster5 isshown in red and invisible matter in blue$


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Bullet Cluster and Cosmological Models


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Dark -atter and &CD- -odel

0o account for the invisible mass in the universe which isre/uired to ex*lain flatness of the galaxy rotation curves5thestability of galaxy clusters etc5 *resence of non-luminousnon-baryonic dark matteris *ostulated in the standard *+,!+osmology$+9:$H' of the current universe is dar! matter.

0he invisible mass in the bullet cluster which is about '0 timesthe mass of the hot gasin it5!nown from lensing studies in thegeneral relativistic framewor! could then *ossibly be the darkmatter of *+,!$

1nfact the detection of such mass in the cluster is a very goodevidence in favour of the dark matter hypothesis$

owever not everybody is pleased$


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Can -O.D take a BU&&/T? !/,was *ro*osed by Milgrom in %)H3 to ex*lain galaxy rotation

curves by modifying


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OT)/$ )U$D&/0

Mastro*ietro J Bur!ert +9&&H. have shown that an initialrelative velocity of the two colliding clusters would need tobe around 3&&& !m=s in order to ex*lain the observed shoc!velocity5 -ray and the lensing data

4owever5 Kounghun and 6iichiro +9&%&. have shown thatsuch a high infall velocity is incompatible with thepredictions of the *+,! model.0he *robability that suchan event could occur is roughly one in %& billionL1heexistence of the bullet cluster poses a serious

challenge to *+,! cosmology unless a lower infallvelocity solution &'$00 kms is found$

So instead of being a *roof for the #C8M5it could as well bein direct conflict with it$


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RECENT STUDES ON !BU""ET

"#E$ C"USTERS


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0he four merging clustersfor which the ?C 8avis grou*members have led the observation and analysis +Bradac etal$ 9&&:> Kee et al$ 9&%9> Bradac et al$ 9&&H> 8awson et al$

9&%9.$ 0hese mergers re*resent a diverse masses5collision velocities5 and observed times since collisions$0he fifth cluster on the right is from a simulation of twoS18M haloDs *ost collision +Rocha et al$ 9&%95 in *re*.$

Credit;Merging Clusters Collaboration ?C 8avis Grou* and ?C 1rvine Grou*


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CU$$/.T WO$1

0hese are cluster mergers ust li!e the bullet cluster which areundergoing highly energetic collisions$

1n all of them we have observed a seperation of the totalmass peaks from the hot gas-implying invisible mass in

each one of them. Some limits have also been obtained on the dar!-matter

reaction cross sections by studying them$

eo*le have also *ostulated dark matter with strong self-interactionsto resolve a number of conflicts between

observations and simulations on the galactic scale and smaller

0his wor! is done by the!erging +lusters +ollaboration at?C 8avis and ?C 1rvine$


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Conclusions


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0he total mass *ea! and the baryonic mass *ea! offset at Hsigma significance clearly indicates missing mass in the bulletcluster$

4owever it is not absolutely certain that the missing mass isindeed cold dar! matter of #C8M$1t could be un!nown invisiblebaryons$

Studies from similar mergers and better estimates for reactioncross-section of the invisible mass in them may be able to shedmore light on dar!" matter$

0he results of Kounghun and 6iichiro+ 9&%&. need to be reviewedand investigated *ro*erly as it could be a serious threat to #C8M$


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$eferences

Wi!i*edia entry on Bullet Cluster et al$ +9&&:.$ N( 8irect 6m*irical roof of the 6xistence of 8ar!MatterN$ 0he (stro*hysical Kournal #etters :2H +9.

Chandra ray Observatory htt*;==chandra$harvard$edu

Kounghun #ee> Pomatsu +9&%&.$ NBullet Cluster; ( Challenge to #C8M CosmologyN$(*K ,%H

0he Mond ages htt*;==www$astro$umd$edu=Issm=mond=motiQbullet$html

Merging Clusters Collaborationwww$mergingclustercollaboration$org

rofessor Marusa Bradacs tal! on Bullet Galaxies and 8ar! Matter at S601

rofessor Suchetana Chatterees class lecture-11 and slides
http://chandra.harvard.edu/http://www.astro.umd.edu/~ssm/mond/moti_bullet.htmlhttp://www.mergingclustercollaboration.org/http://www.mergingclustercollaboration.org/http://www.astro.umd.edu/~ssm/mond/moti_bullet.htmlhttp://chandra.harvard.edu/


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,cknowled!eents

0han!s to rofessor Suchetana Chatteree for being ourcosmology instructor5for arranging this tal!$

Sincere than!s to those who have contributed to thewi!i*edia article$

Gratitude to


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T%an& 'ou

bullet cluster talk

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