1. Dark and Luminous Matter in Cluster Mergers
Anja von der Linden
KIPAC / Stanford
July 8, 2008
SLAC Annual Program Review

2. SLAC Annual Program Review
The Standard Model
-energy density according to cosmological standard model
-will focus on dark matter, compare to baryonic matter in gas, stars
July 8, 2008
SLAC Annual Program Review

3. SLAC Annual Program Review
The Standard Model
Cold Dark Matter (+ Dark Energy) successfully describes many observations:
mass of galaxy clusters
galaxy rotation curves
Cosmic Microwave Background
Structure Formation
dark matter very successful, esp. cold dark matter
postulated in 30s to explain high velocities in galaxy clusters
and in 70s to explain galaxy rotation curves
observations of CMB, structure formation have confirmed and refined standard model
alternative theories (e.g. modified theories of gravity) have troubled explaining these sets of observations at once
July 8, 2008
SLAC Annual Program Review

4. SLAC Annual Program Review
astrophysics has motivated Dark Matter
particle nature of DM likely to be disclosed by Particle Physics
What else can we learn about DM from astrophysics?
our laboratory: (merging) Clusters of Galaxies
form at the nodes of the cosmic web
“bottom up”: small structures form first, merge into larger ones
matter content: ~ 85% DM, ~ 13% (hot) gas, ~ 2% stars/galaxies
DM was postulated from astrophysics
nature of DM, i.e. what particles constitute it, now a particle physics problem, in the sense that DM is not an unseen form of known, baryonic matter, e.g. many small brown dwarfs, black holes
can we learn more about DM from astrophysics?
yes, we can!
by studying galaxy clusters. from steve’s talk: clusters are important cosmological probes, used in several ways
here: focus on what we can learn about the nature of DM from merging clusters
structure formation is hierarchical, i.e. clusters grow by accreting other clusters, galaxy groups
cluster mass budget, like the universe, dominated by DM, hence good laboratory
July 8, 2008
SLAC Annual Program Review

5. SLAC Annual Program Review
cluster mergers fascinating processes
simulation, showing dark matter top left, gas density top middle, gas temperature top right, shocks in the gas bottom right
simulation starts early, can see filamentary structure
small objects form, merge to larger ones
gas follows dark matter pretty good
z~0.5 (maybe stop?) DM clump without gas peak
further on: of astrophysical interest: shocks, cold cores of infalling clumps
credit: Volker Springel, MPA
July 8, 2008
SLAC Annual Program Review

6. Observing Cluster Mergers
Strategy:
combine high-quality, multi-wavelength observations
X-rays:
hot intracluster gas
Chandra, XMM
Optical:
cluster galaxies
Total mass distribution via strong and weak lensing
Hubble Space Telescope, Subaru 8m telescope
Team:
-simulation of course very nice, but what can we actually observe?
-...
Steve Allen
Glenn Morris
Evan Million
Marusa Bradac (now UCSB)
Anja von der Linden
Doug Applegate
plus more collaborators at KIPAC, in Santa Barbara, Hawaii, ...
July 8, 2008
SLAC Annual Program Review

7. SLAC Annual Program Review
The Bullet Cluster
X-ray Dark Matter
about two years ago: we published a really interesting case, the bullet cluster
as we recall, this was very exciting news from KIPAC/ SLAC, with a lot of media attention
why was that again?
shown in red: X-rays -> hot gas typically in clusters
shown in blue: mass distribution as reconstructed from gravitational lensing
matter distribution coincides with galaxies, but not with gas
July 8, 2008
SLAC Annual Program Review

8. SLAC Annual Program Review
The Bullet Cluster
Head-on collision of two clusters
Smaller cluster has cool, dense gas core (“Bullet”)
Shock front allows velocity estimate: ~ 4500 km/s
Dark Matter coincides with galaxies
Both (nearly) collisionless
Gas separated from Dark Matter
Inelastic collision
what we believe is happening: head-on collision of 2 cluster in the plane of the sky
“bullet” in fact a cool, dense gas core in the smaller cluster
in front of bullet: shock front
galaxies small compared to extend of cluster -> do not interact in cluster collision
fact that DM coincides with galaxies indicates that DM also very nearly collisionless!
gas, on the other hand, is separated from DM, and trails the DM peaks -> gas not collisionless, inelastic collision, gas clouds slow each other down
KIPAC papers on the Bullet Cluster:
Bradac et al. 2006, Clowe et al. 2006, Randall et al. 2008
July 8, 2008
SLAC Annual Program Review

9. SLAC Annual Program Review
The Bullet Cluster
- visualization of the merger
as in previous image, blue represents dark matter, red hot gas
start with two spherical systems with both DM and gas
as they collide, DM clumps pass through each other, gas clouds undergo inelastic collision, slow each other down
July 8, 2008
SLAC Annual Program Review

10. SLAC Annual Program Review
The Bullet Cluster
the Bullet Cluster has been labeled as “Direct Proof of Dark Matter”
very hard to explain without (cold, nearly collisionless) Dark Matter
But is it unique?
Are there systematics in the analysis / geometry?
Significant uncertainties from using only a single object?
...
but is it unique?
is this really what is happening? did we get the geometry right? could there be residual systematics in the analysis?
how much can we trust a result based on a single object?
July 8, 2008
SLAC Annual Program Review

11. SLAC Annual Program Review
Hot off the press: MACS0025
selected in X-rays from the MACS sample (MAssive Cluster Survey, Ebeling et al. 2001, 2007)
characteristic merger signatures:
highly elliptical
shifting of X-ray centroid
distant: z=0.586
observed with Chandra, HST
hot off the press: another merging system
has been accepted for publication, press release soon
part of our distant cluster sample, the MACS sample
selected in X-rays to be a likely merger, i.e. it appears elliptical, centroid shifts
has been observed with Chandra in X-rays, and with Hubble in multiple bands
X-ray contours supposed on HST image
X-ray contours elliptical, centroid shift
however, in optical, two clumps of cluster galaxies
south-eastern clump with two central galaxies, north-western clump with one - but all at the same redshift
-> system is in the plane of the sky
in the vicinity of central galaxies: multiple images of background galaxies
Bradac et al. 2008, ApJ, in press
Chandra press release coming up
July 8, 2008
SLAC Annual Program Review

12. SLAC Annual Program Review
Multiple Images
AB: z=2.8 (spectroscopically confirmed)
system A and B: spectra confirm this is the same background galaxy
system C: colors the same, redshift estimated from colors
same for system D
what is happening is strong gravitational lensing: mass of the cluster bends light, light can go “left” or “right” around cluster
this allows reconstruction of mass distribution in lensing cluster
also use weak lensing: sources not directly behind cluster are not mulitply imaged, but still distorted, at least statistically
D: z~2.8 (photometric)
C: z~1.0 (photometric)
July 8, 2008
SLAC Annual Program Review

13. Gravitational Lensing: Total mass
this allows a non-parametric reconstruction of the total mass in the system (which is of course dominated by dark matter)
dark matter follows galaxy distribution! just as in bullet cluster
July 8, 2008
SLAC Annual Program Review

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Mass + gas + light
compare DM distribution to X-ray map (in orange)
also light from galaxies in white
gas separated from DM and galaxies: only one peak, between the mass clumps
as pretty picture, analogous to bullet cluster: DM and gas clearly separated
July 8, 2008
SLAC Annual Program Review

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Bullet vs. MACS0025
separation of DM and gas
z=0.296
1.5 :1 merger
one “fuzz-ball”, one bullet
separation of DM and gas
z=0.586
equal-mass merger
two “fuzz-balls”
now compare bullet cluster to MACS0025
images matched in physical size: separation of DM clumps similar
in both, DM and gas clearly separated
quite different redshifts
bullet cluster has unequal-mass merger, MACS0025 about equal mass
and of course, the smaller subcluster in the bullet cluster has a “bullet”, whereas the MACS0025 merger between 2 fuzz-balls, i.e. gas is hot, less dense
July 8, 2008
SLAC Annual Program Review

16. Dark Matter Cross Section
Dark Matter clumps have passed through each other
-> typically less than one interaction per particle
projected mass surface density (from lensing):
number of DM particles:
-> Dark Matter self-interaction cross section:
July 8, 2008
SLAC Annual Program Review

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Conclusions
The Bullet Cluster and MACS0025 have different initial conditions and different merger dynamics,
yet both are easily explained by the presence of cold, collisionless Dark Matter.
Such cluster mergers are thus strong evidence for the validity of the Standard Model.
Are there more such systems?
Stay tuned...
July 8, 2008
SLAC Annual Program Review