Hubble Space Telescope
909
Cycle 19 GO Proposal
Ghost, Dark, Stripped, and Bullet Clusters Unleashed by
Pandora’s Cluster, Abell 2744
Principal Investigator: Dr. Dan Coe
Institution: Space Telescope Science Institute
Electronic Mail: [email protected]
Scientific Category: COSMOLOGY
Scientific Keywords: Clusters Of Galaxies, Dark Matter, Gravitational Lensing, Intracluster Medium
Instruments: ACS
Proprietary Period: 0
Orbit Request
Prime
Parallel
Cycle 19
8
0
Abstract
The Bullet Cluster presents us with a simple paradigm regarding collisions between galaxy clusters: gas is selfcollisional causing it to be stripped from galaxies and dark matter which are not. However some cluster
mergers appear to deviate from this prescription, likely because of more complicated merger physics yet to be
fully understood or perhaps, as some suggest, due to dark matter self-collisionality. Abell 2744 is a merger of
four galaxy clusters, making it one of the most active mergers known, and the only to feature a Mach ~3 shock
front aside from the Bullet Cluster. Our recent analysis of this merger helps reveal several features which
challenge our current understanding of galaxy cluster mergers. We find gas apparently leading rather than
trailing mass in our "ghost" cluster, perhaps by far the largest "ram-pressure slingshot" yet observed. Mass
appears to be offset from galaxies in our "dark cluster" (perhaps the first of its kind). And gas appears to be
stripped more cleanly and to a greater distance (> 250 kpc) from one of our clusters than any other yet known.
As we demonstrate below, HST imaging and additional analysis are required to verify and understand these
strange behaviors unleashed by "Pandora's Cluster". We waive any proprietary period to this data.
Dr. Dan Coe : Ghost, Dark, Stripped, and Bullet Clusters Unleashed by Pandora’s Cluster,
Abell 2744
Investigators:
Investigator
Institution
Country
PI
Dr. Dan Coe
Space Telescope Science Institute
USA/MD
CoI
Dr. Renato A. Dupke (Co-PI)
Eureka Scientific Inc.
USA/CA
CoI
Dr. Julian Manuel Merten
Universitat Heidelberg
Germany
CoI
Dr. Richard J. Massey
University of Edinburgh, Institute for Astronomy
UK
CoI
Mr. Adi Zitrin
Tel Aviv University - Wise Observatory
Israel
CoI
Dr. Matt Owers
Swinburne University of Technology
Australia
CoI
Dr. Leonidas Moustakas
Jet Propulsion Laboratory
USA/CA
CoI
Dr. Jason Rhodes
Jet Propulsion Laboratory
USA/CA
CoI
Dr. Massimo Meneghetti
INAF, Osservatorio Astronomico di Bologna
Italy
CoI
Dr. Narciso Benitez
Instituto de Astrofisica de Andalucia (IAA)
Spain
CoI
Prof. Brenda L. Frye
University of San Francisco
USA/CA
CoI
Dr. Laerte Sodre
Universidade de Sao Paulo
Brazil
CoI
Dr. Jessica Krick
California Institute of Technology
USA/CA
University of Michigan
USA/MI
CoI
Prof. Joel N. Bregman
Number of investigators: 14
Target Summary:
Target
RA
Dec
Magnitude
ACO-2744CLUSTER
00 14 6.7724
-30 22 47.38
V = 16.7 +/- 0.5
Observing Summary:
Target
Config Mode and Spectral Elements
Flags
Orbits
ACO-2744CLUSTER
ACS/WFC Imaging F435W
3
ACO-2744CLUSTER
ACS/WFC Imaging F606W
2
ACO-2744CLUSTER
ACS/WFC Imaging F814W
3
Total prime orbits: 8
● Scientific Justification
Each “bullet cluster” presents us with a unique and rare opportunity to improve our
understanding of structure formation and test our understanding of dark matter
The Bullet Cluster gave us for the first time direct empirical evidence of the existence
of dark matter as well as upper limits on its self-collisional cross-section (Markevitch04,
Clowe06, Randall08). In the aftermath of this galaxy cluster merger, we find gas stripped
from galaxies due to collisional pressure, while dark matter and galaxies appear to have
passed cleanly through.
Abell 2744 (hereafter, A2744; z = 0.308) appears to include a very similar cluster
collision of Mach ~3, including a gas shock front, but as viewed from an angle closer to
our line of sight (based on analysis of Chandra images by Owers11). Our analysis of this
collision yields constraints on dark matter particle self-interaction cross section (σ/m < ~3
cm2/g; (Merten11) which are very similar to those obtained from the Bullet Cluster. Two
additional clusters also appear to have participated in this merger, making it one of the
most complex known. As revealed in part by our analyses, these additional clusters exhibit
puzzling and rich phenomenology yet to be fully explained.
While the Bullet Cluster provided a straightforward explanation regarding the
interplay between galaxies, gas, and dark matter, other cluster mergers do not always
follow this narrative. The “Baby Bullet” (Bradac08) is one example that does appear to
behave as the Bullet Cluster with gas being stripped from galaxies and mass. A possible
counterexample was found in Abell 520 “the cosmic train wreck” (Mahdavi07), which
appeared to reveal a dark matter core devoid of, and perhaps collisionally stripped from, gas
and galaxies (although see Okabe08). A recent paper (Williams11) has also suggested that
collisional dark matter may have been stripped from a galaxy merging with the core of the
nearby cluster Abell 3827.
These suggestions of collisional dark matter are tempered with possible alternative
explanations. In each case, we are challenged us to either attain a new understanding of
how mergers may proceed (perhaps by recreating the observed behavior in simulations) or
else question our underlying theories about dark matter as a virtually collisionless particle.
A2744 appears to exhibit several such challenging cases, yet further observations are
required to confirm this.
Collisions which exhibit shock fronts are especially valuable, providing us with
information about merger dynamics and plasma physics, as well as perhaps dark matter
collisionality. Such shock fronts have only been detected so far in a handful of clusters (~58; Markevitch10 and references therein). Thus each presents a rare and unique opportunity
to study such an energetic event.
Ghost, Dark, Stripped, and Bullet Clusters in A2744
Only A2744 so far appears to exhibit a shock front with a velocity similar to that
of the Bullet Cluster (Mach ~3 in three dimensions, Owers11). Shocks detected in other
cluster mergers are significantly weaker (Mach ~1.6 -- 2, Markevitch10). In addition to
this “bullet”, A2744, which we dub “Pandora’s Cluster”, appears to have unleashed a “ghost”
cluster, a “dark” cluster, and perhaps the most significantly stripped cluster yet observed
(see Fig. 1). These configurations, which reside in the less well-studied Western half of the
merger (where we propose imaging), have yet to be well explained, though progress has been
made (including Owers11, Merten11).
We detected four separate mass clumps of ~1014 M (within 250 kpc) with our
gravitational lensing analysis of recently acquired HST/ACS images supplemented by VLT
and Subaru images (Merten11). Participating in this bullet merger are two or three of these
clumps, including our “dark cluster” in the NW. Contratry to expectations, we find the
brightest galaxies in this NW region appear to be leading this mass clump by significant
distances (~150 and ~300 kpc). Just as surprisingly, a gas clump, our “ghost cluster”, is
leading still further ahead (~450 kpc). Though long assumed to be trailing the galaxies,
Owers11 present evidence that this gas clump (which they call the “interloper”) is indeed
leading rather than trailing, and they suggest it has undergone a “ram-pressure slingshot”. In
this scenario, the gas previously trailing the mass has since caught up and been flung around
to the other side (Markevitch07, Ascasibar06). Such an effect appears to have been observed
but on a much smaller scale in Abell 168 (Hallman04). If confirmed, the A2744 “slingshot”
would be by far the largest observed to date. HST observations and dynamical simulations
will reveal whether some mechanism, perhaps involving multiple mass clump accretion may
have enhanced this separation.
Directly West of the core we detect a fourth ~1014 M mass clump (“W” in Fig. 1).
This clump is coincident with bright galaxies but significantly stripped of gas. Gas appears
to have been stripped > 250 kpc off of this mass clump, the largest separation between gas
and mass yet observed (compared to ~200 kpc for the Bullet Cluster; see Shan10). A faint
X-ray trail does appear to lead back from this clump back to the core of dense X-ray gas
(Merten11).
HST imaging significantly improves our lensing-based mass models
Toward understanding this mysterious behavior, we propose HST imaging of the
Western half of this complex cluster merger. This region has received less attention in the
literature to date, but our new lensing analysis has revealed significant mass clumps in this
region, warranting follow-up study.
As we clearly demonstrate in Fig. 3 (and discuss below), HST imaging significantly
improves our mass models in both recovered amplitude and resolution. Where we lack this
coverage, our lensing signals may well be diluted, yielding underestimated masses or even
undetected mass clumps. Our NW galaxies, for example, lie just outside our Cycle 17 HST
FOV. Our proposed HST coverage of these galaxies will either a) confirm their mass deficit
and the strange offset, or b) reveal previously underestimated mass clumps well aligned with
the galaxies.
Using the mass map reconstruction method of Merten09 (see Bradac05 for a similar
method), we compare our results with and without HST data. The HST images yield
improved constraints from weak lensing measurements as well as strong lensing features
which could not be identified in the ground-based images. Each individually contributes
additional robust signal which increases the amplitude of our mass model. Without this
data, our estimate of the mass of the central core, for example, is ~60% lower. This lower
mass density is clearly incorrect, as it would fail to reproduce the strong lensing features we
observe.
Our Cycle 17 ACS images yield ~60 galaxies / arcmin2 for our weak lensing analysis,
a three-fold improvement compared to our deep VLT images (Cypriano04 reanalyzed).
Space-based galaxy shape measurements are also higher precision due to the smaller, more
stable PSF (Kasliwal08).
With our Cycle 17 ACS imaging, we also identified 34 multiple images of 11
strongly lensed galaxies (Fig. 4). None of these had been previously identified in groundbased images. As shown in Fig. 3, this strong lensing data also significantly improves our
mass model recovery. Similar improvement has been demonstrated in analyses of simulated
clusters with known input masses (Meneghetti10b).
Resolving the mysteries surrounding this complex and active cluster merger
Our analysis of Cycle 17 HST observations, combined with data from other facilities,
raised important new questions regarding the history of this merger. By obtaining HST
images of the Western half of “Pandora’s Cluster” we will learn:
● Does the NW “dark cluster” in fact lag (surprisingly) behind its associated galaxies?
● Does it have a double mass peak which might help explain a large ram-pressure
slingshot of the “ghost cluster”?
● Does the “ghost cluster” have any significant associated mass?
● What is the precise location of the W mass clump and has its gas been stripped more
cleanly and to a greater distance than any other such mass clump known?
Once these facts have been established, we will work to explain any confirmed odd
behaviors. Dynamical simulations will be performed in Heidelberg in an attempt to recreate
and better understand the physics behind the merger (as in Springel07). We note we will also
be proposing for data from other facilities to help further establish these facts. Deeper Xray observations will further probe the interplay between gas and mass. Spectra of the lensed
arcs will help normalize our mass models. And deeper spectroscopy of cluster members will
enable us to detect and characterize dynamical substructure in the western region.
The unique opportunity of Pandora
An HST Multi-Cycle Treasury program (CLASH) is currently underway to study 25 galaxy
clusters. The majority of these clusters are relaxed, as one of the main science goals is
to study their mass profiles. Five of the CLASH clusters were instead selected based on
their strength as gravitational lenses. One of these, MACS J0717, is a merger nearly as
complex as A2744, though lacking a “bullet” shock front. We stress the uniqueness of
A2744 and thus the importance of our proposed observations. There exist only a handful of
opportunities in our universe to study these rare glimpses into such actively merging clusters.
Fig. 1 -- A2744 is likely a quadruple
cluster merger.
Shown here are
mass contours from our gravitational
lensing analysis (Merten11). Mass
is also shaded blue and (Chandra) Xray gas red, overlaid on a VLT color
image (VRI filters).
A2744 includes a Bullet Cluster
configuration (S -- N and/or NW), as
well as clumps which we call “dark”
(dark matter only) and “ghost” (gas
only).
All clumps appear to be
stripped of gas, though the W clump
appears to have its gas stripped by
>250 kpc, the greatest such distance
yet observed. The NW configuration
is especially puzzling, as gas appears
to be leading galaxies, which in turn
appear to be leading mass.
Fig. 2 -- We propose to image
the Western half of the merger,
as outlined in green at right.
This region has received less
attention to date in the literature,
yet we have detected significant
mass clumps there warranting
further study.
These new
observations will enable us to
recover significantly greater
lensing signals, improving our
mass models in these regions.
For example, HST coverage
will enable us to make more
definitive claims about any
separation between mass and
galaxies in the NW clump.
Orange: Cycle 17 ACS imaging
Green: proposed ACS imaging
Yellow: objects of interest
Color image as in Fig. 1; Black & white: Subaru i-band
Each square field is 10′ (2.7 Mpc) on a side
Fig. 3 -- HST/ACS imaging significantly improves our mass models of A2744. Shown
are our mass reconstructions (in units of critical lensing density) based on various subsets
of our data: 1) weak lensing from VLT and Subaru; 2) adding weak lensing from ACS
(observed area outlined in white); 3) adding strong lensing from ACS (all data); 4) As in
3, but higher resolution, as enabled by the additional data (and requiring significantly more
CPU time). Note the mass map from ground-based weak lensing alone is significantly
underestimated, yielding, for example, a ~60% lower mass of the cluster core. This lower
mass density is clearly incorrect as it would be insufficient to produce the strong lensing
features we observe. Similar results have been obtained applying this analysis method to
simulated clusters of known mass (Meneghetti10b).
ACS BVi color image of the cluster core
100″ ~ 450 kpc on each side
Multiple images of strongly lensed galaxies
and lensing critical curve labeled in white
References
Ascasibar06 ApJ 650, 102
Benitez09 ApJL 692, 5
Bradac05 A&A 437, 39
Bradac06 ApJ 652, 937
Bradac08 ApJ 687, 959
Clowe06, ApJ 648, L109
Cypriano04 ApJ 613, 95
Hallman04 ApJ 610, L81
Kasliwal08 ApJ 684, 34
Mahdavi07 ApJ 668, 806
Markevitch04 ApJ 606, 819
Markevitch07 Phys. Rep 443, 1
Fig. 4 -- In our HST/ACS
BVi imaging, we identified
34 multiple images (labeled
here) of 11 strongly-lensed
galaxies using the method of
Zitrin09. As shown in Fig. 3,
this information significantly
improves our ability to resolve
mass clumps in this region,
both in amplitude and positional
precision. We expect similar
gains from strong lensing
features revealed in our
proposed HST imaging of the
W and NW clumps. Note that
none of these multiple image
systems could be identified
in lower resolution groundbased imaging. Multiband HST
imaging is essential to provide
resolved color information
toward correctly matching these
faint objects.
Markevitch10 arXiv:1010.3660
Meneghetti10a A&A 519, 90
Meneghetti10b A&A 514, A93+
Merten09 A&A 500, 681
Merten11 MNRAS, submitted
Okabe08 PASJ 60, 345
Owers11 ApJ 728, 27
Randall08 ApJ 679, 1173
Shan10 MNRAS 406, 1134
Springel07, MNRAS 380, 911
Williams11 arXiv:1102.3943
Zitrin09 MNRAS 396, 1985
● Description of the Observations
In Cycle 17 we obtained ACS observations in two pointings with ~50% overlap each
with 8 orbits divided among 3 filters. We have demonstrated that these observations yield
significant improvements in our ability to map the mass of these clusters. Here we aim to
extend the area over which we can obtain consistent results.
Thus we propose one additional pointing to image the remaining (and more
intriguing) massive components in the Western half of this cluster merger (see Fig. 2). For
consistency, we propose the same filter set to the same depths as in Cycle 17: F435W (3
orbits), F606W (2.5 orbits), F814W (2.5 orbits). (Apologies that depths are rounded to full
orbits in the APT file.) As we argued in our previous proposal, this filter set allows one to
efficiently identify cluster galaxies as they are well segregated in color-color space. And in
practice, we find these filters yielded surprisingly good photometric redshifts for galaxies
at z < 0.7. Of 118 galaxies with spectroscopic redshifts (all z < 0.7) within our FOV, 99
had confident photo-z and these proved accurate to Δz ~0.06(1+z) RMS with no significant
outliers (all Δz < 0.3).
As with our Cycle 17 data, key analysis tasks will be led by the following people:
● D Coe -- image reduction, photometric redshifts
● R Massey -- weak lensing analysis of ACS images, including CTE/I corrections
● A Zitrin -- strong lensing analysis: image identification and modeling
● J Merten -- strong + weak lensing mass modeling
R Dupke and M Owers will lead analysis and interpretation of the Chandra X-ray data in
context with our mass model results.
● Special Requirements
None.
● Coordinated Observations
None
● Justify Duplications
A2744 has been previously observed with WFPC2 in Cycles 4 and 16 and by our group with
ACS in Cycle 17 (see Fig. 2). The WFPC2 observations are centered on the cluster core and
do not overlap significantly with our proposed observations. Our previous ACS observations
imaged the Eastern half of this complex merger, and here we propose to image the Western
half. Small areas of overlap are necessary to include all regions of interest.
● Past HST Usage and Current Commitments
As a member of the ACS GTO science team, the P.I. (Coe) has worked on multiband Hubble
ACS images of galaxy clusters since the inception of that instrument. Specifically he worked
on A1689 (GO 9289), MS1358 (GO 9717, 9292, 10325), and CL0024 (GO 10325). He also
analyzed ACS (and NICMOS) images of the UDF.
Coe supervised and contributed to the analysis of our Cycle 17 ACS observations of A2744
(GO 11689). He is currently a member of CLASH (GO 10265). The analysis tools for the
proposed project are largely in place, as described above. If this proposal is accepted we will
allot time and resources to rapidly perform and publish these analyses.
Selected recent HST publications:
Coe10, ApJ 723, 1678 “A High-resolution Mass Map of Galaxy Cluster Substructure:
LensPerfect Analysis of A1689”
Coe06, AJ 132, 926 “Galaxies in the Hubble Ultra Deep Field. I. Detection, Multiband
Photometry, Photometric Redshifts, and Morphology”
Merten11, MNRAS, submitted (based on our Cycle 17 observations)
Merten09, A&A 500, 681, “Combining weak and strong cluster lensing: applications to
simulations and MS 2137”
Massey10, MNRAS 409, 109 “Charge transfer inefficiency in the Hubble Space Telescope
since Servicing Mission 4”
Massey07, Nature 445, 286 “Dark Matter Maps Reveal Cosmic Scaffolding”
Zitrin11, MNRAS 410, 1939 “Strong-lensing analysis of a complete sample of 12 MACS
clusters at z > 0.5: mass models and Einstein radii”
Zitrin09, MNRAS 396, 1985, “New multiply-lensed galaxies identified in ACS/NIC3
observations of Cl0024+1654 using an improved mass model”