Hubble Space Telescope Cycle 19 GO Proposal 909 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 self- collisional 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 CoI Prof. Joel N. Bregman University of Michigan USA/MI 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 (~5- 8; 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). 14 We detected four separate mass clumps of ~10 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. 14 Directly West of the core we detect a fourth ~10 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).