The Dissection of Abell 2744: A Rich Cluster Growing Through Major and Minor Mergers

Abstract

New Chandra X-ray data and extensive optical spectroscopy, obtained with AAOmega on the 3.9 m Anglo-Australian Telescope, are used to study the complex merger taking place in the galaxy cluster Abell 2744. Combining our spectra with data from the literature provides a catalog of 1237 redshifts for extragalactic objects lying within 15 ′ of the cluster center. From these, we confirm 343 cluster members projected within 3 Mpc of the cluster center. Combining positions and velocities, we identify two major substructures, corresponding to the remnants of two major subclusters. The new data are consistent with a post core passage, major merger taking place along an axis that is tilted well out of the plane of the sky, together with an interloping minor merger. Supporting this interpretation, the new X-ray data reveal enriched, low entropy gas from the core of the approaching, major subcluster, lying ∼ 2 ′ north of the cluster center, and a shock front to the southeast of the previously known bright, compact core associated with the receding subcluster. The X-ray morphology of the compact core is consistent with a Bullet-like cluster viewed from within ∼ 45 • of the merger axis. An X-ray peak ∼ 3 ′ northwest of the cluster center, with an associated cold front to the northeast and a trail of low entropy gas to the south, is interpreted as the remnant of an interloping minor merger taking place roughly in the plane of the sky. We infer approximate paths for the three merging components.

VLA Radio Observations of the HST Frontier Fields Cluster Abell 2744: The Discovery of New Radio Relics

“…It is interesting to note that the SE boundary of the radio halo is relatively well defined and aligns with the southern shock reported by Owers et al (2011). Macario et al 2011).…”

“…We note that the surface brightness edge we find is not the same one as reported by Owers et al (2011). The orientation of the shock reported by Owers et al and the one corresponding to R2 are roughly parallel; however, they are not directly related given that the separation between the two is ∼0.5Mpc.…”

“…From X-ray analysis it was suggested that the merger mass ratio of the subclusters is roughly equal (Kempner & David 2004), though it was unclear whether the dominant massive cluster core was located to the south (Owers et al 2011) or the north (Kempner & David 2004). The role of the northwest subcluster is also debated, with recent analysis of Chandra data by Owers et al (2011) suggesting that it is actually likely in a post, off-center, core passage phase heading toward the north/northeast.…”

Mass and magnification maps for the Hubble Space Telescope Frontier Fields clusters: implications for high-redshift studies

“…Thanks to the existing HST data for all six clusters, a minimum of two colours is available for each of them. Therefore, following the methodology described in J12, we used the existing spectroscopic and photometric redshifts (Owers et al 2011;Ebeling, Ma & Barrett 2014) to calibrate a colour-colour selection to identify the foreground galaxies and cluster members (the combination of filters used for each cluster is given in Table 7). …”

“…10 shows the galaxies selected by this process as well as all spectroscopically confirmed cluster members. We compile the latter from various catalogues of spectroscopic redshifts: for A2744, we consult Owers et al (2011); for MACS J0416, MACS J0717, and MACS J1149, we use the redshifts published by Ebeling et al (2014); and for AS1063 and A370, we rely on spectroscopic redshifts compiled in the NASA Extragalactic Database. We also use redshifts obtained by Balestra et al (2013, in preparation) for MACS J0416.…”