Internal dynamics of Abell 1240: a galaxy cluster with symmetric double radio relics

Abstract

Context. The mechanisms giving rise to diffuse radio emission in galaxy clusters, and in particular their connection with cluster mergers, are still debated. Aims. We aim to obtain new insights into the internal dynamics of the cluster Abell 1240, which appears to contain two roughly symmetric radio relics, separated by ∼2 h −1 70 Mpc. Methods. Our analysis is based mainly on redshift data for 145 galaxies mostly acquired at the Telescopio Nazionale Galileo and on new photometric data acquired at the Isaac Newton Telescope. We also use X-ray data from the Chandra archive and photometric data from the Sloan Digital Sky Survey (Data Release 7). We combine galaxy velocities and positions to select 89 cluster galaxies and analyze the internal dynamics of the Abell 1237 + Abell 1240 cluster complex, Abell 1237 being a close companion of Abell 1240 in its southern direction. Results. We estimate similar redshifts for Abell 1237 and Abell 1240, z = 0.1935 and z = 0.1948, respectively. For Abell 1237, we estimate a line-of-sight (LOS) velocity dispersion of σ V ∼ 740 km s −1 and a mass of M ∼ 6 × 10 14 h −1 70 M . For Abell 1240, we estimate a LOS σ V ∼ 870 km s −1 and a mass in the range M ∼ 0.9−1.9 × 10 15 h −1 70 M , which takes account of its complex dynamics. Abell 1240 is shown to have a bimodal structure with two galaxy clumps roughly aligned along its N-S direction, the same as defined by the elongation of its X-ray surface brightness and the axis of symmetry of the relics. The two brightest galaxies of Abell 1240, associated with the northern and southern clumps, are separated by a LOS rest-frame velocity difference V rf ∼ 400 km s −1 and a projected distance D ∼ 1.2 h −1 70 Mpc. The two-body model agrees with the hypothesis that we are looking at a cluster merger that occurred largely in the plane of the sky, the two galaxy clumps being separated by a rest-frame velocity difference V rf ∼ 2000 km s −1 at a time of 0.3 Gyr after the crossing core, while Abell 1237 is still infalling onto Abell 1240. Chandra archive data confirm the complex structure of Abell 1240 and allow us to estimate a global X-ray temperature of T X = 6.0 ± 0.5 keV. Conclusions. In agreement with the findings from radio data, our results for Abell 1240 strongly support the "outgoing merger shocks" model to explain the presence of the relics.

Internal dynamics of Abell 2294: a massive, likely merging cluster

“…Boschin et al 2006;Barrena et al 2007a;Girardi et al 2008). The timescale of a few fractions of Gyr agrees both with the results of other merging clusters showing radio halos/relics (e.g., Markevitch et al 2002;Girardi et al 2008;Barrena et al 2009) and with theoretical expectations for radio halos (Brunetti et al 2009). The morphology of the A2294 radio halo is somewhat intriguing.…”

“…At this point, we have the minimum amount of observationbased information to be able to apply the two-body model (Beers et al 1982;Thompson 1982) following the methodology outlined for, e.g., Abell 1240(Barrena et al 2009). This simple model assumes radial orbits for the clumps with no shear or net rotation of the system.…”

Fossil groups origins

“…Previous analyses on data acquired with the same instrumentation and of comparable quality showed that nominal errors obtained through the RVSAO procedure should be multiplied by a factor ∼2 , and references therein). Following the method of Barrena et al (2009), we compared the determinations coming from different masks and found that the nominal errors were underestimated by a factor 2 in our case, too. Therefore, we assumed that the true errors are larger than nominal cross-correlation error by a factor of 2.…”

Fossil group origins

“…The nominal uncertainties given by the cross-correlation algorithm are known to be smaller than the true ones (see, Malumuth et al 1992;Bardelli et al 1994;Ellingson & Yee 1994;Quintana et al 2000). The uncertainties obtained through the RVSAO procedure were multiplied by a factor 2, following previous analyses (Barrena et al 2009, and references therein) on data acquired with the same instrumental setup and with comparable quality or our own. Moreover, to be conservative, we assumed the largest between the formal uncertainty and 100 km s −1 for the LOS velocities measured with the EMSAO procedure.…”