The Merger Dynamics of the Galaxy Cluster Abell 1775: New Insights from Chandra and XMM-Newton for a Cluster Simultaneously Hosting a WAT and a NAT Radio Sources

Abstract

We present a new study of the merger dynamics of Abell 1775 by analyzing the high-quality Chandra and XMM-Newton archival data. We confirm/identify an arc-shaped edge (i.e., the head) at ∼ 48 kpc west of the X-ray peak, a split cold gas tail that extends eastward to ∼ 163 kpc, and a plume of spiral-like X-ray excess (within about 81 − 324 kpc northeast of the cluster core) that connects to the end of the tail. The head, across which the projected gas temperature rises outward from 3.39 +0.28 −0.18 keV to 5.30 +0.54 −0.43 keV, is found to be a cold front with a Mach number of M ∼ 0.79. Along the surfaces of the cold front and tail, typical KHI features (noses and wings, etc.) are found and are used to constrain the upper limit of the magnetic field (∼ 11.2 µG) and the viscosity suppression factor (∼ 0.01). Combining optical and radio evidence we propose a two-body merger (instead of systematic motion in a large-scale gas environment) scenario and have carried out idealized hydrodynamic simulations to verify it. We find that the observed X-ray emission and temperature distributions can be best reproduced with a merger mass ratio of 5 after the first pericentric passage. The NAT radio galaxy is thus more likely to be a single galaxy falling into the cluster center at a relative velocity of 2800 km s −1 , a speed constrained by its radio morphology. The infalling subcluster is expected to have a relatively low gas content, because only a gas-poor subcluster can cause central-only disturbances as observed in such an off-axis merger.

Nonthermal phenomena in the center of Abell 1775

“…1) that is reminiscent of a "remnant-core" cold front, rather than a sloshing cold front (see Tittley & Henriksen 2005;Markevitch & Vikhlinin 2007;ZuHone & Roediger 2016 for reviews). Furthermore, the presence of two elliptical galaxies with similar luminosity and high radial-velocity difference (Chincarini et al 1971;Jenner 1974) is difficult to reconcile with the scenario of a relaxed cluster with a sloshing core.…”

“…In a "slingshot scenario", low-entropy or low-temperature gas spirals and arc-shaped features in the ICM can trace slingshot tails produced by off-axis mergers (e.g., Sheardown et al 2019). The presence of a semi-disrupted core at the center of A1775 together with the two elliptical galaxies with a radial-velocity difference of ∼1800 km s −1 (Chincarini et al 1971;Jenner 1974) possibly suggest that A1775 underwent a more violent merger compared to that required for the sloshing scenario. In this case, the core that we are observing at the center of A1775 is that of a subcluster that fell into the system along the line of sight and with a nonzero impact parameter.…”

“…Optical studies on the velocity distribution of cluster galaxies have revealed that it is a binary (Oegerle et al 1995;Kopylov & Kopylova 2009) or possibly ternary (Zhang et al 2011) system. Here, we focus on its main component, Abell 1775B (hereafter simply A1775), located at z = 0.072, which is known to host two giant ellip-tical galaxies at its center with a radial-velocity difference of ∼1800 km s −1 (Chincarini et al 1971;Jenner 1974;Hintzen 1979;Kirshner et al 1983;Zabludoff et al 1990). Both galaxies are active in the radio band, with the southeast one, B1339+266B, whose radial-velocity deviates from the bulk of the velocities of the other cluster members, hosting an extended headtail radio source that has been studied at different frequencies and resolutions (Owen & Ledlow 1997;Giovannini & Feretti 2000;Giacintucci et al 2007;Terni de Gregory et al 2017).…”

“…LOFAR (van Haarlem et al 2013) Note added in proof. After this paper was accepted, Hu et al (2021) made available results based on the analysis of Chandra and XMM-Newton observations of Abell 1775. Their findings are in good agreement with those presented in this paper.…”