A shock front in the merging galaxy cluster Abell 754: X-ray and radio observations

📝 Abstract

We present new Chandra X-ray and Giant Meterwave Radio Telescope (GMRT) radio observations of the nearby merging galaxy cluster Abell 754. Our X-ray data confirm the presence of a shock front by obtaining the first direct measurement of a gas temperature jump across the X-ray brightness edge previously seen in the imaging data. A754 is only the fourth galaxy cluster with confirmed merger shock fronts, and it has the weakest shock of those, with a Mach number M=1.57+0.16-0.12. In our new GMRT observation at 330 MHz, we find that the previously-known centrally located radio halo extends eastward to the position of the shock. The X-ray shock front also coincides with the position of a radio relic previously observed at 74 MHz. The radio spectrum of the post-shock region, using our radio data and the earlier results at 74 MHz and 1.4 GHz, is very steep. We argue that acceleration of electrons at the shock front directly from thermal to ultrarelativistic energies is problematic due to energy arguments, while reacceleration of preexisting relativistic electrons is more plausible.

💡 Analysis

We present new Chandra X-ray and Giant Meterwave Radio Telescope (GMRT) radio observations of the nearby merging galaxy cluster Abell 754. Our X-ray data confirm the presence of a shock front by obtaining the first direct measurement of a gas temperature jump across the X-ray brightness edge previously seen in the imaging data. A754 is only the fourth galaxy cluster with confirmed merger shock fronts, and it has the weakest shock of those, with a Mach number M=1.57+0.16-0.12. In our new GMRT observation at 330 MHz, we find that the previously-known centrally located radio halo extends eastward to the position of the shock. The X-ray shock front also coincides with the position of a radio relic previously observed at 74 MHz. The radio spectrum of the post-shock region, using our radio data and the earlier results at 74 MHz and 1.4 GHz, is very steep. We argue that acceleration of electrons at the shock front directly from thermal to ultrarelativistic energies is problematic due to energy arguments, while reacceleration of preexisting relativistic electrons is more plausible.

📄 Content

SUBMITTED TO The Astrophysical Journal Preprint typeset using LATEX style emulateapj v. 11/10/09 A SHOCK FRONT IN THE MERGING GALAXY CLUSTER ABELL 754: X-RAY AND RADIO OBSERVATIONS GIULIA MACARIO1,2, MAXIM MARKEVITCH2, SIMONA GIACINTUCCI1,2, GIANFRANCO BRUNETTI 1, TIZIANA VENTURI1, STEPHEN S. MURRAY2 submitted to The Astrophysical Journal ABSTRACT We present new Chandra X-ray and Giant Meterwave Radio Telescope (GMRT) radio observations of the nearby merging galaxy cluster Abell 754. Our X-ray data confirm the presence of a shock front by obtaining the first direct measurement of a gas temperature jump across the X-ray brightness edge previously seen in the imaging data. A 754 is only the fourth galaxy cluster with confirmed merger shock fronts, and it has the weakest shock of those, with a Mach number M= 1.57+0.16 −0.12. In our new GMRT observation at 330 MHz, we find that the previously-known centrally located radio halo extends eastward to the position of the shock. The X-ray shock front also coincides with the position of a radio relic previously observed at 74 MHz. The radio spectrum of the post-shock region, using our radio data and the earlier results at 74 MHz and 1.4 GHz, is very steep. We argue that acceleration of electrons at the shock front directly from thermal to ultrarelativistic energies is problematic due to energy arguments, while reacceleration of preexisting relativistic electrons is more plausible. Subject headings: galaxies: clusters: general — galaxies: clusters: individual (A754) — intergalactic medium — radio continuum: galaxies — X–rays: galaxies: clusters

1. INTRODUCTION Galaxy clusters form via mergers of smaller substructures. During such mergers, most of the kinetic energy of the gas belonging to the colliding subclusters is dissipated by shocks and turbulence into thermal energy of the intracluster medium (ICM) of the resulting system. Shocks and turbulence are also expected to amplify the cluster magnetic fields and ac- celerate cosmic ray particles from the thermal ICM, or reac- celerate preexisting relativistic particles. These non-thermal components manifest themselves as diffuse synchrotron radio sources, known as radio halos and relics (e.g., Ferrari et al. 2008; Cassano 2009 for recent reviews), and inverse Comp- ton X-ray emission at high energies (e.g., Fusco-Femiano et al. 2004; Rephaeli & Gruber 2002; see, however, Wik et al. 2009). Shock fronts represent a unique observational tool to study the physical processes in the ICM. They create sharp discon- tinuities in the cluster X-ray surface brightness images and al- low to measure the gas velocities in the sky plane using X-ray imaging spectroscopy (e.g., Markevitch, Sarazin, & Vikhlinin 1999). Shock-heated regions are routinely observed in merg- ing clusters. However, known shock fronts are still rare, be- cause one has to catch the front when it has not yet moved to the outer, low surface brightness regions of the cluster, where the background X-ray emission dominates. Moreover, the merger has to be occurring nearly in the plane of the sky, otherwise projection effects could hide the gas density and temperture jumps. Until now, reliable detections of merger shock fronts have been reported in only three galaxy clusters. One is the Bul- let cluster (1E 0657–56, Markevitch et al. 2002), another is Abell 520 (Markevitch et al. 2005), and the two fronts have been recently discovered in Abell 2146 (Russell et al. 2010). Only in these clusters both the sharp gas density edges and 1 INAF/Istituto di Radioastronomia, via Gobetti 101, I-40129, Bologna, Italy; g.macario@ira.inaf.it 2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA the unambiguous temperature jumps were found, allowing the identification of the brightness feature as a shock and deter- mination of the gas velocity. In this paper, we report on the new shock front in the merging cluster Abell 754. We present the analysis of Chandra observations and study the connec- tion between the shock and the diffuse cluster radio emission, through Giant Metrewave Radio Telescope (GMRT) observa- tions at 330 MHz and Very Large Array (VLA) archival data at 1.4 GHz. We assume a flat cosmology with H0 = 70 km s−1 Mpc−1 and Ω0 = 0.3, in which 1′′ is 1.054 kpc at the redshift of A754. We adopt the convention S ∝ν−α for the synchrotron spectrum. Uncertainties are 68%, unless stated otherwise.
2. THE MERGING CLUSTER A754 A754 is a rich nearby cluster at z = 0.0542 (Struble & Rood

1999. in the stage of a violent merger. It has been actively studied in the optical and X-ray bands, and is considered the prototype of a major cluster merger. Previous studies revealed that the cluster has a complex galaxy distribution (Fabricant et al. 1986, Zabludoff & Zaritzki 1995), X-ray morphology, and gas temperature structure (Henry & Briel 1995; Hen- rikesen & Markevitch 1996; Markevitch et al. 2003, here- after M03; Henry et al. 2004). These data indicate that A7

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