MNRAS 000,1–20 (0000) Preprint 19 November 2020 Compiled using MNRAS LATEX style file v3.0 The Hercules cluster in X-rays with XMM-Newton and Chandra Juhi Tiwari¢ and Kulinder Pal Singhy Department of Physical Sciences, Indian Institute of Science Education and Research Mohali, Knowledge city, Sector 81, Manauli, Sahibzada Ajit Singh Nagar, Punjab 140306, India 19 November 2020 ABSTRACT We present a detailed X-ray study of the central subcluster of the nearby (I ∼0.0368) Hercules cluster (Abell 2151) identified as A2151C that shows a bimodal structure. A bright clump ∼ ¸0.02 × 43 of hot gas with X-ray emission extending to radius A 304 kpc and !- = 3.03−0.04 10 erg s−1 in the 0.4−7.0 keV energy range is seen as a fairly regular subclump towards the west (A2151C(B)). An irregular, fainter and cooler subclump with radius A ∼364 kpc is seen 43 −1 towards the east (A2151C(F)) and has !- = 1.13 ± 0.02 × 10 erg s in the 0.4−7.0 keV energy band. The average temperature and elemental abundance of A2151C(B) are 2.01±0.05 keV and 0.43 ± 0.05 Z respectively, while these values are 1.17 ± 0.04 keV and 0.13 ± 0.02 Z for A2151C(F). Low temperature (1.55 ± 0.07 keV) and a short cooling time (∼0.81 Gyr) within the central 15 arcsec region confirm the presence of a cool core in A2151C(B). We identify several compact groups of galaxies within A2151C(F). We find that A2151C(F) is a distinct galaxy group in the process of formation and likely not a ram-pressure stripped part of the eastern subcluster in Hercules (A2151E). X-ray emission from A2151C shows a region of overlap between A2151C(B) and A2151C(F) but without any enhancement of temperature or entropy in the two-dimensional (2D) projected thermodynamic maps that could have indicated an interaction due to merger between the two subclumps. Key words: X-rays: galaxies: clusters – galaxies: clusters: intracluster medium – galaxies: clusters: general – galaxies: clusters: individual (A2151) 1 INTRODUCTION Substructures within clusters usually coincide with smaller groups of galaxies which are known to have gas properties that are Clusters of galaxies are the largest gravitationally collapsed concen- different from their larger counterparts (Mulchaey 2000; Lovisari trations of matter in the Universe and are believed to form through et al. 2015). Galaxy groups are not simple scaled-down versions of the continuous merging of smaller galaxy groups (Voit 2005). They the bigger clusters. A number of X-ray studies of the ICM of different are, therefore, very crucial for understanding the large-scale struc- cluster samples (Ponman et al. 1999; Helsdon & Ponman 2000a; ture formation and evolution. Galaxy clusters are generally thought Maughan et al. 2012) suggest that low temperature systems (< 3.5 of as relaxed and evolved systems with total masses of ∼1014 −1015 keV;mainly galaxy groups) show a significant departure from simple M (McNamara & Nulsen 2007; Kravtsov & Borgani 2012) and power-law relationships like ! / ¹:)º2 where ! and :) are the large sizes of several Mpc. Observations of clusters, however, reveal - - X-ray gas luminosity and temperature, respectively. These power- that they contain a significant degree of substructure, indicating that law relations are predicted for clusters under the assumptions that arXiv:2011.08850v1 [astro-ph.GA] 17 Nov 2020 they are dynamically evolving systems (Fabian 1992; Oegerle & Hill they form via a single gravitational collapse and the only source of 2001). The substructures can be seen in the galaxy surface number energy input into the ICM is gravitational (Kaiser 1986). Departure density distribution (Geller & Beers 1982; Flin & Krywult 2006) from power-law relations is believed to be a consequence of non- and X-ray surface brightness (SB) maps of clusters (Gómez et al. gravitational processes (cooling, AGN feedback, star formation, and 1997; Jones & Forman 1999; Schuecker et al. 2001) in the form galactic winds) which play a very important role in the formation of multiple peaks. The merging of substructures into larger cluster and evolution of galaxy groups due to their shallow potential wells units is often visible as a sudden rise in the intra-cluster medium (Eckmiller et al. 2011; Lovisari et al. 2015). The study of individual (ICM) temperature and entropy, or as a discontinuity in the X-ray substructures is, therefore, crucial to understand the role of these SB profiles (Markevitch & Vikhlinin 2007; Böhringer & Werner processes in cluster formation. 2010). Hercules cluster of galaxies (Abell 2151 – hereafter A2151) at a redshift, I, of 0.0368 (Zabludoff et al. 1993) is a significantly ¢ e-mail: tiwarij[email protected] substructured system. Its optical centre as determined by Agulli y e-mail: [email protected] et al.(2017) is at R.A. (J2000) = 16ℎ05<23B.369; Dec. (J2000) = © 0000 The Authors 2 Tiwari & Singh 17◦4500400.33, and it has ∼360 galaxies belonging to it within a coincided with the two components (A2151C(B) and A2151C(F)) 1 radius of ∼1.3R200 . The cluster contains a high fraction of spiral seen in the X-ray emission of A2151C. A more recent spectroscopic galaxies (& 50 per cent; Giovanelli & Haynes(1985); Bird et al. study of A2151 by Agulli et al.(2017) using data from the William (1995); Maccagni et al.(1995)) and has a clumpy gas distribution Herschel Telescope (WHT) has confirmed the presence of substruc- (Magri et al. 1988; Bird et al. 1995). A2151, along with A2147 and ture within A2151C (§3.2 and fig. 5 of their paper), identifying 20 A2152, is part of a much larger Hercules supercluster (Barmby & members within A2151C(B) and 57 members in A2151C(F). The Huchra 1998). mean velocity and velocity dispersion were estimated to be 10116 A2151 has previously been studied in X-rays by Bird et al. km s−1 and 441 km s−1 respectively for A2151C(B), and 10299 km (1995) using data from the Position Sensitive Proportional Counter s−1 and 711 km s−1 respectively for A2151C(F). (PSPC) aboard the Roentgen Satellite (ROSAT), and by Huang & Here, we present a study of hot X-ray emitting gas within the Sarazin(1996) who used ROSAT’s High Resolution Imager (HRI). central subcluster of Hercules (A2151C) using all available archival The PSPC has a field of view (fov) ∼2° across while the HRI has a data from observations with the European Photon Imaging Cam- fov ∼38 arcmin across. The PSPC image of Bird et al.(1995) (fig. 1 era (EPIC) aboard the XMM-Newton observatory and the Advanced of their paper) revealed the presence of three subclusters in A2151 CCD Imaging Spectrometer (ACIS) aboard the Chandra X-ray Ob- – a bright central bimodal gas clump (A2151C), a fainter eastern servatory (CXO). This study exploits the superior spatial and en- clump (A2151E) and a northern clump (A2151N) with negligible ergy resolution of XMM-Newton (∼5 arcsec; ∼120 eV) and Chandra X-ray emission. The HRI image confirmed the two component X- (∼0.5 arcsec; ∼120 eV) over the ROSAT PSPC (∼25 arcsec, ∼550 ray structure of A2151C – a bright central subclump, A2151C(B), eV) and ROSAT HRI which had a good spatial resolution (∼2 arc- and a faint subclump, A2151(F). These subclumps were noticed sec) but no spectral resolution. A detailed X-ray spectral analysis earlier in an X-ray image observed with the Einstein Observatory’s of A2151C was, therefore, not possible with ROSAT. Besides, the Imaging Proportional Counter (IPC) (Magri et al. 1988). ROSAT ROSAT was sensitive only to soft X-rays between 0.1 to 2.0 keV PSPC image of A2151 showing its different components is displayed in contrast to the wider energy bandwidth of Chandra (∼0.2−10.0 in Figure 1 (top panel). keV) and XMM-Newton (∼0.2−10.0 keV). We use the capabilities A global X-ray spectral analysis of A2151C(B) (for a ra- of Chandra and XMM-Newton to investigate the thermodynamic −1 = dius (r) of 210 ℎ75 kpc from the X-ray peak), A2151C(F) (r properties of the ICM of A2151C by performing a detailed spectral −1 = −1 = analysis within the two substructures, A2151C(B) and A2151C(F), 200 ℎ75 kpc), A2151E (r 130 ℎ75 kpc) and A2151N (r −1 which are now optically associated with two different galaxy groups 151 ℎ75 kpc) was performed by Bird et al.(1995) in the energy band 0.1-2.4 keV using low spectral resolution of ROSAT PSPC (∼550 (Huang & Sarazin 1996; Agulli et al. 2017). We obtain radial pro- eV at 2 keV). They found that A2151C(B) and A2151C(F) have files of the properties of the gas in the two groups, and 2D projected X-ray temperatures of 1.67¸0.47 keV and 1.03¸0.60 keV, and over- maps of metallicity and thermodynamic (TD) properties of the gas −0.25 −0.06 in A2151C. all elemental abundances of 0.56¸0.31 and 0.32¸0.15 times solar, −0.20 −0.10 The paper is organized as follows. The observations used here respectively. The X-ray luminosities of A2151C(B) and A2151C(F) and details of the data reduction are provided in §2. The imaging in the 0.1-2.0 keV band were estimated to be 8.7±0.7×1042 erg s−1 and spectral analyses along with the radial profiles of the properties and 3.6¸0.68 × 1042 erg s−1 respectively. A spectral analysis of dif- −0.61 of the gas are presented in §3. Estimates of the cooling time, gas ferent regions within the individual subclusters was not carried out mass and total mass of the two groups are also given in this section.