MACS J0553.4-3342: A young merging galaxy cluster caught through the eyes of Chandra and HST
M.B.Pandge, Joydeep Bagchi, S. S. Sonkamble, Viral Parekh, M.K.Patil, Pratik Dabhade, Nilam R Navale, Somak Raychaudhury, Jacob Joe
aa r X i v : . [ a s t r o - ph . C O ] A ug Mon. Not. R. Astron. Soc. , ?? – ?? (2017) Printed 24 September 2018 (MN L A TEX style file v2.2)
MACS J0553.4-3342: A young merging galaxy clustercaught through the eyes of Chandra and HST
M. B. Pandge ⋆ , Joydeep Bagchi , S. S. Sonkamble , Viral Parekh , M.K. Patil, Pratik Dabhade , Nilam R. Navale , Somak Raychaudhury , , Joe Jacob DST INSPIRE Faculty, Dayanand Science College, Barshi Road, Latur 413512, Maharashtra, India Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pun´e 411007, India School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431606, India Raman Research Institute, C. V. Raman Avenue, Sadashivnagar, Bengaluru 560080, India Department of Physics, Presidency University, 86/1 College Street, Kolkata 700073, India Newman College, Thodupuzha, Kerala, 685584, India.
24 September 2018
ABSTRACT
We present a detailed analysis of a young merging galaxy cluster MACS J0553.4-3342 (z=0.43), from
Chandra
X-ray and
Hubble Space Telescope archival data. X-rayobservations confirm that the X-ray emitting intra-cluster medium (ICM) in this sys-tem is among the hottest (average T = 12 . ± . ∼ ∼ ′′ ( ∼
320 kpc) and ∼ ′′ ( ∼
640 kpc) to the east of SC1. Theinner edge appears to be associated with a merger-driven cold front, while the outerone is likely to be due to a shock front, the presence of which, ahead of the cold front,makes this dynamically disturbed cluster interesting. Nearly all the early-type galaxiesbelonging to the two subclusters, including their BCGs, are part of a well-defined redsequence.
Key words: galaxies:active-galaxies:general-galaxies:clusters:individual:MACSJ0553.4-3342-inter-cluster medium-X-rays:galaxies:clusters
Galaxy clusters are among the most massive gravitation-ally bound systems in the Universe, assembled from thehierarchical merging of smaller sub-haloes over cosmictime. Evidence of such interactions among the smaller sys-tems linger in present day clusters in the form of sub-clustering in the distribution of galaxies, and in the hotintra-cluster medium (ICM) in the form of cold fronts,shock heating (Markevitch & Vikhlinin 2007; Plagge et al.2010), turbulence, and sub-structure (Ogrean et al. 2015;Dasadia et al. 2016; Botteon et al. 2016). Signatures of suchmergers can also be observed as diffuse non-thermal syn-chrotron radio emission in the form of radio haloes andrelics (e.g Bagchi et al. 2002, 2006, 2011; Feretti et al. 2012; ⋆ E-mail: [email protected]
Bonafede et al. 2012). Cluster mergers provide ideal set-tings for detailed studies to understand important aspectsof the physical processes involved in these mergers, in-cluding the thermodynamics of the hot gas, magnetic fieldamplification, and high-energy particle acceleration (cos-mic rays) by shocks and turbulence (Randall et al. 2008;van Weeren et al. 2009; Bonafede et al. 2012; ZuHone et al.2015), and offsets between the gas and the dark matter (DM)subclusters.In this paper, we present the results from the anal-ysis of an 83 ks
Chandra
X-ray observation, along witharchived Hubble Space Telescope (HST) optical observa-tions of the extremely hot, massive and X-ray luminousmerging galaxy cluster MACS J0553.4-3342 (z=0.43,Mann & Ebeling 2012). X-ray and radio studies of thiscluster have been reported earlier by Mann & Ebeling(2012) and Bonafede et al. (2012), using the shallower c (cid:13) Pandge et al.
Table 1.
Chandra
Observation log for MACS J0553.4-3342ObsID Observing Mode CCDs on Starting Date Total Time (ks) Clean Time (ks)12244 VFAINT 0,1,2,3,6 2011-06-23 74.06 73.285813 VFAINT 0,1,2,3,6 2005-01-08 9.94 9.86
Figure 1.
Background subtracted, exposure-corrected central 5 ′ × ′ Chandra image (0.7-2.0 keV) of MACS J0553.4-3342. This imagehas been smoothed with a Gaussian kernel of width σ = 3 ′′ . Arrows in this figure indicate the presence of an X-ray tail-like feature,seen along the north-east direction, appearing to originate from the centre of eastern subcluster (SC1). The two subclusters identified byMann & Ebeling (2012) are highlighted by blue crosses. Chandra (9.86 ks) and Giant Metrewave Radiotelescope(GMRT) observations, respectively, where a disturbedX-ray structure and a radio halo extending over ∼ §
2, we de-scribe the X-ray data reduction and imaging. § § §
4, while § H = 73 km s − Mpc − , Ω M =0.27& Ω Λ =0.73, translating to a scale of 8.091 kpc arcsec − atthe redshift z=0.43 of MACS J0553.4-3342. All spectralanalysis uncertainties are reported at the 90% confidencelevel, while all other uncertainties are given at 68% confi-dence level. c (cid:13) , ?? – ?? ACS J0553.4-3342 Figure 2.
Tri-colour image of MACS J0553.4-3342 obtained using 0.7 − Chandra
X-ray data (shown in blue colour), HST optical I band (F814W) data (red colour) and the GMRT 323 MHz data (green colour). This figure reveals the optical counterparts of the twosubclusters (SC1 and SC2).c (cid:13) , ?? – ?? Pandge et al.
Figure 3.
HST I -band (F814W) images of three regions of the MACS J0553.4-3342 system, centred on SC0, SC1 and SC2 respectively,each 30 ′′ × ′′ in size. A disk galaxy with a prominent dust lane dominates SC0. Figure 4.
An HST image of (F814W) the central ∼ ′ × . ′ region of MACS J0553.4-3342. The subclusters SC1 and SC2 areindicated, and their BCGs and neighbouring members from the inner 30 ′′ are shown by black dotted circles. The red circles indicatethe galaxies with I -band magnitude in the range of 18.5 −
27, while the blue circles mark the galaxies with V − I colours in the range V − I = 1 . − .
25. The point “x” indicates the position of the X-ray peak associated with the subcluster SC1.
MACS J0553.4-3342 has been observed twice by the
Chan-dra
X-ray Observatory, once in January 2005 and later inJune 2011, for an effective combined exposure of 83 ks (Ob- sID 5813 and 12244; for details see Table 1). Both the ob-servations were reprocessed using the
CHANDRA_REPRO task c (cid:13) , ?? – ?? ACS J0553.4-3342 BCG1BCG2 ( V - I) −0.500.511.52 I18 20 22 24 26 28 SC1SC2SC0 ( V - I ) Figure 5.
Left panel:
The colour-magnitude diagram for all the galaxies that have been identified from within the field by the Hubbleextended source catalogue (Whitmore 2015). The filled red circles show galaxies detected within the ∼ ′ × . ′ field of view aroundthe centre of MACS J0553.4-3342, while the blue circles are the red-sequence galaxies within the colour range V − I = 1 . − . Rightpanel:
A zoomed colour-magnitude diagram of all galaxies within the central 30 × ′′ of SC0 (filled yellow circles), SC1 (red triangles)and SC2 (open blue circles). The colour-magnitude diagram of SC0 indicates that the prominent galaxies in this sub-cluster are bluer incolour than the red sequence in SC1 and SC2, and therefore possibly closer to us. available within CIAO Chandra
X-ray Cen-ter (CXC). We followed the standard
Chandra data reduc-tion threads for the analysis of these observations. Periodsof high background flares exceeding 20% of the mean back-ground count rates were identified and removed from fur-ther analysis using the lc sigma clip algorithm. We used theCIAO REPROJECT_OBS task to reproject the event files andthe exposure maps in the energy range 0.7 − FLUX_OBS script.The
ACIS_BKGRND_LOOKUP script within CIAO was usedto identify the suitable blank sky background fields cor-responding to each of the event files. The X-ray back-ground files were modelled using the “blank-sky” datasets,and were reduced following the standard procedure out-lined in http://cxc.harvard.edu/contrib/maxim/acisbg. Wereprojected these sky background fields to match the coordi-nates of the observations and scaled them appropriately sothat their hard band (9 −
12 keV) count rates matched thosein the science frames before subtraction.Point sources identified from the resultant image usingthe
WAVDETECT algorithm were excluded from the subsequentanalysis. For the spectral analysis of the ICM, selected fromdifferent regions of interest, we generated corresponding Re-distribution Matrix Files (RMF), Ancillary Response Files(ARF) using the task
SPECEXTRACT available within CIAO.These spectra were then exported to XSPEC (version 12.9.1,Arnaud 1996) for further analysis. The exposure-corrected, http://cxc.harvard.edu/ciao http://cxc.harvard.edu/ciao/threads/index.html background-subtracted, 0.7 − Chandra image of thecentral 5 ′ × ′ region of MACS J0553.4-3342 is shown inFig. 1. The blue crosses in this figure indicate the positionsof the two previously identified subclusters (SC0 and SC1)(Mann & Ebeling 2012). To investigate the nature of optical counterparts of the twopossible subclusters SC0 and SC1 (Bonafede et al. 2012),associated with the two peaks of X-ray emission (shown bythe blue crosses in Fig. 1), we used the three broad bandimaging observations of MACS J0553.4-3342, taken in fil-ters F435W ( B ), F606W ( V ) and F814W ( I ), with effectiveexposure times of 4572 sec, 2092 sec and 4452 sec respec-tively, from the HST archives. Among these, we use theF814W image for finding optical counterparts of the pos-sible subclusters. We created a tri-colour map by combiningthe optical F814W (shown in red), GMRT 323 MHz radio(in green) (Bonafede et al. 2012) and 0.7 − Chandra
X-ray image (in blue) observations. The resultant compositeimage is shown in Fig. 2.In Fig. 3, the HST images of central 30 ′′ × ′′ ∼ (250 ×
250 kpc) regions centred around the brightest galaxies ofpossible subclusters SC0, SC1 and SC2 are shown from leftto right. In these images, we find a compact subcluster ofgalaxies, dominated by a BCG, in the heart of the X-ray haloSC1 at the centre of the system (Bonafede et al. 2012). Inaddition, we find another subcluster, ∼
650 kpc away to the c (cid:13) , ?? – ?? Pandge et al. west of SC1, marked as SC2 in this figure, also falling withinthe diffuse X-ray emission, but not showing any bright X-ray peak (this subcluster is not mentioned in Bonafede et al.(2012)). In Fig. 4, we show the HST F814W image of anextended 3 . ′ × . ′ region, where all galaxies with 18 . .
5. This plotclearly shows the red-sequence of early-type galaxies in thecore of MACS J0553.4-3342, with the I band magnitudesin the range between 18.5 − V − I colour-cutbetween ∼ − V − I colour values in this range arepart of the same cluster. Interestingly, the brightest clustergalaxies (BCGs) associated with the subclusters SC1 andSC2, highlighted by black circles in Fig. 2, appear close toone another confirming their membership. Their position inCMD plane is also shown in Fig. 5 ( left panel ). It is alsonoted that nearly all the members within 30 ′′ of the BCGs(marked by dotted black circles in Fig. 4) strictly follow thered-sequence of the ETGs.In Fig. 5 ( right panel ), the colour-magnitude informa-tion for all the member galaxies extracted from 30 ′′ circularregion is highlighted by yellow filled circles for SC0, red filledtriangles for SC1 and open blue circles for SC2. The “sub-cluster” earlier identified as SC0, placed to the east of SC1,(Fig. 1) corresponds to a compact group of galaxies visible inthe HST image, dominated by an edge-on disk galaxy with aprominent dust lane. The I -band magnitudes of these galax-ies are between 19.09 to 21.89 and their V − I colour is in between 0.97 to 0.99 shown in Fig. 5 ( right panel ). The CMDshows clearly that these galaxies of SC0 lie significantly be-low the red sequence corresponding to the sublcusters SC1and SC2, and therefore the galaxy group SC0 can not beat the same redshift as SC1 and SC2. Therefore, in the restof this paper, we will consider this galaxy group to be aprojected foreground system, and not part of the MACSJ0553.4-3342 cluster. The bright X-ray peak near to SC0 hasthe high luminosity (0.5 − ∼ . × erg s − (formore detail see § X-ray surface brightness profiles are crucial ingredients forthe investigation of shocks and cold fronts, as indicators ofthe merging processes occurring on the scale of galaxy clus-ters (Ogrean et al. 2015; Dasadia et al. 2016; Botteon et al.2016). To identify such features in the environment ofMACS J0553.4-3342 we have derived azimuthally averagedsurface brightness profiles of the X-ray emitting gas dis-tribution in this cluster, by extracting X-ray counts fromwithin the circular annuli, with their centres as indicated inFig. 6 ( left panel ). The extracted surface brightness profilewas then fitted with the one-dimensional β -model followingthe χ statistics of Gehrels (1986),Σ( r ) = Σ (cid:20) (cid:16) rr (cid:17) (cid:21) − β +0 . , (1)where Σ( r ) represents the X-ray flux at the projecteddistance r , Σ the central surface brightness, r the coreradius and β the slope parameter of the profile. The best fit1D β surface brightness profile is shown by the continuousline in Fig. 6 ( right panel ) with the best fit parameters β and r being 0.78 and 304 kpc, respectively.Unlike in the case of the cool core clusters (Pandge et al.2013; Sonkamble et al. 2015; Vagshette et al. 2017), the datapoints in the central region of this cluster lie below the best-fit model. A comparison of the β -model and the data pointsreveal an edge or discontinuity at a radius of ∼ ′′ . Anotherprobable discontinuity is seen at ∼ ′′ . To examine the global X-ray emission characteristics of theICM in the environment of MACS J0553.4-3342, we haveextracted a cumulative 0.7 − R =3. ′
09 ( ∼ ∼
25 counts per spectral bin and was imported to
XSPEC for further fitting using χ statistics. We tried to constrainthe spectrum with an absorbed single temperature plasmamodel APEC (Smith et al. 2001), with the Galactic absorp-tion fixed at N GalH = 0 . × cm − (Dickey & Lockman1990), letting all other parameters (e.g. temperature, metal-licity and normalization) vary. The best-fit minimum gives c (cid:13) , ?? – ?? ACS J0553.4-3342 Figure 6. left panel:
The 0.7 − Chandra image used for the extraction of the surface brightness profile of the distribution of theICM within MACS J0553.4-3342. The highlighted wedge shaped arcs are for extracting profiles for the identification of the discontinuitiesin the surface brightness distribution. right panel:
Projected radial surface brightness distribution in the energy range 0.7 − β -model to the data points (black crosses).
485 kpc
Tail
R2R1c b a 54321
Figure 7.
The
Chandra image of MACS J0553.4-3342 (en-ergy range of 0.7 − σ -wideGaussian after the removal of point sources. χ = 364 .
15 for 383 degrees of freedom (dof) with theelemental abundance 0.15 ± Z ⊙ and the ICM tem-perature amounting to 12.08 ± . − . , keV] (ROSAT-like)band, the X-ray luminosity within the R region equals L , [0 . − . , keV] =1 . ± . × erg s − .
500 kpc HTR Figure 8.
2D temperature map of the ICM distribution withinthe central 5 ′ × ′ region of MACS J0553.4-3342. Temperaturevalues of the gas from different regions marked in this figure arelisted in Table 2. Note the temperature peak (shown as HTR) inarc 13. We have derived a two-dimensional temperature map of thehot ICM within MACS J0553.4-3342, following the ‘contourbinning’ technique of Sanders (2006). This was achieved bygenerating a contour binned image of 15 different regions,with a minimum signal to noise ratio (S/N) ∼
40 (i.e. 1600counts). The regions were constrained to the geometrical fac-tors of 2 so that they would not be too elongated. Spectra c (cid:13) , ?? – ?? Pandge et al.
Figure 9.
Profiles of the thermodynamical parameters temperature (kT), electron density ( n e ), pressure (P) and entropy (S) (respectivelyfrom top to bottom) for the extracted spectra from regions 1, 2, 3, 4, and 5 are shown in the left panel , while those for regions a, b andc are shown in right panel . and response files were extracted separately from individ-ual bins. The spectra were then grouped to have at least20 counts per energy bin and were fitted with an absorbedsingle temperature APEC model as above. The best-fit tem-perature values from this analysis are shown in the form ofthe temperature map (Fig. 8) and are also summarized inTable 2.This map reveals that the ICM temperature varies sub-stantially within the scale of the cluster, indicating its com-plex nature. In same figure a high temperature region (here-after HTR, region 13) is indicated. Another jump in thetemperature of the ICM is also evident along the east of thiscold front and is probably due to the presence of a shock.Detailed properties of these cold and shock fronts are dis-cussed below. Notice the complexity and extended nature ofthe ICM in the central region. It appears to be in homoge-neously extended along the east-west direction likely due tothe interactions between the two subclusters SC1 and SC2.It is possible that the cold and shock fronts exist alongthe south and west directions of the X-ray centre of thecluster. To look for them, we have extracted separate spec-tra from the regions a, b and c (white semicircular arcs inFig. 7) and regions 1, 2, 3, 4 and 5 (blue arcs in Fig. 7). Theextracted spectra were treated with an absorbed single tem-perature plasma code
APEC with the absorption fixed at theGalactic value and the abundance at Z = 0.20 Z ⊙ . The bestfit thermodynamical parameters temperature (kT), electrondensity ( n e ), pressure ( P ) and the entropy ( S = kT × n − / e )for different regions are shown in Fig. 9 and are also tabu-lated in Table 3. The entropy, the key parameter that recordsgain of the thermal energy through the shocks and/or AGNfeedback while remaining insensitive to the adiabatic com-pressions and expansions, exhibits a significant increase,while moving from region 1 through 5 (Fig. 9 left panel ).Similar rise in the entropy is also evident in the regions a, band c (Fig. 9 right panel ). This analysis failed to detect any Table 2.
Best fit spectral properties of the ICM extracted from15 different regions of the 2D temperature map (Fig. 8).Reg. Net χ (d.o.f.) kT Norm (10 − )Counts (keV) (cm − )0 1940 79.01 (77) 9 . ± .
25 3 . ± .
141 1980 86.23 (80) 13 . ± .
98 3 . ± .
102 4866 122.92 (128) 11 . ± .
86 3 . ± .
133 3055 103.73 (120) 10 . ± .
69 3 . ± .
184 2354 100.99 (94) 8 . ± .
23 6 . ± .
365 1961 73.65 (76) 12 . ± .
83 3 . ± .
096 1990 66.79 (78) 11 . ± .
49 3 . ± .
147 1988 70.59 (79) 13 . ± .
36 3 . ± .
098 1987 74.75 (80) 9 . ± .
39 4 . ± .
119 2083 92.20 (83) 12 . ± .
80 3 . ± . . ± .
14 3 . ± . . ± .
72 3 . ± . . ± .
30 3 . ± . . ± .
13 3 . ± . . ± .
70 3 . ± . compression due to the presence of shocks and fronts. Simi-lar results were also found in the surface brightness analysisalong these regions. The
Chandra image of the cluster MACS J0553.4-3342 (Fig. 1) has also revealed a prominent tail-like structurethat extends in the north-east direction of the subclusterSC1 up to a distance of about 130 ′′ ( ∼ σ ). Thismight be the longest tail, originating from a stripping pro-cess, ever observed in the cluster environment. To examine c (cid:13) , ?? – ?? ACS J0553.4-3342 ] - a r c m i n - S B [ c oun t s s − − Distance [arcmin] 1 χ − − − ] - a r c m i n - S B [ c oun t s s − − Distance [arcmin] −
10 1 χ − − Figure 10.
Projected surface brightness profile extracted from the wedge shaped sector with opening angles between 130 ◦ − ◦ aroundthe region indicated by E1 ( left panel ), while that around the edge E2 is shown in right panel . Both these profiles were fitted with thedeprojected broken power-law density model whose 3D simulations are shown in the insets. Note the jumps in the surface brightnessnear both the edges E1 and E2. Table 3.
Best fit thermodynamical parameters (temperature,electron density, pressure, entropy) of the ICM extracted fromdifferent regions in Fig. 7.Reg. kT n e P S( keV) (10 − cm − ) ( keV cm − ) ( keV cm )1 11 . ± .
20 5 . ± .
06 0 . ± .
018 367 ±
572 13 . ± .
20 2 . ± .
01 0 . ± .
006 679 ±
653 12 . ± .
50 1 . ± .
01 0 . ± .
006 984 ± . ± .
00 0 . ± .
01 0 . ± .
004 1156 ± . ± .
80 0 . ± .
01 0 . ± .
003 1440 ± . ± .
29 10 . ± .
01 0 . ± .
003 194 ±
28b 12 . ± .
20 1 . ± .
01 0 . ± .
022 804 ± . ± .
50 0 . ± .
01 0 . ± .
004 1060 ± the thermal properties of the ICM in this tail-like structure,we analysed the spectra extracted from the long magentapolygon and its neighbouring regions R1 and R2 (white el-lipses), as shown in Fig. 7. The extracted spectra were inde-pendently fitted with an absorbed single temperature APEC model, with the abundance fixed at 0.2 Z ⊙ . The best-fit tem-perature values of the gas appearing in the tail region andits neighbouring regions R1 and R2 are tabulated in Table 4,and are found to be equal to 11.86 ± ± ± Table 4.
Best fit parameters of the X-ray tail and its neighbour-ing regions
Regions Counts kT Z(fixed) L [0 . − . , keV] χ Z ⊙ ) 1043erg s − ± ± ± ± ± ± merger of two equally massive subclusters (Reiprich et al.2004; Eckert et al. 2014; Schellenberger & Reiprich 2015). The surface brightness distribution of the ICM in Fig. 1clearly shows an edge (E1) at ∼ ′′ on the east of the X-ray centre of the cluster MACS J0553.4-3342 (SC1). Thiswas also evident in the radial surface brightness profile de-rived above. The radial profile also indicated the presenceof another edge or discontinuity (E2) at ∼ ′′ beyond SC1.To confirm the presence and to examine the significance ofthese edges relative to the neighbouring regions, we have ex-tracted two separate surface brightness profiles of the X-rayemission from the regions close to E1 and E2. The wedgeshaped regions selected for this extraction have the openingangles of 130 ◦ − ◦ and are shown in Fig. 6 ( left panel ).The extracted surface brightness profiles in the energyrange 0.7 - 4.0 keV are shown in Fig. 10 ( left panel ) and( right panel ), corresponding to the edges E1 and E2 respec-tively. These figures clearly indicate a sharp discontinuitynear the inner edge E1 (Fig. 10 left panel ), while that nearthe outer edge E2 is marginally indicative (Fig. 10 rightpanel ). To compute the compression parameters, and hence c (cid:13) , ?? – ?? Pandge et al.
Table 5.
Parameters obtained from the best fit broken power-law density modelRegions α α r sh n C χ /dof Mach No. ( M )(arcmin) (10 − cm − )E1 0 . ± .
08 1 . ± .
13 0 . ± .
02 7 . ± .
30 1 . ± .
10 29.17/21.00 −− E2 1 . ± .
18 0 . ± .
05 1 . ± .
04 0 . ± .
04 1 . ± .
16 48.96/48 1 . ± . the Mach numbers corresponding to the ICM compression atthese edges, we fitted these profiles with deprojected brokenpower-law density models, using the PROFFIT (V 1.4) pack-age of Eckert et al. (2011). These best-fit broken power-lawdensity models are represented by the continuous lines inthe insets of both the figures and are parametrised as: n ( r ) = C n ( rr sh ) − α , if r < r sh n ( rr sh ) − α , if r > r sh (2)where n ( r ) represents the electron number density at dis-tance r , n the density normalization, C the density com-pression factor of the shock, α and α the power-law indices,while r sh represents the radius corresponding to the putativeedge or cold/shock front. We allowed all the parameters tovary during the fit. The best fit parameters yielded by fittingthe broken power − law density model are listed in Table 5.According to the Rankine-Hugoniot relations(Landau & Lifshitz 1959), the density compression factor C at the location of the compression is related to the Machnumber M as M = (cid:20) Cγ + 1 − C ( γ − (cid:21) / . (3)Here, γ is the adiabatic index of the gas and we assume γ = 5 / in and E1 out for E1 andE2 in and E2 out for E2. All the four spectra were then fittedindependently with a single temperature APEC model withthe redshift fixed at 0.43. The best fit temperature values ofthe ICM across the edge E1 are 9.49 ± ) and15.34 ± ) at E1 in and E1 out , respectively. Here,the gas on the inner side of the edge E1 appears denserand exhibits a sharp boundary, probably due to the pres-ence of a merger driven cold front. The measured valuesof the ICM pressures on either side are the same withinthe uncertainties, thereby confirming that the edge E1 isformed by a merger driven cold front. Similarly, we also com-pute the best fit temperature values of the ICM across theedge E2 and are found to be equal to 15.34 ± ± in and E2 out , respectively. Then wecompute the corresponding Mach numbers using the rela-tion (Landau & Lifshitz 1959) M = (cid:0) T T − (cid:1) + h(cid:0) T T − (cid:1) + 15 i / M computed using Eqs. 3 and 4at the edge E2 are 1.33 ± ± Bullet cluster
Markevitch et al. (2002) and the
Toothbrushcluster van Weeren et al. (2016), justifying this renewed in-terest in this cluster.
We have already seen that the cluster MACS J0553.4-3342represents a highly disturbed, merging system. The
Chan-dra image (Fig. 1) confirms that the large-scale X-ray emis-sion associated with this cluster appears to be elongated to-wards the west. Further, the HST I band image reveals twoclose subclusters SC1 and SC2 separated by about ∼ . ′′ ( ∼
650 kpc), pointing towards an ongoing merging process.Therefore, it is of great interest to compare the dynamicalstate of MACS J0553.4-3342 with other clusters that repre-sent different stages of their dynamical phases ranging fromthe highly disturbed systems to the most relaxed ones.For this purpose, we have made use of the three non-parametric morphology parameters
Gini , M and Concen-tration index ( C ) to characterise the degree of disturbancesin these clusters (Parekh et al. 2015), found to be useful incharacterising galaxy clusters according to their level of dy-namical disturbance. The Gini coefficient parametrises theflux distribution among the image pixels, such that for therelaxed and cool-core clusters, where the X-ray flux is con-centrated only in a small number of image pixels, its valueis closer to 1, while in non-relaxed clusters, where the flux ismore widely distributed among the image pixels,
Gini takesvalues close to 0 (e.g. Lotz et al. 2004). The moment of light M is the normalized second order moment of relative con-tribution of the brightest 20% pixels (Lotz et al. 2004) and isa measure of the spatial distribution of the bright cores andsubclusters in the cluster. Typically, the value of the momentof light parameter M is found to vary in the range between − . − . C is a measure of theconcentration of the flux in the cluster and depends on theratio of the radii at which 80% and 20% of the cluster fluxesare measured (Conselice 2003). It takes the minimum valueof 0.0 for the most disturbed clusters.For estimating these morphological parameters in thecase of MACS J0553.4-3342 we have made use of thecleaned, background and exposure-corrected Chandra im-age. The computed parameters from within the 500 kpc re-gion around the cluster centroid in this figure are listed inTable 6. To compare the dynamical state of MACS J0553.4-3342 with those for the sample clusters of Parekh et al.(2015), we plot different correlations among the morpholog-ical parameters and are known as the morphological planes c (cid:13) , ?? – ?? ACS J0553.4-3342 Gini C o n c e n t r a t i o n Gini -2.5-2.0-1.5-1.0-0.5 M -2.5 -2.0 -1.5 -1.0 -0.5 M C o n c e n t r a t i o n Figure 11.
The morphological parameter planes for the control sample of galaxy clusters taken from Parekh et al. 2015 for identifyingtheir dynamical states. Open circles in all the three plots represent the ‘most relaxed’ clusters, diamonds the ‘relaxed’ clusters, plusesthe ‘non-relaxed’, while the ‘most disturbed’ clusters are indicated by crosses. The squares are clusters with radio halos and known tobe merging clusters. The position of MACS J0553.4-3342 in these plots is indicated by a green star. (Giovannini et al. 2009).
Figure 12.
Morphology parameters vs. temperature for the sample clusters as in Fig. 11. We subdivided the morphology parameter vstemperature plot into three regions: (1) dynamically relaxed clusters, (2) radio-quiet (no radio halo) merger clusters, and (3) radio-loud(with radio halo) merger clusters. The green star represents MACS J0553.4-3342 while a square within a green circle shows the ‘BulletCluster’ 1E 0657-56.c (cid:13) , ?? – ?? Pandge et al.
Table 6.
Morphology parameters for MACS J0553.4-3342 as discussed in § Gini M Concentration
MACS J0553.4-3342 0.40 ± ± ± (Fig. 11). The control sample comprises 49 low-redshift( z = 0 . − .
3) and 36 high-redshift ( z = 0 . − .
8) clustersof different dynamical states, representing relaxed as well asdisturbed phases. Open circles in this figure represent themost relaxed clusters, diamonds the relaxed, pluses ‘+’ thenon-relaxed and the crosses ‘x’ the most disturbed systemsfrom the sample. The squares in all three plots represent thegalaxy clusters with radio halos, known to be merging clus-ters and are taken from Giovannini et al. (2009). This figurereveals that the the relaxed and disturbed systems takes po-sitions on extreme ends, while the clusters with intermediatedynamical stages occupy positions in between them. Theseplots segregate clusters using different combinations of themorphological parameters with their limits ranging between-0.65
200 kpc east ofthe eastern subcluster SC1. From our spectral analysisfrom X-ray photons extracted from a ∼ ′′ ( ∼
120 kpc) cir-cular region of this peak, fitted in the same way as dis-cussed in § − ± ∼ . ± . × erg s − (minimum χ = 122 .
74 for 96degrees of freedom). Comparing with the SC1 gas temper-ature, it is evident that the gas temperature in this X-raybright peak is cooler than that of SC1 (see. region 2 in Ta-ble 3 13 . ± . In this paper, we have presented the analysis of a total of83 ks of
Chandra
X-ray observations, along with HST opti-cal observations, of MACS J0553.4-3342, one of the hottestsystems known representing a merging cluster. The main ob-jectives of the study were to identify and confirm the pres-ence of different subclusters in the environment of MACSJ0553.4-3342, and also to investigate discontinuities or edgesin the X-ray surface brightness distribution that remainedundetected in the previous studies. The present study hasclearly demonstrated that the ICM in this cluster hosts twomerging sub-clusters, whose merger axis lies along the east − west direction of the cluster. Important results from thisstudy are summarized below. • Optical identification of the member galaxies in the fieldof MACS J0553.4-3342 cluster confirms that this systemactually hosts two different merging subclusters SC1 andSC2 separated by a projected distance of ∼
650 kpc. • The exposure corrected background subtracted imageshows an X-ray tail-like structure extending up to a pro-jected distance of 130 ′′ or ∼ σ confidence) fromthe centre of SC1. The gas along this tail appears to be sim-ilar to its neighboring region within the uncertainties. • X-ray surface brightness profiles extracted from thewedge shaped regions with opening angles of 130 ◦ − ◦ in-dicate two sharp surface brightness edges (E1 & E2) at ∼ ′′ ( ∼
323 kpc) and ∼ ′′ ( ∼
647 kpc) east of the centre of thecluster, respectively. The inner edge E1 represents a merger-driven cold front, while the outer edge E2 is due to a shockfront. The Mach numbers M associated with the compres-sion due to the shock at E2 are estimated to be 1 . ± . . ± .
36, from a density compression jump analysisand from the temperature measurement on either sides ofthe shock front. A shock front ahead of the merger drivencold front is very similar to those seen in the
Bullet and the
Toothbrush clusters. c (cid:13) , ?? – ?? ACS J0553.4-3342 • Spectral studies reveal that the ICM in MACSJ0553.4-3342 to be at an average temperature of T =12 . ± .
63 keV, with average metallicity of Z =0.15 ± . Z ⊙ and luminosity L , [0 . − . =1.02 ± . × erg s − . This makes it one of the hottest and brightestclusters known. • The dynamical state of MACS J0553.4-3342 is exam-ined using the morphological parameters, as well as a sub-cluster analysis. This indicates that MACS J0553.4-3342represents a case of a dynamically disturbed cluster. • The colour-magnitude diagram plotted for MACSJ0553.4-3342 demonstrates that nearly all the early-typegalaxies, including BCGs at SC1 and SC2, within 30 ′′ of thecentres of the subclusters SC1 and SC2 are part of the samesystem, and lie within its well-defined red-sequence. ACKNOWLEDGMENTS
MBP gratefully acknowledges the support from follow-ing funding schemes: Department of Science and Tech-nology (DST), New Delhi under the SERB Young Scien-tist Scheme (sanctioned No: SERB/YSS/2015/000534), De-partment of Science and Technology (DST), New Delhiunder the INSPIRE faculty Scheme (sanctioned No:DST/INSPIRE/04/2015/000108). SSS acknowledges finan-cial support under Minority Fellowship program, Ministryof Minority Affairs, Government of India, (Award No F1-17.1/2010/MANF-BUD- MAH-2111/CSA-III). JB, PD andJJ gratefully acknowledge generous support from the Indo-French Centre for the Promotion of Advanced Research(Centre Franco-Indien pour la Promotion de la RechercheAvan´cee) under programme no. 5204-2. JJ wishes to ac-knowledge with thanks the support received from IUCAA,India in the form of visiting associateship. This researchhas made use of the data from
Chandra
Archive. Part ofthe reported results are based on observations made withthe NASA/ESA Hubble Space Telescope, obtained from theData Archive at the Space Telescope Science Institute, whichis operated by the Association of Universities for Researchin Astronomy, Inc.,under NASA contract NAS 5-26555. Thisresearch has made use of software provided by the Chan-dra X-ray Center (CXC) in the application packages CIAO,ChIPS, and Sherpa. This research has made use of NASA’sAstrophysics Data System, and of the NASA/IPAC Ex-tragalactic Database (NED) which is operated by the JetPropulsion Laboratory, California Institute of Technology,under contract with the National Aeronautics and SpaceAdministration. Facilities: Chandra (ACIS), HST (ACS).
REFERENCES
Arnaud, K. A., 1996, in Astronomical Society of the Pacific Con-ference Series, Vol. 101, Astronomical Data Analysis Software andSystems V, G. H. Jacoby & J. Barnes, ed., p. 17Bagchi, J., Durret, F., Neto, G. B. L., & Paul, S., 2006, Science, 314,791Bagchi, J., Enßlin, T. A., Miniati, F., Stalin, C. S., Singh, M., Ray-chaudhury, S., & Humeshkar, N. B., 2002, New A, 7, 249Bagchi, J., Sirothia, S. K., Werner, N., et al., 2011, ApJ, 736, L8Bonafede, A., Br¨uggen, M., van Weeren, R., et al., 2012, MNRAS,426, 40 Botteon, A., Gastaldello, F., Brunetti, G., & Dallacasa, D., 2016,MNRAS, 460, L84Conselice, C. J., 2003, ApJS, 147, 1Dasadia, S., Sun, M., Morandi, A., et al., 2016, MNRAS, 458, 681De Lucia, G., Poggianti, B. M., Arag´on-Salamanca, A., et al., 2007,MNRAS, 374, 809Dickey, J. M. & Lockman, F. J., 1990, ARA&A, 28, 215Ebeling, H., Qi, J., & Richard, J., 2017, ArXiv e-printsEckert, D., Molendi, S., Owers, M., et al., 2014, A&A, 570, A119Eckert, D., Molendi, S., & Paltani, S., 2011, A&A, 526, A79Feretti, L., Giovannini, G., Govoni, F., & Murgia, M., 2012,A&A Rev., 20, 54Gehrels, N., 1986, ApJ, 303, 336Giovannini, G., Bonafede, A., Feretti, L., Govoni, F., Murgia, M.,Ferrari, F., & Monti, G., 2009, A&A, 507, 1257Gladders, M. D. & Yee, H. K. C., 2005, VizieR Online Data Catalog,215Holden, B. P., Stanford, S. A., Eisenhardt, P., & Dickinson, M.,2004, AJ, 127, 2484Kodama, T. & Bower, R. G., 2001, MNRAS, 321, 18Landau, L. D. & Lifshitz, E. M., 1959, Fluid mechanicsLotz, J. M., Primack, J., & Madau, P., 2004, AJ, 128, 163Macario, G., Intema, H. T., Ferrari, C., et al., 2014, A&A, 565, A13Mann, A. W. & Ebeling, H., 2012, MNRAS, 420, 2120Markevitch, M., Gonzalez, A. H., David, L., Vikhlinin, A., Murray,S., Forman, W., Jones, C., & Tucker, W., 2002, ApJ, 567, L27Markevitch, M. & Vikhlinin, A., 2007, Phys. Rep., 443, 1Mei, S., Blakeslee, J. P., Tonry, J. L., et al., 2005, ApJ, 625, 121Nantais, J. B., Flores, H., Demarco, R., Lidman, C., Rosati, P., &Jee, M. J., 2013, A&A, 556, C4Ogrean, G. A., van Weeren, R. J., Jones, C., et al., 2015, ApJ, 812,153Pandge, M. B., Vagshette, N. D., Sonkamble, S. S., & Patil, M. K.,2013, Ap&SS, 345, 183Parekh, V., van der Heyden, K., Ferrari, C., Angus, G., & Holwerda,B., 2015, A&A, 575, A127Plagge, T., Benson, B. A., Ade, P. A. R., et al., 2010, The Astro-physical Journal, 716, 1118Postman, M., Franx, M., Cross, N. J. G., et al., 2005, ApJ, 623, 721Randall, S. W., Markevitch, M., Clowe, D., Gonzalez, A. H., &Bradaˇc, M., 2008, ApJ, 679, 1173Reiprich, T. H., Sarazin, C. L., Kempner, J. C., & Tittley, E., 2004,ApJ, 608, 179Sanders, J. S., 2006, MNRAS, 371, 829Schellenberger, G. & Reiprich, T. H., 2015, A&A, 583, L2Shimwell, T. W., Brown, S., Feain, I. J., Feretti, L., Gaensler, B. M.,& Lage, C., 2014, MNRAS, 440, 2901Smith, R. K., Brickhouse, N. S., Liedahl, D. A., & Raymond, J. C.,2001, ApJ, 556, L91Sonkamble, S. S., Vagshette, N. D., Pawar, P. K., & Patil, M. K.,2015, Ap&SS, 359, 21Stanford, S. A., Eisenhardt, P. R., Brodwin, M., et al., 2005, ApJ,634, L129Stanford, S. A., Eisenhardt, P. R., & Dickinson, M., 1998, ApJ, 492,461Vagshette, N. D., Naik, S., Patil, M. K., & Sonkamble, S. S., 2017,MNRAS, 466, 2054van Weeren, R. J., Brunetti, G., Br¨uggen, M., et al., 2016, ApJ, 818,204van Weeren, R. J., R¨ottgering, H. J. A., Bagchi, J., et al., 2009,A&A, 506, 1083Whitmore, B., 2015, IAU General Assembly, 22, 2247054ZuHone, J. A., Kunz, M. W., Markevitch, M., Stone, J. M., & Biffi,V., 2015, ApJ, 798, 90 c (cid:13) , ?? ––