Chandra observations of the Abell S0295 cluster
DDraft version February 20, 2019
Preprint typeset using L A TEX style emulateapj v. 12/16/11
CHANDRA
OBSERVATIONS OF THE ABELL S0295 CLUSTER
Aurelia Pascut and John P. Hughes Draft version February 20, 2019
ABSTRACTWe present deep (205ks),
Chandra observations of the AS0295 binary merging cluster ( z = 0 . ∼ ∼ . . +0 . − . . We found other merger signaturessuch as a plume of cool gas emerging from the primary cluster and a cold front and a possible bowshock (Mach number of 1 . +1 . − . ) leading the secondary cluster. Based on the observed properties incomparison to binary merger simulations from the literature we propose for AS0295 a low mass ratio,off-axis merging scenario, with secondary close to first apocentre. Comparison of our results withstrong lensing observations of AS0295 from Cibirka et al. (2018) shows an offset between the totalmass peak and the bulk of the gas distribution in the primary cluster. The properties of the mergerand the existence of the offset between mass peak and gas make AS0295 a promising candidate forthe study of mergers involving non-cool core clusters and the nature of dark matter. INTRODUCTIONIn the context of the standard hierarchical scenario ofstructure formation, mergers between clusters are keyevents through which clusters grow (Voit 2005; Kravtsov& Borgani 2012). However, the merging process is notan instantaneous phenomenon and the time it takes fortwo interacting systems to merge and form a morpholog-ically relaxed, more massive system is typically ∼ − Department of Physics and Astronomy, Rutgers Univer-sity, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019, USA;[email protected] S , tefan cel Mare University, Astronomical Observatory andFaculty of Electrical Engineering and Computer Science, Uni-versit˘at , ii 13, Suceava, Romania; [email protected] different entropies (Markevitch & Vikhlinin 2007). Thesecontact edges can take two forms: cold fronts, which arean increase in surface brightness by a factor of about 2over a distance of several kpc accompanied by a drop intemperature by a similar magnitude, and shock fronts,in which both, the surface brightness and temperatureshow a drop across the edge. Moreover, the gas pressureover cold fronts is nearly continuous, while shocks arecharacterized by discontinuous pressure profiles.Cold fronts represent the most common feature seenin X-ray observations of merging clusters (Owers et al.2009a; Ghizzardi et al. 2010; Botteon et al. 2018), withsome clusters displaying more than one cold front (Ros-setti et al. 2013; Werner et al. 2016). Although coldfronts have been observed predominantly in mergingclusters, they also have been found in a significant num-ber of apparently relaxed ones (Markevitch et al. 2001;Mazzotta et al. 2001; Sanders et al. 2005; Dupke et al.2007). The abundance of cold fronts in merging clusters,may be explained by the fact that the cold front structurecan persist for gigayears and that even a minor mergercan displace the low entropy gas and create multiple coldfronts (Ascasibar & Markevitch 2006). Numerical simu-lations of idealized cluster mergers have shown two differ-ent mechanisms which can bring two regions of gas withdifferent entropies in contact and form cold fronts: 1) bydisplacing low entropy gas in regions with higher entropythrough sloshing (Tittley & Henriksen 2005; Ascasibar &Markevitch 2006) and 2) through gas stripping followedby adiabatic cooling (Ricker & Sarazin 2001; Nagai &Kravtsov 2003; Poole et al. 2006; ZuHone 2011).Besides cold fronts, most cluster merging simulationspredict the formation of a pair of shocks shortly beforethe moment of first pericentric passage, shocks which willpropagate in opposite directions towards the cluster out-skirts. The morphology of the shocks is dependent on themass ratio of the merging clusters and the geometry ofthe merger (Paul et al. 2011). The expected Mach num- a r X i v : . [ a s t r o - ph . C O ] F e b ber (defined as the ratio between the shock speed andthe speed of sound in the pre-shock gas) of these shocksis typically ≤ M ∼ − M ∼ Chandra and
XMM-Newton for more detailed stud-ies of this system. A very recent strong-lensing study(Cibirka et al. 2018) of this cluster shows the system hasa clear bimodal mass distribution which follows the clus-ter galaxy concentration.In this paper we present an X-ray investigation of themerging galaxy cluster AS0295, using 205 ks
Chandra ob-servations. We study the spatial and spectral propertiesof the intracluster medium, with the aim of understand-ing the relationship between the observed properties andthe dynamical state of the cluster.
Table 1
Global properties of AS0295. Redshift and cluster members arefrom Ruel et al. (2014) and BCG positions for primary (BCG E )and secondary (BCG W ) are from (Cibirka et al. 2018) andZenteno et al. (2016), respectively.Optical propertiesRedshift 0.3001Confirmed Members 30R.A. BCG E h m . s Dec BCG E − d m . s R.A. BCG W h m . s Dec BCG W − d m . s X-ray derived propertiesR.A. centroid 02 h m . s Dec centroid − d m . s R . ± .
04 MpcM . ± . (cid:12) kT r < R . ± .
32 keVkT . < r < R . ± .
48 keVCentroid shift 0.05 R
We assume H = 70 km s − Mpc − , Ω m = 0 . Λ = 0 .
7. Uncertainties are quoted at 68% confidencelevel. All coordinates are referenced to the J2000 epoch.All
Chandra
X-ray images presented are unbinned images(the pixel size is 0 . (cid:48)(cid:48) ). DATA REDUCTIONThis study uses all available
Chandra observations ofthe AS0295 cluster (6 observations with ObsIDs: 12260,16127, 16524, 16525, 16526, 16282). The cluster wasobserved for a total exposure of 205.7 ks.All observations were taken using the Advanced CCDImaging Spectrometer (ACIS; Garmire et al. (1992);Bautz et al. (1998)), with the ACIS-I array at the focalplane and in very faint (VFAINT) telemetry format. TheVFAINT mode allows a better screening of particle back-ground events, therefore reducing significantly the levelof particle background. Data reduction was performedusing CIAO (version 4.7),
Chandra ’s data analysis sys-tem, and calibration files from the
Chandra
CalibrationDataBase (CALDB; version 4.6.7). All observations havebeen reprocessed starting from level1 event files using theCIAO chandra repro script which creates an observa-tion specific bad pixel file and applies corrections usingthe latest calibration files. It also applies the VFAINTmode filtering of particle background events and includesonly events with grades = 0,2,3,4,6 and status=0 in thefinal filtered event file used for data analysis.The cleaning of background for potential anomalousperiods of very high background rates (flares) has beendone by applying a 3 sigma clipping algorithm to the lightcurves extracted from the entire field of view, excludingbright sources, in the energy band of 0.5-12 keV andbinned in a time interval of 250 s. We found that noneof the observations was affected by serious flares and thefinal exposure time, after flare filtering is 205 ks. DATA ANALYSIS3.1.
Image analysis
The X-ray image obtained after applying the data re-duction steps described in Section 2 and merging all indi-vidual observations is presented in the left panel of Figure1. The merged image of all six individual observationswas obtained from a merged event file, created by repro-jecting each event file to the same tangent point, whichis the tangent point of the observation with the longestexposure time. No absolute astrometry corrections wereapplied to individual observations before merging. Theimage in Figure 1 is a soft band (0.5-2.0 keV), back-ground included, exposure corrected image, smoothedwith a 1 . (cid:48)(cid:48) Gaussian kernel.The central panel of Figure 1 shows a 2 ks optical im-age taken with the H ubble Space Telescope ( HST ), us-ing the Advanced Camera for Surveys (ACS) instrumentand Wide Field Channel (WFC) detector. The level ofimage processing is the one applied as part of the
HST calibration pipeline, and includes: bias and dark currentsubtraction and flat-fielding. Since during this processno astrometric corrections have been applied, the posi-tional uncertainty is ∼ − (cid:48)(cid:48) . With an uncertainty for Chandra data of ≤ (cid:48)(cid:48) , the combined Chandra and
HST positional accuracy is within 2.2 (cid:48)(cid:48) (which is 10 kpc atthe cluster’s redshift of 0.3).The right panel presents the projected surface mass-density distribution obtained from the strong lensinganalysis of AS0295 by Cibirka et al. (2018).To enhance the existing X-ray surface brightness fea-tures, we have created an unsharp-masked image by sub-tracting a highly smoothed image (using a Gaussian ker-nel of size σ = 9 . (cid:48)(cid:48) ) from the image shown in Figure1, which has been smoothed with a 1 . (cid:48)(cid:48) Gaussian. Theunsharp-masked image is shown in Figure 2.3.2.
Spectral analysis
To investigate the variation of gas temperature acrossthe cluster, a 2D map of projected gas temperature wascreated (Figure 3).To create the map, the full band (0.5-7.0 keV) imageof the cluster is divided into multiple regions, using theadaptive binning algorithm of Diehl & Statler (2006).This algorithm assigns each pixel in the image to a re-gion such as to obtain in each region a chosen targetsignal-to-noise ratio. We choose a signal-to-noise ratioof 40 for the binning, a value high enough to allow forsufficient numbers of counts per bin for accurate spectralfitting. For the adopted signal-to-noise, the adaptive bin-ning algorithm results in 38 regions with the net numberof counts varying between 1300 and 3900 among the re-gions, and a mean of ∼ . × cm − (Dickey& Lockman 1990). The 0.3 value chosen for the abun-dance represents the typical metallicity observed in lowredshift clusters (Mushotzky & Loewenstein (1997),DeGrandi & Molendi (2001),Snowden et al. (2008)). The same spectral fitting method is adopted for creating thetemperature map and temperature profiles which will beused in the following sections to investigate temperaturevariations across several interesting regions in the cluster. GLOBAL PROPERTIESEvidence for the presence of an on-going merger inAS0295 comes from the irregular morphology of the X-ray gas as seen in the X-ray image of the cluster and theshape of the X-ray contours (see Figure 1, left panel).The secondary cluster is clearly visible as an X-ray peakin the NW direction. The orientation of the merger axisin the SE-NW direction is suggested by the elongatedmorphology of the X-ray emission along this directionand the “bunching up” of contour lines in front of thesecondary. This accords well with the SE-NW orienta-tion of the two total mass peaks visible in the stronglensing map (Figure 1, right panel), as well as the opti-cal galaxy distribution (Figure 1, central panel).Several X-ray global properties of the cluster derivedfrom our data analysis as presented below, together witha few optical properties taken from the literature aresummarized in Table 1.The position of the centroid is estimated from thesmoothed, exposure corrected image, iteratively, using acircular region with a radius of 3 arcminutes. At the firststep, the center of the circular region is the peak of thecluster and a first estimate of the centroid is obtained. Inthe following steps, the position of the centroid is reesti-mated using the new value for the centroid as the centerfor the circular region and the process is repeated untilconvergence.To quantify the degree of disturbance of the ICM, wehave calculated the centroid shift parameter, followingthe method described in Poole et al. (2006). Centroidshift is the standard deviation of distances between theX-ray peak and centroid measured within several circularregions with radii decreasing from R to 0 . , witha step of 0 . . For each new radius, the circular re-gion is centered on the X-ray peak, while the centroid isreestimated and therefore a new separation between peakand centroid is calculated. Following Poole et al. (2006),when estimating the centroid, we excluded a circular re-gion of 30 kpc radius, centered on the peak position.In general, clusters are characterized by centroid shifts(as a fraction of R , the radius enclosing 500 times thecritical density of the Universe) ranging from ∼ .
001 to ∼ .
15 and a value of 0.01-0.02 has been adopted empir-ically as a threshold between relaxed and disturbed clus-ters (O’Hara et al. 2006; Maughan et al. 2008; B¨ohringeret al. 2010; Cassano et al. 2010; Weißmann et al. 2013;Donahue et al. 2016). With a centroid shift of 0.05,AS0295 can be clearly classified as a disturbed cluster.R has been estimated using the M − Y X relationfrom Vikhlinin et al. (2009), where Y X is the productbetween gas temperature and mass. Following their it-erative prescription, we have estimated at each step thegas temperature from a spectra extracted within 0.15-1R , and the gas mass by fitting the emissivity profileobtained from the same region with a gas density model(Vikhlinin et al. 2009) projected along the line of sight.In the first step, the value assigned to R is 0 . is obtained. As in Vikhlinin et al. (2009), at all but RA (J2000) D e c ( J ) Primary Secondary
100 kpc
RA (J2000)
100 kpc
RA (J2000)
100 kpc
Figure 1.
Left:
Chandra soft band (0.5-2.0 keV), exposure corrected, smoothed image of AS0295 cluster. The secondary cluster (locatedin the NW part of the image) is clearly visible as a surface brightness peak while the primary cluster (in the SE) has a flatter surfacebrightness distribution. Center:
HST archival image taken in the ACS/WFC instrument/detector configuration (PI: F. Pacaud). Right:Projected surface mass-density distribution obtained from strong lensing analysis of AS0295 by Cibirka et al. (2018). All images arematched for the same coordinates. The black diamond symbols (center panel) and square symbols (right panel) mark the positions of thebrightest cluster galaxies (see also Table 1) and the approximate positions of two radio relics detected by Zheng et al. (2018).
RA (J2000) D e c ( J ) C o l d F r o n t P l u m e S h o c k ? S h o c k ?
100 kpc
Figure 2.
Unsharp-masked image created by subtracting a σ =9 . (cid:48)(cid:48) Gaussian smoothed image of the cluster from a σ = 1 . (cid:48)(cid:48) Gaussian smoothed image. Green regions are used for the creationof surface brightness and temperature profiles shown in Figures 4and 6 the first step we have excluded the central 0.15R fromtemperature estimation to avoid contamination from apossible cool core, which are known to introduce scatterin scaling relations. Since the secondary has cool gas as-sociated with its core, we have treated the secondary as acontaminating source and exclude a circular region, cen-tered on the secondary’s X-ray peak and with a radiusof ∼ . We obtain very similarresults for R when we treat the secondary as a con-taminating source compared to the case when secondaryis not excluded from the analysis.Results from Table 1 show that AS0259 is a morpho- logically disturbed (centroid shift of 0.05), hot ( ∼ ∼ × M (cid:12) ) cluster. DIFFERENT TYPES OF SURFACEBRIGHTNESS FEATURESSince mergers between galaxy clusters leave their im-print on the ICM in the form of shocks, cold frontsand other transient features observed in the gas surfacebrightness and temperature distribution, we next lookfor these kind of features for AS0295 in our X-ray imageand temperature map.A qualitative analysis of the unsharp-masked image(see Section 3.1 and Figure 2) shows two clear featuresin the form of surface brightness edges (one near the pri-mary, labeled “plume” and the other in the secondarylabeled “cold front” in Figure 2). The most striking fea-ture is the edge seen in the NW direction, very close tothe core of the secondary cluster. Another noticeablefeature, which is also apparent in the X-ray image andcontours (Figure 1) is the sharp edge seen close to thecore of the main cluster, in the S direction. Two otherless obvious features labeled “Shock?” in Figure 2 arethe locations of possible shocks that are studied in detailbelow.Before investigating in detail the nature of these fea-tures seen in surface brightness, we look at the temper-ature distribution in the cluster as shown in Figure 3.The first thing to notice is the presence of a cold regionsituated in the NW direction (region 2 in the tempera-ture map) and which coincides with the subcluster. Thehigh temperature difference seen in the temperature mapfor this region compared to an adjacent region (region 5)and the high contrast in surface brightness seen in Figure2 suggests this might be a cold front.Related to the other two features seen in surface bright-ness, the temperature map does not give more informa-tion since the size of the bins used to extract temper-atures are larger than the size of the edges seen in thesurface brightness images. However, we note that theedge seen to the SE of the primary is in the vicinity ofthe hottest region found in the temperature map (region
38 37 363534 33 32 31 3029 282726 25 24 2322 21 20 19181716 151413 1211 1098 76 5 4 32 1 k T ( k e V ) Figure 3.
Top panel: Temperature map created using
Chandra data over the 0.5-7.0 keV band. The numbered polygonal regions inthe figure were generated by an adaptive binning algorithm (Diehl& Statler 2006) so as to enclose a target signal-to-noise ratio of 40.Bottom panel: Estimated projected temperature for each regionshown in the temperature map. The horizontal line at 9.5 keVmarks the global mean temperature of the cluster estimated withina circular region with radius equal to R . Cold front in secondary cluster
The most striking feature seen in the unsharp-maskedimage of the cluster (Figure 2) is the sharp drop in sur-face brightness in front of the secondary cluster. Morequantitatively, this sudden change in surface brightnessis seen as a discontinuity at ∼
100 kpc in the surfacebrightness profile which is shown in the top panel of Fig-ure 4. The profile was extracted within the sphericalannular sector shown with a solid green line in Figure 2and the three inscribed sectors are used to extract thetemperature profile, presented in the bottom panel ofFigure 4.We fit first the surface brightness profile with a modelthat assumes a density distribution that follows a bro-ken power law, a model commonly used to represent theobserved jumps in the distribution of surface brightness.In our case, such a model represents a poor match tothe data (blue curve in Figure 4), especially between200 − . SB ( pho t/ s / c m / p x ) −11 −10 −9 −8 Radius (kpc)
50 100 200 300 400 500 600 k T ( k e V ) Figure 4.
Surface brightness (top panel) and temperature (bot-tom panel) profiles extracted within annular sectors across the sur-face brightness drop seen in front of secondary cluster. Verticallines mark the radii of annular regions used to create the tempera-ture profile. Blue and red lines represent the fit of a density modelrepresented by two and three power laws, respectively, connectedby density jumps.
From the fit results we find a jump in density of2 . ± .
29 at 104 kpc. A jump with a similar amplitudeis also found for the projected gas temperature. Thegas temperature increases by a factor of 1 . +0 . − . , from5 . +1 . − . keV in the region interior to the discontinuityto 11 . +3 . − . keV in the region situated just beyond thediscontinuity (see bottom panel of Figure 4). This sud-den decrease in surface brightness accompanied by anincrease in temperature, with a similar amplitude, is anindication of the presence of a cold front.For completeness, we mention that for the second sur-face brightness discontinuity, situated at a distance of246 kpc from the cold front, the amplitude of the densityjump obtained from the fit is 2 . ± .
87. The nature ofthis feature is investigated in Section 6.2.5.2.
Plume in primary cluster
Apart from the cold front detected in front of the sec-ondary cluster, another significant feature seen in theunsharp-masked image (Figure 2) is the sharp surfacebrightness edge found to the south of the primary clus-ter’s core. This feature is also clearly visible in the sur-face brightness contours.What is remarkable about this structure, whose lengthis roughly 240 kpc, is the linear-like shape of its edge.We note that this feature is not an artifact due to a chipgap, since in all observations this region of the clusterfalls entirely on a single chip.The left panel of Figure 5, represents an azimuthal sur-face brightness profile obtained using sectors centered onthe main cluster and with opening angles of 20 degrees.The center of all sectors has been selected such that theedge seen in the unsharp-masked image marks the sideof one of the sectors (this sector is marked in the in-set in Figure 5 at angles between 205 and 225 degrees).As already obvious from the cluster X-ray image (theinset in the Figure), the profile shows that the highestsurface brightness is in the direction joining the two sub-clusters (265 - 5 degrees). The surface brightness dropsrapidly with increasing position angle and in the oppo-site direction (85 - 205 degrees) it drops by a factor of2 . ± .
05. A sudden jump in surface brightness is seenfor the 205-225 degree sector, whose side matches thesurface brightness edge.The right panel of Figure 5 shows the temperature pro-file obtained by fitting spectra extracted from similar re-gions to those used for surface brightness profile, butwith twice the size of the sector’s opening angle. It canbe seen that the region delineated by the surface bright-ness edge (205-245 degree) has the lowest temperature of7 . +0 . − . keV. Compared with the mean temperature ofthe two adjacent sectors, the temperature in this regiondrops by a factor of 1 . +0 . − . . The almost constant temper-ature of regions enclosed between 270-90 degrees, whichinclude the gas in the direction joining the two clusters,matches very well the mean temperature of the clusterof 9 . SE discontinuity
Parallel to the plume extending to the south of theprimary cluster there is another surface brightness dis-continuity visible in the unsharp-masked image (Figure2). Although this feature is the weakest one compared tothe plume found in the vicinity of the primary and thecold front in front of secondary, its existence and mostimportantly its position close to the hottest region of thecluster (region 32 in Figure 3) led us to investigate in de-tail its properties and the possible nature of this feature.Figure 6 shows the surface brightness (top panel) andtemperature profile (bottom panel) in the sector shownin Figure 2.A sudden change in the distribution of surface bright-ness is seen at about 250 kpc. Assuming an underly-ing density profile which follows a broken power law andspherical symmetry, we fitted the projected density alongthe line of sight to the observed surface brightness. Thebest fit is shown as a red curve in the upper panel. Thefit corresponds to a model in which the density jump hasan amplitude of 1 . ± .
03. Similarly, comparing theprojected temperature found in the immediate vicinityof the discontinuity, we find a temperature jump by afactor of 1 . +0 . − . . The detection of a sudden increase indensity accompanied by a rise in temperature is indica-tive of a shock.If we explain the observed density discontinuity bythe presence of a shock, then the shock’s Mach num- ber, estimated from the amplitude of the density jump,is M = 1 . ± . M can be done using thejump in temperature across the shock. Unfortunately,our data do not allow the extraction of a high resolutiondeprojected temperature profile to estimate the jump intemperature. Using the jump of 1 . +0 . − . in projectedtemperature we estimate a M = 1 . +0 . − . . DISCUSSIONS6.1.
Comparison with simulations
To learn more about the dynamical state of AS0295,we compare our observations with the results of hydro-dynamical simulations of binary galaxy cluster mergersavailable in the literature (Poole et al. 2006; Ascasibar &Markevitch 2006; ZuHone 2011). These simulations haveshown that different sets of merger parameters such ascluster mass ratio, inclination angle, impact parameter,stage of merging process lead to unique morphologies inthe surface brightness and temperature distribution. Insome cases, these features are stable against significantvariations in the values of dynamical parameters.Figure 7, which is adapted from the results of Pooleet al. (2006), shows in the central row of two panelsthe surface brightness (left) and temperature (right) pro-files corresponding to one of their hydrodynamical sim-ulations of binary mergers. From all the available re-sults, obtained for a large range of merger parameters,this simulation has been found to agree the best, in aqualitative way, with our observations. For this particu-lar simulation, the mass ratio between primary and sec-ondary cluster is 3:1, with the primary having a mass of10 M (cid:12) . It is an off-axis merger, caught at a time dur-ing the secondary’s first travel between pericentre andapocentre. The secondary moves from SE to NW (in therotated image). The top and bottom row of panels showthe surface brightness distribution for the same merger,but at different stages during the merging process: thefirst pericentric passage and several epochs before this(top panels) and the apocentric passage and the secondclosest encounter of the two clusters (bottom panels).Looking firstly at the surface brightness map (left panelin the central row of panels) we see a good agreement be-tween the morphology of AS0295 as shown by the X-raysurface brightness contours and the surface brightnessobtained from simulations. We see the same elongatedcentral surface brightness which joins the two subclus-ters. The plume observed to the south of the primarycluster matches very well the position of a surface bright-ness edge seen in the simulated image. At larger radii,the cluster has a relaxed morphology, with no obvioussurface brightness features. Also, there is a steepeningof surface brightness gradient in front of the secondarycluster. A significant difference between the propertiesof AS0295 and the simulated results is the absence of acool core in the primary of AS0295, while Poole’s simu-lations, like most cluster merger simulations, follow themerger of clusters in which both systems have a centralcool core, visible as a significant surface brightness excessand temperature drop at the cluster center.Our observed temperature map lacks the necessaryspatial resolution to make a detailed comparison withsimulations. However, the cool region associated with Position angle (degree) S u r f a c e b r i gh t ne ss ( pho t/ s / c m / p x ) Position angle (degree)
50 100 150 200 250 300 350 G a s t e m pe r a t u r e ( k e V ) Figure 5.
Left: Azimuthal surface brightness profile extracted using sectors of a circular region centered on the primary cluster. Thiscircular region, overlaid on the X-ray image of the cluster is shown in the inset. The edge in surface brightness is seen at around 210degrees. Right: Temperature profile created using spectra extracted from regions similar to those used for surface brightness profile butwith twice the opening angle for sectors. The dotted horizontal line marks the global temperature of the cluster of 9 . S u r f a c e b r i gh t ne ss ( pho t/ s / c m / p x ) −10 −9 −8 Radius (kpc)
10 100 1000 T e m pe r a t u r e ( k e V ) Figure 6.
Surface brightness (top panel) and temperature (bot-tom panel) profile estimated from a sector across the edge found inunsharp-masked image, in the SE part of primary cluster. The redcurve represents the result obtained by fitting the surface bright-ness profile with a line-of-sight integrated density model composedof a broken power law. the secondary’s core (regions 1 and 2) and the hot regionto the SE of the main cluster (region 32) are the mainfeatures in the observed data for which we found a matchin the simulated temperature map.The presence of a cold front in the secondary cluster,as seen in our data (see Section 5.1), is in good agree-ment with predictions from these simulations. In mostbinary cluster, off-axis merger simulations a cold front starts forming in front of the secondary shortly beforethe first pericentric passage and it survives for a longtime, typically until the second pericentric passage af-ter which it mixes with the gas in the primary cluster.During its lifetime, the cold front suffers changes in itsmorphology and strength depending on the merger stateand parameters. Immediately after first pericentric pas-sage, the sudden release of ram pressure exerted on thegas in the secondary by the primary’s gas triggers thegravitational slingshot process (Hallman & Markevitch2004). The gas will overshoot the dark mater compo-nent, will expand adiabatically and cool further, makingthe cold front increase its radius of curvature to valueswhich can reach several kiloparsecs and enhance its tem-perature jump.While there is general agreement with the simulationspresented in Figure 7, there is a significant difference be-tween the morphology of the cold front in AS0295 andthat seen in the simulations. Such a difference is notsurprising since the simulations presented in the Figureare not tailored to match AS0295 and the morphology ofcold fronts is dependent on the merger parameters. How-ever, the sole detection of a cold front in the secondaryand its properties can be used to put some constraintson the cluster dynamics.One feature that matches very well the simulated re-sults is the plume seen to the south of the primary clus-ter. It can be seen as an edge in surface brightness as wellas a drop in temperature. This feature is seen in mostsimulations of binary cluster mergers and is interpretedas gas displaced from the primary core as a result of theinteraction between the two clusters, close to the time ofthe first pericentric passage. At this time, the gas in theprimary expands as it goes through the slingshot process.The expansion of the gas will be confined to one side bythe shock in front of the secondary, hence the linearityof the plume.
Figure 7.
Surface brightness (middle row, left panel) and gas temperature distribution (middle row, right panel) obtained from simulationsof an off-axis, 3:1 binary cluster merger, caught at 0.4 Gyr after pericentric passage. At this moment, our observations represent a goodmatch to the simulated data. Overlaid are Chandra X-ray surface brightness contours of AS0295. The top and bottom row of panels showthe surface brightness from simulated data corresponding to different instances of time measured relative to t , the time when the secondarytraverses a circular region of radius R centered on the primary: for top row - 0 Gyr, 0.3 Gyr, 0.5 Gyr (moment of pericentric passage),0.7 Gyr; for middle row - 0.9 Gyr; for bottom row - 1.1 Gyr, 1.5 Gyr (moment of apocentric passage), 1.9 Gyr, 2.5 Gyr (moment of secondpericentric passage). The figure in each panel was scaled and rotated such as to match the observed data. All figures were adapted fromPoole et al. (2006). As most simulations follow the merging of two cool coreclusters, there is already a reservoir of cool gas in the pri-mary which might represent the source for the gas in theplume. This raises the question whether a cool core isnecessary for the formation of this feature. Ascasibar &Markevitch (2006) have shown that this feature can formeven if the primary has a flat distribution of central tem-perature profile. This is also the case of AS0295, in whichno cool gas has been found at the core of the primary.However, in addition to the plume, their results predictthat a cold front, of slingshot origin, should be visiblein the primary, a feature which has not been detected inour data.Although this feature has been seen for a wide rangeof impact parameters and mass ratios for off-axis simu-lations, observationally, there is a very limited numberof plumes detected in merging clusters. The best exam-ple is Abell 2146 (Russell et al. 2010), which has manysimilarities with AS0295. Very large mass ratios, impactparameters or angle of merger axis may be amongst thereasons for a lack of observation of plumes in most merg-ing clusters.Since a large inclination angle of the merger axis caneasily hide the plume due to projection effects, the de-tection of this feature in AS0295 constrains the inclina-tion of the merger axis with respect to the plane of thesky to relatively small values. Moreover, the presenceof the plume rules out very large impact parameters forthe merger, since this would not be able to significantlydisturb the core of the primary.6.2.
Shocks in AS0295?
Two important features predicted by the Poole et al.(2006) simulations are the two outwardly propagatingshocks which form shortly before first pericentric pas-sage: a bow shock in front of the secondary and a reverseshock in the vicinity of the primary. The two shocks areclearly seen as the hottest regions in the temperaturemap obtained from simulations and presented in Figure7. In AS0295 we see little to no indication of a bowshock and some weak evidence in favor of the presenceof a reverse shock.Regarding the bow shock, we have shown in Section5.1 that besides the surface brightness discontinuity as-sociated with the cold front, there is another break, at adistance of 246 kpc from the cold front, where the den-sity drops by a factor of 2 . ± .
87 (see also Table 2).Although the temperature profile shows a drop in tem-perature across this discontinuity (1 . +1 . − . ), it is notsignificant enough (1 sigma) to confirm the presence of ashock. If this density and temperature jump correspondto a shock, then its Mach number, as estimated from theamplitude of density and temperature jump, is 1 . ± . . +1 . − . , respectively.Additional evidence supporting the presence of a bowshock comes from radio observations of AS0295 (Zhenget al. 2018), which show diffuse radio emission in thevicinity of the detected surface brightness discontinuity(compare the position of the radio halo in Figure 1, rightpanel, with Figure 2). From the properties of this emis-sion, interpreted as a radio relic, the authors derived aMach number for the bow shock of 2 .
04. Their resultsare consistent with our finding that the bow shock inAS0295 is in the weak shock regime. With respect to the reverse shock, we found some weakevidence in favor of the existence of a shock (see Section5.3). The Mach numbers estimated from the temperatureand density jump (1 . +0 . − . and 1 . +0 . − . , respectively)agree within the errors and correspond, like in the caseof the bow shock, to a weak shock. Zheng et al. (2018)claim the presence of another radio relic at the position ofthis shock. However, the characterization of this diffuseemission as a radio relic is hindered by the difficulty inthe removal of the contribution of a complex radio pointsource embedded in the diffuse emission.6.3. Possible merger scenario
Based on the observed X-ray data and the comparisonof the two most significant features detected in the sur-face brightness distribution (see Sections 5.1 and 5.2) aswell as the comparison of the global proprieties of AS0295with simulation results (see Section 6.1), we propose anoff-axis, binary major merger scenario for AS0295 inwhich the secondary is in its way to the apocentre, afterhaving its first closest encounter with the core of primary.More constraints on this merging scenario can be ob-tained by adding information from the strong lensinganalysis of AS0295 by Cibirka et al. (2018). Their resultsshow two well separated mass peaks, associated with theindividual subclusters (see Figure 1, right panel), thusclearly confirming the binary merger scenario. Compari-son of the strong lensing mass map with the optical image(Figure 1, central panel) shows that the distribution ofcluster galaxies traces the mass distribution peaks.Numerical simulations (Ricker & Sarazin (2001); Pooleet al. (2006); Ascasibar & Markevitch (2006); ZuHone(2011)) show that in most binary mergers, an offset be-tween the position of the gas and dark matter in theprimary and/or secondary is created at some particularstages during the merger. While around the moment ofcore passage, the high ram pressure created by the pri-mary’s gas on the secondary leads to an offset betweenthe secondary’s gas and dark matter, with the dark mat-ter component leading the gas, immediately after corepassage, the ram-pressure drops quickly. As a result,the gas goes through the gravitational slingshot process(Hallman & Markevitch 2004) and starts overtaking thedark matter. The exact moment when the gas reachesthe dark matter depends on the merging parameters suchas mass ratio, impact parameter and initial relative ve-locities of the two subclusters.Similarly to the secondary, the gas in the core of theprimary feels the effect of the high ram pressure gener-ated at core passage which leads to an offset between thegas and dark matter.These gas-dark matter offsets in the primary and sec-ondary can be used to put constraints on the mergingscenario. For AS0295 we found that the position of thesecondary’s mass peak (coincident with the position ofthe BCG) matches that of the peak of X-ray emissionwithin the core of secondary. Although there is no X-raypeak associated with the primary cluster, we see that themass peak is well separated from the high surface bright-ness region corresponding to the bulk of the gas in thecore. Such a large offset is not seen in merging simula-tions of cool core clusters, where the compact cool coretends to follow more closely the DM component. How-ever, when the primary has a flat central surface bright-0
Table 2
Proprieties of three detected surface brightness jumps (Section 5). Columns contain information about the region where the jump wasdetected (col 1), the proposed nature of the feature (col 2), ratio between temperature (col 3) and density (col 4) across the jump and, incase the feature is characterized as a shock, the estimated Mach number, calculated as weighted mean from Mach numbers estimatedfrom temperature and density jumps.Region Feature type kT kT n e2 n e1 Mach numberNW cold front 0 . +0 . − . . +0 . − . –NW shock? 1 . +1 . − . . +0 . − . . +1 . − . SE shock? 1 . +0 . − . . +0 . − . . +0 . − . ness distribution, the gas is easily separated from thedark matter (Ascasibar & Markevitch 2006; Mastropi-etro & Burkert 2008). Based on the above mentionedobservations, we propose a merger scenario in which thesecondary is caught at a time shortly after the pericentricpassage.6.4. Limits on DM self-interaction cross-section
Cluster mergers, especially those for which there is aclear offset between the gas and the DM component, areideal sources for the study of DM properties. In thesesystems, the existence of this offset implies different scat-tering depths for the two components, and therefore aconstraint on the DM can be obtained by comparison ofits optical depth with that of the gas (Markevitch et al.2004). In AS0295, we see a displacement of the DMfrom the bulk of the gas in the primary, while for thesecondary, the positions of the DM and gas peaks areidentical. In the following we apply the usual formalismto the DM/gas offset of the primary component.Following Markevitch et al. (2004), we have estimatedthe DM self-interaction cross-section under the conditionthat the scattering depth of dark matter ( τ ) cannotbe much greater than 1. The scattering depth is given by τ = σm Σ , where σ is the DM self-interaction cross-section, m is theDM particle mass and Σ is the DM surface density. Un-der the assumption of spherical symmetry, the surfacemass density along the collision direction equals the sur-face mass density along the line of sight. We estimatedthe surface mass density from lensing maps of Cibirkaet al. (2018) using a circular region, with radius of 150kpc, centered on the primary component’s centroid, al-though the numerical value is not very sensitive ( < ∼ .
35 gcm − , we obtain an estimate for the self-interactioncross-section of σ/m < g − . The derived up-per limit is comparable to other cross-section estimates(ranging between 3 − g − ) based on the scatteringdepth method and reported in different studies of clus-ter mergers ( Bullet cluster - Markevitch et al. (2004);MACS J0025.4-1222 - Bradaˇc et al. (2008); Abell 2744- Merten et al. (2011); DLSCL J0916.2+2951 - Dawsonet al. (2012) ). CONCLUSIONSWe have presented
Chandra observations of AS0295cluster, a hot (9 . .
3, goingthrough a merging process.The binary nature of the merger is suggested by themorphology of the cluster, with an evident surface bright-ness peak associated with the secondary and well sepa-rated from the bulk of the primary’s emission. The distri-bution of galaxies in the optical image and the presenceof two separate mass peaks visible in the strong lens-ing map from Cibirka et al. (2018) reinforce the binarymerger scenario. The orientation of the merger axis inthe SE-NW direction is indicated by the position of thesecondary relative to the primary cluster and the elonga-tion of the surface brightness distribution in the SE-NWdirection.In the temperature map, we found gas as cool as 6 keVassociated with the core of the secondary cluster, whilethe primary has a central gas temperature comparableto the cluster mean, which is typical for a non-cool corecluster. We found a small region of hot gas ( ∼
20 keV) inthe primary, which is offset from the central bulk of thegas toward a low surface brightness region. We interpretthe hot region, together with the surface brightness dis-continuity detected in its close vicinity, as a reverse shockcreated when the clusters reached their closest approach.For this shock we derive a Mach number of 1 . +0 . − . .We also found weak evidence for the existence of an-other shock leading the secondary, for which we deriveda Mach number of 1 . +1 . − . .In addition to these probable shocks, we detect twoother merger signatures: a cold front ahead of the sec-ondary and a plume of cool gas emerging from the pri-mary. While cold fronts are a commonly detected fea-ture in merging cluster observations, the plume, whichis a feature expected in off-axis, binary cluster simula-tions has been reported in a more limited number of cases(the best example is Abell 2142 Russell et al. (2012) withwhich AS0295 has many similarities).Good agreement was found when comparing the sur-face brightness and temperature distribution of AS0295with simulations of binary mergers. Based on our dataand the simulations, we propose a merging scenario inwhich the cluster is an off-axis merger, with the sec-ondary on its way to apocentre or close to reaching it.In the primary cluster component there is a significantspatial offset between the peaks of the gas (from the X-1ray image) and the dark matter (from a strong-lensingsurface-mass image) from which we obtain an estimatefor the self-interaction cross-section of σ/m < g − .ACKNOWLEDGMENTSThis research has made use of data obtained from theChandra Data Archive and the Chandra Source Cata-log, and software provided by the Chandra X-ray Cen-ter (CXC) in the application packages CIAO, ChIPS,and Sherpa. JPH acknowledges support from the Na-tional Science Foundation through Astronomy and As-trophysics Research Program award number 1615657.This study used observations made with the NASA/ESAHubble Space Telescope, and obtained from the HubbleLegacy Archive, which is a collaboration between theSpace Telescope Science Institute (STScI/NASA), theSpace Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre(CADC/NRC/CSA).REFERENCES.ACKNOWLEDGMENTSThis research has made use of data obtained from theChandra Data Archive and the Chandra Source Cata-log, and software provided by the Chandra X-ray Cen-ter (CXC) in the application packages CIAO, ChIPS,and Sherpa. JPH acknowledges support from the Na-tional Science Foundation through Astronomy and As-trophysics Research Program award number 1615657.This study used observations made with the NASA/ESAHubble Space Telescope, and obtained from the HubbleLegacy Archive, which is a collaboration between theSpace Telescope Science Institute (STScI/NASA), theSpace Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre(CADC/NRC/CSA).REFERENCES