The Astrophysical Journal, 596:170–180, 2003 October 10# 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.

SPATIAL DISTRIBUTIONS OF A3558 IN THE CORE REGION OF THE SHAPLEY SUPERCLUSTER
F. Akimoto, K. Kondou, A. Furuzawa, Y. Tawara, and K. Yamashita
Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
Received 2002 November 27; accepted 2003 June 19

ABSTRACTThe core region of the Shapley supercluster is dominated by three rich Abell clusters and two poorclusters. Since these member clusters are expected to be evolving rapidly in comparison to nonmember clusters because of the high merging rate, it is important to study the member clusters for understanding of thecluster evolution. Since the spatial distributions of gas temperature and metal abundance in each member
cluster provide us with information on the interactions and motions of member clusters, they are useful forunderstanding their dynamics. From the results of eight ASCA pointing observations (total 300 ks) of thecore region, we obtained parameters of gas temperature, metal abundance, and X-ray luminosity for ﬁve
member clusters and found that they are similar to the other ﬁeld clusters not belonging to superclusters
observed with ASCA. This result and the mean gravitational mass density of the core region indicate that themembers are growing in the same way as the nonmember clusters, and the core of the supercluster is just on
the way to contraction. Based on analyses of detailed spatial structures with a 40  40 scale, the two poor clusters show nearly isotropic temperature distributions, while the three Abell clusters are asymmetric. A3558
was analyzed with a 20  20 scale, owing to the statistical advantage, and it was revealed that A3558 has clear
asymmetric distributions of gas temperature and X-ray surface brightness. This is thought to be caused bycluster-cluster mergings and/or group infallings. A metal-rich region with the size of 320 h1
50 kpc was also
found to the southeast, 120 away from the cluster center of A3558. It is expected that either a remnant of amerged core has been left after a major merging or a group of galaxies has been recently infalling. Thus, thehigh dynamical activity of A3558 is proved.
Subject headings: galaxies: clusters: individual (A3558) — X-rays: galaxies: clusters
mated to be 1015–1016 h1
50 M from galaxy radial velocity
dispersion (Metcalfe, Godwin, & Peach 1994) and X-ray
observation (Raychaudhury et al. 1991; Ettori et al. 1997).
The central galaxy cluster in the core region of theShapley supercluster, A3558, is one of the richest clusters ofgalaxies. Observational results in the X-ray band of A3558
estimate the gas temperature of the overall region to be 3–4
keV (Bardelli et al. 1996) with ROSAT and 5–6 keV (Dayet al. 1991; Markevitch & Vikhlinin 1997; Hanami et al.
1999) with Ginga and ASCA. There is a diﬀerence among
the satellites because of the limitation of the observableenergy band. The metal abundance and gravitational massof A3558 are estimated to be 0.3 solar (Day et al. 1991;
Bardelli et al. 1996; Hanami et al. 1999) and ð3 6Þ  1014h1
50 M (Bardelli et al. 1996; Ettori et al. 1997), respectively.
The temperature and abundance of the whole region are
typical values for a galaxy cluster.
A3558 has a galaxy density distribution elongated in thenorthwest-southeast direction, and many groups and clusters of galaxies are aligned along this major axis (Bardelli
et al. 1994). The X-ray surface brightness distribution also
elongates in the same direction (Bardelli et al. 1996). Therefore, in this core region of the Shapley supercluster, theinfall of matter along this axis is thought to be dominant.
From these characteristics and the two-dimensional distributions of gas temperature and metal abundance obtainedby ASCA observations, we discuss the dynamical structureof A3558.
This paper comprises six sections: In x 2 the details of the
ASCA observations and analysis method are described. We
present the results of the ASCA data analysis for A3558 in
x 3 and for other member clusters of the Shapley core in x 4.
In x 5 the resulting two-dimensional distributions of X-ray

1. INTRODUCTION

Superclusters have the highest number density of galaxy
clusters in the universe. Compared to ﬁeld galaxy clusters,
the member clusters in a supercluster are thought to merge
easily with each other, or with galaxies and groups of galaxies within the potential well of the supercluster. Thus, thedynamical state of supercluster members is a clue tointerpreting the evolution of large-scale structures in theuniverse.
The Shapley supercluster, SCL 124 (Einasto et al. 1997),
has 25 Abell clusters of galaxies within a radius of 50 h150
Mpc and is the region with the highest known number density of member clusters. In particular, the core region of theShapley supercluster is deﬁned to be a region lying east to
west over 15 h1
50 Mpc (Bardelli et al. 1998) that contains ﬁveclusters of galaxies. Easternmost are A3562, SC 1329313,
and SC 1327312, A3558 is in the center position, andwesternmost is A3556, as shown in Figure 1.
Since the core region was observed in the optical wavelength by Shapley (1930), many multiwavelength observations have been done. The radial velocity and distribution of
the galaxies have been investigated with optical observations
in detail (Bardelli et al. 1998, 2000). X-ray observations havealso been done by various satellites: Ginga (Day et al. 1991),
Einstein (Raychaudhury et al. 1991; Breen et al. 1994),
ROSAT (Ettori, Fabian, & White 1997; Ettori et al. 2000),and ASCA (Markevitch & Vikhlinin 1997 for A3558;
Hanami et al. 1999 for the ﬁve core members). The temperature distributions of A3558 and SC 1329313 were investigated by Markevitch & Vikhlinin (1997) and Hanami et al.
(1999), respectively, and they reported asymmetric distributions. The total gravitational mass of the core region is esti170

TEMPERATURE AND ABUNDANCE MAP OF A3558

171

Fig. 1.—ASCA image of the core region in the Shapley supercluster. The energy band is 0.7–10 keV. The circles show ASCA ﬁelds of view. The adopted
equinox year is J2000.0.

surface brightness, gas temperature, and metal abundance
are discussed, and ﬁnally, in x 6 a summary of our work isprovided. Throughout this paper we use the solar abundance table given by Anders & Grevesse (1989), H0 ¼ 50
km s1 Mpc1, and q0 ¼ 0, and the quoted errors of thespectral parameters are at the 90% conﬁdence level.

2. OBSERVATIONS

Eight points in the core region of the Shapley supercluster
were observed with ASCA. The longest observation among
these pointings was done for A3558, which is the brightest
X-ray source among the ﬁve targets in the core. In this paper
we present the results of a detailed study of this target. Data
from the other four X-ray sources are also analyzed with thesame method. The observational log of these data is shownin Table 1. Hanami et al. (1999) and Markevitch &Vikhlinin (1997) have already reported the results ofanalyses for the core without the additional observation of
A3558 (observation 8 in Table 1).
We analyzed ASCA data using LHEASOFT, version
5.0.1, supplied by the NASA Goddard Space Flight Center.
After data screening with the standard criterion, a rise-time
discrimination technique was applied for data of the Gas
Imaging Spectrometer (GIS) to reject particle events and asisclean for data of the Solid-State Imaging Spectrometer(SIS) to exclude hot pixels. We estimated backgrounds frompoint-source–removed data of GIS blank-sky ﬁles1 andstandard data (1994 November) of SIS blank-sky ﬁles.
1 See
http://heasarc.gsfc.nasa.gov/docs/asca/mkgisbgd/mkgisbgd.html.

TABLE 1
Log of ASCA Observations
Exposure(ks)
Number1.........................
2.........................
3.........................
4.........................
5.........................
6.........................
7.........................
8.........................

Target Name
A3558
A3556
SC 1327312
IC regionb
A3562
SC regionc
SC 1329313
A3558

Observed Date
1994 Jul 14–15
1995 Jan 23–24
1995 Jan 24–25
1995 Jul 23–24
1996 Jul 27
1996 Jul 27–29
1996 Jul 29
2000 Jan 21–24

Position (J2000.0)
a

GIS

SIS





20.8
39.1
30.4
31.9
19.8
45.0
27.7
71.7

20.4€

13 27 56.9
13 24 06.0
13 29 45.4
13 29 00.0
13 33 31.8
13 31 52.8
13 31 35.9
13 27 54.8

31 29 38.4
31 38 60.0
31 36 11.9
31 33 50.0
31 39 36.0
31 30 25.2
31 48 44.6
31 29 31.9

39.9€
29.4}
30.8}
20.2}
44.9}
28.5}
66.5|

Notes.—Units of right ascension are hours, minutes, and seconds, and units of declination are degrees,arcminutes, and arcseconds. We used three pointing data from observations 1, 4, and 8 for our investigationof A3558.
a Here spade (€) symbols are for the 4 CCD mode, diamonds (}) for the 2 CCD mode, and clubs (|) for
the 1 CCD mode.
b The midpoint between A3558 and SC 1327312.
c Oﬀ-center region near A3562, SC 1329313, and SC 1327312.

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AKIMOTO ET AL.
3. RESULTS OF THE ASCA DATAANALYSIS OF A3558

A3558, located at the center of the Shapley supercluster
core, has been observed three times with ASCA (observations 1, 4, and 8 in Table 1). The total exposure time of theseobservations is 120 ks for each GIS detector, and the total
number of photons within a radius of 120 (1 h1
50 Mpc) is
1:6  105 counts per detector.
Figure 2 shows an X-ray image obtained by these
ASCA GIS observations. After the background subtraction, a correction of surface brightness was done using
the exposure map and GIS detector eﬃciency map. The
central X-ray peak in Figure 2 coincides with A3558, andthe east peak is a poor cluster, SC 1327312. X-ray
energy spectra of GISs within a radius of 120 (1 h150Mpc) at the center of A3558 were evaluated with anabsorbed thermal (Raymond-Smith) model. The ﬂuxweighted gas temperature and abundance are estimatedto be 5:59  0:06 keV and 0:34  0:02 solar, respectively.
The 2–10 keV luminosity is ð6:8  0:5Þ  1044 h2
50 ergss1. The redshift and galactic hydrogen column density
for absorption are ﬁxed at 0.0482 (Abell, Corwin, &Olowin 1989) and 3:89  1020 cm2 (Dickey & Lockman1990), respectively. The spectral parameters obtained
from ASCA GIS and SIS data are consistent within a
90% conﬁdence level.
As seen in the ROSAT image, ASCA also shows that
the X-ray surface brightness distribution of A3558 iselongated in the northwest-southeast direction, even in
the higher energy band. This structure is thought to beobservational evidence that a cluster merging has alreadyhappened in A3558 one or more times. In order to conﬁrm this scenario and investigate the physical processes
in detail, two-dimensional distributions of temperature
and abundance were made. These maps are powerfultools for understanding the dynamical activity andevolution of the galaxy cluster.

Fig. 2.—ASCA GIS image for the 0.7–10 keV energy band from observations 1, 4, and 8 in Table 1. The radius of the circle is 1 h1
50 Mpc (see text for
details).

Vol. 596

3.1. Assessment of the Merging Eﬀect in Temperature
and Abundance Maps
The merging process distorts not only the distribution of
the X-ray surface brightness but also the distributions oftemperature and abundance. Some authors have alreadyrevealed azimuthally asymmetric temperature maps, evenfor clusters with symmetric surface brightness distribution,
which had been thought to have been isolated for a longtime (Watanabe et al. 1999). It takes typically several gigayears to smooth the X-ray surface brightness, although it
depends on the mass ratio of the clusters or the impact
parameter of the merging. On the other hand, the distributions of temperature and abundance can keep information
about the merging longer than that of surface brightness.
Therefore, temperature and abundance maps are very
important for studying a merger history. We derived the distributions of the temperature and abundance of A3558 byusing some hardness ratio maps. A detailed description of
the method is below.
A hardness ratio is an indicator of gas temperature. For
an X-ray cluster with a temperature of 5.59 keV, the hardness ratio of the 2.5–10 keV energy band (the hard band) tothe 0.7–2.5 keV energy band (the soft band) is 0.57 in thecase of the ASCA GIS. After background subtraction, a
two-dimensional hardness ratio plot was made by dividing
the hard-band image by the soft-band image with an angular resolution of 20  20 . This map shows that the gradient of
the hardness ratio coincides with the direction of the major
axis of the X-ray surface brightness of A3558. The hardness
ratio map is peaked at 40 northwest from the central regionof A3558 and decreases in the northwest and southeast
directions.
A two-dimensional temperature map is obtained from thehardness ratio map in the following way: Assuming isothermal spherical symmetry of the surface brightness, which is
ﬁtted by a single -model with parameters of a core radiusof 4<2 and  of 0.611 from Bardelli et al. (1996), hardness
ratio maps are simulated using an ASCA full simulation
program at a given temperature from 3.5 to 7.5 keV with astep of 0.1 keV. From these maps, the relation between thesimulated hardness ratio and the assumed temperature ateach position on the detectors is derived. The hardness ratio
map is converted according to this relation into a temperature map. An estimated temperature map within the radiusof 120 at the center of A3558 is shown in Figure 3. The superposed contour is an X-ray image in the 0.7–10 keV band,corrected by the exposure map and the ASCA GIS detector
eﬃciency map as shown in Figure 2. The error shown in thisﬁgure is the largest error of the temperature at 90% conﬁdence within the radius of 120 . This temperature map also
shows the gradient along the major axis of A3558 as seen in
the hardness ratio map. The peak value of the temperature
map is 6.5 keV at the position of 40 northwest of the centerof A3558, and the temperature decreases by 1 keV farther
northwest. This map also shows a possible temperaturedrop in the southeast direction from the cluster center.
Although these characteristics are consistent with the result
shown by Markevitch & Vikhlinin (1997), our derived temperature map enables us to investigate spatial structure indetail. The derived temperature is ranging mainly over 5.0–
6.5 keV. No remarkable high-temperature regions are seen
within the radius of 120 , such as the hot regions with temperatures of over 10 keV seen in typical merging clusters, e.g.,

No. 1, 2003

TEMPERATURE AND ABUNDANCE MAP OF A3558

173

0
Fig. 3.—ASCA temperature map of A3558. The units are keV. The radius of the red circle is 1 h1
50 Mpc (=12 ), and the dashed line shows the major axis.
The superposed contours show the X-ray surface brightness of the 0.7–10 keV energy band. The pixel size is 20  20 . The error shown by the arrow is the
maximum error of the temperature within a radius of 120 . The blue circle shows region 1.

the 0.8 h1
50 Mpc oﬀ-center region of the Coma Cluster(Watanabe et al. 1999) and the 1.0 h1
50 Mpc oﬀ-center regionof the Ophiuchus Cluster (Watanabe et al. 2001). Moreover,there are also no clear, striking low-temperature regions,such as a cool region (cold front) 2 keV lower than the otherregions in the merging cluster, e.g., A3667 (Vikhlinin,Markevitch, & Murray 2001) and A2142 (Markevitch et al.
2000), which were revealed by Chandra observations.
Because the gravitational settling of heavy elements takes
much longer than the age of the universe (Sarazin 1988), theobserved abundance map can constrain the amount of mixing due to a small-scale merging or turbulence that can takeplace in a galaxy cluster. Moreover, even if large-scale
cluster-cluster merging has occurred, their abundance distributions have been disturbed by the merging process less eﬀectively than their temperature maps. That is, not only thechemical evolution but also the dynamical evolution can berevealed from an abundance map. To get a two-dimensional
distribution of metal abundance, an intensity ratio of the iron
K line and the neighboring continuum emission is estimated.
First, images in two energy bands, 5.0–6.0 and 6.0–7.0 keV,are made from ASCA data at the spatial resolution of 40  40 .
The image of the 5.0–6.0 keV energy band can be safely
expected to include only continuum emission. On the other
hand, the 6.0–7.0 keV image includes iron lines in addition to
the continuum emission. Next, the two images of the 6.0–7.0
keV continuum emission and the 5.0–6.0 keV continuum
emission are simulated using the ASCA full simulation program. In this simulation, the temperature is assumed to be
6.75 keV, which is obtained from ASCA GIS and SIS 4–10
keV spectra within the radius of 40 . The assumed parametersof the -model are the same as the ones used for estimating
the temperature map (Bardelli et al. 1996). From these simulated images, the two-dimensional continuum intensity ratio

174

AKIMOTO ET AL.

map, f ¼ I6 7 keV =I5 6 keV , is obtained. By multiplying the
ASCA observed 5.0–6.0 keV image by this ratio map f, we
can guess the 6.0–7.0 keV continuum image. An iron-line
intensity map is derived by subtracting this estimated continuum image from the ASCA observed 6.0–7.0 keV image.
Dividing this iron-line map by the simulated 6.0–7.0 keV
continuum image gives a ratio map of iron-line intensity tocontinuum, corresponding to an equivalent-width map.
Finally, an abundance map is obtained by normalizing thevalue averaged within a circle of radius 40 . The obtained
abundance map is shown in Figure 4. Although the above
described simulation was done on the assumption ofisothermal distribution, the nonisothermality mentioned inthis section causes an uncertainty of the abundance of10%.
In Figure 4, 4 pixels within the southeast region 120 awayfrom the center show high abundance, and this is also impliedin the south region at the same distance from the center, whileother regions are almost the same within the error.

Vol. 596
3.2. Detailed Structure of X-Ray Surface
Brightness Distribution

To know the relation of the X-ray surface brightness distribution to some interesting features seen in the temperature and abundance maps, and moreover to get higher order
structures of the surface brightness distribution, we investigated the deviation of the X-ray surface brightness from thesmooth function described with an elliptical -model. The
0.7–10 keV ASCA image was simulated using the ASCA full
simulation program, assuming a core radius of 4<9 and 3<6,a  of 0.61 (Bardelli et al. 1996), a temperature of 5.8 keV,and an abundance of 0.32 solar. The temperature and abundance were obtained from only the ASCA GIS spectrum
within a radius of 40 .
By subtracting the simulated image from the observed
0.7–10 keV image, we got substructures as shown in
Figure 5. This residual image is shown in units of the brightness ﬂuctuation of the observed 0.7–10 keV image, that is,Fig. 4.—ASCA abundance map of A3558. The units are in solar metallicity. The red circle and superposed contour are the same as for Fig. 3. The error
shown by the arrow is the maximum error of the abundance within a radius of 120 . The pixel size is 40  40 . The blue circle shows region 3.

No. 1, 2003

TEMPERATURE AND ABUNDANCE MAP OF A3558

175

Fig. 5.—ASCA residual image of A3558. The units are in . The dashed line, red circle, and superposed contour are the same as for Fig. 3. The blue circle
shows region 2.

. Excess emission over the 3  level is seen in the central
region and expands to the southeast. On the other hand,
there are two drops of over the 3  level in regions 50
northwest and 100 southeast along the spheroidal majoraxis. These structures resemble the sharp edge shown inmany merging clusters. The central excess ﬂux within aradius of 2<5 is estimated to be 9% of the whole cluster.
Since there is a dominant galaxy at the center of A3558, corresponding to the central excess region, it can be thoughtthat the excess ﬂux is associated with this galaxy. If it is so,
the corresponding luminosity of this excess is estimated to1
be 1:4  1043 h2
50 ergs s . The spectrum of this region is wellﬁtted with a single-temperature thermal (Raymond-Smith)model with a temperature of 5:8  0:1 keV and an abundance of 0:35  0:04 solar at the reduced 2 of 1.17. Twotemperature model ﬁtting has been done, and the obtained
þ1:2
parameters are temperatures of 6:4þ0:2
0:3 and 2:00:6 keV andan abundance of 0:35  0:04 solar, although the reduced 2value is not improved signiﬁcantly.

According to a comparison of this residual image withthe temperature map and the abundance map, no clear
excess has appeared at any regions showing peculiarities inthe temperature and abundance maps. On the contrary, inthe hot region 40 northwest from the cluster center, thesurface brightness decreases. Distributions of surface
brightness and temperature in this region resemble those of
the cold front in A2142 and A3667 revealed by Chandra
observation (Markevitch et al. 2000; Vikhlinin et al. 2001),
which is thought to be a boundary of the remaining core of
the merger component, moving through the hotter ambient
intracluster medium. Their hot faint regions have cold
fronts that lie close to the center of these clusters, because
the core has already passed through the central region ofanother merged cluster.
3.3. Peculiar Regions within A3558
As described in xx 3.1 and 3.2, peculiar regions in the spatial distributions of temperature, abundance, and residual

176

AKIMOTO ET AL.

Vol. 596

TABLE 2
Position and X-Ray Spectral Parameters
Position (J2000.0)

Spectral Parameters

RegionCharacteristics

kT (keV)

Abundance (Solar)

1........................
2........................
3........................

13 27 46.4
13 28 05.5
13 28 37.8

31 26 03
31 33 02
31 37 27High temperature, depression
Excess emission
Metal-rich

6.12  0.18
5:51þ0:10
0:09
5:17þ0:27
0:26

0.32  0.04
0.33  0.03
0.58  0.11

Notes.—Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes,
and arcseconds. The radii of these regions are 40 , and their redshift and absorption are ﬁxed at 0.0482 and 3:89  1020cm2, respectively.

X-ray surface brightness are detected. To study theseregions in detail by spectra, we deﬁne three circular regionswith a radius of 40 (corresponding to 320 h1
50 kpc ) named
region 1, region 2, and region 3. These regions are shown by
blue circles in Figures 3, 4, and 5. Their characteristics are a
hot faint region, a bright region with a southeast tail of central surface brightness excess, and a high-abundance region,
respectively. The central positions of these peculiar regions
are shown in Table 2. Each spectrum integrated in theseregions was ﬁtted with an absorbed thermal (RaymondSmith) model, in order to conﬁrm their characteristics oftemperature and abundance. Results are also shown in
Table 2. The temperature of region 1 and abundance ofregion 3 are 6:12  0:18 keV and 0:58  0:11 solar, respectively. Thus, region 1 indicates a signiﬁcantly high temperature and region 3 has a signiﬁcantly high abundancecompared to the spectral parameters of the whole region.
According to the spectral ﬁtting, it is noted that anotherhigh-abundance region in the south 120 from the center
does not show a signiﬁcantly high abundance. Therefore,
we do not focus on this region.
On region 3 we performed further analysis to investigate
the origin of the abundance excess. The redshift estimatedfrom the center energy of the iron K line is 0:047þ0:010
0:004 , whichcoincides with the average value obtained for the whole
region of A3558, 0:051þ0:005
0:002 , and the average redshift formember galaxies, 0.0482 (Abell et al. 1989), within the error.
Therefore, the newly detected high-abundance region is likely
to be within the same gravitationally bound system as A3558.
In order to know the extent of the metal-rich region, theabundances within the various integrated radii were estimated. Figure 6 shows the dependence of the abundance on
the integrated radius. The region within a radius of 50 has asigniﬁcantly high abundance at a 90% conﬁdence level. The
signiﬁcantly metal-rich region is thought to be within a radiusof 40 , that is, 320 h1
50 kpc, considering the point-spread function of the ASCA X-ray Telescope. The spectrum of region 3
is shown in the right panel of Figure 7. Because the abundance estimation is inﬂuenced by temperature, doubts arecast on the reliability of the high abundance. However, thetemperature in this region, 5:17þ0:27
0:26 keV, is slightly lower
than the most eﬀective iron-emitting temperature, 6.6 keV.
Therefore, even if the temperature is 6.6 keV, the abundance
decreases to only 0.55 solar. Thus, the high abundance in thisregion is conﬁrmed. It should be noted that both spectra in
Figure 7 show high-energy tails. Although these structures
are thought to be due to additional components, the spectral
parameters do not suﬀer much from them.
It is thought to be clear that there is an additional metalrich component at the same redshift as A3558. The excessiron abundance within a radius of 40 is 0:3  0:1 solar. However, because excess brightness in the whole energy band of
0.7–10 keV is not seen in Figure 5, it is thought that there isno signiﬁcant additional gas cloud. Then, the mean gas den3sity of this region is estimated to be 0:3  103 h0:5
50 cm
on the assumption of a -model gas distribution for themain cluster (core radius and  from Bardelli et al. 1996)
3
with a central electron density of 2:3  103 h0:5
50 cm . The
gas mass and the excess iron mass of the sphere within a
11 andradius of 320 h1
50 kpc are estimated to be 8  10
M,
respectively.
5  109 h2:5
50
4. FIVE GALAXY CLUSTERS IN THE CORE REGION
OF THE SHAPLEY SUPERCLUSTER

Fig. 6.—Relation between the integrated radius and the abundance ofregion 3. The line shows the upper limit of the average abundance ofA3558.

All ASCA data of A3562, SC 1329313, SC 1327312,
and A3556 were analyzed with the same method as A3558.
The whole obtained image of the core region of the supercluster is shown in Figure 1. Not all positions of the X-ray
peaks of these ﬁve clusters are coincident with Figure 1 ofHanami et al. (1999) (the discrepancy of the peak position
for SC 1327312 is over 40 ) but are consistent with the positions obtained from galaxy distribution (from the NASA/
IPAC Extragalactic Database [NED]) within 10 . The temperature maps were not investigated by Hanami et al.
(1999), except for SC 1329313. Therefore, we estimatedNo. 1, 2003

TEMPERATURE AND ABUNDANCE MAP OF A3558

177

Fig. 7.—ASCA GIS spectra of the central region and high-abundance region within a radius of 40 . The temperature (keV) and abundance (solar) obtainedby ﬁttings with a thermal (Raymond-Smith) model are shown. Right: Spectrum of region 3. Left: Spectrum of the central region shown for comparison.

the maps for all member clusters with 40  40 pixels. Three
Abell clusters in the core region have asymmetric spatial distributions of the gas temperature, as shown in the hardness
ratio map of Figure 8. For A3556 and A3562, the hardness
ratio at the side toward A3558 seems to be higher than thattoward the opposite side. Practically, spectra of the east andwest sides of A3556 within a 12<5 radius show that the temperature of the east side, 3:8þ1:1
0:7 keV, is higher than that of
the west side, 2:6þ0:5
0:4 keV. The spectrum of the southwest
region of A3562 within a radius of 12<9 shows a temperatureof 5:9þ1:1
0:7 keV, which tends to be higher than that of thenortheast region, 4:6þ0:8
0:6 keV, although it is not signiﬁcant.
It is thought to be due to a cluster-cluster merging or agroup infalling. On the other hand, the two poor clusters
show relatively isotropic temperature distributions on thescale of 0.3 h1
50 Mpc.
Gas temperature, abundance, and luminosity within aradius of 120 for each cluster in the Shapley core are shownin Table 3. The total gravitational and gas mass of this coreregion can be estimated from these results and -model
parameters from those published (Bardelli et al. 1996; Ettori
et al. 1997, 2000). The total gas mass is obtained to beM, and the total gravitational mass is2  1014 h2:550M
2  1015 h1. Therefore, the mean gravitational
50
density within a radius of 6.5 h1
50 Mpc is 25 times larger thanthe critical density c .

Fig. 8.—ASCA hardness ratio map of the core region in the Shapley supercluster. The hardness ratio is deﬁned as the ratio of the 2.5–10 keV ﬂux to the
0.7–2.5 keV ﬂux. The superposed contours show the X-ray surface brightness for 0.7–10 keV and are the same as in Fig. 1. The pixel size is 40  40 .

178

AKIMOTO ET AL.

Vol. 596

TABLE 3
X-Ray Spectral Parameters of Five Galaxy Clusters
Parameter

A3562

SC 1329313

SC 1327312

A3558

A3556

Redshift .............................
kT (keV).............................
Abundance (solar) .............
LX (1044 ergs s1)................

0.0490þ0:23
5:040:22
0.40  0.08
þ0:86
2:860:07

0.0482
þ0:25
4:220:23
þ0:11
0:240:10
þ0:28
0:920:03

0.0495þ0:14
3:650:11
0.32  0.07
þ0:50
1:660:04

0.0482
5.59  0.06
0.34  0.02
þ2:05
6:830:04

0.04793:21þ0:25
0:24
0:38þ0:21
0:18
0:33þ0:10
0:03

Note.—These are estimated from 0.7–10 keV spectra within a radius of 120 (1 h1
50 Mpc) of each target. The
LX is the luminosity of the 2–10 keV band.

Both the luminosity and temperature relation and thegravitational mass and temperature relation for these ﬁveclusters are consistent with those of the nearby clusters witha redshift of less than 0.1 investigated by Akimoto et al.
(2001) and Matsumoto et al. (2000) within the deviations of
their data. This result suggests that these ﬁve clusters are
not peculiar clusters of galaxies.
5. DISCUSSION

The two-dimensional distributions of surface brightness,
temperature, and abundance of A3558 indicate that thiscluster has a high dynamical activity. This result is expectedfrom the situation that this cluster resides at the center of asupercluster core with a high number density of member
clusters. According to the distributions we investigated, thiscluster is most probably suggested to at least have experienced mergings twice in the past and possibly to have had arecent group infalling.
The X-ray surface brightness distribution of A3558,
showing elongation in the northwest and southeast direction, suggests that a merging of two large galaxy clusters
with equal mass (major merger) has taken place along the
major axis in the past, according to the results of an N-bodysimulation of head-on mergers (Roettiger, Loken, & Burns
1997). Although the spatial distribution of the temperatureof A3558 is asymmetric, a hot region 30 away from the cluster center northeastward does not spread out much, and thediﬀerence of temperature at each position within A3558 issmall. Therefore, we think that it has been 5 Gyr since thismajor merging (Roettiger et al. 1997). In the case of the
Coma Cluster, there is a large temperature ﬂuctuation, eventhough it shows a smooth surface brightness (Watanabe
et al. 1999). This focusing cluster is a contrary case to theComa Cluster. It is not clear whether this merger had asmall impact parameter or a large one. An N-body simulation of an oﬀset merger for A3266 (Roettiger & Flores 2000)
also gives a distribution of surface brightness that resembles
A3558. Therefore, a scenario that a merging with a large
impact parameter has taken place along the major axis in
the past is not able to be rejected.
The characteristics of temperature and surface brightness
in region 1 are discussed. They look like a cold front due tocluster merging reported by some authors based on Chandra
observations (Markevitch et al. 2000; Vikhlinin et al. 2001;
and so on). Considering the passage of time from the merging, it is diﬃcult to think that these small-scale structures
could have been formed by the above-described large-scale
merger and still survive. It is plausible that after the largescale merger occurred, the main cluster and a subcluster
infalling from the northwest merged with a small impact

parameter, and an area similar to a cold front was formed;
however, no striking high- or low-temperature regions were
made. According to the simulation of oﬀset mergings with amass ratio of 1 : 4 by Takizawa (2000), a bow shock and amerged cold core are seen for 0.25 Gyr after the closest contraction epoch of two merging clusters. At the end of thesimulated time span (0.7 Gyr), although the distribution of
surface brightness is still asymmetric, the ﬂuctuation of temperature disappears. Therefore, it is thought that only lessthan 0.7 Gyr has passed after this small-scale merger. It isnoted that the large elongated gas proﬁle of A3558 would
not have formed within 0.7 Gyr after the merging of one
small-scale merger such as this, according to comparison in
the cases of various mass ratios presented by Takizawa(2000) and Roettiger et al. (1997). The large ellipticity islikely to be due to the large-scale merging (major merger)
mentioned above. Thus, the result that two cluster mergings
happened is beyond a doubt.
On the origin of the high-abundance region (region 3),
the most probable one of some possibilities is that it is aremnant of a merged subcluster core, which often showsabundance enhancement at an evolved stage. The diﬀusion
time of iron is more than 15 Gyr. Therefore, the iron canpersist for a long period in the remnant core after any
dynamical event. Gas density ﬂuctuations, on the other
hand, fade away on the sound-crossing timescale, and any
excess gas density is smeared out. In this way, regions with atypical size of a few hundred kiloparsecs with a signiﬁcantiron excess but without a noticeable density excess canform.
In this cluster, the second small-scale merging with thepassage of time of less than 0.7 Gyr cannot make the remnant of a merged core because this region is far from thecluster center. On the other hand, it is thought that the ﬁrst
major merging can form the iron remnant. After the ﬁrst
major merging, only the iron core can survive, since thediﬀusion time of iron is longer than 5 Gyr, that is, thepassage of time after the large-scale merging.
To examine our scenario of a remnant of a merged cluster
core with high metallicity, we constructed a simple toymodel assuming an initial size of the merged cluster of 300h1
50 kpc, an abundance of 0.58 solar, and a central density ofseveral 103 cm3, which is 10 times larger than the densityof the ambient intracluster medium. After 0.3 Gyr, theradius of the core spreads at the sound speed of 1150 km s1
and reaches 650 h1
50 kpc. The gas density of the core comesto be the same value as that of the ambient intracluster
medium, and then the brightness excess is smeared. The core
size of iron ions, however, can increase to only 20 h1
50 kpcwithin 0.3 Gyr, since iron ions expand through random
walk with a thermal velocity of 130 km s1. The size of theNo. 1, 2003

TEMPERATURE AND ABUNDANCE MAP OF A3558

spread can be described by the product of the mean freepath and square root of the number of collisions. The averaged number of collisions is 4.4. Thus, a region with no electron density excess, that is, no surface brightness excess, butwith a high metal abundance will be able to be formed. Afterthe formation, large-scale merging and mixing may havenot been induced. Nonetheless, thermal conduction,
dynamic viscosity, ram-pressure stripping, and RayleighTaylor and Kelvin-Helmholtz instabilities make the time
for an iron core to survive shorter, unless these processes aresuppressed by magnetic ﬁelds.
Another suggestion is the existence of an active galactic
nucleus (AGN) such as a type II Seyfert galaxy. The spatial
distribution cannot determine whether the residual structure is caused by a point source or not, but the line center
energy of iron K, that is, the redshift coincident within theerror with the mean value of the overall region of the cluster
within a radius of 120 , and the time variability of this region
are not detected signiﬁcantly. No point source has beenfound in images of ROSAT, and no nonthermal component
is needed to describe the ASCA spectra. These facts do notsupport the idea of an AGN.
The other suggestion is the existence of a metal-rich groupof galaxies. If there is a galaxy group that is binding the gas
within 320 h1
50 kpc, it means that this region does not show
strong interactions with A3558, and this idea is able toexplain that the size of 320 h1
50 kpc is the typical size ofgroups of galaxies. If there is a group of galaxies, it has a gas
M and an excess iron mass ofmass of 8  1011 h2:5
50
M
withinasphere of this radius. The gas mass
5  109 h2:5
50
was derived assuming that the sphere has a uniform density.
Because there is no clear electron density excess, the electron
number density was set to be 0.0003 cm3 at the center ofthis region. Thus, the gas mass was obtained. On the other
hand, the gas mass within a column with a projected radius
M. Therefore, the iron mass isof 40 is 7  1012 h2:550
1  1010 h2:5
50 M for an abundance of 0.58. The excess ironmass is 41% of the iron mass, that is, 5  109 h2:5
50 M. If theabundance for other regions of this sphere is 0.34, the sphereis then thought to have excess iron mass. These values arereasonable for a group of galaxies. For example, the radii ofthe X-ray extent of 78 groups and the intragroup medium
masses of 24 groups are shown in Mulchaey et al. (1996).
The iron abundance of galaxy groups varies signiﬁcantly
from group to group: 0–1 solar metallicity (Renzini 1997);
therefore, the iron mass also varies between 108 and109 h2:5
50 M.
In this paper, the iron K line is used, which is advantageous. There are 38 galaxies within the projected radiusof 40 (320 h1
50 kpc) according to the NED. Since not allof their redshifts are known, it is assumed that all arewithin this region. Because their total B-band luminosity
is 2:3  1011 LB, the luminous mass is 2  1012 M on
the assumption that the mass-to-luminosity ratio is 8 foran elliptical galaxy. The ratio of the iron mass to thetotal optical luminosity of the galaxies is obtained to be
0.02 M L1
B , which is also acceptable for galaxy groups.
The ratio of galaxy groups varies over 0.0001–0.02 M
L1
B (Renzini 1997). Because not all of the 38 galaxies are
members of the group, the total B-band luminosity andluminous mass are upper limits, and the ratio is a lower
limit. Therefore, several other possible interpretations
cannot be ruled out, such as bursts of star formation
occurring because of group infalling, stars ejecting a large

179amount of iron into the intragroup medium and thendisappearing, or dust grains with condensed iron being
evaporated.
Finally, the mean density of the core region in the Shapley
supercluster is 25 times larger than the critical density.
Although it is much smaller than needed to virialize, that is,
178 times critical density, it is larger than needed to decouple from the cosmic expansion and contract, that is, 5.6
times critical density (Peebles 1980). Therefore, the coreregion of the Shapley supercluster is on the way to contraction and is collapsing to form a virialized halo. However,
because the characteristics of each member cluster in thecore region are consistent with other typical nearby galaxy
clusters, the core region of the Shapley supercluster is probably in the early phase of contraction. This scenario is supported by the fact that the core region is plotted at the rightedge (the largest scale radius rs and the lowest characteristic
density c ) in Figure 4 of Sato et al. (2000).

6. CONCLUSIONS

The results of the ASCA observations of ﬁve member
clusters in the core region of the Shapley supercluster,
mainly about A3558, are shown. The X-ray surface brightness distribution of A3558 is elongated in the northwest andsoutheast directions and is furthermore asymmetric to theaxis. The temperature map shows that the central region
within a radius of 1 h1
50 Mpc has a temperature gradientalong the major axis. The range of temperature is 5.0–6.5
keV, and there are not extremely hot or cool regions. The
obtained abundance map shows that the range ofabundance is 0.3–0.5 solar and nearly isotropic. Spatial distributions of X-ray surface brightness, temperature, andabundance indicate a hot depression region 40 northwest(region 1), X-ray excess 40 southeast (region 2), and a0metal-rich region within 320 h1
50 kpc, 12 southeast fromthe cluster center of A3558 (region 3).
The overall temperature and elongated distributions ofsurface brightness are thought to have arisen from a largescale merging of two large galaxy clusters that occurred 5
Gyr ago. The hot depression region (region 1) implies that asmall-scale merging with the main cluster and a subcluster
infalling from the northwest occurred less than 0.7 Gyr ago
and after a large-scale merging. The anisotropic distributions of surface brightness and temperature indicate thatmass infall along the major axis is dominant in the coreregion of the Shapley supercluster. As concerns the metalrich region (region 3), it is probably the remnant of a coreregion in a galaxy cluster that merged 5 Gyr ago. If it is
not a remnant, it is possible that an infalling group of galaxies binding X-ray–emitting gas within 320 h1
50 kpc exists
in this region. Thus, two mergers and a possible infalling ofa galaxy group are suggested. It is thought that thiscluster has a high dynamical activity due to the particularenvironment in the supercluster core.
Moreover, the gas temperature, abundance, luminosity,
gas mass, and gravitational mass of the other four member
clusters belonging in core region of the Shapley supercluster
have been estimated. The values of these ﬁve galaxy clusters
show the same correlations of LX -kT and Mgas -kT relations
as nearby galaxy clusters with a redshift of less than 0.1.
Even though these ﬁve clusters of galaxies are in the highest
density circumstance and some clusters show an anisotropic

180

AKIMOTO ET AL.

temperature distribution, they have no diﬀerences withﬁeld clusters of galaxies; that is, the contribution of eachinteraction of the member clusters to overall spectral
parameters is thought to be weak. The sum of the gravitational mass of the ﬁve galaxy clusters is 2  1015 M.
There are no more signiﬁcant X-ray sources bounding dark
matter within the core region. Therefore, the mean densityof the core region of the Shapley supercluster is about 25
times the critical density, which is smaller than 178 to startvirializing but is larger than 5.5 to start contracting. The
core region is just on the way to the contraction.
In order to make our discussion clear, observations with
Chandra, XMM, and Astro-E II, which will be launched in
2005, are needed.
We would like to thank all the members of the ASCAteam for their operation of the satellite. We are grateful tothe referee for useful suggestions and valuable comments.

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