XMM-Newton Observation of the Coma Galaxy Cluster

Astronomy & Astrophysics manuscript no.

(will b e inserted by hand later)

?;??

The temp erature structure in the central region

1 2 3 1 4 5 6 7

M. Arnaud , N. Aghanim ,R. Gastaud , D.M. Neumann ,D.Lumb ,U.Briel , B. Altieri , S. Ghizzardi ,

8 9 10

J. Mittaz , T.P. Sasseen , W.T. Vestrand

CEA/DSM/DAPNIA Saclay, Service d'Astrophysique, L'Orme des Merisiers B^at 709., 91191 Gif-sur-Yvette,

France

IAS-CNRS, UniversiteParis Sud, , B^atiment 121, 91405 Orsay Cedex, France

CEA/DSM/DAPNIA Saclay, Service d'Electronique et d'Informatique, 91191 Gif-sur-Yvette, France

Space Science Dept., Europ ean Space Agency, ESTEC Postbus 299, 2200AG No ordwijk, Netherlands

Max-Planck-Institut fur extraterrestrische Physik, D-85740 Garching, Germany

XMM-Newton Science Op erations Centre, ESA Space Science Department, P.O. Box 50727, 28080 Madrid,

Spain

IFC/CNR, Via Bassini 15, Milano,I-20133, Italy

Department of Space and Climate Physics, UCL, Mullard Space Science Lab oratory, Holmbury St. Mary,

Surrey,UK

University of California, Santa Barbara, CA 93110, USA

NIS-2, MS D436,Los Alamos National Lab oratory Los Alamos, NM 87545, USA

Received 2 Octob er 2000 / Accepted 2 Novemb er 2000

Abstract. We present a temp erature map and a temp erature pro le of the central part (r<20 or 1/4 virial radius)

of the Coma cluster. We combined 5 overlapping p ointings made with XMM/EPIC/MOS and extracted sp ectra

0 0

in b oxes of 3:5 3:5 . The temp erature distribution around the twocentral galaxies is remarkably homogeneous

(r<10 ), contrary to previous ASCA results, suggesting that the core is actually in a relaxed state. At larger

distance from the cluster center we do see evidence for recent matter accretion. We con rm the co ol area in the

direction of NGC 4921, probably due to gas stripp ed from an infalling group. We nd indications of a hot front

in the South West, in the direction of NGC4839, probably due to an adiabatic compression.

Key words. Galaxies: intergalactic medium { Cosmology: observations { Cosmologie: dark matter { Cosmology:

large-scale structure of the Universe { X-rays: general

1. Intro duction In particular the accretion of a sub-cluster, a common

phenomenon in standard hierarchical formation scenario,

Numerical simulations of cluster evolution (e.g. Evrard

should manifest itself bycharacteristic features in the tem-

1990;Schindler & Muller 1993) suggest that the temp er-

p erature map, like heated gas b etween the two units just

ature structure of the Intra-Cluster Medium (ICM) is a

b efore the collision.

powerful indicator of the evolutionary state of clusters.

Recent studies of the Coma cluster with the ASCA

Send o print requests to : [email protected]

satellite (Donnelly et al. 1999, Watanab e et al. 1999)

Based on observations obtained with XMM-Newton, an

revealed complex temp erature variations in this mas-

ESA science mission with instruments and contributions di-

sive cluster. They were interpreted as indicative of re-

rectly funded by ESA Memb er States and the USA (NASA)

cent mergers, con rming earlier evidence based on optical

EPIC was develop ed by the EPIC Consortium led by

dynamical studies (Colless & Dunn 1996 and references

the Principal Investigator, Dr. M. J. L. Turner. The con-

therein) and X{ray morphological analysis (Briel et al.

sortium comprises the following Institutes: University of

1992; White et al. 1993, Vikhlinin et al. 1994, 1997). ASCA

Leicester, University of Birmingham, (UK); CEA/Saclay,IAS

covered a broad energy band, which is essential for pre-

Orsay, CESR Toulouse, (France); IAAP Tuebingen, MPE

cise temp erature estimate, but the observations su ered

Garching,(Germany); IFC Milan, ITESRE Bologna, IAUP

from a relatively large energy dep endent PSF. Therefore

Palermo, Italy. EPIC is funded by: PPARC, CEA, CNES, DLR

temp erature structure determination with ASCA might and ASI

2 Arnaud et al.: The temp erature structure in the central region of Coma

Table 1. Observations

Obs. Rev RA. DEC. MOS1 MOS2 MOS1&2 counts

(J2000.0) (J2000.0) Exp. Exp. [0.3-10] keV [5-10] keV

(ksec) (ksec) Source Bkgd Source Bkgd

h m s 0 00 5 4 4 4

Pc 86 12 59 47 27 57 00 16.4 16.3 7:70 10 4:210 3:52 10 1:00 10

h m s 0 00 5 4 4 4

P5 86 12 59 28 27 46 53 20.9 21.4 6:63 10 5:510 3:23 10 1:30 10

h m s 0 00 5 4 3 3

P6 93 12 58 50 27 58 52 7.4 7.3 1:81 10 1:910 8:53 10 4:52 10

h m s 0 00 5 4 4 4

P9 93 13 00 33 27 56 59 20.8 21.0 7:03 10 5:410 3:08 10 1:29 10

h m s 0 00 5 4 4 4

P10 98 12 59 38 28 07 40 20.6 20.9 5:26 10 5:410 2:43 10 1:27 10

2.2.1. Correction for vignetting e ects have b een sub ject to systematic errors. Furthermore the

spatial resolution was insucient to resolve precisely the

The e ective area at a given energy dep ends on p osition.

temp erature radial pro le in the very core of clusters.

When extracting the sp ectrum of a region, weweight each

The EPIC instrument (Turner et al. 2001) on board

photon falling at p osition (x ;y ) of the detector and of

j j

XMM (Jansen et al. 2001) combines a high sensitivity

energy E by the ratio of the e ective area at that p osi-

with go o d spatial and sp ectral resolution, on a wide energy

tion, to the central e ective area (for this energy). This

range. In this pap er, we use this unique capability to study

weighting is taken into account in the error estimate. The

the temp erature structure in the central (<20 =0:78

`corrected' sp ectrum obtained is an estimate of the sp ec-

Mp c) region of Coma. We present further XMM results

trum one would get if the detector was at. A detailed

in two other pap ers of this issue: the large scale morphol-

description of the metho d and of the vignetting calibra-

ogy of Coma (Briel et al. 2001) and the dynamics of the

tion data used are given in Arnaud et al. (2001). Note that

infalling NGC 4839 group (Neumann et al. 2001). In the

the vignetting due to the RGA is included, but is assumed

following, we assume H = 50km=s=Mp c and q =0:5.

0 0

to b e energy indep endent (the variations are less than 1%

below6 keV).

2. Data Analysis

2.2.2. Background estimate

We generated EPIC MOS background event les (one for

2.1. The data

each MOS camera) by combining several high galactic

The central part of Coma was observed with 5 overlap-

latitude pointings. The data are cleaned for bright pix-

ping p ointingsinFull Frame mo de with the EPIC/MOS

els, background ares and regions corresp onding to bright

camera (medium lter) and in extended Full Frame mo de

point sources are excluded. The integrated exp osure time

with the pn camera. As CTE correction in this pn mo de

is 94.3 ksec for MOS1 and 78.9 ksec for MOS2. These event

is still b eing studied, we considered only MOS data in the

les can b e used for a prop er estimate of the cosmic ray

present sp ectroscopic analysis.

(CR) induced background but not the X-ray background,

which dep ends on pointing p osition and lter. However,

We generated calibrated event les with SASv4.1, ex-

bright cluster emission, like the one observed in the Coma

cept for the gain correction. Correct PI channels are ob-

center, usually dominates the background except at high

tained by interp olating gain values obtained from the

energy (see also Arnaud et al. 2001). Wethus are mostly

nearest observations of the on board calibration source.

sensitive to CR induced background. Note also that the

Data were also checked to remove any remaining bright

o set p ointings considered here always include the cluster

pixels. We excluded periods of high background induced

p eak emission, so scattered light is not a problem. The to-

by solar are protons. We discarded all frames corresp ond-

tal estimated numb er of source and background photons,

ing to a count rate greater than 15 ct/100s in the [10 12]

in the [0:3 10] keV and [5 10] keV energy ranges, are

keVband, where the emission is dominated by the particle

listed in Tab.1 for eachpointing. Note the degradation of

induced background. Finally, only events in the nominal

the S/N ratio at high energies as a consequence of the very

FOV are considered.

hard CR induced background.

The central p osition of each p ointed observation is

It is known that the CR background changes slightly

listed in Table 1, together with the revolution number and

in the FOV. It is thus b etter to consider the same extrac-

remaining observing time after cleaning.

tion regions in detector co ordinates for the source and the

background. Furthermore, if one wants to combine sp ectra

of a given physical Coma region obtained from di erent

2.2. Spatially resolved sp ectroscopy

pointings, it is mandatory to de ne extraction regions in

sky co ordinates. To alleviate this practical problem, we Sp ectra in various regions were considered to study tem-

simply generated a sp eci c background event le for each p erature variations. Each p ointing and each MOS camera

Coma p ointing and camera by mo difying the sky co ordi- are rst treated separately.

Arnaud et al.: The temp erature structure in the central region of Coma 3

Fig. 2. EPIC/MOS1 (green) and EPIC/MOS2 (blue) sp ectra

extracted from within 10 in radius of the galaxy NGC 4874.

Fig. 1. The EPIC/MOS mosaic image of the central region of

Red line: b est t redshifted isothermal mo del: kT =8:25 keV

Coma (5 overlapping p ointings) in the [0:3 2] energy band.

and an abundance of 0:25. Bottom panel: residuals between

The iso contours are the residuals (in ) after subtracting the

mo del and data.

b est t 2{D mo del. The step size is 4 and the lowest iso-

contour corresp onds to 3 signi cance. The p osition of the

bright galaxies are marked.

in each pro jected region considered. Since the sp ectra are

`corrected' for vignetting e ects we can use the on axis

MOS resp onse le, which is considered to b e the same for

nates of the background event le using the asp ect solution

MOS1 and MOS2 (version v3.15). Only data ab ove 0.3

of the considered Coma observation.

keVare considered due to remaining uncertainties in the

For consistency, the background sp ectra were obtained

MOS detector resp onse below this energy. Unless other-

using the same correction metho d as used for the source.

wise stated errors are with a 90% con dence level.

The background comp onent induced by CR is not vi-

gnetted, but as we extract the background and source

sp ectra from the same region in detector co ordinates the 2.3. Imaging analysis

correction factor is the same and do es not intro duces bias.

The MOS mosaic image in the [0:3 2] keV energy band is

Further details on the characteristic of the EPIC/MOS

presented in Fig. 1. The count images for each camera and

background and subtraction metho d can be found in

eachpointing are extracted using the weighting pro cedure

Arnaud et al. 2001.

describ ed ab oveto correct for vignetting. They are then

pro jected on a common sky reference axis, summed and

divided by the mosaic exp osure map (which takes into

2.2.3. Sp ectra extraction and sp ectral tting

account the exp osure time and FOV coverage for each

The source and background sp ectra of a given region (de-

pointing). The images are not background subtracted. In

ned in sky co ordinates) are rst extracted separately for

that energy range and in this central area of the cluster,

each MOS camera and each p ointing, using the metho d de-

the particle background is negligible for MOS.

scrib ed ab ove. As the sp ectra are corrected for vignetting,

the sp ectra of the same physical region observed with dif-

3. Results

ferent o -axis angle (from di erentpointings) and di er-

ent camera can be simply added to maximize the signal

3.1. Core morphology

to noise ratio. The errors are propagated using quadratic

summation.

Toidentify signi cant substructures in the core, we tted

the MOS image with a 2{D ellipsoidal mo del plus back-

Before mo del tting, the source sp ectra are binned so

ground and built up the map of the residuals of the data

that the S/N ratio is greater than 3 in each bin af-

over the b est t mo del. The metho d is discussed in detail

ter background subtraction. The sp ectra are tted with

in Neumann & Bohringer (1997) and Neumann (1999).

XSPEC using isothermal mekal mo dels (with xed red-

The iso-contours of signi cance of the excess (number of

shift z =0:0231). Although the thermo dynamic state of

over background plus cluster mo del) are overlaid on the

the plasma could be more complex (e.g. see the isobaric

MOS image in Fig. 1.

multiphase mo del of Nagai et al. 2000), this is adequate to

Weunambiguously con rm the excess emission around study temp erature spatial variations, the derived b est t

the two central galaxies (NGC4874 and NGC 4889) and temp erature b eing an estimate of the mean temp erature

4 Arnaud et al.: The temp erature structure in the central region of Coma

11 14 12 10 10 8 28.2 6 4 2 0 9 012345678910

28 8

7 27.8 P5 P6 Declinaison (Degrees) Pc 6 P9

P5 Temperature offset Pointings (keV) P10 P6 5 P9 567891011 27.6 P10 Temperature central Pointing (keV)

195.2 195 194.8 194.6

Fig. 4. Temp erature determined from the o set p ointings (P5,

Right Ascension (Degrees)

P6, P9 and/or P10) versus the temp erature obtained, for the

same region, using the central p ointing (Pc) data. Errors are

Fig. 3. Position of the 3:5 3:5 boxes, in which we derive

at 1 -level. The insert shows the histogram of the di erences

EPIC/MOS sp ectra. A di erent symb ol is used for eachofthe

in term of (see text for details).

5 p ointed observations. When the same region is observed in

twoormorepointings, the corresp onding sp ectra are summed

(see text). The sp ectrum of each region is tted with an isother-

21cm value and is unchanged. In the following wethus

mal mo del to build the temp erature map displayed in Fig. 5.

x the N value to the 21 cm value.

Circles: central <10 FOV of each p ointed observation.

When tted separately the MOS1 and MOS2 temp era-

tures are consistent and the relative normalization is 1.05.

In the following analysis we will thus sum MOS1 and

the tail of gas in the direction of NGC4911, revealed bythe

MOS2 sp ectra.

wavelet analysis of Vikhlinin et al. (1997). However, con-

trary to Vikhlinin et al. result, this lamentary structure

The b est t overall temp erature in this central (R<

do es not seem to b e directly connected to the Coma center,

10 ) region is in remarkable agreement with the overall

but originates 0.5 Mp c South of it. Note also that part of

GINGA value of kT = 8:21 0:09 keV (Hughes et al.

the excess is due to resolved galaxy emission (NGC 4911,

1993) and is marginally consistent with the ASCA value

NGC 4921, QSO1259+281, and 5 other p oint sources).

of 9: 0:6keV ( Donnelly et al. 1999) obtained for the

central (R < 9 ) region. Note that the hard excess seen

Di use excess emission is clearly detected in the South-

by SAX (Fusco-Femiano et al. 1999) could not b e seen by

West in particular in the direction of the NGC4839 group

XMM, since it app ears ab ove20 keV.

(see Briel et al. 2001, for full discussion).

3.3. Temp erature map

3.2. Overall sp ectrum

To study the cluster temp erature structure, we next ex-

To compare with results obtained with other satellites we

0 0

tracted sp ectra in 3:5 3:5 contiguous regions in sky

extracted the overall MOS1 and MOS2 sp ectra from a cir-

co ordinates. The b ox size was chosen so that the twocen-

cular region of 10 arcmin in radius centered on NGC4874.

tral galaxies fall approximately in the center of a box,

Only data from the central pointing are used. The data

and that a sucient S/N ratio is reached for each box.

are tted in the [0:3 10] keV range, with indep endent

Circular regions (20 in radius) around bright sources (in

normalizations for MOS1 and MOS2, and common tem-

particular NGC 4889 and NGC 4911) are excised from

p eratures and abundances. The sp ectra are plotted on

the b oxes. The overall region considered for this spatially

Fig. 2, the bottom panel gives the residuals. Fixing the

19 2

resolved sp ectroscopic analysis is ab out 20 in radius. We

(N value to the 21 cm value (N =8:95 10 cm ,

H H

only considered b oxes at o -axis angles smaller than 10 in

Dickey&Lockman 1990), we obtain a b est t temp erature

eachpointing (the vignetting factor b eing more uncertain

of kT =8:25 0:10 keV and an abundance of 0:25 0:02.

2 2

beyond this radius). The central < 10 region of each

The reduced is 1:38 ( = 1457 for 1058 d.o.f ). The t

pointing, delineated as circle, and the central p osition of

is satisfactory. The residuals are concentrated around the

the various b oxes (95 in total) are plotted on Fig. 3.

instrument edges, with residual ratios b etween data and

The validity of our vignetting correction can be as- mo del of ab out 5%, consistent with our presentknowl-

sessed by comparing the tted temp erature of the same edge of the instrument resp onse (Fig. 2). If we let the N

region in various p ointings. The vignetting e ect (decrease value free we get kT = 8:20 keV , an abundance of 0:25

19 2

of e ective area with o -axis angle) increases with en- and N =9:4 0:9 10 cm , in agreement with the H

Arnaud et al.: The temp erature structure in the central region of Coma 5

Fig. 6. Excess emission over a mo del overlayed on the tem-

p erature map. Iso contours are as in Fig. 1.

Fig. 5. Color co ded temp erature map. Note the hot frontin

the south-west (white) and the cold region in the South-East

wn/dark red). The iso contours of the PN image in the

(bro -20¡ < θ < 180¡ or -110¡ < θ < -70¡

:3 2:]keV band (Briel et al. 2001 ) are sup erimp osed. The

[0 12

3 2

lowest contour corresp onds to 6:3 10 ct=s=arcmin and the

3 2

step size is 4: 10 ct=s=arcmin 8

ergy. An understimate (overestimate) of this energy de-

Temperature (keV)

p endence would yield to underestimate (overestimate) of

-180¡ < θ < -110¡ arious p ointings of the same the temp erature. Since the v θ

12 -70¡ < < -20¡

region corresp ond to di erent o -axis angles, an improp er ould translate in systematic di er-

vignetting correction w 10

ences b etween temp erature estimates for the same region.

ws the temp eratures determined from the o set

Fig. 4 sho 8

pointings T versus the temp erature obtained, for the o 6

Temperature (keV)

same region, using the central p ointing T (errors are at

1 -level). The insert shows the histogram of the di erences

2 2 2

2 0 5 10 15 20

in term of =(T T ) =( (T ) + (T ) ) computed

o c o c

2 Distance (arcmin)

for each pair of measurements. Three outliers ( > 4or

more than 2 discrepancy in estimates) are clearly appar-

Fig. 7. Temp erature of each region of the temp erature map

ent. They corresp ond to the points at (6.7,8.4),(6.5,7.3)

with 90% con dence error bars. The temp erature is plotted

versus the distance to the brightest central galaxy NGC 4874

and (7.6,8.5). We do not see any particular problem in

and the data are splitted in three subsamples. Bottom panel:

the corresp onding sp ectra (the statistic and t are go o d)

the S-E sector encompassing the lamentary structure towards

and failed to nd anyobvious reason for the discrepancy.

NGC 4911 (op en squares) and the S-W sector along the direc-

However, these outliers corresp ond to isolated regions (the

tion towards NGC4839 group ( lled squares). Top panel: the

agreementbetween estimates is go o d in adjacent regions)

rest of the regions ( lled circles). The angle (counterclo ckwise

and the overall = 44 for 36 d.o.f is satisfactory when

from west) de ning the sectors are indicated in the gure.

these outliers are excluded. This suggests that the vi-

gnetting correction is basically correct. We thus sum all

sp ectra obtained for a given physical region and build up

NGC 4911 and NGC 4921. It includes the cold regions

the temp erature map presented in Fig. 5.

put into evidence by Donnelly et al. (1999) in that area

(region 1 and part of region 20 in their Fig. 2), that we There is no strong evidence of temp erature variations,

thus con rm. The hot regions in the S/W app ear as a hot except for a cold area in the South-East (contiguous re-

front p erp endicular to the direction connecting the clus- gions colder than average) and a hot area in the South-

ter center to the NGC4839 group, just ahead of the excess West. It is instructive to compare these temp erature fea-

emission in that direction. This excess emission extends tures with the X-ray image substructures (Fig. 6). The

somewhat further to the North, where no sp eci c temp er- S/E cold region in the temp erature map generally coin-

ature feature is apparent. However, the temp erature map cides with the lamentary substructure originating near

6 Arnaud et al.: The temp erature structure in the central region of Coma

As already prop osed by Colless & Dunn (1996), our data tral galaxy of the

9 suggest that NGC4874 is simply the cen

main Coma cluster, rather than being asso ciated with a

second subgroup in an early merging stage with the main

y Donnelly et al. 1999). Several facts

7 cluster (as prop osed b

supp ort this picture. First the remarkably homogeneous 0

Temperature (keV)

distribution within the central < 10 re-

6 temp erature

gion suggests the gas in that region is basically in a relaxed vious evidence of a third p eak

0.4 state. Second, there is no ob

in b etween NGC4889 and NGC4874, whichwould b e asso-

ter. Actually, apart from

0.3 ciated with the `true' cluster cen

the excess around NGC4889, the X{ray morphology can

Abundance 0.2

b e classi ed as an o set-center cluster morphology (varia-

troid with scale). The excess (size 3

0.1 tion of isophote cen

mo del

0 5 10 15 20 in radius) around NGC4874, when subtracting a

tative of the large scale morphology) is a natural

Distance (arcmin) (represen

consequence for this typ e of morphology. Moreover, part of

Fig. 8. Radial temp erature (top panel) and abundance (b ot-

the excess is certainly due to the contribution of the halo

tom panel) pro les. The rings are centered on NGC 4874.

of the galaxy itself. The very signi cant drop in temp era-

ture within 1 = 40kp c of NGC4874 (Fig. 8) is natural in

is sp ecially noisy in that direction. The statistical signi -

that context, as well as the increase in abundance, which

cance of the temp erature variations can b e seen in Fig. 7

could b e due to an enriched ISM.

where we plotted the temp erature of the various b oxes ver-

It might be surprising that substructures survive in

sus their distance to NGC 4874. We split the data in three

the gas density distribution (excess around NGC4889 and

subsamples: i) one S-E sector encompassing the lamen-

centroid shift for the main cluster) while the temp erature

tary structure towards NGC 4911 ii) one S-W sector along

distribution app ears homogeneous. Wemust rst empha-

the direction towards NGC4839 group iii) the rest of the

size that the spatial resolution and accuracy of the tem-

sample. The hot front(kT 11 keV ) in the S-W lo cated

p erature and imaging data is not comparable. However,

at ab out 15 from the center clearly stands out, as well

our results may also indicate that the gas simply follows

as the colder region (kT 6keV ) b eyond 10 in the S-E

the dark matter distribution. Dark matter substructures

sector. Otherwise the temp erature do es not deviate signif-

can survive for a long time after mergers, as indicated by

icantly from the 8:25 0:75 keV temp erature range (less

high resolution simulations (Mo ore et al. 1998). High res-

than 9% variation). In particular, we see no evidence of

olution hydro dynamic simulation, and more sophisticated

the hot sp ot seen by Donnelly et al. (1999)3 north of the

morphology analysis, are essential to b etter understand

NGC 4874 galaxy.We further extracted the sp ectrum (us-

this issue.

ing the central p ointing) corresp onding to this hot ASCA

At larger scale we do see evidence of recent merger

region: from Fig 1 of Donnelly et al. (1999) we consid-

activity. The cold lamentary structure in the South-

0 0

ered a rectangular region of size 7:4 3:2 centered on

East can b e naturally explained by a merging group (see

h m s 0 00

=12 59 40 ; =28 00 30 .WegetkT =8:4 0:4keV

Vikhlinin et al. 1997 and Donnelly et al. 1999). The p osi-

consistent with the mean value and inconsistentwiththe

tion and extent of the cold substructure and the core prop-

+3:6

keV). ASCA value of Box11(kT =12:7

erties outlined ab ove suggest that the merging group is as-

2:0

so ciated with NGC4911 and NGC4921, rather than b eing

due to gas stripp ed from a group centered on NGC4874

3.4. Temp erature pro le

as prop osed by Donnelly et al. (1999). Note that an ex-

We further extracted sp ectra in concentric rings centered

cess in the galaxy distribution is also observed around

NGC4921/NGC4911 (Mellier et al. 1988). on NGC 4874 (all the data are summed). The region

Our analysis revealed for the rst time a hot frontin around NGC 4889 (40 in radius) and bright p oint sources

were excluded. The temp erature and abundance pro les

the South-West, just ahead of the excess emission that

we see at the edge of the MOS mosaic and whichextends are shown on Fig. 8.

further awaytowards NGC4839 (see Briel et al. 2001). It

is situated roughly at the b oundary of the group asso ci-

4. Discussion and Conclusion

ated with this galaxy, as de ned from the optical (Colless

&Dunn 1996, Fig. 9) and is p erp endicular to the direc- The dynamical state of the core of Coma has b een much

tion connecting the center of Coma and NGC4839. This debated, in particular the nature of the merging unit(s)

temp erature structure is likely to b e due to adiabatic com- and their link with the dominant cluster galaxies. If there

pression, caused by the infall of matter asso ciated with the is a consensus that a merging group is asso ciated with

NGC4839 group. The feature we nd is indeed very sim- NGC4889, the situation of NGC4874 is less clear (see in

ilar to the one displayed in Fig.6c of Schindler & Muller particular Colless & Dunn 1996; Donnelly et al. 1999).

Arnaud et al.: The temp erature structure in the central region of Coma 7

(1993), where at this time of the merger event no accretion

sho ckhasyet formed. To de nitively characterise the fea-

ture we need to know the temp erature structure at larger

radii, to quantify the transition conditions. It has already

b een noted that the NGC4839 group is lo cated along the

large scale lament connecting Coma and A1367 (e.g West

et al. 1995). It is commonly thought that clusters form

preferentially through anisotropic accretion of sub-clusters

along large scale laments. Our nding supp orts this sce-

nario. The merger activity in that direction, particularly

interesting for our understanding of cluster formation, is

further discussed in Briel et al. (2001) and Neumann et

al. (2001).

Except for the very center as discussed ab ove, the

abundance is constant and the temp erature radial pro-

le is very weakly decreasing with radius. The slightdrop

beyond 10 is likely to b e due to the cold S/W structure.

The implications of this pro le for the distribution of dark

matter in the core will b e studied in a forthcoming pap er.

Acknow ledgements. Wewould like to thank J. Ballet for sup-

p ort concerning the SAS software and J.-L. Sauvageot for

providing the gain correction. We thank S.Schindler and the

anonymous referee for useful comments, which improved the

pap er.

References

Arnaud, M., Neumann, D.M., Aghanim, N., Gastaud, R.,

Ma jerowicz, S. 2001, A&A, 365 (this issue)

Briel, U.G, Henry,J.P., Bohringer, H. 1992, A&A 259, L31

Briel, U.G, et al. 2001 A&A, 365 (this issue)

Colless, M., Dunn, A. 1996, ApJ 458, 435

Dickey, J.M., Lo ckman, F.J. 1990, ARA&A, 28, 215

Donnelly, R.H., Markevitch, M., Forman, W., Jones, C.,

Churazov, E., Gilfanov, M. 1999, ApJ, 513, 690

Evrard, A.E. 1990, ApJ, 363, 349

Fusco-Femiano, R., dal Fiume, D., Feretti, L., Giovannini, G.,

Grandi, P., Matt, G., Molendi, S., Santangelo, A. 1999,

ApJ, 513, L21

Hughes, J.P., Butcher, J.A., Stewart, G.C., Tanaka, Y. 1993,

ApJ, 404, 611

Jansen, F., Lumb, D., Altieri, B. et al. 2001, A&A, 365 (this

issue)

Mellier, Y., Mathez, G., Mazure, A., Chauvineau, B., Proust,

D. 1988, A&A, 199, 67

Mo ore, B., Governato, F., Quinn, T., Stadel, J., Lake, G. 1998,

ApJL, 499, 5

Nagai, D., Sulkanen, M.E., Evrard, A.E. 2000, MNRAS 316,

120

Neumann, D.M., Bohringer, H. 1997, MNRAS, 289, 123

Neumann, D.M. 1999, ApJ, 512, 630

Neumann, D.M., et al. 2001 ,A&A, 365 (this issue)

Schindler, S., Muller, E. 1993, A&A, 272, 137

Turner, M.J.L., Abb ey, A., Arnaud, M. et al. 2001, A&A, 365

(this issue)

Vikhlinin, A., Forman, W., Jones, C. 1994, ApJ, 435, 162

Vikhlinin A., Forman W., Jones C., 1997, ApJ 474, L7

Watanab e, M., Yamashita, K., Furuzawa, A., Kunieda, H.,

Tawara, Y., Honda, H. 1999, ApJ, 527, 80

West, M.J., Jones, C., Forman, W. 1995, ApJ, 451, L5

White, S.D.M., Briel, U.G., Henry,J.P. 1993, MNRAS, 261, L8