A&A 377, 428–441 (2001) Astronomy DOI: 10.1051/0004-6361:20011134 & c ESO 2001 Astrophysics
The cluster of galaxies Abell 970
L. Sodr´eJr.1,D.Proust2,H.V.Capelato3,G.B.LimaNeto4,H.Cuevas5, H. Quintana6, and P. Fouqu´e7
1 Departamento de Astronomia do IAG/USP, Av. Miguel Stefano 4200, 04301-904 S˜ao Paulo, Brazil 2 Observatoire de Paris - Section de Meudon, DAEC, 92195 Meudon Cedex, France e-mail: [email protected] 3 Divis˜ao de Astrof´ısica INPE/MCT, 12225-010 S˜ao Jos´e dos Campos, Brazil e-mail: [email protected] 4 Departamento de Astronomia do IAG/USP, Av. Miguel Stefano 4200, 04301-904 S˜ao Paulo, Brazil e-mail: [email protected] 5 Departamento de F´ısica, Universidad de La Serena, Benavente 980, La Serena, Chile e-mail: [email protected] 6 Departamento de Astronomia y Astrof´ısica, Pontificia Universidad Cat´olica de Chile, Casilla 104, Santiago 22, Chile e-mail: [email protected] 7 European Southern Observatory, Casilla 19001, Santiago 19, Chile and Observatoire de Paris – Section de Meudon, DESPA, 92195 Meudon Cedex, France e-mail: [email protected]
Received 13 September 2000 / Accepted 14 June 2001
Abstract. We present a dynamical analysis of the galaxy cluster Abell 970 based on a new set of radial velocities measured at ESO, Pic du Midi and Haute-Provence observatories. Our analysis indicates that this cluster has a substructure and is out of dynamical equilibrium. This conclusion is also supported by differences in the positions of the peaks of the surface density distribution and X-ray emission, as well as by the evidence of a large-scale velocity gradient in the cluster. We also found a discrepancy between the masses inferred with the virial theorem and those inferred with the X-ray emission, which is expected if the galaxies and the gas inside the cluster are not in hydrostatic equilibrium. Abell 970 has a modest cooling flow, as is expected if it is out of equilibrium. We propose that cooling flows may have an intermittent behaviour, with phases of massive cooling flows being followed by phases without significant cooling flows after the accretion of a galaxy group massive enough to disrupt the dynamical equilibrium in the centre of the clusters. A massive cooling flow will be established again, after a new equilibrium is achieved.
Key words. galaxies: clusters: general – clusters: individual: A970 – distances and redshifts; large-scale structure of Universe; X-ray: general
1. Introduction study of galaxy clusters may offer important informations for observational cosmology, because cluster properties In the present accepted paradigm of structure formation, depend on cosmological parameters and can be used to small structures are the first to collapse, then merging constrain cosmological scenarios. For instance, the clus- hierarchically to build larger objects. In this framework, ter mass function, which may be described by the Press– clusters of galaxies, the largest (nearly) virialized struc- Schechter formalism (Press & Schechter 1974), depends tures, may be accreting galaxies and/or “dark haloes” on the density parameter Ω0 andonthepowerspectrum even at z = 0 (e.g. Lanzoni et al. 2000). Therefore, the amplitude and shape parameter (e.g., Lacey & Cole 1994; Bahcall & Fan 1998). Also, the morphological and dy- Send offprint requests to:L.Sodr´eJr., namical state of clusters allow us to infer their history e-mail: [email protected] and, again, to constrain cosmological theories of large- Based on observations made at ESO, La Silla (Chile), scale structure formation (e.g., Kauffmann et al. 1999). Haute-Provence and Pic du Midi Observatories (France).
Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20011134 L. Sodr´e Jr. et al.: The cluster of galaxies Abell 970 429
The mass of a cluster may be estimated by several published. Here we examine some properties of the cluster methods: the optical virial mass, from the positions and Abell 970, using a set of 69 new radial velocities. The radial velocities of the cluster galaxies; the X-ray mass, observations of radial velocities reported here are part of from the X-ray emission of the hot intracluster gas; the a program to study the dynamical structure of clusters of gravitational lensing mass, from the distortions produced galaxies, started some years ago and with several results on background object images by the gravitational field of already published (see e.g. Proust et al. 1987, 1988, 1992, the cluster. However, a discrepancy between these estima- 1995, 2000; Capelato et al. 1991). tors are often found (e.g., Mushotzky et al. 1995; Girardi This paper is organized as follows. We present in et al. 1998; Wu et al. 1998). Virial mass estimates rely on Sect. 2 the details of the observations and data reduction. the assumption of dynamical equilibrium. X-ray mass esti- In Sects. 3 and 4 we discuss the galaxy and the X-ray dis- mates also depend on the dynamical equilibrium hypoth- tributions, respectively. In Sect. 5 we analyze the velocity esis and on the still not well-constrained intra-cluster gas distribution of the cluster galaxies. In Sect. 6 we present temperature gradient (Irwing et al. 1999). Finally, mass mass estimates for the central region of the cluster, derived estimates based on gravitational lensing are considered from the optical and X-ray observations. In Sect. 7 we dis- more reliable than the others (e.g., Mellier 1999) because cuss the dynamical status of Abell 970. Finally, in Sect. 8 they are completely independent of the dynamical status we summarize our conclusions. We adopt here, whenever −1 −1 of the cluster, and their discrepancies with other methods necessary, H0 =50h50 km s Mpc ,Ω0 =0.3and may be due to non-equilibrium effects in the central region ΩΛ =0.7. of the clusters (Allen 1998). An important source of departure from equilibrium (that may affect mass estimates) are the substructures. Their very existence supports the current view that clus- 2. Observations and data reductions ters grow hierarchically by accreting nearby groups and galaxies. Note that even the frequency and degree of The new velocities presented in this paper have been ob- clumpiness in the central regions of the clusters depends tained with the 1.52 m ESO telescope at La Silla (Chile), on the cosmology (e.g., Richstone et al. 1992). In many the 2.0 m telescope at Pic du Midi (France), and with the cases, substructures are loosely bound and can survive 1.93 m telescope at Haute-Provence Observatory (France). only a few crossing times in the hostile environment of Observations with the 1.52 m ESO telescope were car- rich clusters. However, they seem to be very common in ried out in February 1996. We used the Boller and Chivens present-day clusters. A recent estimate by Kolokotronis spectrograph at the Cassegrain focus, equipped with a et al. (2000) indicates that at least 45% of rich clusters 600 lines/mm grating blazed at 5000 A˚ and coupled to an present optical substructures. Substructures are detected RCA CCD detector (1024×640 pixels) with a pixel size of in both optical and X-ray images in 23% of the clusters. 15 µm. The dispersion was 172 A/mm,˚ providing spectral This last number may then be considered a lower limit coverage from 3750 to 5700 A.˚ The exposure times ranged on the fraction of real substructures in clusters, and it between 30 and 60 min, according to the magnitude of the is large! Indeed, it implies that one in four clusters may object. During the run, calibration exposures were made be out of equilibrium due to the presence of a substruc- before and after each galaxy observation using an He-Ar ture. The dynamical status of individual clusters should source. therefore be examined in detail before being used in other Observations with the 1.93 m Haute-Provence studies. Observatory telescope were carried out in November 1997, In this paper we present a study of the dynamical sta- November 1998 and April 2000. We used the CARELEC tus of the cluster Abell 970, from the analysis of the posi- spectrograph at the Cassegrain focus, equipped with a tions and velocities of cluster galaxies, as well as from the 150 lines/mm grating blazed at 5000 A˚ and coupled to intra-cluster gas X-ray emission. Abell 970 has a richness an EEV CCD detector (2048×1024 pixels) with a pixel class R = 0 and type B-M III (Abell et al. 1989). Together size of 13.5 µm. The dispersion of 260 A/mm˚ allowed a with a few other clusters (A979, A978 and A993), it is spectral coverage from 3600 to 7300 A.˚ Wavelength cali- member of the Sextans supercluster (number 88 in the bration was done using exposures of Hg-Ne lamps. catalogue of Einasto et al. 1997; and number 378 in the catalogue of Kalinkov et al. 1998). It has a moderate cool- Part of the velocities were obtained during an observ- ing flow (White et al. 1997). ing run at the 2.0 m Bernard Lyot telescope at Pic du A search in the NED database1 indicates that only Midi Observatory in January 1997. Despite the declina- 4 radial velocities are known in the field of the cluster tion of Abell 970, we used the ISARD spectrograph in its ˚ (see Postman et al. 1992) which, however, have not been long-slit mode with a dispersion of 233 A/mm with the TEK chip (1024×1024 pixels) of 25 µm, corresponding 1 ˚ The NASA/IPAC Extragalactic Database (NED) is oper- to 5.8 A/pixel. Typically, two exposures of 2700 s each ated by the Jet Propulsion Laboratory, California Institute of were taken for fields across the cluster. Wavelength cali- Technology, under contract with the National Aeronautics and bration was done using Hg-Ne lamps before and after each Space Administration. exposure. 430 L. Sodr´e Jr. et al.: The cluster of galaxies Abell 970
Table 1. Heliocentric radial velocities for galaxies.
cosmos GALAXY R.A DEC. TYPE bJ VELOCITY R N (2000) (2000) V ±∆V 01 10 17 25.7 –10 41 21 E/D 16.57 17 525 52 9.72 e 02 10 17 24.6 –10 41 22 S0 18.00 16 209 37 7.14 e 03 10 17 28.3 –10 40 59 S0/S 18.67 18 424 74 5.48 e 04 10 17 29.7 –10 40 31 S0 17.62 16 270 52 9.67 e 05 10 17 26.3 –10 41 34 S0 17.51 18 357 54 6.60 e 06 10 17 23.9 –10 42 16 S0/S 18.41 16 435 69 5.61 e 07 10 17 27.3 –10 41 51 E/S0 18.36 18 145 53 5.89 e 08 10 17 29.5 –10 42 17 S 18.38 17 404 71 7.42 e 09 10 17 21.7 –10 42 56 S 17.96 17 631 61 6.91 e 10 10 17 24.3 –10 43 29 S 17.95 16 341 81 6.76 e 11 10 17 31.8 –10 42 59 S0/S 18.83 17 769 81 5.79 e 12 10 17 30.0 –10 43 09 S0 18.45 17 327 35 5.57 e 13 10 17 32.9 –10 43 52 S 18.26 16 974 36 5.14 e 14 10 17 29.4 –10 44 34 Sa 17.69 18 909 75 5.61 e 15 10 17 28.0 –10 44 18 S0 17.67 18 030 29 8.46 e 16 10 17 20.7 –10 44 16 S 18.56 17 711 26 5.85 e 17 10 17 15.6 –10 43 28 Sa 17.51 19 503 96 2.62 e 18 10 17 12.9 –10 42 47 S0 17.83 19 450 51 9.10 e 19 10 17 35.6 –10 39 55 Sb 17.15 18 789 24 9.41 e 20 10 17 28.1 –10 39 27 Sa 18.16 16 842 76 4.79 e 21 10 17 27.6 –10 39 12 S0 18.63 16 404 92 4.48 e 22 10 17 21.0 –10 40 13 S0 17.18 18 788 45 9.41 e 23 10 17 23.2 –10 40 18 E 18.26 19 381 40 10.12 e 24 10 17 22.0 –10 39 45 E 18.00 16 847 47 6.37 e 25 10 17 22.5 –10 39 50 E/S0 17.09 19 483 54 9.32 e 26 10 17 25.2 –10 41 07 E 18.87 17 081 93 3.15 e 27 10 17 28.5 –10 41 13 E 17.52 33 586 98 2.84 e 28 10 17 21.9 –10 43 12 E/S0 18.71 17 764 64 5.17 e 29 10 17 14.4 –10 39 16 S 18.46 18 415 94 3.44 e 30 10 17 12.6 –10 40 05 E/S0 17.69 16 533 80 5.36 e 31 10 17 33.6 –10 38 46 SB 17.88 17 779 109 4.68 e 32 10 17 44.6 –10 39 14 S0/S 18.09 17 992 33 7.55 e 33 10 17 32.4 –10 34 27 S0 19.01 17 606 98 2.75 e 34 10 17 30.7 –10 36 24 S 17.43 16 722 134 3.55 e 35 10 17 41.6 –10 35 46 S0/S 17.63 21 637 37 8.62 e
Data reduction was carried out with IRAF2 using the 3. declination (J2000); longslit package. The spectra were rebinned uniformly in 4. morphological type determined from a visual inspec- log wavelength, with a scale of 1 A/bin.˚ Radial veloci- tion of the Palomar sky survey (POSS) images; ties were determined using the cross-correlation technique 5. bJ magnitude from the COSMOS/UKST Southern Sky (Tonry & Davis 1979) implemented in the RVSAO pack- Object Catalogue; age (Kurtz et al. 1991; Mink et al. 1995), with radial ve- 6. heliocentric radial velocity with its error (km s−1); locity standards obtained from observations of late-type 7. R-value derived from Tonry & Davis (1979); stars and previously well-studied galaxies. 8. telescope and notes- e: 1.52 m ESO telescope, o:1.93m Table 13 lists positions and heliocentric velocities for OHP telescope; p: 2.0 m Pic du Midi telescope. 69 individual galaxies in the field of the cluster. The entries in the table are: For the analysis in the next sections it is important to es- 1. identification number; timate the completeness level of the velocity sample as 2. right ascension (J2000); a function of the magnitude. To do that (and also to study the cluster galaxy projected distribution) we ex- 2 IRAF is distributed by the National Optical Astronomy tracted from the ROE/NRL COSMOS UKST Southern Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooper- Sky Object Catalogue, supplied by the Anglo-Australian ative agreement with the National Science Foundation. Observatory (Yentis et al. 1992), a sample of galaxies in 3 Table 1 is also available in electronic form at the the direction of the cluster. An examination of Table 1 CDS via anonymous ftp to (130.79.128.5) or via indicates that, with one exception, all velocities were http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/377/428. measured in a region of ∼27 × 22 (in RA and DEC, L. Sodr´e Jr. et al.: The cluster of galaxies Abell 970 431
Table 1. continued.
cosmos GALAXY R.A DEC. TYPE bJ VELOCITY R N (2000) (2000) V ± ∆V 36 10 17 51.1 –10 35 00 E 17.29 16 661 62 4.42 e 37 10 17 36.9 –10 46 01 S 16.8 7 11 957 79 4.97 e 38 10 17 36.8 –10 46 05 S0/S 16.8 7 12 221 77 4.70 e1 39 10 16 55.8 –10 38 51 Sa 17.83 17 570 74 7.53 e 40 10 16 58.5 –10 38 07 S0/S 16.42 17 184 44 6.33 e 41 10 16 59.7 –10 37 39 E 18.62 17 873 59 6.44 e 42 10 17 00.6 –10 37 12 E 19.16 17 194 66 5.58 e 43 10 17 03.9 –10 37 38 E/S0 18.11 17 690 52 7.14 e 44 10 17 11.8 –10 36 07 E/S0 16.43 21 998 84 8.33 e2 45 10 16 57.5 –10 40 16 E 17.46 17 903 48 9.11 e 46 10 17 02.0 –10 39 59 E 19.10 18 843 98 4.53 e3 47 10 17 07.6 –10 45 46 S0 16.89 17202 86 15.72 e4 48 10 17 10.2 –10 46 26 E/S0 17.13 17 475 46 12.24 e5 49 10 16 49.9 –10 47 23 Sb 16.91 17 487 64 6.73 e 50 10 17 00.5 –10 47 17 Sa 17.38 21 166 107 3.77 e 51 10 16 54.1 –10 43 10 S0/S 18.45 17 257 85 4.71 e 52 10 16 53.2 –10 43 43 E 18.83 17 351 85 6.70 e 53 10 17 47.0 –10 45 19 S 19.14 16 771 138 2.96 p 54 10 18 18.0 –10 46 48 E/S0 15.95 11 579 63 8.04 p 55 10 18 15.6 –10 45 03 S0 17.69 12 056 81 3.26 p 56 10 17 51.6 –10 44 47 E 19.44 16 372 89 2.53 p 57 10 17 54.6 –10 43 47 S0 19.44 45 962 93 2.61 p 58 10 18 06.9 –10 42 43 S 19.13 51 814 76 2.41 p 59 10 18 04.0 –10 41 44 SBc 19.68 48 068 73 3.01 p 60 10 17 57.7 –10 33 48 S 18.52 16 228 44 4.53 p 61 10 18 13.0 –10 34 42 S 18.47 11 675 69 5.34 o6 62 10 16 55.2 –10 30 49 S 18.16 18 371 133 3.30 o 63 10 16 54.7 –10 33 22 S 17.86 17 737 75 4.47 o 64 10 16 36.9 –10 32 38 S0 17.58 17 982 82 5.62 o 65 10 16 51.9 –10 36 15 S 17.53 18 255 84 5.39 o 66 10 16 48.8 –10 39 10 S0 18.67 18 293 63 6.48 o 67 10 17 42.0 –10 45 49 E 18.60 12 101 114 3.22 o 68 10 16 37.3 –10 32 39 ? 18.77 17 589 136 3.04 o 69 10 16 41.3 –10 52 38 S 17.37 17 649 42 8.74 o Emission lines: 1: [OIII], Hα, [SII] = 12 084 km s−1; 2:Hα = 22 087 km s−1; 3:Hα = 18 858 km s−1; 4:Hα = 17 234 km s−1; 5:Hα = 17 523 km s−1; 6:measuredonHβ, [OIII], Hα; 7: estimated magnitude (see Sect. 4). respectively), centered on the position of galaxy num- The adaptive kernel density map (Silverman 1986) cor- ber 1 in Table 1, which assured a good level of com- responding to this sample is given in Fig. 2. This fig- pleteness in the core of the cluster. Indeed, by comparing ure indicates that the galaxy distribution in the field of the velocity and the COSMOS catalogues we verify that, Abell 970 is approximately regular, with the projected within this region, our velocity sample is ∼92% complete density peaking at the position of the E/D galaxy. There cosmos cosmos at bJ = 18, ∼75% at bJ = 19, and ∼51% at is a substructure at NW, near galaxy number 40 in cosmos bJ =19.75. Table 1. This is the brightest cluster galaxy (considering only galaxies with known radial velocities); it is classified cosmos as S0/S and has magnitude bJ =16.42. This figure 3. Galaxy distribution also indicates that the cluster radial extension may attain several Mpc. We present in Fig. 1 the distribution of galaxies in the It is worth noting that all features displayed in this direction of Abell 970, obtained from the COSMOS cata- map are significant. The significance regions are obtained cosmos logue, for galaxies brighter than bJ < 19.75 (235 ob- through a bootstrap resampling procedure applied to the jects). The plot is centered on an E/D galaxy (number 1 in sample coordinate distribution. This allows the construc- −1 Table 1) and has 45 × 45 (i.e., about 4.6 × 4.6 h50 Mpc). tion of a map of standard deviations of the projected den- It is interesting to point out that, amongst the galaxies sity. By subtracting the projected density map from the with known velocities, this is the second brightest cluster standard deviation map (multiplied by a given number, cosmos galaxy (bJ =16.57). say 3), we define the significance regions of the projected 432 L. Sodr´e Jr. et al.: The cluster of galaxies Abell 970
ing this same iterative method to the Einstein IPC data, Jones & Forman (1999) derived a X-ray luminosity of 2.13 × 1044 erg s−1 in the [0.5–4.5 keV] band and a bolo- metric luminosity equal to 3.79×1044 erg s−1, correspond- ing to a temperature in agreement with the one given by Ebeling et al. (1996). However, since the Einstein IPC detector has some spectroscopic capability, it is possible to estimate its tem- perature by a direct fitting of the available spectra. We have thus obtained both the “events” and image (in the 0.2–3.5 keV band, rebinned to 24 arcsec per pixel) files from the HEASARC Online Service. The spectrum of Abell 970 was extracted from the events file with XSELECT and analysed with XSPEC using the PI chan- nels 4–12 (0.5–4.5 keV) within a region of 9.6 arcmin (cor- −1 responding to 1 h50 Mpc). The X-ray emission was fit- ted with a single temperature, absorbed MEKAL model (Kaastra & Mewe 1993; Liedahl et al. 1995). We have also used the recipe given by Churazov et al. (1996) for cosmos computing the weights (available in XSPEC), based on Fig. 1. Galaxies brighter than bJ =19.75 in the field of Abell 970. The plot is centered on galaxy number 1 in Table 1. the smoothed observed spectrum. With these weights, one can still use the least-square minimisation and the χ2 statistics to estimate the confidence interval of the fitted parameters. density map as those regions for which the resulting sub- With only 9 bins covering the 0.5–4.5 keV band, it is tracted map is positive. impossible to constrain the metallicity. Therefore we have fixed Z to the “canonical” value of ∼0.3 Z ,whichisthe 4. The X-ray emission and the gas distribution mean value obtained for 40 nearby clusters by Fukazawa et al. (1998). Also, with only 3 bins with energy less than Abell 970 is an X-ray source, first observed with Einstein 1 keV, it is difficult to constrain independently the tem- IPC in June 1980 (Ulmer et al. 1981). It was also observed perature and the hydrogen column density. This happens during the rosat all-sky survey (Voges 1992), in 1990, because they are anti-correlated (e.g. Pislar et al. 1997). and is included in the X-ray Brightest Abell type Cluster Therefore, we have also fixed the hydrogen column density 20 −2 catalogue (XBACs; Ebeling et al. 1996). at NH =5.3 × 10 cm , which is the galactic value at Some properties of the X-ray emission of Abell 970 the Abell 970 position (Dickey & Lockman 1990). Figure 3 were derived by White et al. (1997) from its Einstein shows the fitting of the X-ray spectrum using the 9 avail- IPC image. They applied a deprojection analysis that, able energy bins. The results are summarised in Table 2. to constrain the cluster gravitational potential, requires The fitting shown in Fig. 3 presents large residuals at low an X-ray temperature (TX) and velocity dispersion (σ) energies (∼<1 keV), but beyond ∼2 keV, which is the re- for the cluster. Due to the absence of temperature mea- gion of the spectra most important for temperature de- surements for this cluster, these authors estimated it termination, the residuals are acceptable. Nevertheless, −1 from a σ–TX relation, assuming σ = 832 km s .The it is worth stressing that the estimated temperature is +0.2 result of the analysis is TX =4.1−0.6 keV. Moreover, strongly dependent on the number of energy bins em- White et al. (1997), using this same data, suggests that ployed in the analysis. For instance, keeping only the Abell 970 has a weak cooling-flow, with a mass deposition 8 bins with energy ∼>0.7 keV, the estimated temperature +32 −1 rate M˙ =20 M yr (see also Loken et al. 1999). +2.7 2 −20 increases to 4.9 kev, with χ /dof = 11.6/6, while with Ebeling et al. (1996) determined the X-ray flux, luminos- −2.1 the 7 bins with energy larger than ∼1 keV the tempera- ity and temperature using an iterative method running +∞ ture is essentially unconstrained: kT =15.0 ,witha roughly as follows: assuming an initial X-ray temperature −12.2 2 of 5 keV, the bolometric luminosity was computed. Then, χ /dof = 4.2/5. with this luminosity and using the TX−LX relation from The value of the gas temperature in Table 2, kT = White et al. (1997), a new estimate of the temperature 3.3 keV, is well below the IPC upper energy cutoff was made which, in turn, was used to compute a new at 4.5 keV. Although slightly cooler, this value of tem- luminosity and so on. Thus, with rosat data and the perature is in agreement with those derived by Ebeling above mentioned TX−LX relation, Ebeling et al. (1996) et al. (1996) and White et al. (1997), because its er- quoted a flux equal to 9.9 × 10−12 erg cm−2 s−1,lumi- ror is large (∼1 keV at the 68% confidence level; at the nosity of 1.50 × 1044 erg s−1 (both in the 0.1–2.4 keV 90% confidence level our standard solution gives an upper band), and gas temperature kTX =3.6 keV. By apply- limit for kT of 5.4 keV), due mainly to the small number L. Sodr´e Jr. et al.: The cluster of galaxies Abell 970 433
20 1Mpc
10
0 DEC (arcmin)
-10
-20
-20 -10 0 10 20 RA (arcmin)
cosmos Fig. 2. Projected density map of the galaxies brighter than bJ =19.75 in the field of Abell 970, together with the positions −2 2 of the 15 brightest ones. The peak density of this map corresponds to about 210 galaxies Mpc h50. All density levels in this map are significant. Other symbols are as follows- dots: galaxies kinematically linked to the cluster; pluses: galaxies projected in this field but not belonging to the cluster; crosses: galaxies with no measured velocities. The central rectangle displays the area covered by the X-ray image of Fig. 4. The substructure at NW, at ∼(7,3), is centered near galaxy number 40 of Table 1 (see text).
Table 2. X-ray spectral fitting results. kT is the gas tem- In Fig. 4 we display the X-ray isophotes of an Einstein perature, NH is the hydrogen column density, Z is the metal- IPC image in the [0.2–3.5 keV] band. This image has licity, LX is the [0.5–4.5 keV] non-absorbed luminosity inside 24 arcsec per pixel. As it can be seen, the X-ray isophotes h−1 χ2 1 50 Mpc, and the last column gives the and number of are also regular but, interestingly, their peak is not coinci- degrees of freedom. Errors are at a confidence level of 68%. dent with the peak of the projected density distribution, being slightly displaced towards the NW substructure as- 2 kT NH ZLX χ /dof 20 −2 44 −1 sociated to the cluster brightest galaxy (see Fig. 2). (keV) (10 cm )(Z )(10ergs s ) Figure 4 also suggests that there is no X-ray emission +1.1 +0.09 3.3−0.7 5.3 0.3 1.74−0.10 18.1/7 excess near this substructure. To verify whether this is