c1eus and almost encircling it. Our suppo cal and IR spectrum (Oliva et al. 1994, in not be explained by pure photoioniza sition that the brightest extranuclear Ha preparation) are well fitted by a combina tion models. This cone originates in a emission is from normal HII regions is tion of late-B main-sequence stars and nucleus whose visible and infrared po also supported by the very low values of late K supergiants, i.e. typical of a star sitions are spatially shifted suggesting 8 [SII]/Ha+[NII] (Fig. 7). burst which is much older (many x 10 that the true nucleus suffers A v ':::' 20 Between the nucleus and the outer yr) than the starburst ring. magnitudes of extinction. Our observa starburst there are regions with remark The above results are consistent with tions are thus consistent with the pres ably strong [SII] emission (Fig. 5) which a simple model in which a starburst prop ence of a torus which both obscures the most probably traces shocks from super agates out of the nucleus. At the outer Seyfert nucleus from direct view and col nova remnants. edge we see the most massive 0 stars limates its UV continuum emission. In A remarkable feature is the series of from the latest generation (those pho addition, a starburst ring which mayor 'spots' visible in the continuum and low toionizing the HII regions) while closer may not be related to the Seyfert nu excitation lines (Figs. 1, 4, 5, 7 and 10) inside less massive (early B) stars are cleus is also present. Older starburst ac which are aligned along a chain ':::' 20/1 still producing supernovae and remnants tivity, which is perhaps more relevant to south of the nucleus. These are the low responsible for the [SII] emission. In the the possible evolutionary connection be est excitation objects (i.e. those with the central regions (nucleus excluded) the tween starbursts and Seyfert activity, ap highest [SII]/Ha+[NII] ratio ) which, to high density of late B stars is responsi pears to have occurred closer to the nu gether with their sizes (':::' 30 pc), sug ble for the unusually blue colours while K cleus. A puzzling discovery is the chain of gests that they are individual supernova supergiants dominate the near IR emis compact, low excitation, objects,:::, 20/1 S remnants, possibly in small OB associ sion. of the nucleus which are probably super ations dominated by B stars. Additional nova remnants and associated B stars spectroscopy and/or radio observations Conclusions but require further study. are needed to clarify their exact na ture. As their orientation does not corre Line and continuum images of the spond to any obvious morphological fea Circinus galaxy have revealed a promi References ture we can only presume that they trace nent ionization cone with coronal gas the location of a gaseous spiral feature ([FeXI]) close to its apex and lower ex Antonucci, R., 1993, ARAA, 31, 473. Bergeron J., Petitjean P., Durret F., 1989, A&A in which starburst activity has already citation [0111] emission further out which 213,61. ceased. is consistent with photoionization by a Moorwood A.F.M., Glass I.S., 1984, MNRAS Within the central R ::; 2/1 (4 pc) the power law central source. Low excita 135,281. optical continuum images and the stellar tion [SII] emission observed between the Oliva E., Salvati M., Moorwood A.F.M., Mar absorption features observed in the opti- [FeXI] and [0111] peaks, however, can- coni M., 1994, A&A 288,457.
Multi-Wavelength Study of ROSAT Clusters of Galaxies
M. PIERRE, CEAlOSMIOAPNIA CESaclay, France, andMax-Planck-lnstitutfOrExtraterrestrische Physik, Garching, Germany R. HUNSTEAO, A. REIO, G. ROBERTSON, University of Sydney, Australia Y MELLlER, G. SOUCAIL, Observatoire de Toulouse, France H. BOHRINGER, H. EBELING, W VOGES, Max-Planck-Institut fOr Extraterrestrische Physik, Garching, Germany C. CESARSKY, J. OUKBIR, J.-L. SAUVAGEOT, L. VIGROUX, CEAlOSMIOAPNIA CE Saclay, France
1. Clusters of Galaxies ties known in the universe, clusters should ideally be reflected in the evolu as Cosmological Probes of galaxies are key objects for testing tion of the cluster mass function. While the predictions of the various cosmo the mass of a cluster is not a directly Among the 60,000 X-ray sources de logical scenarios. They originated from observable quantity, a number of other tected by the ROSAT All-Sky Survey, the highest peaks in the initial den physical parameters can be measured about one tenth are expected to be clus sity fluctuations, and are expected to directly and, therefore, provide a detailed ters of galaxies. This represents a con have evolved through characteristic pro picture of the dynamical cluster environ siderable potential for cosmological stud cesses, namely, "top-down" or "bottom ment as a function of redshift. ies and has motivated numerous iden up" depending on the nature of the tification campaigns, especially at ESO; dark matter ("hot" or "cold"). Practically, 2. Observational Tests we describe here such a programme to these alternatives correspond to situa gether with associated observations at tions in which clusters formed either from Relevant observations of clusters en other wavelengths. the fragmentation of large "pancakes" compass the whole electromagnetic As the most massive bound enti- or from the merging of sub-groups, and spectrum; a short overview includes: 24 • In the optical, velocity dispersions 3. The Cluster Sample Observed ray flux limited sample of clusters over a obtained through extensive spectros at ESO large area which will constitute a unique copy of cluster galaxies provide tool for studying luminosity andclustering mass estimates, while the pres In 1990 we started an observing evolution. ence of sub-groups in the velocity programme at ESO, aiming at identi distribution may be the signature of fying all clusters detected by ROSAT 4. Combined Observations merging events in the past history of in a single contiguous area covering the cluster. about 1700 deg2 around Hydra (a sim The analysis of the optical/X-ray prop • In X-rays, the cluster temperature func ilar programme is being pursued at the erties of the clusters discovered in the tion could in principle yield an almost AAT around the SGP). The main goals Hydra region revealed some very inter direct determination of the mass func of the project are to derive the clus esting objects which motivated a more tion if virialization is assumed. How ter X-ray luminosity function and study detailed multi-wavelength follow-up start ever, temperatures are known for very the large-scale structures by the spa ing in 1992. We selected a subsam few clusters, and the sensitivity of the tial cluster distribution. The cluster can pie of intrinsically X-ray-bright clusters, collimators (prior to ASCA) restricts didates are selected by cross-correlating which are therefore expected to be such observations to the closest ob the X-ray source lists, issued from the massive and with redshifts preferentially jects. On the other hand, the cluster ROSAT survey standard analysis, with above 0.15. The following observing pro luminosity function is much easier to the ROE/NRL cluster catalogue, the lat grammes are now underway: obtain, and evolution was detected in ter resulting from an automated anal (1) In the optical, the northernmost ob the Einstein Medium Sensitivity Survey ysis of the COSMOS object database jects of the sample have been subse sample (e.g. Henry et al. 1992 and ref derived from the scans of the UK/AAO quently studied at the 3.6-m Canada erences therein). A tight empirical cor Schmidt plates. With this method about France-Hawaii telescope in 1993 and relation between X-ray luminosity and 150 cluster candidates have been se 1994. Deep B, R photometry and ex temperature then enables the shape of lected down to an X-ray flux limit of tensive spectrometry (~ 80 spectra per the initial fluctuation spectrum in CDM ~ 5 x 10-1,3 erg S-1 cm-2 in our area. cluster) - with the MOS/SIS device using scenarios to be constrained (Henry & During the 1990-92 period the pro a 10' x 10' CCD (20482 pixels) - have Arnaud 1991). gramme was allocated a total of 13 nights been obtained in collaboration with the Because the mean free path of the with EFOSC on the 3.6-m telescope. The Toulouse group. hot X-ray emitting gas in the cluster po strategy has been to observe the tar (2) ROSAT PSPC pointings, for a few tential is considerably shorter than that get objects in order of decreasing X-ray selected cases, in order to investigate of the galaxies, the X-ray morphology flux, so that at any stage the current the outer cluster regions, where on-going (and size) of clusters provides insights data set is flux limited. For most clus merging events are expected to be most into subclustering and merging events ters the multi-slit mode is used (other readily detected, and to obtain a first es which are complementary to the opti wise, long slit) so that 5-12 redshifts per timate of the temperature. cal approach. cluster are available, providing in some (3) ROSAT HRI deep pointings, to • The radio emission from a possible cases an estimate of the velocity disper study in detail the gas morphology and cluster halo or particular cluster galax sion. For this first identification step, the the cluster potential at the centre: sub ies may be also a powerful tool for in EFOSC + MOS mode - giving on aver structures and correlation with the cD po vestigating the physical state and mo age 7 redshifts per cluster - presents sition (in progress). tions of the hot intra-cluster medium. the ideal observing set-up, for it allows (4) Radio observations started in 1993 Since the interaction between the hot us to assess directly the membership of in collaboration with the University of plasma and galaxy halos strongly af our cluster candidates. Unfortunately, we Sydney. The selected clusters are first fects the shape of the radio emission, should mention several practical prob imaged at 843 MHz with the Molon the presence of Head-Tailed galax lems encountered in all runs during the glo Observatory Synthesis Telescope ies at the cluster periphery or Wide fabrication of the masks, which turned (MOST; 43/f resolution); subsequently, Angled-Tailed at the cluster centre out to be very time consuming: bad trans higher resolution maps of the cluster ra (usually cDs) can constrain the proper mission between the computer holding dio sources are obtained with the Aus ties of the gas and on the gravitational the slit positions and the punching ma tralia Telescope (AT; 1.4-5 Ghz, 2-7/f potential. chine, punching pin breaking down sev resolution). • As for the infrared emission of clusters eral times, and overall uneven slit quality (5) A subsample of 14 objects will be of galaxies as such, only weak IRAS leading to unrecoverable artefacts in the observed by the ISO satellite (1995) in diffuse emission was detected from a sky subtraction during the subsequent the Central Programme at 7, 14 and 90 small number of poor clusters, but its data analysis. But we acknowledge here I'm (29 hours are allocated to this project: origin is not yet clear (Bregman 1992). the valuable assistance of the local tech Cesarsky et al.). Moreover, clusters are expected to be nical team which enabled us every night the sites where galaxy interactions are to have our masks ready in time. At 5. Multi-Wavelength Maps favoured, leading to enhanced star for present, the redshifts of some 60 clus mation. This is a totally open field, and ters have been measured and a detailed In order to exploit fully the 2-D infor one expects exciting new results es optical/X-ray analysis of the 42 brightest mation provided by the images obtained pecially from the comparison between ones is presented in Pierre et al. (1994). at different wavelengths, the ideal is to infrared and radio properties of clus The most distant cluster in the sample overlay them accurately so that a com ters. found so far has a redshift of ~ 0.31, and prehensive picture of the cluster environ With the coming of large collecting the X-ray sensitivity of the area leads us ment is readily available. Of special in area instruments, providing high spa to expect objects up to .z ~ 0.4. The pro terest is any correlation amongst the op tial and spectral resolution at wave gramme will be continued in the coming tical galaxy positions, particularly the cD, lengths far beyond the optical regime, years in order to complete the identifi the X-ray centroid of the hot ICM (and a new era will open up, enabling the cation of the sample and thus achieve possibly other field X-ray sources), and detailed study of moderately distant our scientific goals. For the first time, we the radio sources. This provides impor clusters. shall have at our disposal a complete X- tant clues: 25 (i) Radio sources can be identified and A1060 z= 1. 14000E-02 localized within the cluster; (ii) It enables a detailed morphological study, such as a comparison of the over ROSAT [0.4-2.4 KeV] MOLONGLO [843 MHz] all shape of the cluster emission in X-rays (elongation, clumps) with the galaxy dis tribution in the optical (alignments, sub groups). This point is especially rele vant for the investigation of the formation -27.481- _ processes of clusters: the presence of structures in the X-ray emission associ ated with (velocity) sub-groups of galax ies is undoubtedly a strong argument in -27.50 favour of the merging hypothesis (see Section 1). Moreover, if both X-ray and radio spatial resolutions are high enough (~ 5"), then for the nearest objects, an even more detailed study of the mo ! -27.52 'J tions within the ICM is possible with the u w combined morphological analysis of in o teractions between the hot X-ray emit ting plasma and the energetic particles -27.54 producing the radio lobes (Bbhringer et al. 1993); (iii) Another interesting point to investi gate is the location of the cD galaxy with -27.56 L respect to the X-ray centroid. In the opti cal, there are evidences for the existence of substructures when the cluster domi nant galaxy velocity shows a significant offset with respect to the cluster's mean 159.20 159.18 159.16 159.14 159.12 159.10 RA (deg.) (Beers et al. 1991). A non-coincidence between the optical and X-ray centres (the latter is supposed to indicate the (cnt/sec/sq.arcmln) (ly/Beam) centre of the cluster potential) may be 1rst contour: 1.72800E-03 5. 39926E-Q4 the signature of merging events in the step: 1. 15200E-03 1.50000E-Q3 cluster's history. ROSAT X-ray (red) and Molonglo radio (blue) contours overlaid on the corresponding CCO (iv) Finally, it allows flagging X optical image. The absolute instrumental positional accuracy is"" 1-2" in the optical and radio, ray emitting point-like objects (stars or and"" 20" in the X-ray band (1cJ); the plotting accuracy is discussed on page 46 in this issue AGNs), which may contaminate the dif of The Messenger. First and step contours are indicated on the figures. l fuse cluster emission (a crucial problem (Left) Abell 1060 (2 = 0.0124) is a nearby cluster with low radio power (log P(WHr ) = 22.3, H = 50 kms- 1 Mpc- l ) and X-ray luminosity ("" 5 x 1043 erg S-I). In this case the in the further determination of the X o number ofX-ray survey photons is greater than toOO; in order to improve the angular resolution, ray cluster luminosity function). Such an we selected only photons falling into the inner part of the detector so that the resulting PSF has a overlay is presented in the Figure, and FWHM of"" l' instead of",,2' (but the number ofphotons is decreased by a factor of",,5.5). Two the procedures followed to produce this large elliptical galaxies occupy the centre of the cluster. The radio emission clearly coincides image are described on page 46 in this with the brighter galaxy (NGC 3309), whereas the X-ray centroid appears to favour the other issue of The Messenger.
6. Preliminary Results
Reduction of the CFH data and over, our sample, which possesses a sig concerned is now underway, in order to ROSAT pointings is in progress, as well nificant fraction of objects at z 2: 0.1, sug characterize their properties. as the processing of the AT observations. gests that the coincidence between X-ray At the present stage it is too early to MOST and AT images have been ob and radio maxima is even stronger for draw definitive conclusions from the X tained for 25 clusters so far (from z ~ high redshift clusters. One explanation ray/radio correlations, but if these pre 0.01-0.3). A preliminary comparison be for this effect could arise from the fact that liminary results are confirmed with better tween X-ray, optical and radio survey im the original sample is close to being X-ray statistics, this may have serious conse ages reveals that for a high fraction of flux limited, which means that the dis quences for the practical determination clusters there is coincidence between the tant clusters are 10-100 times brighter of the cluster X-ray luminosity function. X-ray centroid (or an X-ray peak) and the intrinsically than the nearby ones. Con On the other hand, this is only one as presence of a radio galaxy (see Pierre, sequently, we might have been selecting pect of the entire programme which, once Hunstead & Unewisse 1994 for more de objects in which the X-ray flux is con completed, should provide a unique set tails). This is in agreement with the find taminated significantly by emission from of data for the dynamical study of clus ings of Burns et al. (1994) who attribute an active galactic nucleus. Indeed, in our ters up to z ~ 0.3. In this way, we hope this to a combination of clumped hot gas sample we found the second brightest to be in a position to understand better and AGN emission. We will be investi BL Lac object in the sky. Detailed opti their formation process, and thus to con gating this more closely with the high cal spectroscopy as well as high reso strain the nature and amount of dark mat resolution X-ray and radio images. More- lution radio observations of the galaxies ter present in the universe. 26 A3444 Z=2.541?OE-01 The VLT Site RDSAT [0.4-2.4 keVl MOLONGLO [843 f"IHz 1 at Parana/:
-27.22 September 1994
The centrefold of "The Messenger" shows an aerial view of Cerro Paranal, the site of ESO's Very Large Tele scope. This photo was obtained in mid -27.24 September 1994 andshows the rapid de velopments at the time of the construc tion of the concrete base for the four
OJ telescope enclosures. Also visible is the I1J "d -27.26 complex infrastructure for the various as sociated laboratories. This work is being done by the Skanska/Belfi consortium. The installations for the technical equip ment can be seen to the left. -27.28 The blasting work is now finished and excavations for the various connecting tunnels are clearly visible; they will be covered again when the concrete work is ready To the right, the work on the base -27.30 for Unit Telescope no. 1 is already well under way andin September reached the 155.98 155. 96 155.94 155.92 "floor level" on which the enclosure will RA (deg. ) be placed. The basement concrete floor (cnt/sec/sq.arcmln) U~/Beam) on which the coude focus for Telescope lrst cant-our: 3. 385ZZE-03 5.00000E-03 steo: 2. 86680E-03 1.70000E-03 no. 2 will be installed is in place, and the concrete work will soon start in the holes for Telescopes 3 and 4. To the extreme right and a little lower than the rest of the platform are the ex (NGC 3311). The white contours are from a recent high-resolution image obtained at 13 em with the Australia Telescope; the radio source appears, however, not to be resolved. The CCO cavations for the control building. The image is a 250 s exposure obtained in B band at the AAT platform altitude is about 2640 metres (Right) Abell 3444 (z = 0.254) is one of the most distant clusters in the present sample. above sea level and it measures about At the survey resolution, the X-ray image is point-like. The radio emission is probably slightly 150 metres across. The width of the ac extended to the north (which should not be confused with the natural N-S elongation of the cess road is no less than 12 metres, i.e. beam). Both X-ray and radio intensities are high: '" 3 x 1045 ergs- 1 and log P(WHr 1 ) = 24.4 respectively, and clearly centred on the cluster dominant galaxy. The exact coincidence nearly equal to that of a three-lane high observed here appears to occur in many of the cluster fields, especially for the high redshift way; this is necessary to ensure the safe clusters in the sample. The CCO image is a 3-minute EFOSC exposure in R band. transport ofall telescope parts to the top, especially the four 8.2-m fragile mirrors. In October 1994, the first shipment of steel parts of this enclosure (manufac tured by the SEBIS consortium) with a Acknowledgements Bregman J. N., 1992 in Clusters and Super total weight ofmore than 100 tons left Eu clusters of Galaxies, p. 119, Cambridge, rope for the sea journey to Chile. While M.P. is grateful to the Max-Planck July 1991, Ed. A Fabian, Kluwer. the smaller parts were packed in large Institut fUr Extraterrestrische Physik for Henry J.P., & Arnaud KA,1991, ApJ. 372, 410. containers, special packing was neces long-term financial support. We also ac Henry J.P., Gioia I.M., Maccacaro 1., Morris sary for the very large structures. The knowledge a grant by the EC Human S.L., Stocke J.T, Wolter A, 1992, ApJ. 386, Capital Programme, contract CHRX 408. ship left the port ofMarghera, Italy, and is Pierre M., B6hringer H., Ebeling H., Voges W, expected to dock in Antofagasta towards CT92-003. Schuecker P., Cruddace R., MacGillivray H., the end of December 1994, after which 1994, Astronom. Astrophys. 290, 725. Pierre M., Hunstead R., Unewisse A., 1994, the parts will be transported by truck to Combined X-ray/optical/radio observations the top of Paranal. References of ROSAT clusters of galaxies, in Cosmo This first shipment will be soon fol logical Aspects of X-Ray Clusters of Galax Beers TC., Forman W, Huchra J.P., Jones C., lowed by others. It is expected that con ies, p. 73-77, Munster, June 1993, ed. Seit Gebhardt K., 1991 AJ. 102, 1581. ter, Kluwer. signments ofabout 100 tons each will be B6hringer H., Voges W, Fabian A.C., Edge sent to Chile over the next eight months. AC., Neumann D.M., 1993 MNRAS 264, The enclosure erection will start in Jan L25-28. Burns, J. 0., Rhee G., Owen E, Pinkey, J. uary 1995and will be completed in about 1994 ApJ. 423, 94. 8 weeks. M. Tarenghi
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