A Massive Galaxy Cluster at the Core of the Great Attractor HG. KRAAN-KORTEWEG 1, PA. WOUDT2, V. GAYATTE 1, A.P FAIRALL2, G. BALKOWSKI1, and PA. HENNING 3 10bservatoire de Paris, OAEC, Unite associee au CNRS, 00173, et a l'Universite Paris 7, Meudon, France 20epartment ofAstronomy, University of Cape Town, Rondebosch, South Africa 30epartment of Physics andAstronomy, University of New Mexico, Albuquerque, USA A deep survey for partially-obscured galaxies - leaving, however, the inner­ served properties of the galaxies for the galaxies behind the southern Milky Way most strip of the Milky Way completely foreground extinction would make this using the IIlaJ film copies of the ESO/ opaque. cluster the most prominent overdensity SERC sky survey reveals the cluster The cluster of galaxies Abell 3627 is in the southern skies. A3627 at Galactic co-ordinates 1 ::: c10se to that innermost strip. It had been To arrive at the distribution of the un­ 325°, b ::: -JO to be the most prominent catalogued in 1989 in the extended cat­ covered galaxies in redshift space, ob­ cluster in the southern hemisphere, alogue of rich clusters, because of the servations have been made at the 3.6-m were its galaxies not overlaid by the dust foreground obscuration only about 50 of telescope of ESO using the multifiber and stars of the Milky Way. Subsequent the larger galaxies were noted. The true system MEFOS built at the Paris-Meu­ follow-up observations find this strongly significance of the cluster became un­ don Observatory (cf. Felenbok et al., obscured galaxy cluster to be the near­ veiled with the identification of over 600 1996), individual spectroscopy at the est, rich, massive cluster in the Uni­ additional cluster members, many of 1.9-m telescope of the SAAO (South Af­ verse, comparable to the well-known which are just below the Lauberts diam­ rican Astronomical Observatory), and HI Coma cluster identified al ready by Wolf eter limit of 1 arcmin. Correcting the ob- observations of low surface brightness in 1906. Moreover, this massive cluster lies - within its errors - at the centre of the Great Attractor and most likely con­ stitutes the previously unidentified bot­ tom of this potential weil (cf. Kraan-Kor­ teweg et al., 1996 for further details). Systematic deviations of galaxies from the uniform expansion field indicate the existence of a "Great Attractor" (GA) - a large mass concentration of - 5 x 1016 Mev, at a redshift v - 4500 km/s, close to or behind the southern Milky Way (Lynden-Bell et al., 1988). The na­ ture and extent of the GA has been the quest of many optical and IRAS galaxy surveys, as weil as X-ray searches. While there is a considerable excess of optical-IRAS galaxies in this region, the absence of a dominant rich cluster at its centre remained peculiar. However, our Milky Way cuts across the central part of the large-scale over­ density of the GA (cf. Fig. 3b and 6b in Kolatt et al., 1995). Even if the core of the GA were in the form of luminous matter, it would be overlaid by the dust and the numerous foreground stars of Our Milky Way - our Galaxy blocks the view of about 25% of the extragalactic sky. For some years, two of us (RCKK and PAW) have led a dedicated effort to un­ veil the distribution of galaxies behind the southern Milky Way making use of the lIIaJ ESO/SERC survey. The films are inspected by eye with a 50 times magnification. All galaxies larger than 0 ::: 0.2 arcmin are recorded, Le. a factor A photograph of the innermost part ofthe clusterAGO 3627(25 x 25 arcmin), i.e. approximately the region within the core radius Re 5 10.4 arcmin, as seen on the ESO/SERG lIIaJ fjeld 137. five smaller compared to the ESO/Upp­ The central bright elliptical galaxy (a cD galaxy with blue magnitude BJ = 14.4) is the well-known sala Catalogue of Galaxies of the south­ radio source PKS 1610-60, one of the strongest radio sources in the southern hemisphere. ern hemisphere. Although heavily Another cD galaxy is located to the left of the radio source (BJ = 14.0). Gareful inspection will crowded with the images of the stars of reveal 59 cluster members present in this photograph, a large fraction of those are early-type our Galaxy, careful scrutiny has re­ galaxies. Note furthermore the huge number ofgalactic foreground stars, which will give some vealed over 10,000 previously unknown inkling about the inherent difficulties in mapping the galaxy distribution across the Milky Way. 17 spiral galaxies with the 64-m Parkes Ra­ at the approximate centre of the GA Wall - including the nearby Pavo, Indus dio Telescope. overdensity. Although very massive as a clusters below the Galactic plane and Within the Abell radius of the cluster cluster, its mass contributes only about the shallow overdensity in Vela above A3627, 109 velocities have so far been 10% of the total mass predicted for the the Galactic plane at 6000 km/so This reduced leading to a mean velocity of < Great Attractor. Hence, it is not "the" whole large-scale structure embodies what has been dubbed in 1987 the then vobs > =4882 km/s and a dispersion of 0' Great Attractor as such, but it is the = 903 km/so This puts the cluster weil prime candidate for being the hitherto unseen Great Attractor. within the predicted velocity range of the unidentified centre of this large-scale GA. The large dispersion of A3627 sug­ overdensity. This is supported by the re­ References gests it to be quite massive. Applying the cent analysis of the ROSAT PSPG data vi rial theorem yields a mass of - 5 x 1015 of A3627 by Böhringer et al. (1996) Böhringer, H., Neumann, D.M., Schindler, S., Mev, Le. a cluster on par with the rich, which finds this cluster to be the 6th & Kraan-Korteweg, R.C., 1996, ApJ, in press. well-known Goma cluster - yet consider­ brightest X-ray cluster in the sky for the Felenbok, P, Guerin, J., Fernandez, A., Cay­ ably c1oser. In fact, simulations show, ROSAT spectral band and confirms its atte, V., Balkowski, C., & Kraan-Korteweg, that if Goma were at the position of vi rial mass. R.C., 1996, A&A, submitted. A3627 in velocity space and subjected The results from our programme with­ Kolatt, T., Dekel, A., & Lahav, 0., 1995, to the same foreground obscuration, the in the whole search area in combination MNRAS 275,797. two clusters would be indistinguishable. with the Southern Redshift Gatalogue Kraan-Korteweg, R.C., Woudt, PA, Cayatte, V., Fairall, A.P., Balkowski, C., & Henning, A3627 even has - like Goma - 2 domi­ suggest furthermore that A3627 is the PA, 1996, Nature, 379,519. nant cD galaxies at its core. dominant component of an apparent Lynden-Bell, D., Faber, S.M., Burstein, D., The most intriguing aspect of the rec­ "great wall" structure - similar to the Davies, R.L., Dressler, A., Terlevich, R.J. & ognition of the new cluster is its position Goma cluster in the (northern) Great Wegner, G.,1988, ApJ326,19. 10- and 17-f.lm Test Images of the Galactic Centre: Massive Protostars Near SgrA*? H. ZINNECKER, Astrophysikalisches Institut Potsdam T. STANKE, Astronomisches Institut, Universität Würzburg; H.U. KÄUFL, ESQ-Garching Abstract microns, to a 3-sigma limiting flux of sources at mid-infrared wavelengths, so about 0.3 and 4 Jy per arcsec-2, respec­ that it is easy to find an empty sky posi­ We have obtained 10 and 17 micron tively. tion. The very Galactic Gentre is the ex­ (N and Q band) images of the central ception in this respect; there is no other square-parsec of the Galaxy. The imag­ Technical Details c1ustering of strong mid-infrared sources es were taken with ESO's mid-infrared in the Galaxy that rivals the innermost 64 x 64 pixel array camera TIMMI (Käufl The N band images (A =10.1 micron, parsec. et al., 1992, 1994) at the 3.6-m tele­ 01. =4.0 micron) were taken with a pixel The images shown have not been scope on La Silla. The reduced images scale of 0.66 arcsec/pixel in 1.2 arcsec f1at-fielded, as this appears to be unnec­ are displayed in Figure 1a,b: in total, 4 seeing. Due to the high thermal back­ essary: the overall gradient in sensitivity images are shown, two at each wave­ ground (sky and telescope) the on-chip across the array is less than 10%, and length (i.e. a linear and a logarithmic integration time of the individual frames the pixel-to-pixel variations are even stretch) in order to emphasise the full had to be very short (7 ms). Otherwise smaller (Käufl, 1995). dynamic range. The images represent standard infrared techniques were em­ The star ßGru was used as a point 1.5 and 7 minutes net on chip integration ployed, Le. chopping the secondary mir­ source calibrator (N = -3.55, Q = -3.58) time at each wavelength, respectively. ror (2.5 Hz) and nodding the telescope. and also to measure the seeing at mid­ The images indicate that the morpholog­ Sky frames were taken in a location infrared wavelengths; a Gen (N =-1.52, ical structure of the innermost parsec of clear of 10-micron sources. The Q band Q =-1.55) was used, too. the Galactic Gentre is rather similar at images (A = 17.15 micron, D.A = 1.5 mi­ both wavelengths, but differs significant­ cron) were taken with a smaller pixel Significance of the Images Iy at certain locations, in particular at the scale (0.33 arcsec/pixel), a necessity position of IRS3.