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Astron. Astrophys. 355, 23-36 (2000)

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Mass-detection of a matter concentration projected near the cluster Abell 1942: Dark clump or high-redshift cluster? *

Th. Erben  1, L. van Waerbeke  2, 1, Y. Mellier  3, 6, P. Schneider  1, J.-C. Cuillandre  4 , F.J. Castander  5 and M. Dantel-Fort  6

1 Max-Planck-Institut für Astrophysik, P.O. Box 1523, 85740 Garching, Germany
2 CITA, University of Toronto, 60 St. George Street, Toronto, Ontario, M5S 1A7, Canada
3 Institut d'Astrophysique de Paris, 98bis Boulevard Arago, 75014 Paris, France
4 CFHT Corporation, P.O. Box 1597, Kamuela, Hawaii 96743, USA
5 OMP, 14 Av. Edouard Belin, 31400 Toulouse, France
6 Observatoire de Paris, DEMIRM, 77 avenue Denfert Rochereau, 75014 Paris, France

Received 15 July 1999 / Accepted 25 No vember 1999


A weak-lensing analysis of a wide-field V-band imagecentered on the cluster Abell 1942 ([FORMULA])has uncovered the presence of a mass concentration projected[FORMULA] arcminutes South of the clustercenter. From an additional wide-field image, taken with a differentcamera in the I-band, the pr esence of this mass concentrationis confirmed. A statistical ana lysis, using the aperture masstechnique, shows that the probabil ity of finding such a massconcentration at the observed position from a random alignment ofbackground galaxies is [FOR MULA] and[FORMULA] for the V- and I-bandimage, r espectively. No obvious strong concentration of brightgalaxies i s seen at the position of the mass concentration, but aslight ga laxy number overdensity is present about[FORMULA] away from its center. ArchivalROSAT-HRI data show the presence of a weak extended X-ray source nearto th e mass concentration, but also displaced by about[FORM ULA] from its center, and very close tothe center of the slight galaxy number concentration.

From the spatial dependence of the tangential alignment around th ecenter of the mass concentration, a rough mass estimate can beo btained which depends strongly on the assumed redshift of the le nsand the redshift distribution of the galaxies which are used f ormeasuring the lensing signal. A lower bound on the mass inside asphere of radius [FORMULA] Mpc is[FORMULA], consid erably higher than crude massestimates based on the X-ray data. Shifting the lens to higherredshift increases both the lensing a nd X-ray mass estimates, but doesnot resolve the mass discrepanc y.

Concerning the nature of the mass concentration, no fir m conclusioncan be drawn from the available data. If it were a h igh-redshiftcluster, the weak X-ray flux would indicate that it had an untypicallylow X-ray luminosity for its mass; if the X-ra y emission werephysically unrelated to the mass concentration, e .g. coming from therelatively low-redshift group which shows up in the number density ofgalaxies, this conclusion would be even stronger.

Since the search for massive halos by weak lensi ng enables us forthe first time to select halos based on their m ass properties only, itis possible that new types of objects can be detected, e.g. halos withvery little X-ray and/or optical lu minosity, should they exist. Themass concentration in the field of A1942 may be the first example ofsuch a halo. Possibilities t o establish the nature of this massconcentration with future obs ervations are briefly discussed.

Key words: galaxies: clusters:general – cosmology:observations – cosmology: darkmatter – cosmology: gravitational lensing

* based on observations with the Canada-France-Hawaii Telescope (CFH T) operated by the National Research Council of Canada (CNRC), t he Institut des Sciences de l'Univers (INSU) of the Centre Natio nal de la Recherche Scientifique (CNRS) and the University of Ha waii (UH) and on data obtained through the NASA/GSFC HEASARC Onl ine archive.

Send offprint requests to: (erben@mpa-garching.mpg.de)

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000