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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 November 1999

Abstract

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 presence of this mass concentrationis confirmed. A statistical analysis, using the aperture masstechnique, shows that the probability of finding such a massconcentration at the observed position from a random alignment ofbackground galaxies is [FORMULA] and[FORMULA] for the V- and I-bandimage, respectively. No obvious strong concentration of brightgalaxies is seen at the position of the mass concentration, but aslight galaxy number overdensity is present about[FORMULA] away from its center. ArchivalROSAT-HRI data show the presence of a weak extended X-ray source nearto the mass concentration, but also displaced by about[FORMULA] from its center, and very close tothe center of the slight galaxy number concentration.

From the spatial dependence of the tangential alignment around thecenter of the mass concentration, a rough mass estimate can beobtained which depends strongly on the assumed redshift of the lensand the redshift distribution of the galaxies which are used formeasuring the lensing signal. A lower bound on the mass inside asphere of radius [FORMULA] Mpc is[FORMULA], considerably higher than crude massestimates based on the X-ray data. Shifting the lens to higherredshift increases both the lensing and X-ray mass estimates, but doesnot resolve the mass discrepancy.

Concerning the nature of the mass concentration, no firm conclusioncan be drawn from the available data. If it were a high-redshiftcluster, the weak X-ray flux would indicate that it had an untypicallylow X-ray luminosity for its mass; if the X-ray 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 lensing enables us forthe first time to select halos based on their mass pro © European Southern Observatory (ESO) 2000

Online publication: March 17, 2000

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