Astron. Astrophys. Suppl. Ser. 136, 117-137

A search for gravitational lensing in 38 X-ray selected clusters of galaxies

G.A. Luppino^1,6, I.M. Gioia^1,2,6, F. Hammer³, O. LeFèvre⁴, and J.A. Annis⁵

Send offprint request: I.M. Gioia,
e-mail: gioia@ira.bo.cnr.it, Istituto di Radioastronomia del CNR, Via Gobetti 101, I-40129, Bologna, Italy.

1 - Institute for Astronomy, University of Hawaii, Honolulu HI 96822, U.S.A.
2 - Istituto di Radioastronomia del CNR, I-40129 Bologna, Italy
3 - DAEC, Observatoire de Paris Meudon, F-92195 Meudon Principal Cedex, France
4 - Laboratoire d'Astronomie Spatiale, F-13376 Marseille Cedex 12, France
5 - Experimental Astrophysics Group, Fermilab, Batavia IL 60510, U.S.A.
6 - Visiting Astronomer at CFHT, operated by the National Reasearch Council of Canada, le Centre National de la Recherche Scientifique de France and the University of Hawaii

Received October 2; accepted December 16, 1998

Abstract:

We present the results of a CCD imaging survey for gravitational lensing in a sample of 38 X-ray-selected clusters of galaxies. Our sample consists of the most X-ray luminous ($L_{\rm x}$ $\geq$ 2 10⁴⁴ ergs^-1) clusters selected from the Einstein Observatory Extended Medium Sensitivity Survey (EMSS) that are observable from Mauna Kea ($\delta\gt$ -40$^{\rm o}$). The sample spans a redshift range of $0.15\leq z\leq 0.823$ and includes 5 clusters with z> 0.5. CCD images of the clusters were obtained in excellent seeing. There is evidence of strong gravitational lensing in the form of giant arcs (length $l\geq8''$, axis ratio $l/w\geq10$) in 8 of the 38 clusters. Two additional clusters contain shorter arclets, and 6 more clusters contain candidate arcs that require follow-up observations to confirm their lensing origin. Since the survey does not have a uniform surface brightness limit we do not draw any conclusion based on the statistics of the arcs found. We note, however, that 60% (3 of 5) of the clusters with $L_{\rm x}\gt$ 10⁴⁵ ergs^-1, and none of the 15 clusters with $L_{\rm x}< 4$ 10⁴⁴ ergs^-1 contain giant arcs, thereby confirming that high X-ray luminosity does identify the most massive systems, and thus X-ray selection is the preferred method for finding true, rich clusters at intermediate and high redshifts.

The observed geometry of the arcs, most of which are thin, have large axis ratios (l/w >10), and are aligned orthogonal to the optical major axes of the clusters, indicate the cluster core mass density profiles must be compact (steeper than isothermal). In several cases, however, there is also some evidence, in the form of possible radial arcs, for density profiles with finite core radii.

Key words: galaxies: clusters: general -- gravitational lensing -- X-rays: galaxies -- Cosmology: observations: dark matter