We discuss a method to constrain the intrinsic shapes of galaxy clusters by combining X-ray and SunyaevZel'dovich observations. The method is applied to a sample of 25 X-ray-selected clusters, with measured SunyaevZel'dovich temperature decrements. The sample turns out to be slightly biased, with strongly elongated clusters preferentially aligned along the line of sight. This result demonstrates that X-ray-selected cluster samples may be affected by morphological and orientation effects, even if a relatively high threshold signal-to-noise ratio is used to select the sample. A large majority of the clusters in our sample exhibit a marked triaxial structure; the spherical hypothesis is strongly rejected for most sample members. Cooling-flow clusters do not show preferentially regular morphologies. We also show that identification of multiple gravitationally lensed images, together with measurements of the Sunyaev-Zel'dovich effect and X-ray surface brightness, can provide a simultaneous determination of the three-dimensional structure of a cluster, of the Hubble constant, and of the cosmological energy density parameters.

Abstract. We present a near-infrared spectroscopic survey of a large area centered on the Cygnus OB2 association aimed at constraining its massive star contents. Our goal is to establish a nearly complete list of O-type members of the association, both to examine recent claims based on starcounts that suggest a richer content than previously thought, and to provide a suitable database for further studies of the entire high-mass end of one of the richest associations of the Galaxy. The target selection is based on the JHK photometry published in the 2MASS all-sky survey. We identify 46 new early-type candidates, most of them expected to be O-type stars, plus 16 new stars with emission in Brγ and often in other lines as well, characteristic of evolved massive stars undergoing intense mass loss. We also present spectra of three luminous stars with CO overtone emission, one of them having also intense H2 emission and being associated with compact nebulosity. By considering our findings, those of other authors, and plausible completeness corrections, we estimate the number of O-type stars or stars having evolved from a O-type progenitor to be 90-100, slightly below, but compatible with, most recent starcounts estimates by Knödlseder (2000, A&A, 360, 539). These results support the notion that Cygnus OB2 may be considered as a young globular cluster. The lists of new members that we provide, in particular those with emission lines, should be a useful resource for future investigations of Cygnus OB2 itself, as well as of very massive stellar evolution by providing a nearby, abundant sample of stars sharing a common environment.

We consider the possibility that masses and gravitational potentials of galaxy cluster, estimated at X‐ray wavelengths, could be explained without assuming huge amounts of dark matter, but in the context of f(R) gravity. Specifically, we take into account the weak field limit of such theories and show that the corrected gravitational potential allows to estimate the total mass of a sample of 12 clusters of galaxies. Results show that such a gravitational potential provides a fair fit to the mass of visible matter (i.e. gas + stars) estimated by X‐ray observations, without the need of additional dark matter while the size of the clusters, as already observed at different scale for galaxies, strictly depends on the interaction lengths of the corrections to the Newtonian potential.