Optical Substructures in 48 Galaxy Clusters: New Insights from a Multiscale Analysis

Abstract

We analyze the presence of substructures in a set of 48 galaxy clusters by using galaxy positions and redshifts. The data are taken from literature sources, with the addition of some new data provided by recent observations of galaxy clusters. We use a multiscale analysis that couples kinematical estimators with the wavelet transform. With respect to previous works, we introduce three new kinematical estimators. These estimators parameterize the departures of the local means and/or local dispersions of the measured radial velocities with respect to their global values for the environment. We classify the analyzed clusters as unimodal, bimodal, and complex systems. We find that about 14% of our clusters are strongly substructured (i.e., they are bimodal or complex) and that about 24% of the remaining unimodal clusters contain substructures at small scales. Thus, in substantial agreement with previous studies, about one-third of clusters show substructures. We find that the presence of substructures in unimodal clusters does not affect the estimates of both velocity dispersions and virial masses. Moreover, the galaxy velocity dispersion is generally in good agreement with the X-ray temperature, according to the expectations of the standard isothermal model for galaxy clusters. These facts suggest that unimodal clusters, which are the most frequent cases in the nearby universe, are not too far from a status of dynamical equilibrium. In contrast, the estimates of velocity dispersions and masses for some bimodal or complex clusters strongly depend on whether they are treated as single systems or as sums of different clumps. In these cases the X-ray temperature and the velocity dispersion may be very different

Velocity Dispersions and Cluster Properties in the Southern Abell Redshift Survey Clusters. II.

“…Our values of and D are typical of groups of galaxies. Nevertheless, the average extensions of our groups are larger than those derived by Girardi et al 1997 ($0.2 h À1 Mpc). The KMM technique for substructure detection tends to identify systems at larger distances from the cluster center (DV = 1515 AE 304 km s À1 ).…”

“…Therefore, we expect a significant degree of substructure in clusters of galaxies if the remnants of the accretion of groups in the recent past has not been erased by the dynamical relaxation of the clusters. The substructure in rich clusters has been extensively analyzed in recent years (Dressler & Shectman 1988;West & Bothun 1990;Zabludoff et al 1993;Girardi et al 1997;Solanes, SalvadorSolé, & Gonzáles-Casado 1999). The results are consistent with substructure in most of the cases studied, irrespective of the samples and method of analyses used.…”

A comparison of different cluster mass estimates: consistency or discrepancy?

“…According to the present data sets, it is unlikely that there are significant differences between the two mass estimators. The virial masses are from Carlberg et al (1996) and Girardi et al (1997a).…”

Parsec-scale properties of brightest cluster galaxies

“…This cluster is at z = 0.0205; its internal structure is not clearly understood. Girardi et al (1997) found that it consists of two systems, both very extended, separated in velocity by over 4000 km s −1 , but spatially overlapping. The BCG in the NVSS image is not detected (≤1.4 mJy), which corresponds to an upper limit to the total power of Log P ≤ 21.1 (W/Hz).…”

“…It is a highly structured cluster despite the apparently regular velocity distribution of the main field. This cluster should be considered as an ongoing cluster merging (Girardi et al 1997). This cluster exhibits two peaks in the X-ray emission (Bird et al 1995).…”