Individual Cluster Members Research Articles

Multivariate methods for clustered ordinal data with applications to survival analysis.

Clustered data are the rule in many clinical specialties such as ophthalmology. Methods have been developed for the treatment of clustered continuous or binary outcome data. Less attention has been given to ordinal outcomes which occur frequently in ophthalmology. For example, grading systems of cataract and diabetic retinopathy are commonly used where a photograph is graded by comparison with a series of reference photographs of increasing severity. Some commonly used methods for the analysis of ordered categorical data include the proportional odds and continuation ratio models. It is difficult, however, to incorporate clustering effects into these models. Instead, for clusters of size two, we propose a generalization of the adjacent category model given by log[Pr(i + 1,j)/Pr(i,j)] = ui + (j - 1) lambda + beta' x, where Pr(i,j) denotes the probability that the right (left) eye has grade i(j), x is a vector of (person or eye-specific) covariates for the right eye, u and beta are vectors of location and covariate parameters and lambda is a clustering parameter. Based on this model, and a similar model interchanging the role of i and j, we derived a closed-form expression for Pr(i,j) as a function of u, lambda and beta and use Newton-Raphson method to maximize the likelihood. An extension of the method allows for extra agreement along the diagonal and is then a generalization of the agreement plus linear-by-linear association model proposed by Agresti in the setting of no covariates. We apply these methods to a data set of 43 diabetic subjects from the Harvard Clinical Cataract Research Center, where cortical cataract grade was the outcome. We also extend this methodology to a survival setting, where both censored and uncensored outcomes are available for individual cluster members, and one wishes to take clustering into account. We apply the survival analysis model to a data set of 1807 children (two ears per child) in the greater Boston area, who were followed for the development of otitis media over the first year of life.

Morphological studies of the galaxy populations in distant 'Butcher-Oemler' clusters with HST. 1: AC 114 AT Z = 0.31 and Abell 370 at Z = 0.37

view Abstract Citations (227) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Morphological Studies of the Galaxy Populations in Distant ``Butcher-Oemler'' Clusters with HST. I. AC 114 at Z = 0.31 and Abell 370 at Z = 0.37 Couch, Warrick J. ; Ellis, Richard S. ; Sharples, Ray M. ; Smail, Ian Abstract We present the first results of an ongoing program we are undertaking with the Wide Field Camera of the Hubble Space Telescope (HST) to understand the physical origin of the enhanced star formation seen in moderate-redshift (z ~ 0.3-0.4) cluster galaxies. Deep HST exposures have been obtained for the central regions of two rich compact "Butcher- Oemler" clusters, AC 114 at z = 0.31 and Abell 370 at z = 0.37. Both of these clusters have been subject to extensive ground-based spectroscopic and multiband imaging studies with a significant fraction of confirmed members being seen in the active or ensuing phases of a starburst. We have used the HST images in conjunction with the ground-based data to examine the morphology of individual cluster members, in particular these starburst" objects. We find that those blue members that display spectral evidence of active or recently completed star formation are predominantly disk-dominated systems whose abundance is greater than that seen in the cores of present-day rich clusters. Furthermore ~55% of the galaxies in this category in both clusters show convincing evidence of dynamical interactions, in contrast to a 20%-30% rate of occurrence among the red population. While similar conclusions had been drawn from high-quality ground-based data, we demonstrate the unique role HST can play in identifying interacting galaxy pairs which have separations less than the current resolution limits of the best ground-based images. There is convincing evidence in these first two clusters we have examined that interactions and mergers play a major role in inciting the star formation activity associated with the Butcher-Oemler effect. Of equal significance is the morphological nature of the numerous red members in our HST data set which show various spectroscopic and photometric indications of previous starburst activity, including strong Balmer-line absorption. Most of these galaxies appear to be undisturbed and isolated with a normal E morphology. There is no convincing evidence that these are merger products. Although larger samples are still required, we conjecture that the Butcher-Oemler effect may involve at least two physical processes arising from the hierarchical growth of clusters: galaxy-galaxy interactions and environmentally induced star formation arising from the hierarchical merging of clusters. Much work remains to be done to understand exactly how the fraction of disk galaxies seen in distant rich clusters declines to its present low value. Publication: The Astrophysical Journal Pub Date: July 1994 DOI: 10.1086/174387 Bibcode: 1994ApJ...430..121C Keywords: Astronomical Photometry; Galactic Clusters; Galactic Structure; Hubble Space Telescope; Interacting Galaxies; Spaceborne Astronomy; Star Formation; Balmer Series; Morphology; Red Shift; Starburst Galaxies; Astronomy; GALAXIES: INTERACTIONS; GALAXIES: CLUSTERS: INDIVIDUAL ALPHANUMERIC: AC 114; GALAXIES: CLUSTERS: INDIVIDUAL NAME: ABELL 370; GALAXIES: PHOTOMETRY; GALAXIES: STELLAR CONTENT full text sources ADS | data products SIMBAD (81) NED (39) MAST (1) ESA (1) Related Materials (1) Part 2: 1998ApJ...497..188C

Radial distribution of the integrated light and photometric colours in open star clusters

Mass segregation in the form of preferential concentration of more massive stars in the central regions of a number of open star clusters has been known for some time. In this paper, integratedUBV colours in concentric zones have been estimated for 12 nearby open clusters using the observations of individual cluster members. It is found that the clusters showing pronounced mass segregation also show significant radial variations in the integrated colours. However, the effects of stochastic fluctuations around the massive portion of the mass distribution function on the integrated colours should be taken into consideration, if they are present.

Old Globular Clusters in the Magellanic Clouds

The old globular clusters in the Magellanic Clouds are important links between our understanding of globular clusters in our own galaxy and similar unresolved objects in more distant galaxies. The Cloud clusters spread over a large range in age. Several contain RR Lyrae variable stars. High weight abundance data are needed for individual cluster members as well as deeper color-magnitude diagrams.

Linear clusters of galaxies - A999 and A1016

Redshifts have been measured for galaxies in two of the 'linear' clusters of the sample of Adams, Strom, and Strom (1980), including 44 redshifts in A999 and 40 in A1016. From the data, it is concluded that the galaxies in A999 are probably drawn from a spherically symmetric distribution, while those in A1016 probably are not. Both A999 and A1016 have mass-to-light ratios lower than typical of other clusters. The effect of anisotropy on the determination of cluster masses from the virial theorem is examined, and it is found that if the shortest axes of these clusters are close to the line of sight, the mass-to-light ratio may be underestimated by about 50 percent. No significant evidence is found for alignments of individual cluster members with the cluster axis in the convincing linear cluster A1016. There is similarly no evidence of segregation by luminosity morphological type in A1016.

Extended radio emission in clusters of galaxies: recent Westerbork observations

Whether galaxy clusters, in addition to a number of radio sources associated with individual cluster members, have an extended radio emitting region designated as cluster halo is in an interesting way related to several questions concerning cluster characteristics: