Killing spinor space-times and constant-eigenvalue Killing tensors

A class of Petrov type D Killing spinor space-times is presented, having the peculiar property that their conformal representants can only admit Killing tensors with constant eigenvalues.Comment: 11 pages, submitted to CQ

The Binary Galaxies NGC 147 And NGC 185

Contrary to a previously published claim it is found that the spheroidal galaxies NGC 147 and NGC 185 probably form a stable binary system. Distance estimates place this pair on the near side of the Andromeda subgroup of the Local Group. The fact that this system has probably remained stable over a Hubble time suggests that it does not have a plunging orbit that brings it very close to M 31. It is noted that the only two Local Group galaxy pairs, in which the components have comparable masses, also have similar morphological types. NGC 147 and NGC 185 are both spheroidals, while the LMC and SMC are both irregulars. This suggests that protogalaxies of similar mass that are spawned in similar environments evolve into objects having similar morphologies.Comment: To be published in A.J. October 199

Spiral Structure and Galaxy Environment

Among 330 normal spirals of types Sa-Sc the fraction of objects exhibiting ``ring'', ``intermediate'' and ``spiral'' arm varieties does not correlated with environment. A similar conclusion appears to apply to the arm varieties of 123 barred spirals of types SBa-SBc. It is concluded that, among the northern Shapley-Ames galaxies, the distinction between the spiral and ring varieties of spiral arms is, within the accuracy of presently available data, independent of galaxy environment. This result suggests that the detailed morphology of spiral arms depends primarily on parent galaxy characteristics, rather than on the galactic environment.Comment: 8 pages. no figures. To be published in the August 2202 issue of A

Diameters of Open Star Clusters

The present paper presents a tabulation of data on all 600 Galactic open clusters for which it is presently possible to calculate linear diameters. As expected, the youngest `clusters' with ages < 15 Myr, contain a significant (greater than or equal to 20%) admixture of associations. Among intermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, the median cluster diameter is found to increase with age. Small compact clusters are rare among objects with ages > 1.5 Gyr. Open clusters with ages > 1 Gyr appear to form what might be termed a `cluster thick disk', part of which consistst of objects that were probably captured gravitationally by the main body of the Galaxy.Comment: Astronomical Journal, in pres

Purely radiative irrotational dust spacetimes

We consider irrotational dust spacetimes in the full non-linear regime which are "purely radiative" in the sense that the gravitational field satisfies the covariant transverse conditions div(H) = div(E) = 0. Within this family we show that the Bianchi class A spatially homogeneous dust models are uniquely characterised by the condition that $H$ is diagonal in the shear-eigenframe.Comment: 6 pages, ERE 2006 conference, minor correction

Caltech Faint Galaxy Redshift Survey XIV: Galaxy Morphology in the HDF (North) and its Flanking Fields to z=1.2

Morphological classifications are reported for Hubble Space Telescope (HST) images of 241 galaxies in the Hubble Deep Field (HDF) and its Flanking Fields (FF) with measured redshifts in the interval 0.25 < z < 1.2, drawn from a magnitude-limited redshift survey to R = 24.0. The galaxies are divided into three groups with redshifts in the intervals [0.25,0.6], [0.6,0.8], [0.8,1.2]. R606 images from the first group and I814 images from the second and third groups are compared with B-band images of nearby galaxies. All classifications were therefore made at approximately the same rest wavelength. Selection biases are discussed. We corroborate and extend the results of earlier investigations by observing that: Most intermediate and late-type galaxies with $z\gtrsim0.5$ have morphologies that are dramatically different from those of local galaxies and cannot be shoehorned into the Hubble ``tuning fork'' classification scheme. Grand-design spirals appear to be rare or absent for $z\gtrsim0.3$. Many Sa and Sb spirals with $z\gtrsim0.6$ do not exhibit well-defined spiral arms. The arms of distant Sc galaxies appear more chaotic than those of their nearby counterparts. (abridged) On the basis of these and similar observations, it is inferred that the development of pronounced spiral structure was delayed until \~5 Gyr and that most bulges are probably not formed by disintegrating bars. Major morphological changes were still taking place only ~5 Gyr ago even though changes in the integrated light of most galaxies were then much slower than they were ~10 Gyr ago.Comment: Accepted by the Astronomical Journa