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We describe the properties of a remarkable group of actively star-forming dwarf galaxies and H II galaxies in the A1367 cluster, which were discovered in a large-scale Halpha imaging survey of the cluster. Approximately 30 Halpha-emitting knots were identified in a region approximately 150 kpc across, in the vicinity of the spiral galaxies NGC 3860, CGCG 97-125, and CGCG 97-114. Follow-up imaging and spectroscopy reveal that some of the knots are associated with previously uncataloged dwarf galaxies (M-B = -15.8 to -16.5), while others appear to be isolated H II galaxies or intergalactic H II regions. Radial velocities obtained for several of the knots show that they are physically associated with a small group or subcluster, including CGCG 97-114 and CGCG 97-125. No comparable concentration of emission-line objects has been found elsewhere in any of the eight northern Abell clusters surveyed to date. The strong H emission in the objects and their high spatial density argue against this being a group of normal, unperturbed dwarf galaxies. Emission-line spectra of several of the knots also show some to be anomalously metal-rich relative to their luminosities. The results suggest that many of these objects were formed or triggered by tidal interactions or mergers involving CGCG 97-125 and other members of the group. A Westerbork Synthesis Radio Telescope H I map of the region shows direct evidence for tidal interactions in the group. These objects may be related to the tidal dwarf galaxies found in some interacting galaxy pairs, merger remnants, and compact groups. They could also represent evolutionary precursors to the class of isolated ultracompact dwarf galaxies that have been identified in the Fornax Cluster.

The rotation curves of low surface brightness galaxies provide a unique data set with which to test alternative theories of gravitation over a large dynamic range in size, mass, surface density, and acceleration. Many clearly fail, including any in which the mass discrepancy appears at a particular length scale. One hypothesis, Modified Newtonian Dynamics (MOND), is consistent with the data. Indeed, it accurately predicts the observed behavior. We find no evidence on any scale that clearly contradicts MOND and much that supports it.

The severity of the mass discrepancy in spiral galaxies is strongly correlated with the central surface brightness of their disks. Progressively lower surface brightness galaxies have ever larger mass discrepancies. No other parameter (luminosity, size, velocity, morphology) is so well correlated with the magnitude of the mass deficit. The rotation curves of low surface brightness disks thus provide a unique data set with which to probe the dark matter distribution in galaxies. The mass discrepancy is apparent from R = 0, giving a nearly direct map of the halo mass distribution. The luminous mass is insignificant. Interpreting the data in terms of dark matter leads to troublesome fine-tuning problems. Different observations require contradictory amounts of dark matter. Structure formation theories are as yet far from able to explain the observations.

Through extended integrations using the recently-installed deep depletion CCD on the red arm of the Keck I Low Resolution Imaging Spectrograph, we present new measurements of the resolved spectra of 70 morphologically-selected star-forming galaxies with i_AB2. Remarkably, we find a well-defined Tully-Fisher relation with up to 60% increase in scatter and stellar mass zero-point shift constraint of 0.02+/-0.02 dex since z~1.7, compared to the local relation. Although our sample is incomplete in terms of either a fixed stellar mass or star formation rate limit, we discuss the implications that typical star-forming disk galaxies evolve to arrive on a well-defined Tully-Fisher relation within a surprisingly short period of cosmic history., Comment: 13 pages, 8 figures, accepted for publication at ApJ

It is often speculated that the dark halos of spiral galaxies are triaxial or prolate, with the long axis pointing in the plane of the disk. This would produce a potential that is elongated in the plane of the disk. The signatures produced by such deviations from axisymmetry are explored. Observable effects in the photometry and kinematics of the stars and gas in the disk are found, if substructure like bars and spiral arms can be ignored. These effects include ellipticity gradients and position angle twists in disks, velocity gradients along the apparent minor axis of the disk, differences between kinematically determined inclinations, and systematic residuals in circular orbits fits to the velocity field

Using mean relative peculiar velocity measurements for pairs of galaxies, we estimate the cosmological density parameter and the amplitude of density fluctuations j8 Qm . Our results suggest that our statistic is a robust and reproducible measure of the mean pairwise velocity and thereby the parameter. We get and 0.17 Q Q m m p 0.300.07 . These estimates do not depend on prior assumptions on the adiabaticity of the initial density 0.22 j8 p 1.130.23 fluctuations, the ionization history, or the values of other cosmological parameters.

Using analytic stability criteria we demonstrate that, because of their low surface mass density and large dark matter content, low surface brightness (LSB) disks are quite stable against the growth of global nonaxisymmetric modes such as bars. However, depending on their (poorly constrained) stellar velocity dispersions, they may be only marginally stable against local instabilities. We simulate a collision between an LSB and a high surface brightness (HSB) galaxy and find that, while the HSB galaxy forms a strong bar, the response of the LSB disk is milder, manifesting weaker rings and spiral features. The lack of sufficient disk self-gravity to amplify dynamical instabilities naturally explains the rarity of bars in LSB disks. The stability of LSB disks may also inhibit interaction-driven gas inflow and starburst activity in these galaxies.