Galaxy Evolution Explorer observations of IC 3418, a low surface brightness galaxy in the Virgo Cluster, revealed a striking 17 kpc UV tail of bright knots and diffuse emission. Hα imaging confirms that star formation is ongoing in the tail. IC 3418 was likely recently ram pressure stripped on its first pass through Virgo. We suggest that star formation is occurring in molecular clouds that formed in IC 3418's turbulent stripped wake. Tides and ram pressure stripping (RPS) of molecular clouds are both disfavored as tail formation mechanisms. The tail is similar to the few other observed starforming tails, all of which likely formed during RPS. The tails' morphologies reflect the forces present during their formation and can be used to test for dynamical coupling between molecular and diffuse gas, thereby probing the origin of the star forming molecular gas.

Ram pressure stripping is an important process in the evolution of both dwarf galaxies and large spirals. Large spirals are severely stripped in rich clusters and may be mildly stripped in groups. Dwarf galaxies can be severely stripped in both clusters and groups. A model is developed that describes the stripping of a satellite galaxy's outer H \textsc{i} disk and hot galactic halo. The model can be applied to a wide range of environments and satellite galaxy masses. Whether ram pressure stripping of the outer disk or hot galactic halo occurs is found to depend primarily on the ratio of the satellite galaxy mass to the mass of the host group or cluster. How the effectiveness of ram pressure stripping depends on the density of the inter-group gas, the dark matter halo concentrations, and the scale lengths and masses of the satellite components is explored. The predictions of the model are shown to be well matched to H \textsc{i} observations of spirals in a sample of nearby clusters. The model is used to predict the range of H \textsc{i} gas fractions a satellite of mass $M_{v,sat}$ can lose orbiting in a cluster of mass $M_{v,gr}$.Comment: 13 Pages, 3 figure

We calculate expressions for the quadrupole moments of nonstrange baryons in which the number of QCD color charges is N c . Using only the assumption of single-photon exchange, we obtain 4 relations among the 6 moments, and show how all of them may be obtained from Q ∆ + p up to O(1/N 2 c ) corrections. We compare to the N c = 3 case, and obtain relations between the neutron charge radius and quadrupole moments. We also discuss prospects for the measurement of these moments.

This paper presents a study of the specific major merger rate as a function of group membership, local environment, and redshift in a very large, 500 h −1 Mpc, cosmological N-body simulation, the Millennium Simulation. The goal is to provide environmental diagnostics of major merger populations in order to test simulations against observations and provide further constraints on major merger driven galaxy evolution scenarios. A halo sample is defined using the maximum circular velocity, which is both well defined for subhalos and closely correlated with galaxy luminosity. Subhalos, including the precursors of major mergers, are severely tidally stripped. Major mergers between subhalos are therefore rare compared to mergers between subhalos and their host halos. Tidal stripping also suppresses dynamical friction, resulting in long major merger timescales when the more massive merger progenitor does not host other subhalos. When other subhalos are present, however, major merger timescales are several times shorter. This enhancement may be due to inelastic unbound collisions between subhalos, which deplete their orbital angular momentum and lead to faster orbital decay. Following these results, we predict that major mergers in group environments are dominated by mergers involving the central galaxy, that the specific major merger rate is suppressed in groups when all group members are considered together, and that the frequency of fainter companions is enhanced for major mergers and their remnants. We also measure an "assembly bias" in the specific major merger rate in that major mergers of galaxy-like halos are slightly suppressed in overdense environments while major mergers of group-like halos are slightly enhanced. A dynamical explanation for this trend is advanced which calls on both tidal effects and interactions between bound halos beyond the virial radii of locally dynamically dominant halos.

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