Jeans Instability of Palomar 5's Tidal Tail

Tidal tails composed of stars should be unstable to the Jeans instability and this can cause them to look like beads on a string. The Jeans wavelength and tail diameter determine the wavelength and growth rate of the fastest growing unstable mode. Consequently the distance along the tail to the first clump and spacing between clumps can be used to estimate the mass density in the tail and its longitudinal velocity dispersion. Clumps in the tidal tails of the globular cluster Palomar 5 could be due to Jeans instability. We find that their spacing is consistent with the fastest growing mode if the velocity dispersion in the tail is similar to that in the cluster itself. While all tidal tails should exhibit gravitational instability, we find that clusters or galaxies with low concentration parameters are most likely to exhibit short wavelength rapidly growing Jeans modes in their tidal tails.Comment: sumbmitted to MNRA

Barriers to Action Learning in the United States

This study investigates the contribution barriers to Action Learning play in limiting its use in United States organizations. Reviewing literature on current practices identifies barriers to Action Learning. Seven hypotheses are tested to determine if there is a significant difference between the practices used in US Action Learning programs versus similar programs in non-US organizations. Results of a survey administered to a sample of Action Learning practitioners did not substantiate any of the seven hypotheses

Jeans Instability in a Tidally Disrupted Halo Satellite Galaxy

We use a hybrid test particle/N-body simulation to integrate 4 million massless test particle trajectories within a fully self-consistent 10^5 particle N-body simulation. The number of massless particles allows us to resolve fine structure in the spatial distribution and phase space of a dwarf galaxy as it is disrupted in the tidal field of a Milky Way type galaxy. The tidal tails exhibit nearly periodic clumping or a smoke-like appearance. By running simulations with different satellite particle mass, halo particle mass, number of massive and massless particles and with and without a galaxy disk, we have determined that the instabilities are not due to numerical noise, amplification of structure in the halo, or shocking as the satellite passes through the disk of the Galaxy. We measure Jeans wavelengths and growth timescales in the tidal tail and show that the Jeans instability is a viable explanation for the clumps. We find that the instability causes velocity perturbations of order 10 km/s. Clumps in tidal tails present in the Milky Way could be seen in stellar radial velocity surveys as well as number counts. We find that the unstable wavelength growth is sensitive to the simulated mass of dark matter halo particles. A simulation with a smoother halo exhibits colder and thinner tidal tails with more closely spaced clumps than a simulation with more massive dark matter halo particles. Heating by the halo particles increases the Jeans wavelength in the tidal tail affecting substructure development, suggesting an intricate connection between tidal tails and dark matter halo substructure.Comment: 15 pages, 7 figures, submitted to MNRAS, May 25 201

Galactic spiral patterns and dynamo action I: A new twist on magnetic arms

We generalise the theory of mean-field galactic dynamos by allowing for temporal non-locality in the mean electromotive force (emf). This arises in random flows due to a finite response time of the mean emf to changes in the mean magnetic field, and leads to the telegraph equation for the mean field. The resulting dynamo model also includes the nonlinear dynamo effects arising from magnetic helicity balance. Within this framework, coherent large-scale magnetic spiral arms superimposed on the dominant axially symmetric magnetic structure are considered. A non-axisymmetric forcing of the mean-field dynamo by a spiral pattern (either stationary or transient) is invoked, with the aim of explaining the phenomenon of magnetic arms. For a stationary dynamo forcing by a rigidly rotating material spiral, we find corotating non-axisymmetric magnetic modes enslaved to the axisymmetric modes and strongly peaked around the corotation radius. For a forcing by transient material arms wound up by the galactic differential rotation, the magnetic spiral is able to adjust to the winding so that it resembles the material spiral at all times. There are profound effects associated with the temporal non-locality, i.e. finite `dynamo relaxation time'. For the case of a rigidly rotating spiral, a finite relaxation time causes each magnetic arm to mostly lag the corresponding material arm with respect to the rotation. For a transient material spiral that winds up, the finite dynamo relaxation time leads to a large, negative (in the sense of the rotation) phase shift between the magnetic and material arms, similar to that observed in NGC 6946 and other galaxies. We confirm that sufficiently strong random seed fields can lead to global reversals of the regular field along the radius whose long-term survival depends on specific features of a given galaxy.Comment: Version accepted for publication in MNRAS. 23 pages, 19 figure

Structure in phase space associated with spiral and bar density waves in an N-body galactic disk

An N-body hybrid simulation, integrating both massive and tracer particles, of a Galactic disk is used to study the stellar phase space distribution or velocity distributions in different local neighborhoods. Pattern speeds identified in Fourier spectrograms suggest that two-armed and three-armed spiral density waves, a bar and a lopsided motion are coupled in this simulation, with resonances of one pattern lying near resonances of other patterns. We construct radial and tangential (uv) velocity distributions from particles in different local neighborhoods. More than one clump is common in these local velocity distributions regardless of the position in the disk. Features in the velocity distribution observed at one galactic radius are also seen in nearby neighborhoods (at larger and smaller radii) but with shifted mean v values. This is expected if the v velocity component of a clump sets the mean orbital galactic radius of its stars. We find that gaps in the velocity distribution are associated with the radii of kinks or discontinuities in the spiral arms. These gaps also seem to be associated with Lindblad resonances with spiral density waves and so denote boundaries between different dominant patterns in the disk. We discuss implications for interpretations of the Milky Way disk based on local velocity distributions. Velocity distributions created from regions just outside the bar's Outer Lindblad resonance and with the bar oriented at 45 degrees from the Sun-Galactic center line more closely resemble that seen in the solar neighborhood (triangular in shape at lower uv and with a Hercules like stream) when there is a strong nearby spiral arm, consistent with the observed Centaurus Arm tangent, just interior to the solar neighborhood.Comment: accepted for publication in MNRA

Thymoma and paraneoplastic myasthenia gravis in a cat

Ranger, a five year old male neutered Domestic Shorthair cat was presented to the Cornell University Hospital for Animals (CUHA) for evaluation of lethargy, weakness, and inappetance. Additional historical findings included dysphonia, dysphagia, and regurgitation. A physical examination disclosed diffuse neuromuscular weakness and seborrhea sicca. Intravenous edrophonium was administered, and a marked increase in neuromuscular strength was noted. An acetylcholine receptor antibody titer confirmed a diagnosis of myasthenia gravis. Thoracic radiography, thoracic ultrasound, and computed tomography disclosed a cranial mediastinal mass. A median sternotomy was performed and the mass was excised, with histopathology yielding a diagnosis of thymoma. Pyridostigmine was administered for 2.5 months, and Ranger maintained a significant improvement in neuromuscular strength. However, at serial re-evaluations, exfoliative dermatitis, pemphigus dermatitis, and pleural effusion were documented. The exfoliative dermatitis and pemphigus dermatitis completely resolved and the pleural fluid partially resolved in response to discontinuation of pyridostigmine administration and initiation of prednisolone administration. The pathophysiology and treatment of thymoma and paraneoplastic myasthenia gravis and the associated complications are discussed in the following presentation