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

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

The Science Performance of JWST as Characterized in Commissioning

This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.Comment: 5th version as accepted to PASP; 31 pages, 18 figures; https://iopscience.iop.org/article/10.1088/1538-3873/acb29