Bars and spirals in tidal interactions with an ensemble of galaxy mass models

We present simulations of the gaseous and stellar material in several different galaxy mass models under the influence of different tidal fly-bys to assess the changes in their bar and spiral morphology. Five different mass models are chosen to represent the variety of rotation curves seen in nature. We find a multitude of different spiral and bar structures can be created, with their properties dependent on the strength of the interaction. We calculate pattern speeds, spiral wind-up rates, bar lengths, and angular momentum exchange to quantify the changes in disc morphology in each scenario. The wind-up rates of the tidal spirals follow the 2:1 resonance very closely for the flat and dark matter dominated rotation curves, whereas the more baryon dominated curves tend to wind-up faster, influenced by their inner bars. Clear spurs are seen in most of the tidal spirals, most noticeable in the flat rotation curve models. Bars formed both in isolation and interactions agree well with those seen in real galaxies, with a mixture of "fast" and "slow" rotators. We find no strong correlation between bar length or pattern speed and the interaction strength. Bar formation is, however, accelerated/induced in four out of five of our models. We close by briefly comparing the morphology of our models to real galaxies, easily finding analogues for nearly all simulations presenter here, showing passages of small companions can easily reproduce an ensemble of observed morphologies.Comment: 30 pages, 29 colour figures, accepted for publication in MNRAS. Videos of simulations can be found at http://www.youtube.com/playlist?list=PLQKy--XcWrIVBc1sS2RNc-ekyfeBsGtD

Gasoline2: A Modern SPH Code

The methods in the Gasoline2 Smoothed Particle Hydrodynamics (SPH) code are described and tested. Gasoline2 is the most recent version of the Gasoline code for parallel hydrodynamics and gravity with identical hydrodynamics to the Changa code. As with other Modern SPH codes, we prevent sharp jumps in time steps, use upgraded kernels and larger neighbour numbers and employ local viscosity limiters. Unique features in Gasoline2 include its Geometric-Density-Average Force expression, explicit Turbulent Diffusion terms and Gradient-Based shock detection to limit artificial viscosity. This last feature allows Gasoline2 to completely avoid artificial viscosity in non-shocking compressive flows. We present a suite of tests demonstrating the value of these features with the same code configuration and parameter choices used for production simulations.Comment: 15 pages, 12 figures, Accepted to MNRA

Gas and stellar spiral structures in tidally perturbed disc galaxies

Tidal interactions between disc galaxies and low mass companions are an established method for generating galactic spiral features. In this work we present a study of the structure and dynamics of spiral arms driven in interactions between disc galaxies and perturbing companions in 3-D N-body/smoothed hydrodynamical numerical simulations. Our specific aims are to characterize any differences between structures formed in the gas and stars from a purely hydrodynamical and gravitational perspective, and to find a limiting case for spiral structure generation. Through analysis of a number of different interacting cases, we find that there is very little difference between arm morphology, pitch angles and pattern speeds between the two media. The main differences are a minor offset between gas and stellar arms, clear spurring features in gaseous arms, and different radial migration of material in the stronger interacting cases. We investigate the minimum mass of a companion required to drive spiral structure in a galactic disc, finding the limiting spiral generation cases with companion masses of the order $1\times10^9M_\odot$, equivalent to only 4% of the stellar disc mass, or 0.5% of the total galactic mass of a Milky Way analogue.Comment: 20 pages, 23 figures, accepted for publication by MNRA

The Full Re-Ionization of Helium

Observations of resolved HeII Lyman alpha absorption in spectra of two QSO's suggest that the epoch of helium ionization occurred at z~3. Proximity zones in the spectra of the quasars (z=3.18, 3.285) at 304 A resemble Stromgren spheres, suggesting that the intergalactic medium is only singly ionized in helium. We present models of the proximity effect which include the full physics of the ionization, heating and cooling and an accurately simulated inhomogeneous gas distribution. In these models the underdense intergalactic medium is heated to at least 10,000-20,000 K after cooling to as low as a few 1000 K due to cosmological expansion, with higher temperatures achieved farther away from the quasar due to absorption-hardened ionizing spectra. The quasars turn on for a few times 10^7 years with a fairly steady flux output at 228 A comparable to the 304 A flux output directly observed with HST. The recoveries in the spectra occur naturally due to voids in the IGM and may provide a fairly model-independent probe of the baryon density.Comment: 5 pages, 3 figures, to appear in the proceedings of "After the Dark Ages: When Galaxies were Young (the Universe at 2<z<5)", 9th Annual October Astrophysics Conference in Marylan

Star formation and ISM morphology in tidally induced spiral structures

Tidal encounters are believed to be one of the key drivers of galactic spiral structure in the Universe. Such spirals are expected to produce different morphological and kinematic features compared to density wave and dynamic spiral arms. In this work we present high resolution simulations of a tidal encounter of a small mass companion with a disc galaxy. Included are the effects of gas cooling and heating, star formation and stellar feedback. The structure of the perturbed disc differs greatly from the isolated galaxy, showing clear spiral features that act as sites of new star formation, and displaying interarm spurs. The two arms of the galaxy, the bridge and tail, appear to behave differently; with different star formation histories and structure. Specific attention is focused on offsets between gas and stellar spiral features which can be directly compared to observations. We find some offsets do exist between different media, with gaseous arms appearing mostly on the convex side of the stellar arms, though the exact locations appear highly time dependent. These results further highlight the differences between tidal spirals and other theories of arm structure.Comment: 17 pages, 19 colour figures, accepted for publication in MNRA

Proto-Brown Dwarf Disks as Products of Protostellar Disk Encounters

The formation of brown dwarfs via encounters between proto-stars has been confirmed with high-resolution numerical simulations with a restricted treatment of the thermal conditions. The new results indicate that young brown dwarfs (BDs) formed this way are disk-like and often reside in multiple systems. The newly-formed proto-BDs disks are up to 18 AU in size and spin rapidly making small-scale bipolar outflows, fragmentation and the possible formation of planetary companions likely as have recently been observed for BDs. The object masses range from 2 to 73 Jupiter masses, distributed in a manner consistent with the observed sub-stellar initial mass function. The simulations usually form multiple BDs on eccentric orbits about a star. One such system was hierarchical, a BD binary in orbit around a star, which may explain recently observed hierarchical systems. One third of the BDs were unbound after a few thousand years and interactions among orbiting BDs may eject more or add to the number of binaries. Improvements over prior work include resolution down to a Jupiter mass, self-consistent models of the vertical structure of the initial disks and careful attention to avoid artificial fragmentation.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter

Tidal stirring and the origin of dwarf spheroidals in the Local Group

N-Body/SPH simulations are used to study the evolution of dwarf irregular galaxies (dIrrs) entering the dark matter halo of the Milky Way or M31 on plunging orbits. We propose a new dynamical mechanism driving the evolution of gas rich, rotationally supported dIrrs, mostly found at the outskirts of the Local Group (LG), into gas free, pressure supported dwarf spheroidals (dSphs) or dwarf ellipticals (dEs), observed to cluster around the two giant spirals. The initial model galaxies are exponential disks embedded in massive dark matter halos and reproduce nearby dIrrs. Repeated tidal shocks at the pericenter of their orbit partially strip their halo and disk and trigger dynamical instabilities that dramatically reshape their stellar component. After only 2-3 orbits low surface brightness (LSB) dIrrs are transformed into dSphs, while high surface brightness (HSB) dIrrs evolve into dEs. This evolutionary mechanism naturally leads to the morphology-density relation observed for LG dwarfs. Dwarfs surrounded by very dense dark matter halos, like the archetypical dIrr GR8, are turned into Draco or Ursa Minor, the faintest and most dark matter dominated among LG dSphs. If disks include a gaseous component, this is both tidally stripped and consumed in periodic bursts of star formation. The resulting star formation histories are in good qualitative agreement with those derived using HST color-magnitude diagrams for local dSphs.Comment: 5 pages, 5 figures, to appear on ApJL. Simulation images and movies can be found at the Local Group web page at http://pcblu.uni.mi.astro.it/~lucio/LG/LG.htm