51 research outputs found
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
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
, 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
Spectral reconstruction for radiation hydrodynamic simulations of galaxy evolution
Radiation from stars and AGN plays an important role in galaxy formation and
evolution, and profoundly transforms the IGM, CGM & ISM. On-the-fly RT has
started being incorporated in cosmological simulations, but the complex,
evolving radiation spectra are often crudely approximated with a small number
of broad bands with piece-wise constant intensity and a fixed photo-ionisation
cross-section. Such a treatment is unable to capture the changes to the
spectrum as light is absorbed while it propagates through a medium with
non-zero opacity. This can lead to large errors in photo-ionisation and heating
rates. We present a novel approach of discretising the radiation field in
narrow bands, located at the edges of the typically used bands, in order to
capture the power-law slope of the radiation field. In combination with
power-law approximations for the photo-ionisation cross-sections, this model
allows us to self-consistently combine radiation from sources with different
spectra and accurately follow the ionisation states of primordial and metal
species through time. The method is implemented in Gasoline2 in connection with
Trevr2. We compare our new piece-wise power-law reconstruction to the
piece-wise constant method in calculating the primordial chemistry
photo-ionisation and heating rates under an evolving UVB and stellar spectrum,
and find that our method reduces errors significantly, up to two orders of
magnitude in the case of HeII ionisation. We apply our new spectral
reconstruction method in RT post-processing of a cosmological zoom-in
simulation, including radiation from stars and a live UVB, and find a
significant increase in total neutral hydrogen mass in the ISM and the CGM due
to shielding of the UVB and a low escape fraction of the stellar radiation.
This demonstrates the importance of RT and an accurate spectral approximation
in simulating the CGM-galaxy ecosystem.Comment: Submitted for publication to A&A, 17 pages, 15 figure
TREVR2: Illuminating fast radiative transfer
We present TREVR2 (Tree-based REVerse Ray Tracing 2), a fast, general
algorithm for computing the radiation field, suitable for both particle and
mesh codes. It is designed to self-consistently evolve chemistry for zoomed-in
astrophysical simulations, such as cosmological galaxies with both internal
sources and prescribed background radiation, rather than large periodic
volumes. Light is propagated until absorbed, with no imposed speed limit other
than those due to opacity changes (e.g. ionization fronts). TREVR2 searches
outward from receiving gas in discrete directions set by the HEALPIX algorithm
(unlike its slower predecessor TREVR), accumulating optical depth and adding
the flux due to sources combined into progressively larger tree cells with
distance. We demonstrate execution time with
absorption and many sources. This allows multi-band RT costs comparable to
tree-based gravity and hydrodynamics, and the usual speed-up when active
particles evolve on individual timesteps. Sources embedded in non-homogeneous
absorbing material introduce systematic errors. We introduce transmission
averaging instead of absorption averaging which dramatically reduces these
systematic effects. We outline other ways to address systematics including an
explicit complex source model. We demonstrate the overall performance of the
method via a set of astrophysical test problems.Comment: Submitted to MNRA
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
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