3,213 research outputs found
Astrophysical Dynamics 1999/2000: Merging Research and Education
The workshop `Astrophysical Dynamics 1999/2000' followed a homonymous
advanced research course, and both activities were organized by me. In this
opening paper of the proceedings book, I describe them and document their
strong impact on the academic life of the local institutions. The advanced
research course was open to graduate students, senior researchers, and
motivated under-graduate students with good background in physics and
mathematics. The course covered several multi-disciplinary issues of modern
research on astrophysical dynamics, and thus also of interest to physicists,
mathematicians and engineers. The major topic was gas dynamics, viewed in
context with stellar dynamics and plasma physics. The course was complemented
by parallel seminars on hot topics given by experts in such fields, and open to
a wide scientific audience. In particular, I gave a friendly introduction to
wavelets, which are becoming an increasingly powerful tool not only for
processing signals and images but also for analysing fractals and turbulence,
and which promise to have important applications to dynamical modelling of disc
galaxies. The workshop was open to a wide scientific audience. The workshop
with published proceedings book was, as a matter of fact, the innovative form
of exam that I proposed for the advanced research course. The contributions
were refereed and their quality is high on average, exceptionally high in a few
cases. The advanced research course and the workshop all together produced
great enthusiasm in the students and welcomed the participation of a hundred
different people, which means an order of magnitude more than an average
graduate course at Chalmers University of Technology and G\"oteborg University.Comment: opening paper; the proceedings book is in
http://www.oso.chalmers.se/~romeo/PROCEEDINGS_BOOK_
MY LIFE AS TUTOR: Reflections on Two Recent Experiences
In this final report, I briefly reflect on two parallel teaching experiences
as tutor. Besides, I briefly view such experiences in interaction with my
research work, private life and new teaching position. In harmony with my
conception of teaching, I avoid the standard formal style of reports and try an
interactive dialogue with the reader.Comment: 9 pages (tex
Online Permutation Routing in Partitioned Optical Passive Star Networks
This paper establishes the state of the art in both deterministic and
randomized online permutation routing in the POPS network. Indeed, we show that
any permutation can be routed online on a POPS network either with
deterministic slots, or, with high probability, with
randomized slots, where constant
. When , that we claim to be the
"interesting" case, the randomized algorithm is exponentially faster than any
other algorithm in the literature, both deterministic and randomized ones. This
is true in practice as well. Indeed, experiments show that it outperforms its
rivals even starting from as small a network as a POPS(2,2), and the gap grows
exponentially with the size of the network. We can also show that, under proper
hypothesis, no deterministic algorithm can asymptotically match its
performance
Routing Permutations in Partitioned Optical Passive Star Networks
It is shown that a POPS network with g groups and d processors per group can
efficiently route any permutation among the n=dg processors. The number of
slots used is optimal in the worst case, and is at most the double of the
optimum for all permutations p such that p(i)i for all i.Comment: 8 pages, 3 figure
A double molecular disc in the triple-barred starburst galaxy NGC 6946: structure and stability
The late-type spiral galaxy NGC 6946 is a prime example of molecular gas
dynamics driven by "bars within bars". Here we use data from the BIMA SONG and
HERACLES surveys to analyse the structure and stability of its molecular disc.
Our radial profiles exhibit a clear transition at distance R ~ 1 kpc from the
galaxy centre. In particular, the surface density profile breaks at R ~ 0.8 kpc
and is well fitted by a double exponential distribution with scale lengths R_1
~ 200 pc and R_2 ~ 3 kpc, while the 1D velocity dispersion sigma decreases
steeply in the central kpc and is approximately constant at larger radii. The
fact that we derive and use the full radial profile of sigma rather than a
constant value is perhaps the most novel feature of our stability analysis. We
show that the profile of the Q stability parameter traced by CO emission is
remarkably flat and well above unity, while the characteristic instability
wavelength exhibits clear signatures of the nuclear starburst and inner bar
within bar. We also show that CO-dark molecular gas, stars and other factors
can play a significant role in the stability scenario of NGC 6946. Our results
provide strong evidence that gravitational instability, radial inflow and disc
heating have driven the formation of the inner structures and the dynamics of
molecular gas in the central kpc.Comment: MNRAS, in pres
A simple and accurate approximation for the Q stability parameter in multi-component and realistically thick discs
In this paper, we propose a Q stability parameter that is more realistic than
those commonly used, and is easy to evaluate [see Eq. (19)]. Using our Q_N
parameter, you can take into account several stellar and/or gaseous components
as well as the stabilizing effect of disc thickness, you can predict which
component dominates the local stability level, and you can do all that simply
and accurately. To illustrate the strength of Q_N, we analyse the stability of
a large sample of spirals from The HI Nearby Galaxy Survey (THINGS), treating
stars, HI and H_2 as three distinct components. Our analysis shows that H_2
plays a significant role in disc (in)stability even at distances as large as
half the optical radius. This is an important aspect of the problem, which was
missed by previous (two-component) analyses of THINGS spirals. We also show
that HI plays a negligible role up to the edge of the optical disc; and that
the stability level of THINGS spirals is, on average, remarkably flat and well
above unity.Comment: MNRAS, in pres
Chemodynamic evolution of dwarf galaxies in tidal fields
The mass-metallicity relation shows that the galaxies with the lowest mass
have the lowest metallicities. As most dwarf galaxies are in group
environments, interaction effects such as tides could contribute to this trend.
We perform a series of smoothed particle hydrodynamics (SPH) simulations of
dwarf galaxies in external tidal fields to examine the effects of tides on
their metallicities and metallicity gradients. In our simulated galaxies,
gravitational instabilities drive gas inwards and produce centralized star
formation and a significant metallicity gradient. Strong tides can contribute
to these instabilities, but their primary effect is to strip the outer
low-metallicity gas, producing a truncated gas disk with a large metallicity.
This suggests that the role of tides on the mass-metallicity relation is to
move dwarf galaxies to higher metallicities.Comment: Accepted to Ap
Characterizing gravitational instability in turbulent multi-component galactic discs
Gravitational instabilities play an important role in galaxy evolution and in
shaping the interstellar medium (ISM). The ISM is observed to be highly
turbulent, meaning that observables like the gas surface density and velocity
dispersion depend on the size of the region over which they are measured. In
this work we investigate, using simulations of Milky Way-like disc galaxies
with a resolution of pc, the nature of turbulence in the ISM and how
this affects the gravitational stability of galaxies. By accounting for the
measured average turbulent scalings of the density and velocity fields in the
stability analysis, we can more robustly characterize the average level of
stability of the galaxies as a function of scale, and in a straightforward
manner identify scales prone to fragmentation. Furthermore, we find that the
stability of a disc with feedback-driven turbulence can be well described by a
"Toomre-like" stability criterion on all scales, whereas the classical
can formally lose its meaning on small scales if violent disc instabilities
occur in models lacking pressure support from stellar feedback.Comment: 11 pages, 5 figures, submitted to MNRA
Weak lensing of the Lyman-alpha forest
The angular positions of quasars are deflected by the gravitational lensing
effect of foreground matter. The Lyman-alpha forest seen in the spectra of
these quasars is therefore also lensed. We propose that the signature of weak
gravitational lensing of the forest could be measured using similar techniques
that have been applied to the lensed Cosmic Microwave Background, and which
have also been proposed for application to spectral data from 21cm radio
telescopes. As with 21cm data, the forest has the advantage of spectral
information, potentially yielding many lensed "slices" at different redshifts.
We perform an illustrative idealized test, generating a high resolution angular
grid of quasars (of order arcminute separation), and lensing the
Lyman-alphaforest spectra at redshifts z=2-3 using a foreground density field.
We find that standard quadratic estimators can be used to reconstruct images of
the foreground mass distribution at z~1. There currently exists a wealth of Lya
forest data from quasar and galaxy spectral surveys, with smaller sightline
separations expected in the future. Lyman-alpha forest lensing is sensitive to
the foreground mass distribution at redshifts intermediate between CMB lensing
and galaxy shear, and avoids the difficulties of shape measurement associated
with the latter. With further refinement and application of mass reconstruction
techniques, weak gravitational lensing of the high redshift Lya forest may
become a useful new cosmological probe.Comment: 9 pages, 7 figures, submitted to MNRA
- âŠ