2,391 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
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
Modelling Gravity in N-Body Simulations of Disc Galaxies: Optimal Types of Softening for Given Dynamical Requirements
Modelling gravity is a fundamental problem that must be tackled in N-body
simulations of stellar systems, and satisfactory solutions require a deep
understanding of the dynamical effects of softening. In a previous paper (Romeo
1997), we have devised a method for exploring such effects, and we have focused
on two applications that reveal the dynamical differences between the most
representative types of softened gravity. In the present paper we show that our
method can be applied in another, more fruitful, way: for developing new ideas
about softening. Indeed, it opens a direct route to the discovery of optimal
types of softened gravity for given dynamical requirements, and thus to the
accomplishment of a physically consistent modelling of disc galaxies, even in
the presence of a cold interstellar gaseous component and in situations that
demand anisotropic resolution.Comment: A&A (in press). 7 pages (latex, A&A style), 2 postscript figure
Reply to Melott's Comment on ``Discreteness Effects in Lambda Cold Dark Matter Simulations: A Wavelet-Statistical View'' by Romeo et al
Melott has made pioneering studies of the effects of particle discreteness in
N-body simulations, a fundamental point that needs careful thought and analysis
since all such simulations suffer from numerical noise arising from the use of
finite-mass particles. Melott (arXiv:0804.0589) claims that the conclusions of
our paper (arXiv:0804.0294) are essentially equivalent to those of his earlier
work. Melott is wrong: he has jumped onto one of our conclusions and
interpreted that in his own way. Here we point out the whys and the wherefores
The impact of stellar feedback on the density and velocity structure of the interstellar medium
We study the impact of stellar feedback in shaping the density and velocity
structure of neutral hydrogen (HI) in disc galaxies. For our analysis, we carry
out pc resolution -body+adaptive mesh refinement (AMR)
hydrodynamic simulations of isolated galaxies, set up to mimic a Milky Way
(MW), and a Large and Small Magellanic Cloud (LMC, SMC). We quantify the
density and velocity structure of the interstellar medium using power spectra
and compare the simulated galaxies to observed HI in local spiral galaxies from
THINGS (The HI Nearby Galaxy Survey). Our models with stellar feedback give an
excellent match to the observed THINGS HI density power spectra. We find that
kinetic energy power spectra in feedback regulated galaxies, regardless of
galaxy mass and size, show scalings in excellent agreement with super-sonic
turbulence () on scales below the thickness of the HI
layer. We show that feedback influences the gas density field, and drives gas
turbulence, up to large (kpc) scales. This is in stark contrast to density
fields generated by large scale gravity-only driven turbulence. We conclude
that the neutral gas content of galaxies carries signatures of stellar feedback
on all scales.Comment: 19 pages, 13 figures, 2 tables, accepted for publication in Monthly
Notices of the Royal Astronomical Societ
Angular momentum and local gravitational instability in galaxy discs: does correlate with or ?
We introduce a new diagnostic for exploring the link between angular momentum
and local gravitational instability in galaxy discs. Our diagnostic
incorporates the latest developments in disc instability research, is fully
consistent with approximations that are widely used for measuring the stellar
specific angular momentum, , and is also very
simple. We show that such a disc instability diagnostic hardly correlates with
or , and is remarkably constant across spiral galaxies
of any given type (SaSd), stellar mass
() and velocity
dispersion anisotropy (). The fact
that is tightly correlated with star formation rate
(), molecular gas mass (), metallicity
() and other fundamental galaxy properties thus implies
that nearby star-forming spirals self-regulate to a quasi-universal disc
stability level. This proves the existence of the self-regulation process
postulated by several star formation models, but also raises important caveats.Comment: MNRAS Letters, in press. Minor revision to match the accepted version
(added Fig. 1, Sect. 3.2, the final paragraph of Sect. 4, references and
clarifications
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