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 $\sim 9$ 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" $Q$ stability criterion on all scales, whereas the classical $Q$ 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

Comparative Saving Behavior of Rural and Urban Households in the Philippines

Marginal saving rates for the Philippine households are estimated, distinguishing between rural and urban households, by region and by income group. At a given income level, rural households generally save more, both on the average and on the margin. This contrasts with the higher savings rate for urban households in various regions, attributable for their higher incomes. Marginal savings rate for rural households are higher than for their urban counterparts. Faster growth of rural income need not result in lower aggregate savings.agriculture sector, income, rural sector, savings

Comparative Saving Behavior of Rural and Urban Households in the Philippines

Marginal saving rates for the Philippine households are estimated, distinguishing between rural and urban households, by region and by income group. At a given income level, rural households generally save more, both on the average and on the margin. This contrasts with the higher savings rate for urban households in various regions, attributable for their higher incomes. Marginal savings rate for rural households are higher than for their urban counterparts. Faster growth of rural income need not result in lower aggregate savings.agriculture sector, income, rural sector, savings

N-body simulations with two-orders-of-magnitude higher performance using wavelets

Noise is a problem of major concern for N-body simulations of structure formation in the early Universe, of galaxies and plasmas. Here for the first time we use wavelets to remove noise from N-body simulations of disc galaxies, and show that they become equivalent to simulations with two orders of magnitude more particles. We expect a comparable improvement in performance for cosmological and plasma simulations. Our wavelet code will be described in a following paper, and will then be available on request.Comment: Mon. Not. R. Astron. Soc., in press. The interested reader is strongly recommended to ignore the low-resolution Fig. 3 (and Fig. 4), and to download the full-resolution paper (700 kb) from http://www.oso.chalmers.se/~romeo/Paper_VI.ps.g