Daniel Pfenniger

University of Geneva | UNIGE · Department of Astronomy

Aims. New or refined methods for determining instantaneous scalar and vector pattern speeds from a restricted domain are developed, for applications in N-body simulations or in galaxies, for which the stellar coordinates become increasingly better known. Methods. The general feature used throughout follows from the fact that the time-derivative of...

We analyze the origin of the buckling instability in stellar bars using high-resolution N-body simulations. Previous studies have promoted the nonresonant firehose instability to be responsible for the vertical buckling. We have analyzed the buckling process following the resonant excitation of stellar orbits in the bar. We find that (1) the buckli...

We use high resolution numerical simulations in order to analyze the stellar bar evolution in spinning dark matter (DM) halos. Previous works have shown that the halo spin has a substantial effect on the bar evolution and can lead to bar dissolution following the vertical buckling instability. Here, we invoke the DM spin sequence, $\lambda=0-0.09$,...

The buckling process in stellar bars is full of unsolved issues. We analyze the origin of the buckling instability in stellar bars using high-resolution N-body simulations. Previous studies have promoted the nonresonant firehose instability to be responsible for the vertical buckling. We have analyzed the buckling process in terms of the resonant e...

The cold molecular gas in contemporary galaxies is structured in discrete cloud complexes. These giant molecular clouds (GMCs), with 10⁴–10⁷ solar masses (M⊙) and radii of 5–100 parsecs, are the seeds of star formation¹. Highlighting the molecular gas structure at such small scales in distant galaxies is observationally challenging. Only a handful...

Four expressions involving sums of position and velocity coordinates bounding the total angular momentum of particle systems, and by extension of any continuous or discontinuous material systems, are derived which are tighter for any particle configuration than similar inequalities derived by Sundman (Acta Math 36:105, 1913), Saari (Conference boar...

Four expressions involving sums of position and velocity coordinates bounding the total angular momentum of particle systems, and by extension of any continuous or discontinuous material systems, are derived which are tighter for any particle configuration than similar inequalities derived by Sundman [16], Saari [14], and Scheeres [15]. Eight disti...

The cold molecular gas in contemporary galaxies is structured in discrete cloud complexes. These giant molecular clouds (GMCs), with $10^4$-$10^7$ solar masses and radii of 5-100 parsecs, are the seeds of star formation. Highlighting the molecular gas structure at such small scales in distant galaxies is observationally challenging. Only a handful...

Based on a high-quality N-body simulation of a double-bar galaxy model, we investigate the evolution of the bar properties, including their size, strength, and instantaneous pattern speed derived by using three distinct methods: the Fourier, Jacobi integral, and moment of inertia methods. The interaction of the two bars, which rotate at distinct sp...

Based on a high quality $N$-body simulation of a double bar galaxy model, we investigate the evolution of the bar properties, including their size, strength and instantaneous pattern speed derived by using three distinct methods: the Fourier, Jacobi integral, and moment of inertia methods. The interaction of the two bars, which rotate at distinct s...

Context. Condensation of H 2 in the interstellar medium (ISM) has long been seen as a possibility, either by deposition on dust grains or thanks to a phase transition combined with self-gravity. H 2 condensation might explain the observed low efficiency of star formation and might help to hide baryons in spiral galaxies. Aims. Our aim is to quantif...

Improving the astrometric accuracy by one or two orders of magnitude over ground-based techniques will not only change our raw knowledge about the Galaxy, but it will also modify 1) the fundamental questions that can be addressed, and 2) the stellar dynamical concepts used so far. More detail in Galactic structure, such as the shape and flow in its...

Two mechanisms involving purely dynamical processes can lead to the formation of a bulge after its disc: 1) small bulges (1 – 2 kpc), including box-shaped bulges and mildly triaxial bulges, can result from the formation and destruction of a bar; 2) big bulges (> 2kpc) à la Sombrero can grow following the accretion of small satellites. Fully consist...

A preliminary step towards the construction of self-consistent models for barred galaxies consists in understanding the stellar orbital behaviour in a given axisymmetrical + bar-like potential and, in particular, the influence of various parameters characterizing the bar on the different kinds of possible motions. To do this we undertook a systemat...

We examine the general instability at large amplitude of the radial periodic orbits along the rotation axis of bulges, spheroids and other rotating triaxial ellipsoidal systems.

We measure quantitatively the growth of stochasticity due to various perturbations to a model of elliptical galaxy. This is achieved by computing the Liapunov characteristic exponents of randomly selected orbits.

The effective potential neighboring the corotation resonance region in barred galaxies is shown to be strongly time-dependent in any rotating frame because of the competition of nearby perturbations of similar strengths with differing rotation speeds. Contrary to the generally adopted assumption, that in the bar rotating frame the corotation region...

In search for a synthetic understanding, a scenario for the evolution of the star formation rate and the chemical abundances in galaxies is proposed, combining gas infall from galactic halos, outflow of gas by supernova explosions, and an oscillatory star formation process. The oscillatory star formation model is a consequence of the modelling of t...

Molecular clouds consist typically of 3/4 H2, 1/4 He and traces of heavier elements. In an earlier work we showed that at very low temperatures and high densities, H2 can be in a phase transition leading to the formation of ice clumps as large as comets, or even planets. However, He has very different chemical properties and no phase transition is...

The damping on the fluctuation spectrum and the presence of thermal velocities as properties of warm dark matter particles like sterile neutrinos imprint a distinct signature found from the structure formation mechanisms to the internal structures of halos. Using warm dark matter simulations we explore these effects on the structure formation for d...

The existence of sub-stellar cold H2 globules in planetary nebulae and the mere existence of comets suggest that the physics of cold interstellar gas might be much richer than usually envisioned. We study the case of a cold gaseous medium in ISM conditions which is subject to a gas-liquid/solid phase transition. First the equilibrium of such fluids...

Gravity-bound isolated systems, from stars, planetary systems, star clusters to galaxies, share common properties where evolution is the rule. Typically if they start forming at a well defined epoch they tend to change significantly over a timescale comparable to their present age. So evolution is never truly stopped, it just proceeds slower and sl...

The talk consisted mainly in commenting in a linear way the seminal paper in 1964 by Michel Henon and graduate student Carl Heiles at Princeton University: "The applicability of the third integral of motion: Some numerical experiments" in the field of galactic dynamics. Instead of repeating here the lecture of the paper, we advise the reader intere...

Results from the UV satellite GALEX revealed large extensions of disks in some nearby spiral galaxies, extending out to 3 to 4 times the isophotal radius, r25. M63 is a remarkable example of a spiral galaxy with one of the most extended UV disks, so it offers the opportunity to search for the molecular gas and characterize the star formation in out...

We present preliminary results of the temporal variability of the interstellar extinction in the line of sight of NGC 4833. We detected 10 stars which could vary according to a variation of the reddening.

We analyze the correlations in the cosmological cosmic microwave background radiation and those of point sources, i.e. galaxies and quasars. The Wilkinson Microwave Anisotropy Probe 7-year data are used along with the method developed by Tegmark et al. for obtaining of foreground reduced maps, combined with the Kolmogorov parameter. We obtain the c...

The structure and evolution of the stellar velocity ellipsoid plays an important role in shaping galaxies undergoing bar driven secular evolution and the eventual formation of a boxy/peanut bulge such as present in the Milky Way. Using collisionless N-body simulations, we show that during the formation of such a boxy/peanut bulge, the meridional sh...

We use the Wilkinson Microwave Anisotropy Probe 7-year data (WMAP7) to further probe point source detection technique in the sky maps of the cosmic microwave background (CMB) radiation. The method by Tegmark et al. for foreground reduced maps and the Kolmogorov parameter as the descriptor are adopted for the analysis of WMAP satellite CMB temperatu...

We characterize the radial migration of stars in the disk plane by calculating the diffusion coefficient and the diffusion time-scale for a bulge-disk N-body self-consistent system with a marginally-stable Safronov-Toomre Q_T. We find that diffusion is not constant in time, but follows the evolution of the bar, and becomes maximum near the corotati...

We characterize empirically the radial diffusion of stars in the plane of a typical barred disk galaxy by calculating the local spatial diffusion coefficient and diffusion time-scale for bulge-disk-halo N-body self-consistent systems which initially differ in the Safronov-Toomre-Q_T parameter. We find different diffusion scenarios that depend on th...

As data collection and computers expand their capabilities, modelling changes its characteristics. Not only the purpose of modelling becomes multiple, but thresholds are reached beyond which the best suited methods change. In view of the expected impact of the GAIA mission, the possibilities of modelling the Milky Way and spiral galaxy dynamics ove...

One of the main objectives for a scientist is to sort out the complications of nature and to find out ways to describe its mechanisms with simple comprehensible terms. Disk galaxies belong certainly to the class of complex systems which, however, display regular patterns and correlations across their variety of morphologies. On the one hand, one ne...

The concept of isolated galaxy is useful to sort out the intrinsic and extrinsic factors playing a role in the galaxy structure and evolution. Using the Lagrange-Jacoby identity one can order the global energies and powers at play in galaxies determining their equilibrium, respectively their evolution. Some factors appear then obviously as secondar...

We analyse the radial and tangential velocity fields in the Galaxy as seen from the Sun by using as a first approximation a simple axisymmetric model, which we then compare with the corresponding fields in a barred N-body model of the Milky Way. This provides a global description of these quantities missing in the literature, showing where they tak...

The near proportionality between HI and dark matter in outer galactic disks prompted us to run N-body simulations of galactic disks in which the observed gas content is supplemented by a dark gas component representing between zero and five times the visible gas content. While adding baryons in the disk of galaxies may solve some issues, it poses t...

We study N-body models of disk galaxies which develop long-range correlations during the formation of a central bar (a global macro-scale instability) and we analyze systematically how the strength of the bar correlates with the micro-scale chaos by following orbital exponential sensitivity indicators.

The current practices of modeling the dynamics of galaxies can be projected for the next decade. The exponential growth of computer capabilities has reached a threshold where the particle representation is a superior way to capture subtleties of galactic dynamics than Boltzmann's perfect phase-space fluid representation. The up-to-now often neglect...

The equation of state and the stability of the helium-molecular hydrogen mixture at cryogenic temperature up to moderate pressure are studied by means of current molecular physics methods and statistical mechanics perturbation theory. The phase separation, segregation and hetero-coordination are investigated by calculating the Gibbs energy dependin...

After general considerations about limits of theories and models, where small changes may imply large effects, we discuss three cases in galactic astrophysics illustrating how galactic dynamics models may become insufficient when previously neglected effects are taken into account: 1) Like in 3D hydrodynamics, the non-linearity of the Poisson-Boltz...

$\Lambda$CDM numerical simulations predict that the "missing baryons" reside in a Warm-Hot gas phase in the over-dense cosmic filaments. However, there are now several theoretical and observational arguments that support the fact that galactic disks may be more massive than usually thought, containing a substantial fraction of the "missing baryons"...

After decades of investigations, the interstellar medium (ISM) still appears as extraordinarily complex. Therefore, as a sound strategy, it is appropriate to return considering the simplest situations. The low excitation regions in or around galaxies, such as the galaxy outer disks, the inter-arm regions, the well shielded starless cores in molecul...

We report a new search for 12CO(1-0) emission in high-velocity clouds (HVCs) performed with the IRAM 30 m telescope. This search was motivated by the recent detection of cold dust emission in the HVCs of Complex C. Despite a spatial resolution which is three times better and sensitivity twice as good compared to previous studies, no CO emission is...

We present results of fully self-consistent numerical simulations involving a barred galaxy and a spherical companion. The goal of this study is to calculate the effects of the companion in the bar pattern speed. Our simulations show that only a heavy companion (50% of the target mass) is capable to change substantially (around 15%) the bar pattern...

Today we have numerous pieces of evidence suggesting that galaxies evolve dynamically along the Hubble sequence through various processes, sometimes over much shorter periods than the standard galaxy age of 10–15 Gyr. Linking this to the known mass components provides new indications on the nature of the galaxy dark matter. Bounds on the amount of...

Galactic warps represent an old unresolved problem since the discovery of the Hi warp of the Milky Way at the end of the fifties. In this paper, we propose a new scenario explaining a large fraction of the observed optical warps. Based on N-body simulations, we show that realistic galactic disks, where the dark matter is essentially distributed in...

The collisionless limit of the N-body problem is often justified by the long 2-body relaxation time. Several other possibilities exist by which stellar systems can evolve and relax faster than previously expected. In particular observations and numerical experiments suggest that fractal distribution functions can spontaneously occur. The understand...

We have shown hot to treat consistently a set of rapidly mixing particles. It is possible to devise various dissipative transformations and then correct them to conserve the mass, centers of mass and velocity, and especially the total angular momentum. Furthermore, for simple linear friction laws it is possible to specify in advance the energy diss...

In a first part the scope of classical thermodynamics and statistical mechanics is discussed in the broader context of formal dynamical systems, including computer programmes. In this context classical thermodynamics appears as a particular theory suited to a subset of all dynamical systems. A statistical mechanics similar to the one derived with t...

Context: The widespread view that cosmological neutrinos, even if massive, are well described since the decoupling redshift z~10^10 down to the present epoch by an almost perfectly collisionless fluid of classical point particles is re-examined. Aims: In view of the likely sub-eV rest mass of neutrinos, the main effects due to their fermionic natur...

We explore the possibility that galaxy clusters or halos could be suited environments to host fermions like massive neutrinos in their degenerate state. Equilibrium density profiles for galaxy clusters and halos are calculated where the contribution of fermion particles to an isotropic collisionless component is taken into account. Fermions are sup...

This paper explicates the most fundamental mechanism that rules spiral galaxies. Although spiral galaxies are complex systems for which we do not yet have a complete understanding, the dark matter being the most severe unknown, it is possible to pinpoint the few physical factors that determine their most important properties, such as bars and spira...

Based on N-body simulations, we show that realistic galactic disks are subject to bending insta- bilities of fire-hose type when the disks are substantially self-gravitating, that is, if they contain dark matter distributed in the disk. Depending on the degree of instability, S and U-shaped, as well as asymmetric warps are generated. In some cases,...

During the post Big Bang recombination epoch and subsequent dark age the first molecules are thought to be form from the first neutral (He) and charged atomic species in the growing matter inhomogeneities. The knowledge of the precise molecular abundances given by solving the chemical kinetic equations in the post recombination nucleosynthesis matt...

Based on N-body simulations, we show that realistic galactic disks are subject to bending instabilities of fire-hose type when the disks are substantially self-gravitating, that is, if they contain dark matter distributed in the disk. Depending on the degree of instability, S and U-shaped, as well as asymmetric warps are generated. In some cases, t...

The Baryonic Tully-Fisher relation (BTF) can be substantially improved when considering that the galactic baryonic mass is likely to consist not only from the detected baryons, stars and gas, but also from a dark baryonic component proportional to the HI gas. The BTF relation is optimally improved when the HI mass is multiplied by a factor of about...

Bars and spiral arms have played an important role as constraints on the dynamics and on the distribution of dark matter in the optical parts of disk galaxies. Dynamics linked to the dissipative nature of gas, and its transformation into stars provide clues that spiral galaxies are driven by dissipation close to a state of \textit{marginal stabilit...

We review the different cold dark gas models that have been proposed in the literature, as well as a new variant which addresses their principal stability problems by taking into account the property of molecular hydrogen to become solid or liquid below 33 K and at sufficiently high pressure. This new physical ingredient provides the possibility to...

A substantial fraction of the warps in spiral galaxies may result from bending instabilities if the disks are essentially self-gravitating. With N-body simulations, we show that galaxies with self-gravitating disks as thick as HI disks are subject to bending instabilities generating S-shaped, U-shaped or asymmetric warps. S-shaped warps persist dur...

To address, among other questions, puzzling observations about star forming in the extreme outer HI disk of M 31 (Cuillandre et al. 2001), a scenario of interstellar gas cycling between the visible and a very cold invisible phase is investigated. The key new element sketched here, allowing to maintain the bulk of the gas out of sight, is that molec...

We discuss work in progress about the existence and possible forms of molecular hydrogen as a dark matter component particularly in the outer galactic disks, and in late or low surface brightness galaxies. Mainly, the direct condensation of molecular hydrogen in solid or liquid form during gravitational collapses opens possibilities on the evolutio...

In this work, we explore a simple method with few free parameters, which describes the global dynamical consequences on disk galaxies of a slow cycling of gas between the interstellar, almost collisionless very cold gas and the warm collisional phases, with a secular transformation of a fraction of warm gas into stars.

Below a temperature of 33K molecular hydrogen can condense to liquid, and below 14 K to solid phase. These phase transitions require however a vapor pressure large enough that can be satisfied in the ISM only in particular conditions, such as during the collapse of sub-stellar mass clouds. The change from compressible gaseous to almost incompressib...

The cold universe is the part of the universe in a cold form close to the background radiation temperature. Until recently, this cold state has been neglected because it is hard to observe, although it must inevitably play a crucial role as the original state of most of the hotter observable objects such as planets, stars, and galaxies. Fortunately...

To understand galaxies and their evolution, it is necessary to describe how the different scales interact: how the microscopic physics, such as star formation, or the large scale physics, such as galaxy interactions may modify the galaxy global shapes. The purpose of this review is to point out some general or recent topics related to such scale in...

The fragmentation of cold gas is usually thought to proceed as long as the medium radiates faster that its core heats. Thus ISM cores are among the coldest places until eventually the cores become opaque and heat toward usual star conditions. Now the central pressure for a given column density is the highest for the low mass fragments. At a critic...

Since the first HI observations of the Milky Way warp 50Â years ago numerous hypotheses have been proposed to explain this common galactic phenomenon. While actually most of the works on warps are based on the assumption of a disk embedded in a dark matter halo, the possibility of a heavy self-gravitating disk is often neglected. The aim of this pa...

While most works on warped galaxies are based on the assumption of a spherical or triaxial dark matter halo, none of them are able to clearly satisfying all the constraints provided by the observations. Following another way, this paper explores the hypothesis of a heavy self-gravitating and warped disk. Characteristics of such a model are compared...

The thermochemistry of H2 and HD in non-collapsed, non-reionized primordial gas up to the end of the dark age is investigated with recent radiation-matter and chemical reaction rates taking into account the efficient coolant HD, and the possibility of a gas-solid phase transition of H2. In the standard big-bang model we find that these molecules ca...

Observed self-gravitating systems reveal often fragmented non-equilibrium structures that feature characteristic long-range correlations. However, models accounting for non-linear structure growth are not always consistent with observations and a better understanding of self-gravitating $N$-body systems appears necessary. Because unstable gravitati...

Following Toomre & Kalnajs (1991), local models of slightly dissipative self-gravitating disks show how inhomogeneous structures can be maintained over several galaxy rotations. Their basic physical ingredients are self-gravity, dissipation and differential rotation. In order to explore the structures resulting from these processes on the kpc scale...

It is often assumed that a warped galaxy can be modeled by a set of rings. This paper verifies numerically the validity of this assumption by the study of periodic orbits populating a heavy self-gravitating warped disk. The phase space structure of a warped model reveals that the circular periodic orbits of a flat disk are transformed in quasi annu...

This study is part of the ‘Dynamics of Embedded Bars and Connections with AGN’ (DEBCA) project whose main goal is to provide new insights on the fueling processes of AGN. For that purpose the DEBCA team plans to observe and compare to numerical simulations a sample of 12 galaxies: Seyfert (1 or 2) or not, with or without double-bars. Fueling the ce...

The interstellar medium (ISM) reveals strongly inhomogeneous structures on every scale. These structures do not seem completely random since they obey certain power laws. Larson's (1981)) law σ∝ Rδ and the plausible assumption of virial equilibrium justify the consideration of fractals as a possible description of the ISM. In the following we inves...

In this contribution, we investigate the scenario of major mergers between galaxies of comparable mass to the formation of massive counter-rotating disks. Such a scenario can be ruled out in general because they produce rather ellipticals. Yet for a narrow range of initial conditions, major mergers can nevertheless produce anyway rare galaxies such...

Gravity is scale free. Thus gravity may form similar structures in self-gravitating systems on different scales. Indeed, observations of the interstellar medium, spiral disks and cosmic structures, reveal similar characteristics. The structures in these systems are very lumpy and inhomogeneous. Moreover some of these structures do not seem to be of...