We present velocity fields of nearby disk galaxies observed in Hα using Fabry.Perot interferometry which provides, for a given object, a complete data cube of intensity, velocity and velocity dispersion, with velocity resolution of some 7 km/s and seeing limited angular resolution, of order 1" across the full galaxy image. To illustrate the value of these data we have used the Tremaine-Weinberg method to derive formally the pattern speeds of the bars in several of these galaxies. The results are very encouraging, in that if the position angle of the disk has been correctly chosen the plot of average intensity weighted velocities against average intensity weighted positions shows very nice linearity, allowing an accurate pattern speed to be derived. It is interesting that the technique gives such well defined results, given that one of the constraints is that the continuity equation be valid for the component concerned. In one case, NGC6946, we find two very clearly distinct patterns speeds: 22 km/s/kpc for an outer bar, and 47 km/s/kpc for an inner bar. Both bars were previously well observed, and we can relate well the resonant structure of the bar system to the velocities and velocity gradients observed in the rotational curve, which can be derived cleaned of non circular components from the two-dimensional velocity field of the Fabry-Perot system.

We study the evolution of stellar-gaseous disks embedded on a suitable dark-matter halo evolving in a fully consistent cosmological framework. We study how the star formation can modify the results already obtained analysing simple stellar gaseous disks with cooling. The more massive disks always form long lasting bars in the new and in the old stars components, even in the cases where, without star formation, the bar was gradually destroyed by the central gas concentration. The less massive disks also form small central bars in the old star components but the new star population form a bulge which is hiding the central bar.

Disc galaxies often show spirals, bars or lopsided structures. These structures are presumed to be the result of dynamical instabilities which occur in the stellar component of an initially axisymmetric system. We have developed a computer code to analyse the stability of razor-thin disk galaxies. To describe the stellar component of a disk galaxy, we use the global potential and the distribution function, which gives the density of stars at a given position and with a certain velocity. An instability is described as the superposition of a small perturbation on a time-independent axisymmetric equilibrium configuration. The perturbations are uniformly rotating and exponentially growing in time. The evolution of the distribution function can be calculated with the linearised collisionless Boltzmann equation. Once we know the response of the distribution function to the perturbation, we can compute the perturbed density and corresponding potential. For self-consistent perturbations, the response should be equal to the original perturbing potential. In our code we search the self-consistent instabilities with a matrix method. As a first application of this code, we investigated whether the observed lopsidedness of many galaxies is the result of an instability. Therefore we have examined the influence of counter-rotation on the instability of spiral galaxies. Models with no net rotation show a strong lopsided mode (m=1) and a weaker pair of counter-rotating bars (m=2). Maximally rotating models show a strong two-armed spiral (m=2) and a weaker one-armed spiral (m=1). In the first case, the m=1 mode gets weaker when there is less counter-rotation, while in the second case, it becomes stronger. To unravel the physical origin of the two different m=1 instabilities, we studied the orbits of individual stars that move in the perturbed potential to if they maintain the perturbed density. Apparently, the presence of the perturbation gives rise to a very interesting orbital behaviour, including butterfly and banana orbits.

Satellite accretion events have been invoked for mimicking the internal secular evolutionary processes of bulge growth. However, N-body simulations of satellite accretions have paid little attention to this possible evolutionary process, and to the consistency of this evolution with observations. We have investigated whether satellite accretions indeed drive the growth of bulges, what is the role of the different galactic components in the evolution (in particular, of the primary galactic disc), and whether the remnant characteristics are consistent with global scaling relations of real bulges and discs. We have performed collisionless N-body simulations of the accretion of satellites onto disc galaxies. A Tully-Fisher scaling between primary and satellite ensures that density ratios, critical to the outcome of the accretion, are realistic. We have explored the results using different central density ratios between the two galaxies, luminous mass ratios (1:6, 1:9, 1:18), and orbits. We have carried out a full structural, kinematic and dynamical analysis of the evolution of each galactic component, and of the bulge-to-disc scaling relations. The remnants of the accretion exhibit bulge-disc structure, with normal rotation curves. The satellite is completely disrupted in all the experiments. Both the bulge-to-disc ratio (B/D) and the Sersic index (n) of the remnant bulge increase as a result of the accretion, with moderate final bulge Sersic indices: n = 1.0 to 1.9. Bulge growth occurs no matter the fate of the satellite, which reaches the remnant center only in the experiments with the lowest density and mass ratios. Global structural parameters evolve following trends similar to observations. We show that the dominant mechanism for bulge growth is the inward flow of material from the primary disc to the bulge region during the satellite decay, which is driven by transitory non-axisymmetric distorsions in the primary disc. The models confirm that the growth of the bulge out of disc material, a central ingredient of secular evolution models, may be triggered externally through satellite accretion.

The spiral density waves in M81 are investigated using SPITZER IRAC near infrared data from the Spitzer Infrared Nearby Galaxies Survey (SINGS). The 3.6 (µm IRAC band is used to study the underlying mass distribution, assuming a constant mass-luminosity relation. The non-axisymmetric components of the mass are analysed and clear evidence for a density wave is found. In addition to the mass distribution, the response of the gas can be observed (via dust emission) in the 8 (µm IRAC band. A comparison between the observed angular offsets of the stellar density wave with the gas shock allows the radius of corotation to be determined. From the corotation radius and the rotation curve of the galaxy, the pattern speed is found, and the position of the inner Lindblad resonance located. As a potentially useful aside to this work, it is shown that the underlying mass-distribution of the galaxy can be recreated from both infrared and optical data. We are now extending our analysis to the other spiral galaxies in the SINGS dataset.

Although spiral arms in galactic disks are most commonly observed in the optical domain, their continuations in the neutral gas component extend to way larger radii and allow for a better assessment of spiral arm properties and density wave dynamics. For the analysis of spiral morphologies one usually employs observations of face-on spirals, while for a kinematical analysis edge-on systems are preferred. In order to do both in the same system (even for highly inclined disks), we develop a deprojection software that is able to extract information on the spiral arm pattern of a galaxy from spectroscopic HI data cubes. This pattern is inferred by tracing the disk's geometry and its kinematics as supplied by a tilted-ring model. We present our method and results, mainly focusing on the spiral galaxy NGC5055, and discuss our findings.

Most disk galaxies display graceful spiral patterns, yet over a century and a half, after their first discovery, we still do not fully understand how they originate. To date, following Lin and Shu the majority of experts in the field regard spiral structure in most galaxies as a density wave pattern, which does not remain stationary in a frame of reference rotating around the center of the galaxy at a proper speed. Such a density wave structure may be excited by real instabilities of gravity perturbations (e.g. those produced by a spontaneous disturbance). In the spirit of Lin and Shu, we assume that a density wave is the result of a natural gravitational Jeans-type instability developing in a dynamically cold galactic disk. For the first time in galactic dynamics, we develop the weakly nonlinear (quasi-linear) hydrodynamic theory of nonresonant Jeans instability in a dynamically cold subsystem (identified as the gaseous component) of a disk. This theory leads to the understanding of the secular evolution and transport properties of highly flattened, rapidly rotating galaxies. It is shown that gravitationally unstable systems, such as disks of galaxies, are efficient at mass and angular momentum transfer: already on the timescale of an order of 2-3 rotational periods a violently unstable disk sees a large portion of its angular momentum transferred outward, and mass transferred both inward and outward. We do find evidence of angular momentum transport great enough to be astrophysically important: enhanced accretion generated by spontaneous (that is, intrinsic) gravitational instabilities may dominate the evolution of galactic disks. Certain applications of the theory to actual galaxies are explored as well. In particular, in good conformity with observations the process studied leads to the core-dominated exponential-like mass density profile in the disk galaxies.

The original Tremaine-Weinberg method is based on the assumption that there is one, well-defined, pattern speed in the system. However, attempts were made to extend this method to measurements of multiple pattern speeds. Here we show that even if the relation between the photometric and the kinematic integrals strongly indicates multiple rotating patterns, a naive extension of the Tremaine-Weinberg method returns incorrect pattern speeds. By carefully extending the method, we realize that although its diagnostics for numerical values of the pattern speeds are strongly limited, it clearly discriminates between corotating and counterrotating patterns. For example, the extended Tremaine-Weinberg method can be used in the search for counterrotating cores in galaxies, which would otherwise require integral field spectroscopy, and even then such cores might remain hidden if the counterrotating population were small.

Recent observations indicate that the massive star formation occurred in the Galactic center within a few tens parsecs several million years ago, since a number of young massive stars are observed in this region. We study possible relation between large-scale gas dynamics and the central massive star formation. We especially pay attention to an inner bar that is suggested by Alard (2001) and investigate a role of the nested bars in fueling to the Galactic center region. For this purpose, we have performed two dimensional hydrodynamical simulations and SPH simulations in the gravitational potential models of the Galaxy that contain an inner bar potential component. Because the detail density profile of the inner bar is not observationally clear, we systematically vary the inner bar models, that is, we assume various mass and their elongation. We also assume the two sizes for the inner bar semi-major axis. The first is the case the size of inner bar is smaller than the radius of gas ring that is formed in the no-inner bar case. The other is the larger one than the radius of the gas ring. In all models, the gas ring is formed due to the outer bar and its radius is 200-300 pc, and its mass is ~10⁸ M(sun). This corresponds to the Central Molecular Zone. In no-inner bar case, the gas supply rate is very small. It is difficult to attribute the massive star formation to this gas supply. In cases of small inner bars with large elongation, a small massive gas disk is formed within 20 pc from the Galactic center and its mass is as large as 10⁷ M(sun). The gas supply rate is very larger than the no-inner bar case. We perform SPH simulations to investigate evolution of this gas disk due to the self-gravitational instability and study features of star formation in this region. In large inner bar cases, we cannot find a model which realizes large gas fueling to the Galactic center without destroying the gas ring.

The gradients in age and stellar populations in the bar and bulge region can help us understanding the role played by bars in bulge formation and its subsequent evolution; as well as providing information about the evolution of the bar itself. We have analysed in detail the stellar populations and ages along bars for a sample of 20 galaxies. We find evidence of bars existing in two different evolutionary regimes, one in which the bars are long lasting and likely to secularly form a inner disk/bulge, and a another one in which bars are short lived (~2 Gyr). The main parameter driving this different bar evolution is the central stellar velocity dispersion.

Bars are one of the most important drivers of the evolution of disk galaxies. At least 25% of early-type barred galaxies also harbor secondary bars. The dynamics of such double barred galaxies are still not well understood, partly because of a lack of realistic $N$-body models with which to study them. We discuss the generation of double bars due to the presence of rotating pseudobulges, and the dynamical properties of such long-lived double-barred systems. We find that secondary bars rotate faster than primary ones. The amplitude of a secondary bar pulsates and its pattern speed varies regularly, as it rotates through the primary. When the two bars align, the amplitude of the secondary bar is weaker, and it rotates more rapidly than when they are perpendicular. We also compare the kinematic properties of secondary bars with observations. This self-consistent study supports the invariant loop orbits as the backbone of double-bars. The pulsating nature of secondary bars may have important implications for understanding the central region of double-barred galaxies.

Recent kinematical studies have revealed large non-circular motions in many “nearly normal” disk galaxies. Attempts to use standard perturbed epicycle theory to model them quickly break down as the perturbation becomes large, causing the derived mass distributions to be increasingly inaccurate. We present a new procedure to fit non-circular motions in velocity fields, which does not require them to be small. We focus on a model in which flow pattern asymmetries stem from bar-like or oval distortions, as could arise from a bar or triaxial halo. We apply the model to the high-quality Hα and CO kinematics of the nearby spiral NGC2976 from Simon et al. (2003), and find that it fits the data as well as their model with prohibitively large radial flows. We will discuss the implications of our steeper rotation curve for the structure of this system, our attempts to disentangle its baryonic and dark matter components with self-consistent hydrodynamical models, and the application of our technique to the kinematics of other nearby systems.

Star formation depends strongly both on the local environment of galaxies, and on the internal dynamics of the interstellar medium. To disentangle the two effects, we obtained, in the framework of the AMIGA project, Hα and Gunn r photometric data for more than 200 spiral galaxies lying in very low-density regions of the local Universe. We characterise the Hα emission, tracing current star formation, of the 45 largest and less inclined galaxies observed for which we estimate the torques between the gas and the bulk of the optical matter. We could subsequently study the Hα morphological aspect of these isolated spiral galaxies. Using Fourier analysis, we focus on the modes of the spiral arms and also on the strength of the bars, computing the torques between the gas and newly formed stars (Hα) and the bulk of the optical matter (Gunn r). We interpret the various bar/spiral morphologies observed in terms of the secular evolution experienced by galaxies in isolation. We also classify the different spatial distributions of star forming regions in barred galaxies. The observed frequency of particular patterns brings constraints on the lifetime of the various evolution phases. We propose an evolutive sequence accounting for the transitions between the different phases we could observe. Isolated galaxies appear not to be preferentially barred or unbarred. Through numerical simulations, trying to fit the Hα distributions yields constraints on the star formation law, which is likely to differ from a genuine Schmidt law. In particular, it is probable that the relative velocity of the gas in the bar also needs to be taken into account.