Rings and spirals in barred galaxies. I Building blocks

In this paper we present building blocks which can explain the formation and properties both of spirals and of inner and outer rings in barred galaxies. We first briefly summarise the main results of the full theoretical description we have given elsewhere, presenting them in a more physical way, aimed to an understanding without the requirement of extended knowledge of dynamical systems or of orbital structure. We introduce in this manner the notion of manifolds, which can be thought of as tubes guiding the orbits. The dynamics of these manifolds can govern the properties of spirals and of inner and outer rings in barred galaxies. We find that the bar strength affects how unstable the L1 and L2 Lagrangian points are, the motion within the 5A5A5Amanifold tubes and the time necessary for particles in a manifold to make a complete turn around the galactic centre. We also show that the strength of the bar, or, to be more precise, of the non-axisymmetric forcing at and somewhat beyond the corotation region, determines the resulting morphology. Thus, less strong bars give rise to R1 rings or pseudorings, while stronger bars drive R2, R1R2 and spiral morphologies. We examine the morphology as a function of the main parameters of the bar and present descriptive two dimensional plots to that avail. We also derive how the manifold morphologies and properties are modified if the L1 and L2 Lagrangian points become stable. Finally, we discuss how dissipation affects the manifold properties and compare the manifolds in gas-like and in stellar cases. Comparison with observations, as well as clear predictions to be tested by observations will be given in an accompanying paper.Comment: Typos corrected to match the version in press in MNRA

Relativistic and Newtonian core-shell models: analytical and numerical results

We make a detailed analysis of Newtonian as well as relativistic core-shell models recently proposed to describe a black hole or neutron star surrounded by shells of matter, and in a seminal sense also galaxies, supernovae and star remnants since there are massive shell-like structures surrounding many of them and also evidences for many galactic nuclei hiding black holes. We discuss the unicity of the models in relation to their analyticity at the black hole horizon and also to the full elimination of conical singularities. Secondly, we study the role played by the presence/lack of discrete reflection symmetries about equatorial planes in the chaotic behavior of the orbits, which is to be contrasted with the almost universal acceptance of reflection symmetries as default assumptions in galactic modeling. We also compare the related effects if we change a true central black hole by a Newtonian central mass. The numerical findings are: 1- The breakdown of the reflection symmetry about the equatorial plane in both Newtonian and relativistic core-shell models does i) enhance in a significant way the chaoticity of orbits in reflection symmetric oblate shell models and ii) inhibit significantly also the occurrence of chaos in reflection symmetric prolate shell models. In particular, in the prolate case the lack of the reflection symmetry provides the phase space with a robust family of regular orbits that is otherwise not found at higher energies. 2- The relative extents of the chaotic regions in the relativistic cases (i. e. with a true central black hole) are significantly larger than in the corresponding Newtonian ones (which have just a $-1/r$ central potential).Comment: AASTEX, 22 pages plus 28 postscript figures, to appear in Ap.

Disk heating agents across the Hubble sequence

We measure the shape of the velocity ellipsoid in two late-type spiral galaxies (Hubble types Sc and Scd) and combine these results with our previous analyses of six early-type spirals (Sa to Sbc) to probe the relation between galaxy morphology and the ratio of the vertical and radial dispersions. We confirm at much higher significance (99.9 percent) our prior detection of a tight correlation between these quantities. We explore the trends of the magnitude and shape of the velocity ellipsoid axes with galaxy properties (colour, gas surface mass density, and spiral arm structure). The observed relationships allow for an observational identification of the radial and vertical disk heating agents in external disk galaxies.Comment: 11 pages, 8 figures, 4 tables. Accepted for publication in MNRA

Re-visiting the relations: Galactic thin disc age-velocity dispersion relation

The velocity dispersion of stars in the solar neighbourhood thin disc increases with time after star formation. Nordstrom et al. (2004) is the most recent observational attempt to constrain the age-velocity dispersion relation. They fitted the age-velocity dispersion relations of each Galactic cardinal direction space velocity component, U (towards the Galactic centre), V (in the direction of Galactic rotation) and W (towards the North Galactic Pole), with power laws and interpreted these as evidence for continuous heating of the disc in all directions throughout its lifetime. We re-visit these relations with their data and use Famaey et al. (2005) to show that structure in the local velocity distribution function distorts the in-plane (U and V) velocity distributions away from Gaussian so that a dispersion is not an adequate parametrization of their functions. The age-sigma(W) relation can however be constrained because the sample is well phase-mixed vertically. We do not find any local signature of the stellar warp in the Galactic disc. Vertical disc heating does not saturate at an early stage. Our new result is that a power law is not required by the data: disc heating models that saturate after ~ 4.5 Gyr are equally consistent with observations.Comment: Accepted for publication in MNRAS, 24 pages, 20 figure

Rings and spirals in barred galaxies. III. Further comparisons and links to observations

In a series of papers, we propose a theory to explain the formation and properties of rings and spirals in barred galaxies. The building blocks of these structures are orbits guided by the manifolds emanating from the unstable Lagrangian points located near the ends of the bar. In this paper, the last of the series, we present more comparisons of our theoretical results to observations and also give new predictions for further comparisons. Our theory provides the right building blocks for the rectangular-like bar outline and for ansae. We consider how our results can be used to give estimates for the pattern speed values, as well as their effect on abundance gradients in barred galaxies. We present the kinematics along the manifold loci, to allow comparisons with the observed kinematics along the ring and spiral loci. We consider gaseous arms and their relations to stellar ones. We discuss several theoretical aspects and stress that the orbits that constitute the building blocks of the spirals and rings are chaotic. They are, nevertheless, spatially well confined by the manifolds and are thus able to outline the relevant structures. Such chaos can be termed `confined chaos' and can play a very important role in understanding the formation and evolution of galaxy structures and in galactic dynamics in general. This work, in agreement with several others, argues convincingly that galactic dynamic studies should not be limited to the study of regular motions and orbits.Comment: 17 pages, 12 figures; accepted in MNRA

Structural properties of disk galaxies. II. Intrinsic shape of bulges

(Abridged) The structural parameters of a magnitude-limited sample of 148 unbarred S0-Sb galaxies were analyzed to derive the intrinsic shape of their bulges. We developed a new method to derive the intrinsic shape of bulges based on the geometrical relationships between the apparent and intrinsic shapes of bulges and disks. The equatorial ellipticity and intrinsic flattening of bulges were obtained from the length of the apparent major and minor semi-axes of the bulge, twist angle between the apparent major axis of the bulge and the galaxy line of nodes, and galaxy inclination. We found that the intrinsic shape is well constrained for a subsample of 115 bulges with favorable viewing angles. A large fraction of them is characterized by an elliptical section (B/A<0.9). This fraction is 33%, 55%, and 43% if using their maximum, mean, or median equatorial ellipticity, respectively. Most are flattened along their polar axis (C<(A+B)/2). The distribution of triaxiality is strongly bimodal. This bimodality is driven by bulges with Sersic index n>2, or equivalently, by the bulges of galaxies with a bulge-to-total ratio B/T>0.3. In particular, bulges with n\leq2 and with B/T\leq0.3 show a larger fraction of oblate axisymmetric (or nearly axisymmetric) bulges, a smaller fraction of triaxial bulges, and fewer prolate axisymmetric (or nearly axisymmetric) bulges with respect to bulges with n>2 and with B/T>0.3, respectively. According to predictions of the numerical simulations of bulge formation, bulges with n\leq2, which show a high fraction of oblate axisymmetric (or nearly axisymmetric) shapes and have B/T\leq0.3, could be the result of dissipational minor mergers. Both major dissipational and dissipationless mergers seem to be required to explain the variety of shapes found for bulges with n>2 and B/T>0.3.Comment: 16 pages, 12 figures; accepted for publication in A&

Tracing the vertical composition of disc galaxies through colour gradients

(Abbreviated) Optical observations of a statistically complete sample of edge-on disc galaxies are used to study the intrinsic vertical colour gradients in the galactic discs, to constrain the effects of population gradients, residual dust extinction and gradients in the galaxies' metal abundance. It appears that the intrinsic vertical colour gradients are either non-existent, or small and relatively constant as a function of position along the galaxies' major axes. Our results are consistent with the absence of any vertical colour gradient in the discs of the early-type sample galaxies. In most galaxies small-scale variations in the magnitude and even the direction of the vertical gradient are observed: at larger galactocentric distances they generally display redder colours with increasing z height, whereas the opposite is often observed in and near the galactic centres. For a significant fraction of our sample galaxies another mechanism in addition to the effects of stellar population gradients is required to explain the magnitude of the observed gradients. The non-zero colour gradients in a significant fraction of our sample galaxies are likely (at least) partially due to residual dust extinction at these z heights, as is also evidenced from the sometimes significant differences between the vertical colour gradients measured on either side of the galactic planes. We suggest that initial vertical metallicity gradients, if any, have likely not been accentuated by accretion or merging events over the lifetimes of our sample galaxies. On the other hand, they may have weakened any existing vertical metallicity gradients, although they also may have left the existing correlations unchanged.Comment: 17 pages LaTeX, incl. 5 embedded postscript figures, resubmitted to MNRAS (referee's comments taken into account