Bar diagnostics in edge-on spiral galaxies. I. The periodic orbits approach

We develop diagnostics to detect the presence and orientation of a bar in a n edge-on disk, using its kinematical signature in the position-velocity di agram (PVD) of a spiral galaxy observed edge-on. Using a well-studied barre d spiral galaxy mass model, we briefly review the orbital properties of two -dimensional nonaxisymmetric disks and identify the main families of period ic orbits. We use those families as building blocks to model real galaxies and calculate the PVDs obtained for various realistic combinations of perio dic orbit families and for a number of viewing angles with respect to the b ar. We show that the global structure of the PVD is a reliable bar diagnost ic in edge-on disks. Specifically, the presence of a gap between the signat ures of the families of periodic orbits in the PVD follows directly from th e nonhomogeneous distribution of the orbits in a barred galaxy. Similarly, material in the two so-called forbidden quadrants of the PVD results from t he elongated shape of the orbits. We show how the shape of the signatures o f the dominant x(1) and x(2) families of periodic orbits in the PVD can be used efficiently to determine the viewing angle with respect to the bar, an d to a lesser extent to constrain the mass distribution of an observed gala xy. We also address the limitations of the models when interpreting observa tional data.