1 Introduction

Hydrogen is by far the most abundant component of the interstellar medium (ISM). Studying the structural and kinematic properties of each of its phases helps us understand the physical phenomena which give rise to those phases.

The study of the ISM, its kinematics, and the role it plays in star formation enables us to infer the relation between the optical appearance of a galaxy and the evolution of its spiral or barred structure, in response to the underlying dynamics of its stellar and gaseous components. One alternative to the usual kinematic technique for spirals, which uses the H I 21 cm line, is Fabry-Perot interferometry of H$\alpha$, which permits the acquisition of much data in a short observing time, useful above all for kinematic mapping of a whole galaxy. With this method one can obtain standard information, such as intensity and velocity maps, as well as rotation curves with high spatial and velocity resolution. From this one can go on to analyze the role of ionized hydrogen as a tracer of the star forming process, via its general distribution, and also, using the kinematics, obtain information on specific processes in defined zones of star formation, such as arms and bars. Fabry-Perot interferometry is valuable for studying motion perpendicular to the plane of the galaxy observed, as well as non-circular motion in the plane. These can be streaming motions across the arms or gas flows around the bar, whose analysis yields the underlying dynamics of the galaxy, via the response of the ionized hydrogen to the non-axisymmetric component of its gravitational potential (Knapen 1997, 1998). One can compute maps of residual velocity, after subtracting the projected circular component, and from these estimate the non-circular motions on a global scale. The observations also yield maps of velocity dispersion, needed to understand the vertical equilibrium, and departures from equilibrium in the galaxy, and the kinematics of specific H II regions (Combes & Bequaert 1997; Rozas et al. 1998a). Finally non-circular motions, specifically vertical motions above star formation zones are important to show how the disc is affecting the halo, and to compare with theoretical models (e.g. Norman & Ikeuchi 1988). Therefore the study of the internal kinematic of stars and gas in spiral galaxies continues being an area of active investigation in astrophysics.

This work provides in this field new detailed observations which report the internal gas motions in the galaxy NGC 3359, a barred late type spiral (SBc) galaxy, included in the most relevant grand design galaxy surveys. The complex morphology of this galaxy shows significant features at different wavelengths, such as an apparently inclined nuclear bar misaligned in the I band (Sempere & Rozas 2000) or twin peaks in the central part of the H$\alpha$ distribution of the galaxy, which are out of alignment with the principal bar, underlining the absence of star formation in the nucleus. In this work the kinematic data of this galaxy, useful by itself, are provided. Moreover, a complete analysis of the gas distribution and kinematics in the optical disc is performed for this galaxy.

This article is the second half of a study we have performed on the grand-design spiral galaxy NGC 3359, based on high quality images (H$\alpha$ and complementary U, I, K images in Rozas et al. 1999; hereafter Paper I) obtained in the context of the BARS international time project of the Canary Island Observatories. We summarize the general properties of the galaxy in Paper I and we show some statistical and physical properties of the H II regions and analyze the distributions of the ionized gas. In the present study we obtain and analyze a complete Fabry-Perot kinematic map of the ionized hydrogen in NGC 3359. In Sect. 2 we describe the observational and data reduction procedures. Section 3 contains the kinematic results: we analyze the velocity map, derive the rotation curve, and a model velocity field, via which we obtain the residual map of non-circular velocities. We also show the velocity dispersion map in Sect. 3. Section 4 is devoted to the gas flows round the bar, and in Sect. 5 we set our conclusions.