Abstract. High-resolution Halpha observations reveal the existence of an unusual bar in the disk of NGC 1808 at a position angle of 155 degr, about 20 degr offset from the major axis. The bar, which is dominated by a chain of HII regions along a line of 6 kpc length, rotates with a pattern speed of about 40 km/s/kpc. Its axial ratio appears very large, whereas the bar-to-disk length ratio of about 0.3 is average. A remarkably similar bar has been found in the galaxy NGC 7479. The outer parts of the HII region chain in NGC 1808 coincide with maxima in the HI gas distribution and further out connect to the outer spiral arms. The streaming of gas along the bar is clearly seen in the HI velocity field. Isovelocity contours nearly perpendicular to the leading side of the bar point out shocked gas regions, and contours which are almost parallel to the bar on its trailing side indicate gas is streaming inwards. The shock fronts can also be observed as broad dust lanes emerging near the nuclear region and curved along the leading sides of the bar. Inside the corotation radius the gas accumulates at the inner Lindblad resonances which are found within a radius of 1 kpc where they coincide with a nuclear ring or spiral measured in HI absorption and CO emission. Using a corotation radius of 1.2 times the bar radius the outer Lindblad resonance is found at a radius of about 7 kpc, not coincident with the location of the outer spiral arms.
Keywords. galaxies: general - galaxies: spiral - galaxies: NGC 1808, NGC7479 - galaxies: kinematics and dynamics of - radio lines: 21 cm
Halpha emission overlaid onto the HI distribution of NGC 1808