Vertical dust lanes and magnetic field in spiral galaxies

Dark filaments (dust lanes) vertically emerging toward the halo from the 3-4-kpc molecular ring in the disk plane are found in the spiral galaxies NGC 253 and NGC 7331. The filamentary structure of dust is interpreted as a trace of a vertical magnetic field penetrating the galaxy disk. The vertical field, being twisted by galactic rotation, is suggested to yield an outflow of disk matter into the halo, which we call a 'magnetic fountain' of galaxy scale. The magnetic fountain is suggested to be a possible formation mechanism of radio halos in these galaxies. A more condensed vertical field in the central region may be related to a mass ejection from the nucleus of NGC 253. (author)