The Interacting Galaxies NGC 5394/5395: A Post-Ocular Galaxy and Its Ring/Spiral Companion

Journal article, 1999

H I, radio continuum, Fabry-Perot Hα, and 12CO J = 1 → 0 observations and broadband optical and near infrared images are presented of the interacting spiral galaxies NGC 5395 and NGC 5394. Kinematically, there are three important, separate components to the H I gas associated with this galaxy pair: (1) the main disk of NGC 5395, (2) a long, northern tidal arm of NGC 5395 distinct in velocity from its main disk, and (3) the disk of NGC 5394. The H I northern tidal arm of NGC 5395 has a line of–sight velocity as much as 75–100 km s-1 greater than the main disk of NGC 5395 at the same projected location and thus is not in the same plane as the disk. The velocity field of the disk of NGC 5395 is asymmetric and distorted by large-scale and small-scale noncircular motions. In NGC 5395, the encounter appears to be exciting m = 1 and m = 0 modes in what had been a two-armed spiral. The dominant spiral arm of NGC 5395 forms a large ring or pseudo-ring of Hα, radio continuum, and H I emission, somewhat off center with respect to the nucleus. The H I trough in the center of NGC 5395 is not filled in by molecular gas. The Hα velocity contours exhibit an organized pattern of kinks in crossing the ring and also show streaming motions in a large stellar caustic feature. The eastern side of the ring is brighter in radio continuum and Hα; the western side is brighter in H I and contains massive (108 M☉) H I clouds not associated with the most luminous H II regions. The smaller galaxy NGC 5394 is in an immediate post-ocular phase, with a central starburst, an intrinsically oval disk, two long, fairly symmetric, open tidal arms with high arm-interarm contrast, and very bright inner spiral arms, disjoint from the outer tidal arms. Most of the gas in NGC 5394 is in molecular form and concentrated within 3.8 kpc of the center, so is suitable for fueling the starburst. Despite the presence of H I gas, two of the three optically bright inner spiral arms of NGC 5394 show no evidence of ongoing star formation. A galaxy encounter simulation reproduces some of the main features of this system with a collision that is prograde relative to NGC 5394 and retrograde at a high tilt angle relative to NGC 5395. The model finds that the inner spiral structure of NGC 5394 developed from an eye-shaped ("ocular") structure at slightly earlier times. NGC 5394 and the two ocular galaxies IC 2163 and NGC 2535, studied earlier, form an evolutionary sequence of structures resulting from prograde encounters and thus confirm the generic models of such collisions. The agreement between the model for NGC 5394/95 and the ring/spiral structures seen in NGC 5395 extends our understanding of collisional ring galaxies.