H I and CO observations of Arp 104 : a spiral – elliptical interacting pair

We present data probing the spatial and kinematical distribution of both the atomic (H I) and molecular (CO) gas in NGC 5218, the late-type barred spiral galaxy in the spiral–elliptical interacting pair, Arp 104. We consider these data in conjunction with far-infrared and radiocontinuum data, and N-body simulations, to study the galaxies interactions, and the star formation properties of NGC 5218. We use these data to assess the importance of the bar and tidal interaction on the evolution of NGC 5218, and the extent to which the tidal interaction may have been important in triggering the bar. The molecular gas distribution of NGC 5218 appears to have been strongly affected by the bar; the distribution is centrally condensed with a very large surface density in the central region. The N-body simulations indicate a time-scale since perigalacticon of ∼3 × 108 yr, which is consistent with the interaction having triggered or enhanced the bar potential in NGC 5218, leading to inflow and the large central molecular gas density observed. Whilst NGC 5218 appears to be undergoing active star formation, its star formation efficiency is comparable to a ‘normal’ SBb galaxy. We propose that this system may be on the brink of a more active phase of star formation.