Optical Structure and Evolution of the Arp 104 Interacting Galaxy System
  Arp 104 is a pair of luminous interacting galaxies consisting of NGC 5216, an
elliptical, and NGC 5218, a disturbed disk galaxy and joined by a stellar
bridge. We obtained optical imaging to support photometric and color studies of
the system. NGC 5216 lies on the red sequence, while the unusual distribution
of stellar population properties in combination with intense central star
formation in a dusty region result in NGC 5218 being a nearby example of an
intermediate color (green valley) system. The stellar bridge has remarkably
uniform optical surface brightness, with colors consistent with its stars
coming from the outskirts of NGC 5218, but is relatively gas-poor while the
northern tidal tail is rich in HI. While both galaxies contain shells, the
shell structures in NGC 5218 are pronounced, and some appear to be associated
with counter-rotating gas. This combination of features suggests that Arp 104
could be the product of distinct multiple interactions in a small galaxy group,
possibly resulting from a hierarchical merging process, and likely leading to
the birth of a relatively massive and isolated early-type galaxy.
