Extraordinary diffuse light in Hickson Compact Group 90

A two-color photometric study is presented for the interacting compact system of galaxies Hickson Compact Group 90 (HCG 90). Currently, HCG 90's galaxies are experiencing a strong tidal interaction, and as a result, a significant luminous diffuse light component has formed. Through isophotal definition of the distinction between the galaxy light and diffuse intracluster light, approximately 38%-48% of the total light belongs to the diffuse light component. Moreover, the surface brightness of the tidal debris is much higher than seen in other interacting systems. The intracluster light has a narrow color distribution centered around a mean color of V-R = 0.67 +/- 0.03 mag, which is consistent with an old stellar population. Although a spiral galaxy is heavily involved in an interaction, an associated blue component to the diffuse light is not clearly seen. An interaction of this magnitude would also remove much of the cold gas in the interacting galaxies, which would then be heated by the group potential; however, Chandra observations reveal only a very faint intracluster medium at energy of kT similar to 0.7 keV. The X-ray-emitting gas shows a narrow bridge that connects two of the interacting galaxies. This morphology is similar to that exhibited by the diffuse optical emission. This morphology might be an indicator of a young dynamical age for this system. However, the overall luminosity ratio of X-ray gas emission to the diffuse light emission is less than 10(-3), and this might indicate a dynamical old system (e.g., the gas has cooled). Therefore, HCG 90 is a dynamical paradox in that it has a very large amount of diffuse intracluster light and a very small amount of related hot intracluster gas and has overall features that give conflicting inferences concerning its dynamical age.