This is a color gri from SDSS, overlayed with narrowband Halpha+NII images (visible as the red and green filaments). The giant elliptical on the right is M86, which appears to be the brightest galaxy in a group or sub-cluster that is merging with Virgo. The galaxy on the left is NGC 4438 (also Arp 120), a highly-disturbed spiral. The red filaments are Halpha+NII emission that appear to link the two galaxies, suggesting that they have undergone a high-speed collision. The green filaments are Halpha+NII emission at a higher recessional velocity; it is not clear whether the galaxy in the lower right and the associated line emission are involved in this interaction.
NGC 4438 is very HI-deficient for a spiral galaxy of it's size. If it lost most of it's HI during the collision, then it is expected that a significant fraction of the kinetic energy of that gas went into heating the ISM of M86. This heating would be enough to prevent gas from cooling and forming stars in M86, possibly obviating the need for radio-mode AGN feedback. Thus this interacting system may be a nice example of the "gravitational quenching" mechanism discussed by Dekel & Birnboim (2008).
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