Are Passive Spiral Galaxies Truly “Passive” and “Spiral”? : Near-Infrared Perspective

Passive spiral galaxies — unusual galaxies with spiral morphology without any sign of ongoing star formation — have recently been discovered to exist preferentially in cluster infalling regions (at about the virial radius, or at a local galaxy density of ∼ 1 Mpc). The discovery directly connects the passive spiral galaxies to the cluster galaxy evolution studies such as the Butcher-Oemler effect or the morphology-density relation, i.e., passive spiral galaxies are likely to be transition objects between high-z blue, spiral galaxies and low-z red, cluster early-type galaxies. Thus, detailed study of passive spiral galaxies potentially could bring a new insight on the underlying physical mechanisms governing cluster galaxy evolution. However, in previous work, passive spiral galaxies are selected from the low resolution optical images with ∼ 1.5 arcsec of seeing. Therefore, passive spirals could be a mis-identification of S0 galaxies; or dusty-starburst galaxies which are not passive at all. To answer these questions, we performed a deep, high-resolution, near-infrared imaging of 32 passive spiral galaxies. Our high resolution K band images show clear spiral arm structures. Thus, passive spirals are not S0s. Optical-infrared colour does not show any signs of dusty-starburst at all. Therefore, it is likely that they are truly “passive” and “spiral” galaxies in the midst of cluster galaxy evolution.