Are passive spiral galaxies truly 'passive' and 'spiral'? A near-infrared perspective

Passive spiral galaxies – unusual galaxies with spiral morphology without any sign of on-going star formation – have recently been found to exist preferentially in cluster infalling regions (at about the virial radius, or at a local galaxy density of ∼1 Mpc−2). The discovery directly connects passive spiral galaxies to 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 could potentially bring new insight into the underlying physical mechanisms governing cluster galaxy evolution. However, in previous work, passive spiral galaxies were selected from low-resolution optical images with ∼1.5 arcsec of seeing. Therefore, passive spirals could be a misidentification of S0 galaxies; or dusty starburst galaxies which are not passive at all. To answer these questions, we performed 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.