  Passive spiral galaxies, despite their spiral morphological appearance, do
not have any emission lines indicative of ongoing star formation in their
optical spectra. Previous studies have suggested that passive spiral galaxies
preferentially exist in infall regions of galaxy clusters, suggesting that the
cluster environment is likely to be responsible for creating these galaxies. By
carrying out spatially resolved long-slit spectroscopy on four nearby passive
spiral galaxies with the Apache Point Observatory 3.5-m telescope, we
investigated the stellar populations of passive spiral galaxies separately for
their inner and outer regions. In the two unambiguously passive spiral galaxies
among the four observed galaxies, H$\delta$ absorption lines are more prominent
in the outer regions of the galaxies, whereas the 4000-{\AA} breaks
(D$_{4000}$) are strongest in the inner regions of the galaxies. A comparison
with a simple stellar population model for the two passive spiral galaxies
indicates that the outer regions of the galaxies tend to harbour younger
populations of stars. The strong H$\delta$ absorption observed in the outer
regions of the sample galaxies is consistent with that of galaxies whose star
formation ceased a few Gyrs ago. Because of the large uncertainty in the
absorption indices in our samples, further observations are needed in order to
place constraints on the mechanisms that quench star formation in passive
spiral galaxies.

Astrophysics