anemic spiral galaxy
NGC 4145, an anemic spiral in the Ursa Major cluster of galaxies. While the spiral arms of this galaxy are punctuated by some activity, the rest of the galaxy is quite quiet. The likelihood is that a galaxy like this will soon (in galactic terms) lose the impetus to form stars and settle down to become a lenticular galaxy. Image credit : NOAO.
An anemic spiral galaxy is a galaxy with characteristics that are intermediate between those of a normal spiral galaxy and a lenticular galaxy. Anemic spirals have smooth, weak spiral arms and a reduced rate of star formation linked to their deficiency in neutral hydrogen gas. They tend to occur in rich clusters of galaxies, suggesting that they may have been stripped of interstellar material through multiple encounters with other galaxies. This raises the question of whether anemic spirals mark both a morphological and an evolutionary transition between normal spirals and lenticulars.