This Hubble Space Telescope image shows the galaxy IC 335 in front of a backdrop of distant galaxies. The disk of IC 335 appears edge-on from the vantage point of Earth. This makes it difficult for astronomers to classify it because most of the characteristics of a galaxy’s morphology, the arms of a spiral or the bar across the center, are only visible on its face. It’s possible that the 45,000 light-year-long galaxy could be classified as an S0 type.
Such lenticular galaxies have structure that fall between those of true spiral and of elliptical galaxies. They have a thin stellar disk and a bulge, like spiral galaxies, but unlike typical spiral galaxies, they have used up most of the interstellar medium. Only a few new stars can be created out of the material that is left and the star formation rate is very low. Hence, the population of stars in S0 galaxies consists mainly of aging stars, very similar to the star population in elliptical galaxies. Since S0 galaxies have ill-defined spiral arms, they are easily mistaken for elliptical galaxies if they are seen edge-on. Also, S0 and elliptical class galaxies share some common characteristics, like typical sizes and spectral features.
Both classes of galaxies are evolving passively. However, while elliptical galaxies may be passively evolving when we observe them, they have usually had violent interactions with other galaxies in their past. On the other hand, S0 galaxies are either aging and fading spiral galaxies which never had any interactions with other galaxies or they are the aging result of a single merger between two spiral galaxies in the past.
Image Credit: ESA / NASA