The density-morphology relation

In the previous chapter we saw how some galaxies are in low density environments, some are in higher density groups, and others in dense clusters of many hundred galaxies. Figure 10.1 shows that the fraction of elliptical and S0 galaxies increases rapidly with increasing galaxy density. In other words, in a region of high galaxy density, such as a dense group or cluster of galaxies, most galaxies tend to be S0 or E, and very few are of type S. Vice versa, in regions of low galaxy density, most galaxies tend to be of the type S. This is called the density-morphology relation, since it relates the density of galaxies in a given region with their typical morphology (i.e. S, E, S0). Clearly this is telling us something on the nature of galaxy formation and evolution.

Recall from the movie I showed of the motion of galaxies in a cluster, that encounters between galaxies are very frequent in a region of high galaxy density. Such tidal encounters could be responsible for converting S-type galaxies into S0 and E, but other processes might operate as well. Recall that we already discussed that this cannot be the whole story, since the stellar populations of Ss and Es tend to differ as well.

Figure 10.1: The fraction of Elliptical (E), S0, and Spiral + Irregular (S+I) galaxies as function of the logarithm of the projected density, in galaxies Mpc$^{-2}$. The data shown are for all cluster galaxies in the sample and for the field. Also shown is an estimated scale of true space density in galaxies Mpc$^{-3}$. The upper histogram show the number distribution of the galaxies over the bins of projected density. Es and S0s tend to populated regions of high galaxy density, and Ss regoins of low galaxy density. From Dressler, ApJ 236, p. 351 (1980).