of $\simeq 1\,h_{75}^2\,{\rm
Mpc^{-2}}$ seen in the SFR–Density relation of G[' o]{}mez et al. corresponds to a cluster–centric radius of between $\simeq 1$ to $2$ virial radii. The key question is; Are these two phenomena just different manifestations of the same physical process which is transforming both the morphology and SFR of the galaxies at this critical density? I believe the jury is still out on this question (but see the contributions of Bower and Miller in these proceedings).

I note here that Postman & Geller (1984) also reported a critical density (or “break”) in their [[$T$–$\Sigma$]{}]{}relation at approximately the same density as seen in Figure \[critical\], [*i.e.*]{}, $\simeq 3.5
h^3_{75}{\rm Mpc^{-3}}$. Therefore, this “break” (or critical density) seen in the [[$T$–$\Sigma$]{}]{}relation appears to be universal, as it has been seen in two separate studies, which are based on different selection criteria and analysis techniques.

Strangulation of Star–Formation
-------------------------------

One possible physical model$\footnote{See the reviews of Bower, Mihos
\& Moore for a discussion of other physical mechanisms that can affect
the properties of galaxies in clusters and groups of galaxies}$ for explaining the critical density seen at $\simeq 1 h^2_{75}{\rm
Mpc^{-2}}$ (or $>1$ virial radius) in the SFR–Density and [[$T$–R]{}]{}relations 