The Antennae:

Also, star formation and molecular gas is strongest between the nuclei:

differences in progenitor gas mass distribution (exp vs 1/r)?

gas dynamical issues?

The Cartwheel:

Simulations of ring galaxies with spokes demand dissipation (Mihos & Hernquist 1994):

Models predict lots of gas and star formation in the central ring!

What do we see? (courtesy Higdon etal)

Uh oh....

But wait...

there is molecular gas
(Horellou etal 1998)
M_H2 ~ 1.5-6x10⁹ M_sun

and there is mid-IR emission
indicative of star formation
(Charmandaris etal 1999)

...just not as dominant as in the outer ring.

threshold effects?
other physical influences?

Stephan's Quintet

images courtesy Cong Xu etal 1999

Optical	ISOCAM LW3

Starburst (maybe) occurs at interface between galaxy and hot intragroup medium.

Compare optical, Xray, radio continuum:

(radio: van der Hulst & Rots 1981
Xray: Pietsch etal 1997)

NGC 6240 (etal):

(van der Marel, Mihos, Hernquist, Barnes, in prep)

no way....