From general considerations - From general considerations...

From general considerations, some of the induced star formation must be triggered by processes
not requiring direct contact of disk material from different galaxies; some objects with high SFR
are too far apart, and relatively undisturbed, so that internal effects of tidal stress must be
responsible. Detailed modelling is thwarted by the great range of relevant physical scales in
some of these cases. In testing these proposals, studies of the ISM in interacting systems, and
understanding their dynamics, are crucial. For example, H
2
masses in combination with SFR
estimates can suggest whether the SFR goes up because of creation or accumulation of new
molecular gas (and normal accompanying star formation), or via an enhancement of the
"efficiency" of star formation. A survey of 13 merger candidates by Young et al. (1986 ApJL 311,
L17) suggested that the SFR reflects large molecular gas content; more recent results (Young,
IAU Symp. 146) extend this by suggesting that the H
2
/H I mass ratio is systematically larger in
interacting systems than in normal spirals. CO surveys of complete and well-understood sets of
both interacting and non-interacting galaxies are urgently needed (and in progress).
For very luminous galaxies which are dusty enough that most of their power emerges in the far-
IR (once known as IRAS galaxies, now sharing such acronyms as LIRG, ULIRGs, PIGs, or
ELFs), it can be subtle to tell whether the dominant energy source is a starburst or AGN.
Compact, flat-spectrum radio sources indicate an AGN, but more diffuse radio emission can
come from star-forming nuclei as well. Condon and Broderick (1988 AJ 96, 30) have introduced
a ratio of radio and far-IR flux densities as a discriminant, based on the empirical relation found
for star-forming regions and the fact that powerful AGN are usually more radio-loud. Mid-IR