  High resolution observations of CO and 13CO 1--0 in the Medusa (NGC4194)
minor merger show the CO/13CO 1--0 intensity ratio (R) increasing from normal
values (5-10) in the outer parts of the galaxy to high (>20) values in the
central, extended starburst region. Ratios >20 are otherwise typical of more
luminous mergers. The Medusa L_FIR/L_CO ratio rivals that of ultraluminous
galaxies (ULIRGs), despite the comparatively modest luminosity, indicating an
exceptionally high star formation efficiency (SFE). We present models of the
high pressure ISM in a ULIRG and the relatively low pressure ISM of the Medusa.
We discuss how these models may explain large R in both types of distributions.
Since the HCN emission is faint towards the Medusa, we suggest that the SFE is
not primarily controlled by the mass fraction of dense (n > 10^4 cm-3) gas, but
is probably strongly dependent on dynamics. The bright HCN emission towards
ULIRGs is not necessarily evidence that the IR emission there is always powered
by starbursts.
