  We compare the central mass distribution of galaxies simulated with three
different models of the interstellar medium (ISM) with increasing complexity:
primordial (H+He) cooling down to 10^4K, additional cooling via metal lines and
to lower temperatures, and molecular hydrogen (H_2) with shielding of atomic
and molecular hydrogen, in addition to metal line cooling. In order to analyze
the effect of these models, we follow the evolution of four field galaxies with
V_peak < 120 km/s to a redshift of zero using high-resolution Smoothed Particle
Hydrodynamic simulations in a fully cosmological LCDM context. The spiral
galaxies produced in simulations with either primordial cooling or H_2 physics
have realistic, rising rotation curves. In contrast, the simulations with metal
line cooling and otherwise similar feedback and star formation produced
galaxies with the peaked rotation curves typical of most previous LCDM
simulations of spiral galaxies. The less-massive bulges and non-peaked rotation
curves in the galaxies simulated with primordial cooling or H_2 are linked to
changes in the angular momentum distribution of the baryons. These galaxies had
smaller amounts of low-angular momentum baryons because of increased gas loss
from stellar feedback. When there is only primordial cooling, the star forming
gas is hotter and the feedback-heated gas cools more slowly than when metal
line cooling is included and so requires less energy to be expelled. When H_2
is included, the accompanying shielding produces large amounts of clumpy, cold
gas where H_2 forms. Star formation in clumpy gas results in more concentrated
supernova feedback and greater efficiency of mass loss. The higher feedback
efficiency causes a decrease of low-angular momentum material. (abridged)
