  Baryon and energy densities, which are reached in central Au+Au collisions at
collision energy of $\sqrt{s_{NN}}= 39$ GeV, are estimated within the model of
three-fluid dynamics. It is shown that the initial thermalized mean proper
baryon and energy densities in a sizable central region approximately are
$n_B/n_0 \approx$ 10 and $\varepsilon\approx$ 40 GeV/fm$^3$, respectively. The
study indicates that the deconfinement transition at the stage of
interpenetration of colliding nuclei makes the system quite opaque. The final
fragmentation regions in these collisions are formed not only by primordial
fragmentation fireballs, i.e. the baryon-rich matter passed through the
interaction region (containing approximately 30\% of the total baryon charge),
but also by the baryon-rich regions of the central fireball pushed out to
peripheral rapidities by the subsequent almost one-dimensional expansion of the
central fireball along the beam direction.

Nuclear Theory;High Energy Physics - Experiment;High Energy Physics - Phenomenology