  Lattice studies suggest that at zero baryon chemical potential and increasing
temperature there are three characteristic regimes in QCD that are connected by
smooth analytical crossovers: a hadron gas regime at T < T_ch ~ 155 MeV, an
intermediate regime, called stringy fluid, at T_ch < T < ~ 3 T_ch, and a
quark-gluon plasma regime at higher temperatures. These regimes have been
interpreted to reflect different approximate symmetries and effective degrees
of freedom. In the hadron gas the effective degrees of freedom are hadrons and
the approximate chiral symmetry of QCD is spontaneously broken. The
intermediate regime has been interpreted as lacking spontaneous chiral symmetry
breaking along with the emergence of new approximate symmetry, chiral spin
symmetry, that is not a symmetry of the Dirac Lagrangian, but is a symmetry of
the confining part of the QCD Lagrangian. While the high temperature regime is
the usual quark-gluon plasma which is often considered to reflect
"deconfinement" in some way. This paper explores the behavior of these regimes
of QCD as the number of colors in the theory, N_c, gets large. In the large N_c
limit the theory is center-symmetric and notions of confinement and
deconfinement are unambiguous. The energy density is O(N_c^0) in the meson gas,
O(N_c^1) in the intermediate regime and O(N_c^2) in the quark-gluon plasma
regime. In the large N_c limit these regimes may become distinct phases
separated by first order phase transitions. The intermediate phase has the
peculiar feature that glueballs should exist and have properties that are
unchanged from what is seen in the vacuum (up to 1/N_c corrections), while the
ordinary dilute gas of mesons with broken chiral symmetry disappears and
approximate chiral spin symmetry should emerge.
