  Partonic matter produced in the early stage of ultrarelativistic
nucleus-nucleus collisions is assumed to be composed mainly of gluons, and
quarks and antiquarks are produced at later times. The comparable hydrodynamic
simulations of heavy-ion collisions for (2+1)-flavor and Yang-Mills equations
of state performed by using three different hydrodynamic codes are presented.
Assuming slow chemical equilibration of quarks, the spectra and elliptic flows
of thermal dileptons and photons are calculated for central Pb+Pb collisions at
the LHC energy of $\sqrt{s_{_{\rm NN}}} = 2.76$ TeV. It is shown that a
suppression of quarks at early times leads to a significant reduction of the
yield of the thermal dileptons, but only to a rather modest suppression of the
$p_T$-distribution of direct photons. It is demonstrated that an enhancement of
photon and dilepton elliptic flows might serve as a promising signature of the
pure-glue initial state. Calculations based on Bjorken hydrodynamics suggest
that collisions of small systems at intermediate energies available at RHIC or
future FAIR facilities may show stronger effects associated with initial pure
gluodynamic evolution.
