  The thermal-photon emission from strongly coupled gauge theories at finite
temperature is calculated by using holographic models for QCD in the Veneziano
limit (V-QCD). These emission rates are then embedded in hydrodynamic
simulations combined with prompt photons from hard scattering and the thermal
photons from hadron gas to analyze the spectra and anisotropic flow of direct
photons at RHIC and LHC. The results from different sources responsible for the
thermal photons in the quark gluon plasma (QGP) including the weakly coupled
QGP (wQGP) from perturbative calculations, strongly coupled $\mathcal{N}$=4
super Yang-Mills (SYM) plasma (as a benchmark for reference), and Gubser's
phenomenological model mimicking the strongly coupled QGP (sQGP) are then
compared. It is found that the direct-photon spectra are enhanced in the
strongly coupled scenario compared with the ones in the wQGP, especially at
intermediate and high momenta, which improve the agreements with data.
Moreover, by using IP-glassma initial states, both the elliptic flow and
triangular flow of direct photons are amplified at high momenta ($p_T$>2.5 GeV)
for V-QCD, while they are suppressed at low momenta compared to wQGP. The
distinct results in holography stem from the blue-shift of emission rates in
strong coupling. In addition, the spectra and flow in small collision systems
were evaluated for future comparisons. It is found that thermal photons from
the deconfined phase are substantial to reconcile the spectra and flow at high
momenta.
