Modelling simple stellar populations in the near-ultraviolet to
  near-infrared with the X-shooter Spectral Library (XSL)
  We present simple stellar population models based on the empirical X-shooter
Spectral Library (XSL) from NUV to NIR wavelengths. The unmatched
characteristics of relatively high resolution and extended wavelength coverage
($350-2480$ nm, $R\sim10\,000$) of the XSL population models bring us closer to
bridging optical and NIR studies of intermediate and old stellar populations.
It is now common to find good agreement between observed and predicted NUV and
optical properties of stellar clusters due to our good understanding of the
main-sequence and early giant phases of stars. However, NIR spectra of
intermediate-age and old stellar populations are sensitive to cool K and M
giants. The asymptotic giant branch, especially the thermally pulsing
asymptotic giant branch, shapes the NIR spectra of $0.5-2$ Gyr old stellar
populations; the tip of the red giant branch defines the NIR spectra of
populations with ages larger than that. We construct sequences of the average
spectra of static giants, variable-rich giants, and C-rich giants to include in
the models separately. The models span the metallicity range $-2.2<[Fe/H]<+0.2$
and ages above 50 Myr, a broader range in the NIR than in other models based on
empirical spectral libraries. Our models can reproduce the integrated optical
colours of the Coma cluster galaxies at the same level as other semi-empirical
models found in the literature. In the NIR, there are notable differences
between the colours of the models and Coma cluster galaxies. The XSL models
expand the range of predicted values of NIR indices compared to other models
based on empirical libraries. Our models make it possible to perform in-depth
studies of colours and spectral features consistently throughout the optical
and the NIR range to clarify the role of evolved cool stars in stellar
populations.
