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We present simple stellar population models based on the empirical X-shooterSpectral Library (XSL) from NUV to NIR wavelengths. The unmatchedcharacteristics of relatively high resolution and extended wavelength coverage($350-2480$ nm, $R\sim10\,000$) of the XSL population models bring us closer tobridging optical and NIR studies of intermediate and old stellar populations.It is now common to find good agreement between observed and predicted NUV andoptical properties of stellar clusters due to our good understanding of themain-sequence and early giant phases of stars. However, NIR spectra ofintermediate-age and old stellar populations are sensitive to cool K and Mgiants. The asymptotic giant branch, especially the thermally pulsingasymptotic giant branch, shapes the NIR spectra of $0.5-2$ Gyr old stellarpopulations; the tip of the red giant branch defines the NIR spectra ofpopulations with ages larger than that. We construct sequences of the averagespectra of static giants, variable-rich giants, and C-rich giants to include inthe 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 onempirical spectral libraries. Our models can reproduce the integrated opticalcolours of the Coma cluster galaxies at the same level as other semi-empiricalmodels found in the literature. In the NIR, there are notable differencesbetween the colours of the models and Coma cluster galaxies. The XSL modelsexpand the range of predicted values of NIR indices compared to other modelsbased on empirical libraries. Our models make it possible to perform in-depthstudies of colours and spectral features consistently throughout the opticaland the NIR range to clarify the role of evolved cool stars in stellarpopulations.

Modelling simple stellar populations in the near-ultraviolet to near-infrared with the X-shooter Spectral Library (XSL)