Left panel: colour–colour diagram J − H vs. H − K_s corresponding to the central region of the GALACTICNUCLEUS survey. The black arrows indicate the over-density features that correspond to the spiral arms detected in Sect. 3. The blue dashed lines mark the stellar population belonging to the GC, where the density is significantly different from the rest. Right panel: zoom into the foreground population from the spiral arms and histograms corresponding to the underlying distributions. The red solid line indicates the result of a four-Gaussian fit (green dashed lines for each individual Gaussian). The black arrows show the mean location of the spiral arms. The red arrow indicates the overlap between the first and the second spiral arms in one single feature in the histogram J − H.