Colour–magnitude diagrams K_s vs. H − K_s. Left panel: best model-fit solution obtained for the CMD K_s vs. H − K_s (see Table 2). Different colours indicate the stars contributing from each of the spiral arms. In particular, purple, green, orange, red, and black dots indicate stars from the first, second, third, and fourth spiral arms, and background sources respectively. Black solid parallelograms mark the selection of reference (mainly main-sequence) stars to analyse the extinction corresponding to each of the spiral arms. Right panel: real data considering completeness correction as indicated in Sect. 3.3. The red dashed line indicates the saturation limit of the K_s photometry (K_s = 11.5 mag). The red solid parallelograms depict the reference main sequence stars according to the best model fit as shown in the left panel. The isochrones corresponding to a stellar population of 1 Gyr and [M/H]= − 0.07 are over-plotted in red (see Sect. 3.3). The blue dashed lines indicate the stars added to the reference main sequence stars to create a K_s luminosity function for the second and third features (see Sect. 7).