Distance and extinction to the Milky Way spiral arms along the Galactic
  centre line of sight
  The position of the Sun inside the Milky Way's disc hampers the study of the
spiral arm structure. We aim to analyse the spiral arms along the line-of-sight
towards the Galactic centre (GC) to determine their distance, extinction, and
stellar population. We use the GALACTICNUCLEUS survey, a JHKs high angular
resolution photometric catalogue (0.2") for the innermost regions of the
Galaxy. We fitted simple synthetic colour-magnitude models to our data via
$\chi^2$ minimisation. We computed the distance and extinction to the detected
spiral arms. We also analysed the extinction curve and the relative extinction
between the detected features. Finally, we built extinction-corrected Ks
luminosity functions (KLFs) to study the stellar populations present in the
second and third spiral arm features. We determined the mean distances to the
spiral arms: $d1=1.6\pm0.2$, $d2=2.6\pm0.2$, $d3=3.9\pm0.3$, and $d4=4.5\pm0.2$
kpc, and the mean extinctions: $A_{H1}=0.35\pm0.08$, $A_{H2}=0.77\pm0.08$,
$A_{H3}=1.68\pm0.08$, and $A_{H4}=2.30\pm0.08$ mag. We analysed the extinction
curve in the near infrared for the stars in the spiral arms and found mean
values of $A_J/A_{H}=1.89\pm0.11$ and $A_H/A_{K_s}=1.86\pm0.11$, in agreement
with the results obtained for the GC. This implies that the shape of the
extinction curve does not depend on distance or absolute extinction. We also
built extinction maps for each spiral arm and obtained that they are
homogeneous and might correspond to independent extinction layers. Finally,
analysing the KLFs from the second and the third spiral arms, we found that
they have similar stellar populations. We obtained two main episodes of star
formation: $>6$ Gyr ($\sim60-70\%$ of the stellar mass), and $1.5-4$ Gyr
($\sim20-30\%$ of the stellar mass), compatible with previous work. We also
detected recent star formation at a lower level ($\sim10\%$) for the third
spiral arm.
