  Deep JHK-Brg photometry of the southern arm of the grand-design spiral galaxy
NGC 2997 was obtained by ISAAC/VLT. All sources in the field brighter than K=19
mag were located. Color-color diagrams were used to identify young stellar
complexes among the extended sources. Ages can be estimated for the youngest
complexes and correlated with azimuthal distances from the spiral arms defined
by the K-band intensity variation. The extended sources with Mk <-12 mag
display a diffuse appearance and are more concentrated inside the arm region
than fainter ones, which are compact and uniformly distributed in the disk. The
NIR colors of the bright diffuse objects are consistent with them being young
starforming complexes with ages <10 Myr and reddened by up to 8 mag of visual
extinction. They show a color gradient as a function of their azimuthal
distance from the spiral arms. Interpreting this gradient as an age variation,
the pattern speed Op = 16 km/s/kpc of the main spiral was derived assuming
circular motion. The alignment and color gradient of the bright, diffuse
complexes strongly support a density wave scenario for NGC 2997. Only the
brightest complexes with Mk <-12 mag show a well aligned structure along the
arm, suggesting that a strong compression in the gas due to the spiral
potential is required to form these most massive aggregates, while smaller
starforming regions are formed more randomly in the disk. The sharp transition
between the two groups at Mk = -12 mag may be associated with expulsion of gas
when the first supernovae explode in the complex.
