Surface density of clusters in the region (l ∈ (225°,260°)) from Kharchenko et al. (2013) as a function of the heliocentric distance d_hc. The histogram shows the density of all clusters; lines indicate the distribution of clusters with a certain age (blue: clusters younger than 120 Myr, green: clusters between 120 and 300 Myr, red: clusters older than 300 Myr). Positions of spiral arms (Local and Perseus) are indicated, as well as the limiting distance, where the catalogue is complete.