Searching for distinctive signatures , which characterize different formation channels of binary black holes ( BBHs ) , is a crucial step towards the interpretation of current and future gravitational wave detections .
Here , we investigate the demography of merging BBHs in young star clusters ( SCs ) , which are the nursery of massive stars .
We performed 4 \times { } 10 ^ { 3 } N-body simulations of SCs with metallicity Z = 0.002 , initial binary fraction 0.4 and fractal initial conditions , to mimic the clumpiness of star forming regions .
Our simulations include a novel population-synthesis approach based on the code MOBSE .
We find that SC dynamics does not affect the merger rate significantly , but leaves a strong fingerprint on the properties of merging BBHs .
More than 50 % of merging BBHs in young SCs form by dynamical exchanges in the first few Myr .
Dynamically formed merging BBHs are significantly heavier than merging BBHs in isolated binaries : merging BBHs with total mass up to \sim { } 120 M _ { \odot } form in young SCs , while the maximum total mass of merging BBHs in isolated binaries with the same metallicity is only \sim { } 70 M _ { \odot } .
Merging BBHs born via dynamical exchanges tend to have smaller mass ratios than BBHs in isolated binaries .
Furthermore , SC dynamics speeds up the merger : the delay time between star formation and coalescence is significantly shorter in young SCs .
In our simulations , massive systems such as GW170729 form only via dynamical exchanges .
Finally \sim { } 2 % of merging BBHs in young SCs have mass in the pair-instability mass gap ( \sim { } 60 - 120 M _ { \odot } ) .
This represents a unique fingerprint of merging BBHs in SCs .