ALMA-IMF XV: N_2H^+ kinematic analysis on the intermediate protocluster G353.41

The ALMA-IMF Large Program provides multi-tracer observations of 15 Galactic massive protoclusters at matched sensitivity and spatial resolution. We focus on the dense gas kinematics of the G353.41 protocluster traced by N_2H^+ (1-0), with a critical density of 2×10^5 cm^-3, and spatial resolution ∼0.02 pc. G353.41, at a distance of 2 kpc, is embedded in a larger scale (∼8 pc) filament and has a mass of 2500 M_⊙ within 1.3×1.3 pc^2. We extract the N_2H^+ isolated line component and we decompose it by fitting up to 3 Gaussian velocity components. This allows us to identify velocity structures that are either muddled or impossible to identify in the traditional position-velocity diagram. We identify multiple velocity gradients (VGs) on large and small scales. We find good agreement between the N_2H^+ and the previously reported DCN core velocities, suggesting that cores are kinematically coupled to the dense gas in which they form. We measure 9 converging V-shaped VGs, located in filaments, that are sometimes associated with cores near their point of convergence. The average timescale associated with the V-shapes are ∼67 kyr, or about twice the free-fall time of cores in the same area (∼ 33 kyr) but substantially shorter than protostar lifetime estimates (∼ 0.5 Myr). We interpret these V-shapes as inflowing gas feeding the regions near cores and we derive their mass accretion rates. This feeding might lead to further filament collapse and formation of new cores. We suggest that the protocluster is collapsing on large scales, but the velocity signature of collapse is slow compared to pure free-fall. Thus these data are consistent with a comparatively slow global protocluster contraction under gravity, and faster core formation within, suggesting the formation of multiple generations of stars over the protocluster lifetime.