2025A&A...696A.202S

ALMA-IMF observed 15 massive protoclusters capturing multiple spectral lines and the continuum emission. Here, we focus on the massive protocluster G351.77 (∼2500 M_⊙, estimated from single-dish continuum observations) located at 2 kpc. We trace the dense gas emission and kinematics with N₂H⁺ (1-0) at ∼4 kau resolution. We estimate an N₂H⁺ relative abundance of ~(1.66 ± 0.46) × 10^–10. We decompose the N₂H⁺ emission into up to two velocity components, highlighting the kinematic complexity in the dense gas. By examining the position-velocity (PV) and position-position-velocity (PPV) diagrams on small scales, we observe clear inflow signatures (V-shapes) associated with 1.3 mm cores. The most prominent V-shape has a mass inflow rate of ∼13.45 × 10^–4 M_⊙ yr^–1 and a short timescale of ∼11.42 kyr. We also observe V-shapes without associated cores. This suggests both that cores or centers of accretion exist below the 1.3 mm detection limit, and that the V-shapes may be viable tracers of very early accretion and star formation on ∼4 kau scales. The large-scale PV diagram shows that the protocluster is separated into two principal velocity structures separate by ∼2 km s^–1. Combined with smaller-scale DCN and H₂CO emission in the center, we propose a scenario of larger-scale slow contraction with rotation in the center based on simple toy models. This scenario is consistent with previous lines of evidence, and leads to the new suggestion of outside-in evolution of the protocluster as it collapses. The gas depletion times implied by the V-shapes are short (∼0.3 Myr), requiring either very fast cluster formation, and/or continuous mass feeding of the protocluster. The latter is possible via the Mother Filament that G351.77 is forming out of. The remarkable similarities in the properties of G351.77 and the recently published work in G353.41 indicate that many of the physical conditions inferred via the ALMA-IMF N₂H⁺ observations may be generic to protoclusters.