tokens: for further studies of substructures and galaxy populations. The paper opens with a short description of the 15 constrained simulations used for the study proposed in this paper and of the unique Virgo candidate identified in each one. In a third section, the merger trees and in particular the main and second (second after the main) branches are studied in detail. Finally, we conclude that the constrained scheme proves to be efficient to some extent also at the merger tree level and provides some further indications regarding the mergers that formed the Virgo cluster within the last few gigayears. 2 Virgo halos 2.1 Constrained simulations Sorce et al. (2016b ) described in detail the scheme used to build the constrained initial conditions and to run the simulations. Furthermore the introduction of this paper summarises the various existing techniques. Thus, we summarise in this section only the main steps required to produce the simulations constrained with observational radial peculiar velocity catalogs we use here. We also give a brief description of their purpose with the latest references in the literature of the algorithms used: grouping (e.g., Tully 2015a , b ) of the radial peculiar velocity catalog to remove nonlinear virial motions that would affect the linear reconstruction obtained with the linear method (e.g., Sorce et al. 2017 ; Sorce Tempel 2017 ); minimizing the biases ( Sorce 2015 ) inherent to any observational radial peculiar velocity catalog; reconstructing the cosmic displacement field with the Wiener-Filter technique (linear minimum variance estimator, WF, Zaroubi et al. 1995 , 1999 ) applied to the peculiar velocity constraints; relocating constraints to the positions of their progenitors using the reverse Zel’dovich approximation and the reconstructed cosmic displacement field ( Doumler et al. 2013 ) and replacing noisy radial peculiar velocities by their WF 3D reconstructions ( Sorce et al. 2014 ). This ensures that, after evolving the structures with an N-body code from an early redshift until today, structures are at the same position (within the 2 h −1 Mpc limit of the linear-threshold) to that observed. Keeping constraints at their current position to build initial conditions would indeed result in a shift of ≥10 h −1 Mpc between observed and simulated structures after a complete evolution of the initial conditions until today; producing density fields constrained by the modified observational peculiar velocities combined with a random realization to restore statistically the missing structures using the constrained realization technique (CR, Hoffman Ribak 1991 , 1992 ); rescaling the density fields to build constrained initial conditions and increasing the resolution by adding small-scale features (e.g., GINNUNGAGAP code 1 ). Tully et al. (2013) supplied the observational catalog used as constraints. Since the Virgo cluster is the object of study here, Fig. 1 permits us to grasp the distribution of the constraints
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