Galaxy cluster mass accretion rates from IllustrisTNG

We used simulated cluster member galaxies from the TNG300-1 run of the IllustrisTNG simulations to develop a technique for measuring the galaxy cluster mass accretion rate (MAR) that can be applied directly to observations. We analyzed 1318 IllustrisTNG clusters of galaxies with M-200c > 10(14) M-circle dot and 0.01 <= z <= 1.04. The MAR we derived is the ratio between the mass of a spherical shell located in the infall region and the time for the infalling shell to accrete onto the virialized region of the cluster. At fixed redshift, an approximately one order of magnitude increase in M-200c results in a comparable increase in MAR. At fixed mass, the MAR increases by a factor of approximately five from z = 0.01 to z = 1.04. The MAR estimates derived from the caustic technique are unbiased and lie within 20% of the MARs based on the true mass profiles. This agreement is crucial for observational derivation of the MAR. The IllustrisTNG results are also consistent with (i) previous merger tree approaches based on N-body dark matter only simulations and with (ii) previously determined MARs of real clusters based on the caustic method. Future spectroscopic and photometric surveys will provide MARs of enormous cluster samples with mass profiles derived from both spectroscopy and weak lensing. Combined with future larger volume hydrodynamical simulations that extend to higher redshift, the MAR promises important insights into the evolution of massive systems of galaxies.