We provide a new observational test for a key prediction of the \Lambda CDM cosmological model : the contributions of mergers with different halo-to-main-cluster mass ratios to cluster-sized halo growth .
We perform this test by dynamically analyzing seven galaxy clusters , spanning the redshift range 0.13 < z _ { c } < 0.45 and caustic mass range 0.4 - 1.5 10 ^ { 15 } h _ { 0.73 } ^ { -1 } M _ { \odot } , with an average of 293 spectroscopically-confirmed bound galaxies to each cluster .
The large radial coverage ( a few virial radii ) , which covers the whole infall region , with a high number of spectroscopically identified galaxies enables this new study .
For each cluster , we identify bound galaxies .
Out of these galaxies , we identify infalling and accreted halos and estimate their masses and their dynamical states .
Using the estimated masses , we derive the contribution of different mass ratios to cluster-sized halo growth .
For mass ratios between \sim 0.2 and \sim 0.7 , we find a \sim 1 \sigma agreement with \Lambda CDM expectations based on the Millennium simulations I and II .
At low mass ratios , \lesssim 0.2 , our derived contribution is underestimated since the detection efficiency decreases at low masses , \sim 2 \times 10 ^ { 14 } h _ { 0.73 } ^ { -1 } M _ { \odot } .
At large mass ratios , \gtrsim 0.7 , we do not detect halos probably because our sample , which was chosen to be quite X-ray relaxed , is biased against large mass ratios .
Therefore , at large mass ratios , the derived contribution is also underestimated .