Evaluating bulk flow estimators for CosmicFlows-4 measurements

Understand

For over a decade there have been contradictory claims in the literature about whether the local bulk flow motion of galaxies is consistent or in tension with the $\Lambda$CDM model.

• While it has become evident that systematics affect bulk flow measurements, systematics in the estimators have not been widely investigated.
• In this work, we thoroughly evaluate the performance of four estimator variants, including the Kaiser maximum likelihood estimator (MLE) and the minimum variance estimator (MVE).
• We find that these estimators are unbiased, however their precision may be strongly correlated with the survey geometry.