Distortion of infall regions in redshift space-I

We show that spherical infall models (SIMs) can better describe some galaxy clusters in redshift slice space than in traditional axially convolved projection space. This is because in SIM, the presence of transverse motion between cluster and observer, and/or shear flow about the cluster (such as rotation), causes the infall artifact to tilt, obscuring the characteristic two-trumpet profile; and some clusters resemble such tilted artifacts. We illustrate the disadvantages of applying SIM to convolved data and, as an alternative, introduce a method fitting a tilted 2D envelope to determine a 3D envelope. We also introduce a fitting algorithm and test it on toy SIM simulations as well as three clusters (Virgo, A1459 and A1066). We derive relations useful for using the tilt and width-to-length ratio of the fitted envelopes to analyse peculiar velocities. We apply them to our three clusters as a demonstration. We find that transverse motion between cluster and observer can be ruled out as sole cause of the observed tilts, and that a multicluster study could be a feasible way to find our infall towards Virgo cluster.