Infall Artifacts

Peculiar velocity flows associated with infall towards large galaxy clusters can cause striking distortions in redshift-space. For example, spherical infall produces a diamond-shaped artifact with concave edges, called the Kaiser effect [1]. Similar concave caustics show up around many clusters, like the Virgo cluster at left [2,5].

The figure below illustrates the distortion caused by spherical infall and shows how an observer's peculiar velocity modifies the resulting artifact [3].

The left panel shows a set of concentric nested shells in real-space. Shells inside the turn-around shell are falling towards the center; shells outside are expanding outward, more slowly than they would be if there was no infall. The right panel shows how each shell distorts in redshift-space. Note that shells inside the turnaround shell invert. Also note how the observer's transverse peculiar velocity, shown as an arrow, causes the artifact to tilt downward in the direction the observer is moving [2]. Rotational flow around the cluster can have a similar effect [3].

Although some redshift-space structure around clusters resembles artifacts produced by spherical infall, many clusters show features which cannot be reproduced by spherical infall alone. For example, structure around the Virgo cluster can be matched fairly well by a spherical infall model on one side (Virgo Southern Extension), but not on the other side (Ursa Major group) [2]. However, the appearance of these mystery features suggests that they also are artifacts, produced by some type of velocity field other than spherical infall. Distinguishing between what is real structure and what is artifact in redshift maps is important for such endeavors as the calibration of distance indicators [4].

There is some evidence that the Ursa Major group in the Local Supercluster may have a small finger-of-god embedded within it, at the "pinch point" of the caustics [5]. Since there is a similar redshift-space structure in the Southern Supercluster (Dorado, to one side of the central Fornax cluster), this suggests that a similar type of velocity field produced both structures. However, exactly what sort of flow this might be is not yet clear.

One possibility is a flow with a rotational component about the small side clusters. For example, circular rotational flow around a cluster can produce a bowtie-like redshift-space artifact, bisecting the cluster's finger-of-god at the pinch point [2]. Elliptical flow can produce an asymmetric structure with a similar pinch point, which does not necessarily bisect the central finger [6]. Although neither type of artifact matches the Ursa Major and Dorado structures in detail, that produced by elliptical flow is more similar. Artifacts from filamentary infall do not match at all [6].

It is our group's hope that eventually one may be able to look at a redshift map and read the flows that may be present, by consulting a sufficiently comprehensive catalog of redshift artifacts produced by different types of peculiar velocity fields. Such an approach would complement existing techniques. See reference [6] for some preliminary entries in such a catalog.

References