Radial alignment of elliptical galaxies by the tidal force of a cluster of galaxies

Abstract

Unlike the random radial orientation distribution of field elliptical galaxies, galaxies in a cluster are expected to point preferentially towards the center of the cluster, as a result of the cluster's tidal force on its member galaxies. In this work an analytic model is formulated to simulate this effect. The deformation time scale of a galaxy in a cluster is usually much shorter than the time scale of change of the tidal force; the dynamical process of the tidal interaction within the galaxy can thus be ignored. An equilibrium shape of a galaxy is then assumed to be the surface of equipotential, which is the sum of the self-gravitational potential of the galaxy and the tidal potential of the cluster at this location. We use a Monte-Carlo method to calculate the radial orientation distribution of these galaxies, by assuming the NFW mass profile of the cluster and the initial ellipticity of field galaxies. The radial angles show a single peak distribution centered at zero. The Monte-Carlo simulations also show that a shift of the reference center from the real cluster center weakens the anisotropy of the radial angle distribution. Therefore, the expected radial alignment cannot be revealed if the distribution of spatial position angle is used instead of that of radial angle. The observed radial orientations of elliptical galaxies in cluster Abell 2744 are consistent with the simulated distribution.

Catalog of Ultra-Diffuse Galaxies in the Coma Clusters From Subaru Imaging Data*

“…Several mechanisms have been proposed to make the radial alignment in a cluster, including tidal torque (Ciotti & Dutta 1994;Adami et al 2009;Pereira & Bryan 2010;Rong et al 2015a) and primordial alignment (Plionis et al 2003;Faltenbacher et al 2007;Rong et al 2015b).…”

Time evolution of intrinsic alignments of galaxies

“…Because IA are impacted by the large-scale tidal field and other fields, they are also potentially of interest as a probe of the LSS and its effects on galaxies. This effect has already been studied in simulations [38][39][40] and observationally, on small scales in highly clustered environments [41,42], for red galaxies in SDSS-BOSS [43][44][45] and Mega-Z [46], and for blue galaxies in CFHTLenS [47] and WiggleZ [48]. Observations suggest as well that different galaxy populations exhibit different magnitudes of the IA signal, with more massive galaxies aligning more strongly with the LSS [49] and different color/morphological types displaying different IA amplitudes, underscoring the need for robust models of the physical processes which influence IA.…”

Exploring the origin of ultra-diffuse galaxies in clusters from their primordial alignment

“…The illustrations of the three angles θ, φ, and ξ, which are used to test for the primordial alignment, and radial alignment, as well as distributions of the satellite galaxies, respectively. oriented toward the center of the parent cluster, however, is crucial to study the tidal interaction between the satellites and central dominant galaxy/parent cluster after galaxies falling into a cluster knot along the large-scale filaments, as well as mass distribution and dynamical state of the parent cluster (e.g., Pereira et al 2008;Rong et al 2015aRong et al , 2019bSchneider et al 2013).…”