The Anisotropy in the Galaxy Velocity Field Originated from the Gravitational Pancaking Effect

Abstract

We analyze the Millennium run semi-analytic galaxy catalog to explore quantitatively the gravitational pancaking effect on the orientation of galaxy velocity field. We first calculate the probability density distribution of the cosine of the angle between the velocity of a field galaxy and the direction normal to a local pancake plane which is determined using two nearest neighbor field galaxies. A clear signal of alignment is detected for the case that the pancake scale is in the range of 5 − 8h −1 Mpc. The tendency of the velocity-pancake alignment is found to still exist when the pancakes are determined using three neighbor galaxies, indicating that it has a spatial coherence. The degree of the velocity-pancake alignment is shown to increase with the velocity magnitude and the local density, while it decreases with the separation distance from the galaxy to the pancake and disappears when the pancake has a filamentary shape. A final conclusion is that our work may provide another clue to understanding the large-scale structure in the universe. Subject headings: cosmology:theory — large-scale structure of universe It is an undeniable fact that there are large-scale anisotropies in the universe. In the context of the cold dark matter (CDM) paradigm, the large-scale anisotropies are understood as a nonlinear manifestation of the primordial tidal field (Bond, Kofman, & Pogosyan 1996). The large-scale anisotropy induced in the distribution and the orientation of galaxies has been extensively studied both observationally and theoretically (Flin & Godlowski 1986;