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

We analyze the Millennium Run semianalytic galaxy catalog in order to explore quantitatively the gravitational pancaking effect on the orientation of the 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 that is determined using the two nearest neighbor field galaxies. A clear signal of alignment is detected for the case in which the pancake scale is in the range of 5-8 h super(-1) Mpc. This tendency of velocity-pancake alignment is found to still exist when the pancakes are determined using three nearest 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.