ANDANGULARMOMENTAOF肃DMHALOS.PDFVIP

The Astrophysical Journal, 627:647–665, 2005 July 10 A # 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A. INTERNAL AND EXTERNAL ALIGNMENT OF THE SHAPES AND ANGULAR MOMENTA OF CDM HALOS Jeremy Bailin1,2,3 and Matthias Steinmetz1,2,4 Receivved 2004 August 9; accepted 2005 March 20 ABSTRACT We investigate how the shapes and angular momenta of galaxy and group mass dark matter halos in a CDM N-body simulation are correlated internally and how they are aligned with respect to the location and properties of surrounding halos. We explore these relationships down to halos of much lower mass (1011 h1 M) than previous studies. The halos are triaxial, with c/a ratios of 0:6 0:1 and a mean two-dimensional projected ellipticity of e ¼ 0:24. More massive halos are more flattened. The axis ratios rise out to 0.6r , beyond which they drop. The h i vir principal axes, in particular the minor axes, are very well aligned within 0.6 rvir . High-mass halos show particularly strong internal alignment. The angular momentum vectors are also reasonably well aligned except between the very outermost and very innermost regions of the halo. The angular momentum vectors tend to align with the minor axes, with a mean misalignment of 25 , and lie perpendicular to the major and intermediate axes. The properties of a halo at 0.4 rvir are quite characteristic of the