Anisotropy and characteristic scales in halo density gradient profiles

¹ Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, Anhui 230026, PR China
e-mail: wambyybz@mail.ustc.edu.cn; whywang@ustc.edu.cn
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
³ Department of Astronomy, University of Massachusetts, Amherst, MA 01003-9305, USA

Received: 24 June 2022
Accepted: 7 September 2022

Abstract

We use a large N-body simulation to study the characteristic scales in the density gradient profiles in and around halos with masses ranging from 10¹² to 10¹⁵ M_⊙. We investigate the profiles separately along the major (T₁) and minor (T₃) axes of the local tidal tensor and how the characteristic scales depend on halo mass, formation time, and environment. We find two prominent features in the gradient profiles: a deep “valley” and a prominent “peak.” We use the Gaussian process regression to fit the gradient profiles and identify the local extrema in order to determine the scales associated with these features. Around the valley, we identify three types of distinct local minima, corresponding to caustics of particles orbiting around halos. The appearance and depth of the three caustics depend on the direction defined by the local tidal field, formation time, and environment of halos. The first caustic is located at r > 0.8R₂₀₀, corresponding to the splashback feature, and is dominated by particles at their first apocenter after infall. The second and third caustics, around 0.6R₂₀₀ and 0.4R₂₀₀, respectively, can be determined reliably only for old halos. The three caustics are consistent with the prediction of self-similar gravitational collapse. The first caustic is always the most prominent feature along T₃, but may not be true along T₁ or in azimuthally averaged profiles, suggesting that caution must be taken when using averaged profiles to investigate the splashback radius. We find that the splashback feature is approximately isotropic when proper separations are made between the first and the other caustics. We also identify a peak feature located at ∼2.5R₂₀₀ in the density gradient profile. This feature is the most prominent along T₁ and is produced by mass accumulations from the structure outside halos. We also discuss the origins of these features and their observational implications.