Reconstructing the cosmic velocity and tidal fields with galaxy groups selected from the Sloan Digital Sky Survey

Abstract

Cosmic velocity and tidal fields are important for the understanding of the cosmic web and the environments of galaxies, and can also be used to constrain cosmology. In this paper, we reconstruct these two fields in the Sloan Digital Sky Survey (SDSS) volume from dark matter halos represented by galaxy groups. Detailed mock catalogues are used to test the reliability of our method against uncertainties arising from redshift distortions, survey boundaries, and false identifications of groups by our group finder. We find that both the velocity and tidal fields, smoothed on a scale of ∼ 2 h −1 Mpc, can be reliably reconstructed in the inner region (∼ 66%) of the survey volume.The reconstructed tidal field is used to split the cosmic web into four categories: clusters, filaments, sheets, and voids, depending on the sign of the eigenvalues of local tidal tensor. The reconstructed velocity field nicely shows how the flows are diverging from the centers of voids, and converging onto clusters, while sheets and filaments have flows that are convergent along one and two directions, respectively. We use the reconstructed velocity field and the Zel'dovich approximation to predict the mass density field in the SDSS volume as function of redshift, and find that the mass distribution closely follows the galaxy distribution even on small scales. We find a large-scale bulk flow of about 117 km s −1 in a very large volume, equivalent to a sphere 2 Huiyuan Wang et al.with a radius of ∼ 170 h −1 Mpc, which seems to be produced by the massive structures associated with the SDSS Great Wall. Finally, we discuss potential applications of our reconstruction to study the environmental effects of galaxy formation, to generate initial conditions for simulations of the local Universe, and to constrain cosmological models. The velocity, tidal and density fields in the SDSS volume, specified on a Cartesian grid with a spatial resolution of ∼ 700 h −1 kpc, are available from the authors upon request.

Detection of the Velocity Shear Effect on the Spatial Distributions of the Galactic Satellites in Isolated Systems

“…As stated in Wang et al (2012), the accuracy of the reconstructed density and peculiar velocity fields depends on the filtering scale. On small scales R f < 2l c , the reconstructed fields are expected to be inaccurate owing to the limited spatial resolution.…”

“…To determine the principal axes of the local velocity shears at the positions of the selected host galaxies, we utilize the density and velocity fields reconstructed by Wang et al (2009Wang et al ( , 2012 from the galaxy group catalog based on the SDSS DR7 (Blanton et al 2005;Abazajian et al 2009). Wang et al (2012) estimated the masses of the galaxy groups according to the prescriptions of Yang et al (2007), which is based on the ranking of the stellar masses of the galaxies and a varying mass-to-light ratio model. For a detailed description of the halo-mass estimator, see Yang et al (2007).…”

“…For a detailed description of the halo-mass estimator, see Yang et al (2007). Selecting only those galaxy groups with M 10 12 h −1 M located in the "survey volume" that spans the redshift range of 0.01 z 0.12, and taking into account the redshift distortion effect on the positions of the selected galaxy groups, Wang et al (2012) reconstructed the density field δ(x) with the help of the halo-tracing algorithm. The reconstructed density field δ(x) was defined on the 494 × 892 × 499 cubic cells composing the survey volume, where each cell has a linear size of l c ≈ 0.71 h −1 Mpc and its position x is measured in the equatorial coordinate systems.…”

“…Transforming the density field δ(x) into the Fourier space and assuming that the peculiar velocity field v(x) is curl free and thus related to the density field as v ∝ ∇∇ −2 δ, Wang et al (2012) determined the Fourier amplitudes of the filtered peculiar velocity fieldṽ(k) from those of the density fieldδ(k) asṽ(k) ∝ (k/k 2 )δ (k)W (kR f ), where k is the Fourier-space wave vector with magnitude of k ≡ |k| and W (kR f ) is the Fourier-space Gaussian window function with a filtering radius of R f . The peculiar velocity field smoothed on the scale of R f was reconstructed via the inverse Fourier transformation ofṽ.…”

“…As can be seen, the projected positions of the satellites of the isolated galactic systems are aligned with the projected minor principal axes and antialigned with the projected major principal axes of the local velocity shears. Although the strength of (Wang et al 2012). …”