Generation of vorticity and velocity dispersion by orbit crossing

Abstract

We study the generation of vorticity and velocity dispersion by orbit crossing using cosmological numerical simulations, and calculate the backreaction of these effects on the evolution of large-scale density and velocity divergence power spectra. We use Delaunay tessellations to define the velocity field, showing that the power spectra of velocity divergence and vorticity measured in this way are unbiased and have better noise properties than for standard interpolation methods that deal with mass weighted velocities. We show that high resolution simulations are required to recover the correct large-scale vorticity power spectrum, while poor resolution can spuriously amplify its amplitude by more than one order of magnitude. We measure the scalar and vector modes of the stress tensor induced by orbit crossing using an adaptive technique, showing that its vector modes lead, when input into the vorticity evolution equation, to the same vorticity power spectrum obtained from the Delaunay method. We incorporate orbit crossing corrections to the evolution of large scale density and velocity fields in perturbation theory by using the measured stress tensor modes. We find that at large scales (k ≃ 0.1 h Mpc −1 ) vector modes have very little effect in the density power spectrum, while scalar modes (velocity dispersion) can induce percent level corrections at z = 0, particularly in the velocity divergence power spectrum. In addition, we show that the velocity power spectrum is smaller than predicted by linear theory until well into the nonlinear regime, with little contribution from virial velocities.

The properties of cosmic velocity fields

“…Both, the results of these authors as well as ours indicate that this is mostly due to the slow convergence of the flow vorticity. Somewhat unexpectedly however, Pueblas & Scoccimarro (2009) do not find an obvious resolution dependence of the convergence spectrum (while they do for the vorticity spectrum). Since vorticity is driven by small-scale non-linearities, we find that the slow convergence of vorticity is, at least to some degree, driven by a lack of resolution at the non-linear scale: when running the L1000N512 box with a force resolution of only 8 times the mean inter-particle distance we observe a significant drop of the large-scale vorticity spectrum (not shown).…”

“…based on the particle positions only, which has been employed commonly in the literature (e.g. Icke & van de Weygaert 1987;Bernardeau & van de Weygaert 1996;Pueblas & Scoccimarro 2009;Jennings et al 2011). Such a method does not involve a projection of the actual distribution function, but interpolates between the particles on the fine-grained distribution in configuration space (i.e.…”

“…As shown by Bernardeau & van de Weygaert (1996) and Pueblas & Scoccimarro (2009), Delaunay tessellations of the particle distribution can be used to better control noise and measurement errors of velocity power spectra from N -body simulations. In addition, Jennings (2012) showed that the DTFE estimator (Schaap & van de Weygaert 2000;Pelupessy et al 2003;Schaap 2007), as implemented by Cautun & van de Weygaert (2011), gives much more reliable spectral properties compared to fixed kernel smoothing (confirming a similar finding by Pueblas & Scoccimarro 2009). …”

“…Tessellations allow to define unique volumes around particles (by Voronoi tessellations) or by connecting the particles (Delaunay triangulations: e.g. Icke & van de Weygaert 1987;Bernardeau & van de Weygaert 1996;Pueblas & Scoccimarro 2009), and they have been particularly popular in recent analyses of N -body simulations (e.g. Pandey et al 2013).…”

“…Dekel 1994, for a classic review), of the kinetic Sunyaev-Zeldovich effect, as well as for more theoretically motivated questions regarding the formation and dynamics of the cosmic large-scale structure (e.g. Bertschinger & Dekel 1989;Bernardeau & van de Weygaert 1996;Pichon & Bernardeau 1999;Pueblas & Scoccimarro 2009;Kitaura et al 2012).…”