The Cosmic Mach Number as an Environment Measure for the Underlying Dark Matter Density Field

Romain Meriot*, Sadegh Khochfar, Jose Onorbe, Britton Smith

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Using cosmological dark matter (DM) only simulations of a (1.6 Gpc/h)3 volume from the Legacy simulation project, we calculate Cosmic Mach Numbers (CMN) and perform a theoretical investigation of their relation with halo properties and features of the density field to gauge their use as an measure of the environment. CMNs calculated on individual spheres show correlations with both the overdensity in a region and the density gradient in the direction of the bulk flow around that region. To reduce the scatter around the median of these correlations, we introduce a new measure, the rank ordered Cosmic Mach number (⁠M^g⁠), which shows a tight correlations with the overdensity δ=ρ−ρ¯ρ¯⁠. Measures of the large-scale density gradient as well as other average properties of the halo population in a region show tight correlations with M^g as well. Our results in this first empirical study suggest that M^g is an excellent proxy for the underlying density field and hence environment that can circumvent reliance on number density counts in a region. For scales between 10 and 100Mpch−1⁠, Mach numbers calculated using DM haloes (> 1012 M) that would typically host massive galaxies are consistent with theoretical predictions of the linear matter power spectrum at a level of 10 percent due to non-linear effects of gravity. At redshifts z ≥ 3, these deviations disappear. We also quantify errors due to missing large-scale modes in simulations. Simulations of box size ≤1 Gpc h−1 typically predict CMNs 10–30 per cent too small on scales of ∼100 Mpc h−1.
Original languageEnglish
Pages (from-to)27-40
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date25 Feb 2022
Publication statusPublished - 1 May 2022


  • astro-ph.CO


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