Environment-dependent sign reversal in satellite β(r) in TNG100?
Shivangi Gupta
20 Sep
Hi Dylan,
I’m using IllustrisTNG‑100 (snapshot 99, z = 0) to measure satellite galaxy velocity anisotropy
β(r) = 1 − σ_t²(r)/(2σ_r²(r))
across five host‑halo mass bins (log₁₀ M₂₀₀ ≈ 11.0–14.5).
After validating the pipeline on synthetic halos with known input β, I found the expected mass trend: tangentially biased orbits at group scales, becoming radially biased at cluster scales. I also see a sensible quenched‑fraction gradient with radius.
My question is about a more tentative result. At fixed mass, β shows a statistically significant dependence on large‑scale environment (per‑halo partial Spearman, controlling for mass). More interestingly, splitting each mass bin by median environment gives a sign reversal in Δβ: in denser environments β becomes more tangential at group/intermediate masses, but more radial at cluster masses. The cluster bin is small (N = 127 halos; ~62–65 per environment half), so I’m treating this as suggestive rather than a firm detection.
Has this kind of mass‑dependent sign reversal in the environmental dependence of satellite β shown up before in TNG analyses, or is it something that might be worth following up (e.g., with TNG300 or a more refined environment definition)?
Hi Dylan,
I’m using IllustrisTNG‑100 (snapshot 99, z = 0) to measure satellite galaxy velocity anisotropy
β(r) = 1 − σ_t²(r)/(2σ_r²(r))
across five host‑halo mass bins (log₁₀ M₂₀₀ ≈ 11.0–14.5).
After validating the pipeline on synthetic halos with known input β, I found the expected mass trend: tangentially biased orbits at group scales, becoming radially biased at cluster scales. I also see a sensible quenched‑fraction gradient with radius.
My question is about a more tentative result. At fixed mass, β shows a statistically significant dependence on large‑scale environment (per‑halo partial Spearman, controlling for mass). More interestingly, splitting each mass bin by median environment gives a sign reversal in Δβ: in denser environments β becomes more tangential at group/intermediate masses, but more radial at cluster masses. The cluster bin is small (N = 127 halos; ~62–65 per environment half), so I’m treating this as suggestive rather than a firm detection.
Has this kind of mass‑dependent sign reversal in the environmental dependence of satellite β shown up before in TNG analyses, or is it something that might be worth following up (e.g., with TNG300 or a more refined environment definition)?
Full report and reproducible pipeline (with validation tests and the β(r) atlas):
https://github.com/pikaism/TNG-anisotropy-atlas