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)?
Thanks, Dylan.
I've looked at both papers. The Abdullah et al. (2025) paper is particularly relevant for confirming the mass trend in β(r) for clusters. However, it doesn't split by environment within mass bins. The sign reversal I'm finding in the environmental dependence of β seems to be a different, complementary result. One that I am thinking to test against their Uchuu-based mass trend. The Iannuzzi & Dolag paper is a great reference for the physical evolution of these orbits.
I'm planning to re-run the environment-split analysis on TNG300 to see if the sign reversal holds with a larger cluster sample to check if my results are solid enough.
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
Perhaps you find some related analyses in:
Thanks, Dylan.
I've looked at both papers. The Abdullah et al. (2025) paper is particularly relevant for confirming the mass trend in β(r) for clusters. However, it doesn't split by environment within mass bins. The sign reversal I'm finding in the environmental dependence of β seems to be a different, complementary result. One that I am thinking to test against their Uchuu-based mass trend. The Iannuzzi & Dolag paper is a great reference for the physical evolution of these orbits.
I'm planning to re-run the environment-split analysis on TNG300 to see if the sign reversal holds with a larger cluster sample to check if my results are solid enough.