Deprojecting galaxy-cluster cold fronts: evidence for bulk, magnetized spiral flows

Tangential discontinuities known as cold fronts (CFs) are abundant in groups and clusters of galaxies (GCs). The relaxed, spiral-type CFs were initially thought to be isobaric, but a significant, 10 per cent–20 per cent jump in the thermal pressure P_t was reported when deprojected CFs were stacked, interpreted as missing P_t below the CFs (i.e. at smaller radii r) due to a locally enhanced non-thermal pressure P_nt. We report a significant (∼4.3σ) deprojected jump in P_t across a single sharp CF in the Centaurus cluster. Additional seven CFs are deprojected in the GCs A2029, A2142, A2204, and Centaurus, all found to be consistent (stacked: ∼1.9σ) with similar pressure jumps. Combining our sample with high quality deprojected CFs from the literature indicates pressure jumps at significance levels ranging between 2.7σ and 5.0σ, depending on assumptions. Our nominal results are consistent with P_nt (0.1–0.3)P_t just below the CF. We test different deprojection and analysis methods to confirm that our results are robust, and show that without careful deprojection, an opposite pressure trend may incorrectly be inferred. Analysing all available deprojected data, we also find: (i) small variations around the mean density and temperature CF contrast q within each GC, monotonically increasing with the GC mass M₂₀₀ as q ∝ M₂₀₀^0.23±^0.04; (ii) hydrostatic mass discontinuities indicating fast bulk tangential flows below all deprojected CFs, with a mean Mach number ∼0.76; and (iii) the newly deprojected CFs are consistent (stacked: ∼2.9σ) with a 1.25⁺₋⁰₀⁰⁹₀₈ metallicity drop across the CF. These findings suggest that GCs quite generally harbour extended spiral flows.