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

Abstract

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%-20% 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 nonthermal 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) a nearly constant CF contrast q of density and temperature within each GC, monotonically increasing with the GC mass M 200 as q ∝ M 0.23±0.04 200 ; (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 +0.09 −0.08 metallicity drop across the CF. These findings suggest that GCs quite generally harbor extended spiral flows.

Radio halos and relics from extended cosmic-ray ion distributions with strong diffusion in galaxy clusters

“…For MHs, both hadronic and leptonic models are often considered, but the former is increasing disfavored, for example because observation do not precisely show its steady-state −1.1 α −1.0 spectrum (e.g., Timmerman et al 2021, concerning α = −0.95 ± 0.10 in the Phoenix cluster) or by interpreting the MH-CF association as evidence that MHs arise from particle (re)acceleration in sloshing-induced turbulence (Giacintucci et al 2019, and references therein) -although shear magnetization, anticipated and already observed (Reiss & Keshet 2014;Naor et al 2020) below CFs, suffices to explain the association in a hadronic model.…”

“…MHs are ubiquitously found in the centres of the more relaxed, cool-core clusters (possibly in ∼ 80% of the cores; Giacintucci et al 2017). They extend roughly over the cool region (Gitti et al 2002), often engulfing ⋆ E-mail: ukeshet@bgu.ac.il the more compact radio emission from an active galactic nucleus (AGN), and typically truncate at cold fronts (CFs; Mazzotta & Giacintucci 2008): projected tangential discontinuities that confine spiral flows , which are observed to strongly magnetise the plasma (Reiss & Keshet 2012;Naor et al 2020) and appear to regulate the core (ZuHone et al 2010;Keshet 2012). MHs are typically unpolarised, regular, and spectrally flat, with a typical integrated photon index −1.0 α −1.2 (although cases as soft as α ≃ −1.6 were reported, as detailed below), showing a fairly universal central ratio η ≡ νIν/FX between radio and X-ray surface brightness (Keshet & Loeb 2010, henceforth KL10).…”

Advection by large-scale spiral flows in galaxy clusters

“…Spiral-type CFs are tangential discontinuities, widely believed to arise from large-scale, sloshing motions of the ICM following some violent trigger event. Such CFs show hydrostatic mass jumps that reveal nearly sonic tangential flows below the CFs (Keshet et al 2010), with a typical Mach number M ∼ 0.8 (Naor et al 2020, henceforth N20), and thermal pressure jumps that indicate † Electronic address: naoryos@post.bgu.ac.il ‡ Electronic address: ukeshet@bgu.ac.il magnetization levels enhanced by ∼ 10% below the CFs, probably due to shear (Reiss & Keshet 2014, and N20).…”