[Paper Review] A high-resolution view of the filament of gas between Abell 399 and Abell 401 from the Atacama Cosmology Telescope and MUSTANG-2

Adam D. Hincks, Federico Radiconi+44 more|arXiv (Cornell University)|Jul 9, 2021

Galaxies: Formation, Evolution, Phenomena101 references36 citations

TL;DR

This study presents the first high-resolution detection of a hot gas filament connecting galaxy clusters Abell 399 and Abell 401 using high-sensitivity data from the Atacama Cosmology Telescope (ACT) and the MUSTANG-2 receiver. It confirms a 3.3 ± 0.7 × 10¹⁴ M⊙ gas bridge at 5σ significance, revealing a transverse width of ∼1.9 Mpc and a line-of-sight inclination of ∼17°, implying a true separation of ∼11 Mpc, with gas density of (0.88 ± 0.24) × 10⁻⁴ cm⁻³, consistent with the warm-hot intergalactic medium (WHIM).

ABSTRACT

We report a significant detection of the hot intergalactic medium in the filamentary bridge connecting the galaxy clusters Abell 399 and Abell 401. This result is enabled by a low-noise, high-resolution map of the thermal Sunyaev-Zeldovich signal from the Atacama Cosmology Telescope (ACT) and Planck satellite. The ACT data provide the $1.65'$ resolution that allows us to clearly separate the profiles of the clusters, whose centres are separated by $37'$, from the gas associated with the filament. A model that fits for only the two clusters is ruled out compared to one that includes a bridge component at $>5\sigma$. Using a gas temperature determined from Suzaku X-ray data, we infer a total mass of $(3.3\pm0.7) imes10^{14}\,\mathrm{M}_{\odot}$ associated with the filament, comprising about $8\%$ of the entire Abell 399-Abell 401 system. We fit two phenomenological models to the filamentary structure; the favoured model has a width transverse to the axis joining the clusters of ${\sim}1.9\,\mathrm{Mpc}$. When combined with the Suzaku data, we find a gas density of $(0.88\pm0.24) imes10^{-4}\,\mathrm{cm}^{-3}$, considerably lower than previously reported. We show that this can be fully explained by a geometry in which the axis joining Abell 399 and Abell 401 has a large component along the line of sight, such that the distance between the clusters is significantly greater than the $3.2\,\mathrm{Mpc}$ projected separation on the plane of the sky. Finally, we present initial results from higher resolution ($12.7"$ effective) imaging of the bridge with the MUSTANG-2 receiver on the Green Bank Telescope.

Motivation & Objective

• .
• To directly image and characterize the hot intergalactic medium in the filament connecting Abell 399 and Abell 401.
• To determine the physical properties of the filament, including its mass, width, and geometry, using multi-wavelength data.
• To test whether the filament's structure and dynamics are consistent with a pre-merger cluster system.

Proposed method

• .
• Utilized high-resolution (1.65′) thermal Sunyaev-Zeldovich (tSZ) maps from the Atacama Cosmology Telescope (ACT) to resolve the cluster profiles and detect the filament.
• Combined ACT tSZ data with Suzaku X-ray measurements to constrain gas temperature and infer total mass.
• Fitted phenomenological models (e.g., mesa model) to the filament’s radial profile to estimate transverse width and geometry.
• Used MUSTANG-2 12.7′′ resolution imaging to probe small-scale pressure substructure and turbulence in the bridge.
• Applied Bayesian inference with nested sampling (emcee) to fit cluster and filament models to the data.
• Incorporated HEALPix-based sky mapping and HEALPy for data analysis and visualization.

Experimental results

Research questions

• RQ1 .
• RQ2 What is the total mass and spatial extent of the hot gas filament connecting Abell 399 and Abell 401?
• RQ3 Is the observed signal consistent with a filamentary structure or merely a superposition of cluster emission?
• RQ4 What is the true geometry of the filament, particularly its orientation relative to the line of sight?
• RQ5 How does the measured gas density compare to expectations for the warm-hot intergalactic medium (WHIM)?
• RQ6 What is the nature of the radio emission ridge detected between the clusters, and is it consistent with turbulence or shocks?

Key findings

• .
• The filament is detected at >5σ significance, with a total mass of (3.3 ± 0.7) × 10¹⁴ M⊙, constituting ~8% of the total system mass.
• The filament has a transverse width of ∼1.9 Mpc, significantly narrower than its projected separation of 3.2 Mpc on the sky.
• The gas density is measured at (0.88 ± 0.24) × 10⁻⁴ cm⁻³, substantially lower than previous estimates.
• The line-of-sight inclination is estimated at ∼17°, implying a true physical separation of ∼11 Mpc, not 3.2 Mpc as projected.
• The MUSTANG-2 data show no significant pressure substructure, but detect an excess signal consistent with low-level turbulence in the filament.
• The system is gravitationally bound, with cluster masses and velocities consistent with a pre-merger configuration.