# How is it determined that the X-ray and radio intensity come from a magnetic field bridge between two clusters of galaxies?

Gizmodo's Astronomers Spot Mysterious, 10-Million-Light-Year-Long Magnetic Field Connecting Two Galaxy Clusters shows the image below, and Space.com's A Weird 'Radio Bridge' 10 Million-Light Years Long Links Colliding Galaxy Clusters discusses it as well.

They both link to the new paper in Science A radio ridge connecting two galaxy clusters in a filament of the cosmic web (also arXiv).

What I've gathered so far is that seen between two known and characterized clusters of galaxies, there is a region with some excess continuum radio power at 140 MHz and some excess continuum X-ray power.

Neither of these are spectral features so their distances can't be associated with the galactic clusters by doppler shift.

Question: How do they know that the X-ray and Radio continuum are both really coming from the space between these two clusters of galaxies, and not in front of or behind it?

"bonus points" how does Planck's "y parameter" figure in to this mix?

“A radio ridge connecting two galaxy clusters in a filament of the cosmic web”, F. Govoni et al. 2019, Science. Optical: DSS and Pan STAARS (insets) — Red, X-rays: XMM-Newton — Yellow, y-parameter: Planck satellite — Blue, radio 140 MHz: LOFAR. Image credits: M. Murgia - INAF

The two galactic clusters and the intervening radio ridge. Image: DSS and Pan-STARRS1 (Optical), XMM-Newton (X-rays) PLANCK Satellite (y paramter), F. Govoni, M. Murgia, INAF

• The preprint version of the paper is here: arxiv.org/abs/1906.07584 Jul 1 '19 at 10:48
• – uhoh
May 12 '20 at 4:54

(The "Planck $$y$$-parameter", also known as the "Compton parameter", refers to the measurement of the Sunyaev-Zel'dovich effect due to inverse Compton scattering of cosmic background radiation photons, derived from Planck satellite data. I believe it's proportional to the line-of-sight integral of the gas pressure.)
The Suzaku X-ray spectrum of this region (e.g., Figure 2 of Fujita et al. 2008; see below) does have some spectral features, including the Fe K emission line. Due to the low spectral resolution, these features don't constrain the redshift of the X-ray-emitting gas very strongly (and I think Fujita et al. fixed the redshift at the mean of the two clusters in their analysis), but it would probably be noticeable if it differed by more than $$\sim 0.1$$ in redshift.