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 (and paywalled) paper in Science A radio ridge connecting two galaxy clusters in a filament of the cosmic web.
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