The shocking features in the closest rich galaxy cluster Norma

Abstract

The merger shocks generated by the collision of galaxy clusters elevate the pressure within the intracluster medium, significantly influencing the evolution of embedded cluster galaxies. We detect a merger shock (Mach number $\sim 1.3$) on the northwest side of the closest rich galaxy cluster Norma (A3627), using XMM-Newton and Chandra data. The textbook ram pressure stripping (RPS) galaxy ESO 137-001 appears to be located in the post-shock region. The shock boosts RPS and may induce the formation of the brightest known X-ray tail behind a cluster late-type galaxy. Another prominent head-tail radio galaxy ESO 137-007, with one of the longest radio continuum tails ($> 500$ kpc), is also likely in the post-shock region. The shock may have reversed the upstream jet to a one-sided radio head-tail morphology. Moreover, the shock can strip and roll the jet cocoon into a vortex ring structure like a `smoke ring' behind the end of the jet as observed by the ASKAP data. Therefore, the cluster merger shock can remarkably change cluster galaxies. Furthermore, Norma is the second brightest non-cool-core cluster following the Coma cluster, with a cool core remnant on its southeast side. Its original cool core may be disrupted by cluster mergers and/or active galactic nuclei.

Figures (3)

• Figure 1: Left: optical background image from the Dark Energy Camera Legacy Survey (DECaLS; Dey2019) in the $g, r, z$ bands, overlaid with a red radio image from ASKAP 944 MHz Koribalski2024, and a blue X-ray image from the XMM-Newton 0.5-2 keV mosaic. The green circles mark two BCGs, ESO 137-006 and ESO 137-008, the green box marks the head-tail radio galaxy ESO 137-007, and the two green dashed boxes mark two RPS galaxies, ESO 137-001 and ESO 137-002, which are enlarged in the right panels. Right: zoom-in images of ESO 137-001 and ESO 137-002, with stars removed from the background DECaLS images using StarNet , overlaid with the red radio image from ASKAP 944 MHz, and the blue X-ray image from the Chandra 0.5-2 keV residual image.
• Figure 2: Top left: XMM-Newton 0.5-2 keV mosaic of A3627. The cyan contours are from ASKAP 944 MHz Koribalski2024 and show the WAT radio galaxy ESO 137-006 and the head-tail radio galaxy ESO 137-007 marked by a green box; dashed cyan contours depict the arc-shaped radio filaments resembling a 'smoke ring' beyond the radio tail of ESO 137-007. Green circles mark two BCGs: ESO 137-006 and ESO 137-008. The green 'X' labels the X-ray peak. Green arrows illustrate the direction of motion of two RPS galaxies with X-ray tails: ESO 137-001 and ESO 137-002. We find a merger shock front (marked by the green arc) indicated by a surface brightness edge on the NW side of the cluster. Top right: XMM-Newton residual image with the best-fit ICM model subtracted. Prominent residual features include the CC remnant and the X-ray tails of ESO 137-001 or ESO 137-002. The peak of the CC remnant is marked by a black '+'. The radio lobes of ESO 137-006 likely excavate two X-ray cavities indicated by the X-ray decrements. Bottom left: Chandra 0.5-2 keV mosaic marked with bright X-ray sources and tails. Bottom right: Chandra residual image with the same best-fit ICM model from the XMM-Newton image subtracted. The CC remnant with X-ray cavities and the X-ray tails are also remarkable. The same bar at the bottom right of all panels shows 15$^{\prime}$/300 kpc.
• Figure 3: Top: XMM-Newton temperature map of A3627 with a color bar in units of keV. Regions containing point sources and diffuse substructures are masked as black circles and ellipses. The green sector marks the extraction region for the SBP and the temperature profile shown in the bottom panel. The green dashed line marks the location of the SF. Notable galaxies are marked as in Fig. \ref{['fig:img']}. Bottom: SBP and temperature profiles near the shock. The blue solid line shows the best-fit broken power-law model. Black dots are the projected temperature, while open circles are the deprojected temperature. The vertical error bars show $1\sigma$ uncertainties. The dashed line denotes the location of the SF, where a discontinuity in the density and temperature of the ICM is observed.