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14th of March 2023
HI absorption associated with Norma’s brightest cluster galaxy
by Saraf et al.
ESO 137-G006 is the brightest cluster galaxy (BCG) of the cool-core and dynamically young Norma cluster. Saraf et al. report the discovery of an atomic hydrogen (HI) absorption line associated with this BCG using the Australia Telescope Compact Array. Derived estimates of gas column density and spin temperature are consistent with the HI properties of other early-type galaxies and cool-core cluster BCGs. The results support the scenario that ESO 137-G006 may be a recent arrival to the cluster centre and not the original BCG. This scenario is consistent with the observed spatial alignment of the BCG's wide-angle tail radio lobes with Norma's X-ray sub-cluster and the significant line-of-sight velocity offset between the mean velocity of Norma and that of the BCG.

The plots above show the total flux density spectrum integrated over the HI detection region of the elliptical aperture towards ESO 137-G006’s core. The velocity centroid of the HI absorption profile is marked as the blue dash-dot line. In panel a, the velocities of the BCG (magenta dashed) and Norma cluster (golden dotted) are marked as vertical lines with error-bars. The blue, horizontal grey and vertical golden shaded regions mark the HI profile area, the 3 times RMS spectral noise and the velocity dispersion of the Norma cluster respectively. Zoom-panel b marks the edges (vertical cyan dotted lines) and widths -- W50 (horizontal cyan solid line) and W20 (horizontal cyan dashed line) -- of the absorption profile. The figure shows that the detected HI absorption line is statistically significant and associated with the Norma cluster's BCG.

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