A spectroscopic study of 14 structures behind Holm15A: Detecting a galaxy group candidate at z=0.58

TL;DR

This study spectroscopically identifies and characterizes 14 structures behind Holm15A, revealing a galaxy group at z=0.58 with evidence of past mergers, using spectral modeling and fossil record analysis.

Contribution

First spectroscopic identification and detailed analysis of structures behind Holm15A, including a candidate galaxy group at z=0.58, with insights into their past interactions and merger history.

Findings

14 structures identified behind Holm15A, not associated with it.

A galaxy group at z=0.5814 with estimated scale >146 kpc and velocity dispersion 622 km/s.

Evidence of past mutual interactions and multiple merger trees within the group.

Abstract

Holm15A hosts one of the most massive back holes ever known. Hence, it is important to characterize any structure within its core to avoid any wrong association with its central black hole and, therefore, bias any future study. In this work, we present the first identification and characterization of 14 structures hidden behind the surface brightness of Holm15A. We model and subtract the spectral contribution of Holm15A to obtain the spectral information of these structures. We spectroscopically confirm that the 14 objects found are not associated with Holm15A. Ten objects have a well-defined galaxy spectrum from which we implement a fossil record analysis to reconstruct their past evolution. Nine objects are candidates members to be part of a compact galaxy group at redshift 0.5814. We find past mutual interaction among the group candidates that support the scenario of mutual crossings. Furthermore, the fossil reconstruction of the group candidates brings evidence that at least three different merger trees could assemble the galaxy group. We characterize the properties of the galaxy group from which we estimate a lower limit of the scale and mass of this group. We obtain a scale of $>$146$\pm$3 kpc with a dispersion velocity of 622$\pm$300 km/s. These estimations consider the lensing effects of the gravitational potential of Holm15A. The other five objects were studied individually. We use public archive data of integral field spectroscopic observations from the Multi-Unit Spectroscopic Explorer instrument.