  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.
