High-redshift galaxy groups as seen by Athena/WFI
  The first massive galaxy groups in the Universe are predicted to have formed
at redshifts well beyond two. Baryonic physics, like stellar and active
galactic nuclei (AGN) feedback in this very active epoch, are expected to have
left a strong imprint on the thermo-dynamic properties of these early galaxy
groups. Therefore, observations of these groups are key to constrain the
relative importance of these physical processes. However, current instruments
are not sensitive enough to detect them easily and characterize their hot gas
content. In this work, we quantify the observing power of the Advanced
Telescope for High ENergy Astrophysics (Athena), the future large X-ray
observatory of the European Space Agency (ESA), for discovering and
characterizing early galaxy groups at high redshifts. We used the SImulation of
X-ray TElescopes (SIXTE) simulator to mimic Athena observations, and a
custom-made wavelet-based algorithm to detect galaxy groups and clusters in the
redshift range $0.5 \le z \le 4$. We performed extensive X-ray spectral fitting
in order to characterize their gas temperature and X-ray luminosity. We also
investigate how well Athena will constrain different feedback mechanisms. In
the deep Wide Field Imager (WFI) survey expected to be carried out during part
of Athena's first four years (the nominal mission lifetime) more than 10,000
galaxy groups and clusters at $z\ge 0.5$ will be discovered. We find that
Athena can detect $\sim20$ high-redshift galaxy groups with masses of
$M_{500}\geq$ $5\times 10^{13}$ $M_{\odot}$ and $z\geq2$, and almost half of
them will have a gas temperature determined to a precision of $\Delta T/T \le
25\%$. We demonstrate that high-redshift galaxy groups can be detected very
efficiently as extended sources by Athena and that a key parameter determining
the total number of such newly discovered sources is the area on the sky
surveyed by Athena.
