Athena is a mission proposed to address the Science Theme "The Hot and Energetic Universe", which has been selected by ESA in its Cosmic Vision program. In particular, it undertakes three key scientific objectives:
1) Determine how and when large-scale hot gas structures formed in the Universe and track their evolution from the formation epoch to the present day.
2) Perform a complete census of black hole growth in the Universe, determine the physical processes responsible for that growth and its influence on larger scales, and trace these and other energetic and transient phenomena to the earliest cosmic epochs.
3) Provide a unique contribution to astrophysics in the 2030s by exploring high energy phenomena in all astrophysical contexts, including those yet to be discovered.
From the unique perspective endowed to Athena by its unprecedented spectroscopic and imaging capabilities in the 0.5-12keV range, this mission will lead the quest into solving these questions from its launch in 2028.
Regarding the first question, our current understanding is that most of the ordinary matter (baryons) in the Universe is in a warm-hot state, both inside the potential wells of groups and clusters of galaxies and in the so-called Warm Hot Intergalactic Medium (WHIM) in filaments between the densest regions in the Cosmos. Investigating how such potential wells formed and evolved, and how and when the material trapped in them was energised and chemically enriched, can uniquely be tackled by observations in the X-ray band, combining wide-field images with high resolution spectroscopy, both of high sensitivity. Such capabilities will also serve us well when studying the WHIM, both in emission and in absorption against bright background targets.
The second question has become a most pressing issue since the realisation that all nearby massive galaxies harbour a Super-Massive Black Hole (SMBH, with a mass intriguingly proportional to that of the galaxy bulge) and that the cosmic evolution of galaxy growth through star formation and SMBH growth via accretion (shining as Active Galactic Nuclei -AGN-) follow parallel tracks, increasing back in time from the present time to its heyday at redshifts z~1-3. Is this fascinating set of clues revealing that AGN actually mould their host galaxies? How does this happen? How far back does this start? Again, the ability of wide field X-ray observations to pinpoint AGN among the myriad sources in the sky, even if they are heavily obscured, will enable detecting the elusive obscured and high redshift populations. High resolution X-ray spectroscopy will reveal the workings of the inner parts of the AGN engine (and those of other stellar-mass accreting engines in our Galaxy) as well as the outflows of ionised gas by which it is speculated that the AGN influences its host galaxy. Finally, the fast Target of Opportunity capabilities of Athena will enable studies of Gamma Ray Bursts and other transient phenomena to the earliest cosmic epochs.
Aside from these topics, the singular and outstanding capabilities of Athena as an observatory are expected to make a profound impact in essentially all fields of Astrophysics, from understanding the structure and energetics of stellar winds and their interplay with atmospheres and magnetospheres of planets, or exploring the behaviour of matter under extreme conditions of density and magnetic fields in stellar binaries and neutron stars, to probing the physics of the enrichment and heating of our Galaxy's Inter-stellar Medium by supernova explosions. This will not happen in isolation, but taking full advantage of Athena's synergies with the impressive set of multi-wavelength astronomical facilities then in exploitation (e.g. LOFAR, SKA, ALMA, JWST, E-ELT, LSST, CTA, to name but a few).