The Virgo Cluster as Seen at X-Ray and Optical Energies

How Did the Known Structures in the Universe Evolve?

• The question of evolution of structures in the Universe is perhaps the central question of astronomy: after all, astronomers are "historians of the Universe". Since light travels with finite speed, looking "far in distance" is looking "far back in time".

• Observations of the farthest reaches of the Universe show that it is "lumpy", with well developed structures that are very old: we detect galaxies and clusters of galaxies as far back in time as we can see, when the Universe was about a fifth of its current age - but we still do not understand how these structures condensed out of the "primordial soup". The formation of structure appears much quicker than the current theories would have it!

• These structures - old and new - are tightly bound via gravity, yet we cannot account for all the mass that would provide the gravity - where is that mass?

• Many of these old structures show the presence of a variety of chemical elements - we know this from a study of their spectra. For instance, the diffuse gas permeating the clusters of galaxies contains a comparable fraction of "heavy" elements (other than hydrogen and helium) to that observed presently in our own Galaxy - yet clusters formed long ago.

• We know that the early Universe contained only the lightest elements, hydrogen and helium. All heavier elements have been synthesized from these two light ones, and the only place we know where this can happen is in the centers of stars (which are, after all, "lumps"). Subsequently, these elements were dispersed into the interstellar medium via supernova explosions - and this includes carbon, nitrogen and oxygen, the main ingredients of life as we know it! But - is this the only way of making these heavier elements, or are we possibly oblivious to another avenue of producing them?

• Astronomers are thus literally presented with the " chicken or the egg?" question: Here it is: "the stars or the galaxies?" - making unraveling the riddle of the formation of structure that much more challenging.

• The best way to answer this "stars or galaxies?" question is to look far back in time, with a sensitive instrument with "sharp" optics - to measure the presence of elements in the most distant galaxies and clusters of galaxies. X-ray observatories capable of high-resolution spectroscopy are well posed to answer these questions.