Intergalactic medium The intergalactic medium (IGM) is defined as all the material that lies in between galaxies. The IGM is the main reservoirs of baryonic matter (e.g. protons) at all epochs, and it is from where galaxies get the material to form and evolve.
Galaxies Galaxies are the places where stars are form. Due to their high densities, gas accreted from the IGM can actually cool and condense to form stars. These stars enrich their surroundings (i.e. the interestellar medium, ISM) with elements heavier than helium (i.e. metals). Because of the enriched material it allows the creation of planets and ultimately life. Still, galaxies only account for about ~10% of the baryons in the Universe.
IGM-galaxy connection Because of stellar winds and other energetic phenomena (e.g. active galactic nuclei), some of the enriched material created in the stars will escape their parent galaxy to the IGM. This enriched IGM material can eventually fall again to the same or another galaxy or remain loose for eternity. The continuous interplay between the IGM and galaxies is key into the evolution of baryonic matter in the Universe, and so it is sensible to study these two concepts simultaneously.
Dark matter Dark matter is a type of matter that does not interact through electromagnetism like protons, electrons or neutrons do (or if it does, such interaction has to be extremely weak). Dark matter has been observed through its gravitational signatures on baryonic matter, and its existence is fundamental to forming galaxies as well as to explain the observed properties of the Universe as a whole.
Cosmic web Because of the action of gravity, the distribution of dark matter (and hence baryonic matter) in the Universe is not homogeneous and follows an intrincated pattern of sheets, filaments and nodes. This particular distribution is commonly referred to as the large-scale structure (LSS) or the 'cosmic web'. Galaxies follow this underlying distribution because they form at the matter density peaks, and their properties are partly shaped by LSS environment (i.e. in which part of the cosmic web they reside). We expect the IGM to follow such cosmic web, but its three-dimensional distribution has been much more difficult to observe than that of galaxies.
Absorption line technique Because of its low densities (of the order of 1 proton per cubic meter), the IGM does not emit enough photons for a direct detection in emission. Currently, the only feasible way to observe the IGM is in absorption through the characterization of intervening absorption-line systems in the spectra of bright background sources (e.g. quasars, gamma-ray bursts, galaxies). Although this technique limits the IGM observation to being one-dimensional, an averaged three-dimensional picture can be obtained by combining multiple lines-of-sight (LOS) and galaxy surveys.
During my Ph.D, I studied the relationship between the IGM (traced by neutral hydrogen, HI) and galaxies in a statistical manner, for the second half of the history of the Universe (redshifts < 1). We measured the HI-galaxy two-point cross-correlation function, as a function of different HI/galaxy properties. [under construction...]
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