In 1965, a far-reaching paper entitled "On the Interaction of Radiation from Distant Sources with the Intervening Medium" by J.N. Bahcall and E.E. Salpeter was published in The Astrophysical Journal.

The paper shed light on the possible detection of a non-uniform InterGalactic Medium (IGM) through discrete quasar absorption lines. Earlier in 1965, spectroscopic evidence of a distant quasar at z=2, provided by M. Schmidt, stimulated new remarkable ideas on the possible detection of the IGM, such as that described in the same year by J.E. Gunn and B.A. Peterson. Many other milestone papers, such as those by G.B. Field, also contributed 50 years ago to a new scientific interest on the possible detection of hydrogen and metals in intergalactic space.

Since then, the thermal history of the IGM, expanding with the Universe and pervaded by photons and matter processed in galaxies, has been of central interest. Countless hydrogen and metal absorption lines have been detected in quasar spectra, revealing an inhomogeneous IGM. Large-scale surveys have shown that galaxies are distributed along filaments of what we now call the cosmic web. Today, 50 years later, the IGM is again in the spotlight because our understanding of galaxies is changing. Numerical simulations of cosmic structure formation and evolution are now suggesting that a galaxy-cosmic web connection might be the key ingredient for regulating galaxy evolution, including the morphological type, star formation history and gas content. Star-forming galaxies follow a linear relationship between star formation rate and stellar mass that is redshift-dependent and can be explained as a result of continuous gas accretion from the cosmic web. While accreting, galaxies evolve internally through disk instabilities and interactions with nearby galaxies, but the star formation rate might be just the equilibrium value that balances the accretion rate, averaged over a gas depletion time. Metallicity differences among galaxies might also be related to cosmic accretion if outflows in galactic winds are considered.

Directly detection of cosmic accretion is still very difficult. This is why the most basic questions remain 50 years after the first attempts to detect intergalactic gas: Is the intergalactic medium driving star formation? What are the direct and indirect evidences that the star formation rate and blue galaxy evolution are regulated by gas streams from the IGM? How can the physics of large scale structures in the Universe affect the formation of molecular clouds and the gas collapse on much smaller scales? Are quenched red- and dead-galaxies simply structures disconnected from the cosmic web? What is the mass loading of galactic winds into the circumgalactic medium and how does this factor depend on internal processes in galaxies? What is the impact of different environments, such as voids, groups, and clusters on galaxy evolution? What fraction of accretion comes from satellite galaxies and what fraction is smooth accretion ffrom the cosmic web? And how is this proportion changing over cosmic time?

The aim of the Conference is to discuss the current views about the possible links between intergalactic gas and star formation in galaxies. We will be focusing on the following observational and theoretical topics:

Cosmic web flows into galaxies

  • Characterizing the cosmic web and its evolution
  • Predicted geometries and substructures of cold streams
  • Time dependence of gas flows (long lasting? episodic?)
  • The feeding process for baryons and dark matter
  • Detecting IGM flows into galaxies
  • Metallicity of the IGM

Disk growth across cosmic time
  • Numerical simulations of galaxy growth
  • Formation and physics of the circumgalactic medium
  • Transition from chaotic to spiral galaxies: signature of slowing accretion?
  • Origin of the galaxy fundamental plane and its scatter as a function of redshift
  • Gas accretion in different environments and the Local Group accretion history
  • Accretion and feedbacks in dwarf galaxies

The evolution of star formation in galaxies
  • Star-forming clumps and the gas depletion timescale
  • Origin of the mass-metallicity-star formation relation
  • Violent disk instabilities
  • Evolution from blue- to red-galaxies
  • Signature of quenching in the circumgalactic medium
  • Effects of the environment on gas and star formation rate


Osservatorio Astrofisico di Arcetri