Probing galaxy halos with strong absorption line systems

In our modern view, galaxies are not isolated entities, but they are part of a complex cosmic ecosystem in which gas is continuously exchanged between the inner galactic regions where stars form and the diffuse intergalactic medium that pervades the universe. A pressing and critical goal of modern cosmology is to obtain a comprehensive view of how galaxies acquire the fresh fuel needed for the formation of new stars, and how they disperse gas enriched with heavy elements produced in stars back into the cosmos. In Durham, we are exploiting the world's largest telescopes to shape our current understanding of gas flows next to galaxies during one of the most active phases of cosmic history, between redshift 2 and 3, when the universe was only 2 billion years old.

As these gas flows around galaxies are too tenuous to be seen directly with current telescopes, most of our empirical knowledge on how galaxies exchange gas with their surroundings comes from studies ``in silhouette''. Observers exploit bright quasars as a flashlight to illuminate this gas. As the light of quasars travels to Earth passing next to galaxies, the gas flows associated with these foreground galaxies block some of the light of the background quasars, imprinting characteristic absorption lines. By studying the position, depth and shape of these lines, observers infer the density, chemical composition, and velocity of gas flows in the distant universe, constraining how much material is exchanged by galaxies with their surroundings.

Left: Numerical simulation of a galaxy that is forming in the distant universe, showing characteristic streams of hydrogen that feed the formation of new stars. Centre: Numerical simulation at 100 times higher resolution of the central star forming regions of a galaxy, showing bubbles associated with the explosion of massive stars that are responsible for ejecting gas rich in chemical elements back into the cosmos. Right: Real spectroscopic observations of a pair of two bright quasars (image credit: V. D'Odorico 2016). On its way to Earth, the light of these quasars travels through gas near to a galaxy in the foreground (as schematically represented), giving rise to characteristic absorption lines (vertical features in rightmost panel) that allow us to study ``in silhouette'' the physical properties of gas flows around galaxies. By studying the similarities and differences of these absorption lines in many of the quasar pair spectra, we are mapping the gas distribution around distant galaxies to understand, in synergy with simulations, how gas flows shape the evolution of galaxies through cosmic epochs.

When looking at thin rays in the direction of quasars, however, observers recover the gas properties only in one dimension, along the line of sight, thus limiting our ability to test numerical simulations that make distinctive predictions for the spatial distribution of these inflows and outflows. For a leap forward in this field, we are exploiting the serendipitous alignments of multiple closeby quasars to map the extended morphology of gas near galaxies. Combining spectroscopic observations from the Hubble Space Telescope and the largest optical telescopes (such as the Very Large Telescopes), we are looking at correlations between absorption line systems along pairs of quasar sightlines, a new technique to reconstruct the distribution, chemical composition, and typical velocity of gas around galaxies. With this novel vantage point on gas flows, we are providing some of the most stringent empirical tests on whether galaxies acquire gas in streams as predicted by theory. Moreover, this experiment provides information of the amount of heavy elements that are found in the surroundings of galaxies, thus constraining the efficiency with which stellar explosions eject gas metals back into the cosmos, a critical but ill-understood process of galaxy evolution.

Furthermore, we are connecting the properties of inflows and outflows as seen in absorption to the physical properties of the central star-forming galaxies as seen in emission. To this end, we are leading a VLT large programme to target with MUSE 20 fields that host bright quasars at a redshift of about z~3.5. With MUSE, spectra for all the galaxies aligned in projection with these bright quasars can be efficiently obtained, thus increasing by orders of magnitude samples of galaxies associated to absorption line systems. With this new large dataset, more than 5 times larger than previously available, we are undertaking a comprehensive study of the density and chemical content of gas flows in relation to the stellar properties of galaxies. Furthermore, these observations are planned to reach sufficient depth to search for faint emission originating from these diffuse gas flows, with the potential of opening a new window into the study of the galaxy ecosystem in the distant universe.

Staff involved with this project at Durham are Simon Morris and Tom Shanks and Michele Fumagalli.