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We goals are to study the physical processes governing the formation and evolution of galaxies. Galaxies form out of the "cosmic web", a ubiquitous cosmic-scale gaseous medium that has a remarkably similar structure to a 3D spider web. The connecting regions are called filaments, and these stream (infall) into overdense regions. This gaseous matter forms stars (galaxies). Surrounding each galaxy is a region called the circumgalactic medium in which gas is inflowing and outflowing. The gas flows strongly govern galaxy evolution and so charting the gas chemical make up, dynamical motions, and temperatures and densities is key to increasing our understanding of how galaxies, including our own Milky Way, form and evolve.

(left image) We originally proposed the experiment as described in our Cycle 21 Large Hubble Space Telescope proposal. We subsequently extended the number of target galaxies by searching for and locating observations in the Space Telescope Science Institute archive.

(credit STScI) The gas does not emit light, so it must be observed via the light it absorbs. By taking the spectrum of a luminous distant background quasar (QSO), we can measure the absorption "lines" in the spectrum to measure the gas properties as a function of cosmic time.

(credit Carnegie Observatories) Computer simulations of the gas flow through galaxies are highly sophisticated. Filamentary streams are always infalling into the galaxy and continually supplying additional gas from which new stars can form. As stars form, some die in catastrophic explosions called supernovae. These can generate galactic scale outflowing gaseous winds. It is believed that much of this outflowing gas does not escape the galaxy, but falls back in. The result is a complex and dynamics gaseous region surrounding galaxies.

As powerful and predictive the simulations are, they must be constrained by observational data.

(credit STScI) We have designed an experiment to use the Cosmic Origins Spectrograph on the Hubble Space Telescope. We have carefully selected some 40 galaxies for which we will probe their circumgalactic medium, study the gas conditions in detail, and compare directly to the predictions of cosmological simulations of galaxies.

The otherwise invisible gas is observable as absorbed light in the spectrum of the distant background QSO. We can measure the number of "clouds", their densities, temperatures, relative velocities, and distances from the galaxies.

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(Kacprzak+ Figure 2) The gas does not emit light, so it must be observed via the light it absorbs. By taking the spectrum of a luminous distant background quasar (QSO), we can measure the absorption "lines" in the spectrum to measure the gas properties as a function of cosmic time.

(Kacprzak+ Figure 3) The gas does not emit light, so it must be observed via the light it absorbs. By taking the spectrum of a luminous distant background quasar (QSO), we can measure the absorption "lines" in the spectrum to measure the gas properties as a function of cosmic time.

(Kacprzak+ Figure 4) The gas does not emit light, so it must be observed via the light it absorbs. By taking the spectrum of a luminous distant background quasar (QSO), we can measure the absorption "lines" in the spectrum to measure the gas properties as a function of cosmic time.

(Nielsen+ Figure 3) The gas does not emit light, so it must be observed via the light it absorbs. By taking the spectrum of a luminous distant background quasar (QSO), we can measure the absorption "lines" in the spectrum to measure the gas properties as a function of cosmic time.

(Nielsen+ Figure 6) The gas does not emit light, so it must be observed via the light it absorbs. By taking the spectrum of a luminous distant background quasar (QSO), we can measure the absorption "lines" in the spectrum to measure the gas properties as a function of cosmic time.

(Kacprzak+ Figure B4) The gas does not emit light, so it must be observed via the light it absorbs. By taking the spectrum of a luminous distant background quasar (QSO), we can measure the absorption "lines" in the spectrum to measure the gas properties as a function of cosmic time.

(Kacprzak+ Figure 4) The gas does not emit light, so it must be observed via the light it absorbs. By taking the spectrum of a luminous distant background quasar (QSO), we can measure the absorption "lines" in the spectrum to measure the gas properties as a function of cosmic time.

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