The relationship between the intergalactic medium and galaxies

Abstract

In this thesis we study the relationship between the intergalactic medium (IGM) and galaxies at z<1, in a statistical manner. Galaxies are mostly surveyed in emission using optical spectroscopy, while the IGM is mostly surveyed in absorption in the ultra-violet (UV) spectra of background quasi-stellar objects (QSOs). We present observational results investigating the connection between the IGM and galaxies using two complementary methods:

• We use galaxy voids as tracers of both underdense and overdense regions. We use archival data to study the properties of H I absorption line systems within and around galaxy voids at z<0.1. Typical galaxy voids have sizes
10 Mpc and so our results constrain the very large-scale association. This sample contains 106 H I absorption systems and 1054 galaxy voids.

• We use a sample of H I absorption line systems and galaxies from pencil beam surveys to measure the H I–galaxy cross-correlation at z<1. Our sample is composed of a combination of archival and new data taken by the author and collaborators. This survey covers transverse separations
between H I and galaxies from ∼ 100 kpc (proper) up to ∼ 10 Mpc, filling the gap between the very large scales and those associated with the so-called circumgalactic medium (CGM). This sample contains 654 H I absorption systems and 17509 galaxies.

Our results hint towards a picture in which there are at least three types of association between the diffuse gas in the Universe and galaxies at z 1:

• One-to-one direct association because galaxies do contain diffuse gas.

• Indirect association because both the IGM and galaxies trace the same over-dense underlying dark matter distribution. We provide quantitative evidence for this association. Moreover, we show that not all galaxies are
related to the diffuse gas in the same way. In particular, a non negligible fraction of ‘non-star-forming’ galaxies might reside in environments devoid of diffuse H I.

• No association because there are regions in the Universe that contain a significant amount of diffuse gas but that are devoid of galaxies. In these regions, only the IGM follows the underdense underlying dark matter distribution
because galaxies are not present. We provide quantitative evidence for this scenario.