Dissecting Gas in Galaxies over Cosmic Time – Jens-Kristian Krogager

An important constituent of galaxies is their gas reservoir, most notably the neutral gas. The neutral gas is crucial for star formation, leading one to expect a correlation between the evolution of the average neutral gas mass density and the average star formation rate. However, over the past 10 billion years, while the average rate of star formation has dropped by about a factor of 10, the neutral gas density hasn’t evolved much. To understand this discrepancy, it’s essential to constrain gas properties over cosmic time. Absorption spectroscopy emerges as a powerful probe of the neutral gas which is otherwise extremely elusive beyond the local volume. This technique provides insights into the gas between us and distant background sources, allowing precise measurements of column densities of various atomic and molecular species. Moreover we can constrain the chemical composition, offering valuable information about the cosmic star formation history. This talk highlights how absorption spectroscopy enables the study of neutral gas in galaxies across cosmic distances – from the Milky Way to galaxies at the “cosmic noon” and beyond. These absorption systems provide a unique perspective on the galaxy population, complementing traditional photometric studies sampling mainly the brightest tip of the iceberg. Lastly, I will focus on current efforts towards the first blind quasar survey, emphasizing its importance not only for understanding absorption systems but also for gaining broader insights into the evolution of quasars and galaxies.