ALMACAL: The Evolution of Gas and Dust in Galaxies Using ALMA Calibrator Observations

Abstract

A fundamental question in astronomy is how galaxies form and evolve. How does gas flow into and out of galaxies? What physical processes drive the evolution of the star formation rate history? What is the role of dusty star formation at high redshift? To answer these questions we must understand the complex interplay between galaxies and the surrounding circum-galactic medium and we have to study the evolution of molecular gas and dust in galaxies.

We present the ALMACAL survey utilizing ALMA calibration observations for science. We use this unique dataset to study the evolution of molecular gas and dust in galaxies with cosmic time.

Using this survey, we select a sample of CO emission line detections in gas-rich galaxies first identified as intervening absorbers. From this parent sample we select the three galaxies detected in multiple CO emission lines for a further analysis and follow up observations. Ultimately we are aiming for a better understanding of the population of gas-rich galaxies.

As a pilot study, we use VLT/MUSE to follow up one absorption-selected system at detected in multiple CO transitions. We find in total four galaxies at the absorber redshift, one of which was detected in CO. This provides further evidence that the connection between absorber and host galaxy is more complex than a simple one-to-one relation. We find that most probably the absorbing gas is tracing intra-group medium.

Next we focus on the multiple CO transitions and study for the first time the CO spectral line energy distribution of absorption-selected galaxies. We find evidence for more excited ISM conditions compared to the Milky Way. This indicates that previous studies of absorption-selected systems might overestimate the molecular gas mass in some galaxies. Furthermore, we suggest that absorption-selected galaxies may preferentially trace group environments.

In addition to the local baryon cycle in single objects, we study the global baryon cycle over cosmic time. To understand the processes that drive the evolution of the star formation rate history, we trace the evolution of the molecular gas mass density over cosmic time using intervening molecular absorption. In the currently largest available dataset of quasar spectra in the submillimetre regime, ALMACAL, we do not detect intervening CO absorption. We place constraints on the evolution of the molecular gas mass density. This suggests, combined with complementary measurements from the literature, a strong evolution following that of the star formation rate history.

Finally, half of the star formation activity in the Universe is expected to take place in dusty star-forming galaxies. We use our ALMACAL dataset to search for dusty star-forming galaxies observed at m. We determine the first high-frequency number counts at m free of source blending and cosmic variance effects. At this wavelength we find that we resolve the majority of the extragalactic background light.