PhD defense by Vasily Kokorev

Title: CHARTING THE STELLAR, DUST AND GAS CONTENT OF GALAXIES: A Panchromatic View at the Galaxy Evolution Across Cosmic Time


Since time immemorial the humanity has gazed upon the heavens in fascination. Generations upon generations have attempted to uncover the mysteries of the cosmos and understand the inner-workings of this complex mechanism. At first, the luminous bands of light in the night sky belonging to our home the Milky Way galaxy, have been revealed to be trillions of stars, not unlike our Sun. Soon after, by observing the faint and diffuse nebulae barely visible in the night sky, we have come to understand that a great multitude of galaxies similar to our own exist in the vast expanse that we call the Universe. The light which travels to us from these distant galaxies, has a finite speed, thus allowing us to look far not only in space, but also time. By observing the Universe at different distances, we can therefore reconstruct the history of the galaxy formation by looking at various snapshots spread out in time. In my thesis, I primarily studied the interstellar medium, which consists of two primary ingredients - dust, which attenuates and dims the stellar light, and gas, the fuel for star formation.

First, I developed an algorithm - Stardust, which combines the information from light, emitted by galaxies at different wavelengths, in order to understand their physical properties. These properties include total masses of all stars, dust and gas, fraction of light contributed by the active galactic nuclei, as well as rate of formation of new stars. By applying my algorithm to the deepest and most complete optical and infrared galaxy catalogues to date I have conducted a thorough examination of the evolution of both the dust and gas - to stellar mass ratios over a course of 12 billion years of cosmic history. The scaling relations which I recover, not only agree with the existing studies, but also push our knowledge of dust and gas mass evolution to higher redshifts. In addition to that, my study is one of the most statistically robust and complete investigations of the ISM evolution to date and can provide a perfect test-bed for simulations. During this analysis I have also discovered a population of galaxies with massive gas mass reservoirs, the so called ’gas-giants’, which defy the current observational and theoretical predictions.

Second, I have constructed a set of HST and Spitzer mosaics, and photometric catalogues, covering the ALMA lensing cluster survey (ALCS) fields. My work presents, for the first time, a uniformly combined and reprocessed set of archival HST/Spitzer observations covering 33 lensed cluster fields in Hubble Frontier, RELICS and CLASH, and currently contains the deepest and most complete photometry in those fields. These catalogues will serve as an important tool in aiding the search of the sub-mm galaxies in future ALMA surveys, as well as follow ups of the "HST dark" - IRAC sources. The combination of photometry from two telescopes allows us to place better constraints on photometric redshifts and stellar masses of these galaxies, thus giving us an opportunity to identify high-redshift (z > 7) candidates for spectroscopic follow ups and answer the important questions regarding the epoch of reionisation and formation of first galaxies.

Finally, I conducted a preliminary assessment of source detection feasibility for the MIRI component of the upcoming upcoming JWST Cycle I GO COSMOS - Webb program. By simulating the SEDs and modeling the MIRI mosaics I predict that the survey is expected to recover a significant proportion of optically detected sources. However more work is required to correctly estimate the number and properties of optically dark and high-z galaxies. These sets of simulations will be used to optimise critical source detection algorithms, and aid in the interpretation and reduction of the data.

The work presented in this thesis, including the peripheral studies that I have contributed to, will allow us to expand our understanding of the evolution of dusty star forming galaxies. Moreover, the tools and the catalogues produced during this thesis are made available to the wider scientific community, and can be used to conduct the search and analysis of new and unique galaxy populations.


  • Georgios Magdis, Niels Bohr Institute, University of Copenhagen

Evaluation Committee:

  • James Dunlop, Institute for Astronomy, Royal Observatory, University of Edinburgh
  • Andrew Bunker, Department of Physics, University of Oxford
  • Charlotte Mason, Niels Bohr Institute, University of Copenhagen


There will be a following reception in Aud. C (across the hallway of Aud. A)


ZOOM-link for participants whom are not able to attend at Aud. A :

Meeting ID: 686 1794 2907

Passcode: 500577