Mark Sargent

telescope array

Gas and star formation in galaxies through cosmic time

Supervisor: Mark Sargent


  • Seb Oliver, Stephen Wilkins, Anna Cibinel, Jon Loveday, Peter Thomas (U. of Sussex)
  • Emanuele Daddi (CEA Saclay)
  • Eva Schinnerer, Fabian Walter (MPIA Heidelberg)
  • Alex Karim (Uni. Bonn)
  • Marcella Carollo (ETH Zurich)
  • Vernesa Smolcic (Uni. Zagreb)
  • Marcella Brusa (U. of Bologna)
  • Sara Ellison (UVic)
  • the COSMOS- and GOODS-teams (incl. ~100 partner institutes worldwide, e.g. Caltech, Cornell, Ehime, IPMU, Harvard, Hawaii, Marseille, NOAO, Vienna)

Start date: 1st October 2019

Funding: STFC quota studentship. (SEPnet and EU studentships may also be available.)

Our view of high-redshift star formation is undergoing a paradigm shift. It has been known for a couple of decades that galaxies were more active in the past (a galaxy with a mass similar to our own Milky Way, for example, formed almost ten times more stars when the Universe was half as old as today). Until recently, it was generally believed that this enhanced activity was caused by galaxy interactions that led to vigorous bursts of star formation. In such events almost all gas in the merging galaxies can be converted to new stars on cosmologically very short time scales of less than 100 million years. In the past few years, however, there has been growing evidence that stars in high-redshift galaxies are born under much less extreme conditions and that most of the gas in these early galaxies is converted to stars over 1-2 billion years, similar to the gas depletion time scale of our own Milky Way. In this newly emerging picture of galaxy formation, the steady accretion of pristine gas from intergalactic space regulates the star formation activity throughout the history of the Universe rather than violent and occasional/stochastic processes like interactions between galaxies. Observationally, this is reflected by the fact that instantaneous mass growth and total mass acquired at earlier times (the star formation rate and stellar mass, respectively) of star-forming galaxies correlate strongly over much of cosmic time, implying that there is a general pattern of star formation history which is followed by the majority of galaxies. This uniformity of the star-forming galaxy population has led extragalactic astronomers to speak of the existence of a "main sequence of star-forming galaxies", in analogy to the preferred locus of stars (also called "main sequence") in the Hertzsprung-Russel diagram.

Observations at radio wavelengths are uniquely placed to directly study both the interstellar medium (gas and dust) and star formation activity in galaxies throughout the history of the Universe. Spectroscopy at (sub-)mm and decimeter wavelengths targets rotational transitions of molecular gas and emission from atomic hydrogen. Continuum emission at sub-mm and decimeter wavelengths, respectively, serves as a probe of a galaxy's dust content and star formation activity.
A range of different projects are available that all address questions related to the interstellar medium and star formation activity of galaxies in different environments and at different epochs of the Universe. The observational focus of these projects will lie on radio observations (either from large surveys on JVLAIRAM and ALMA or PI projects on these observatories led by the supervisor and/or his collaborators), analysed and interpreted in the context of ancillary multi-wavelength data sets from a variety of extragalactic surveys (e.g., COSMOSCOSMOSHELPSDSS and ZENS). These projects aim to address the following questions:

  •  What is the relative importance of (a) feedback from active galactic nuclei (AGN) and (b) gas-consumption during starburs events for producing red and dead elliptical galaxies?
  •  How do the different phases of the interstellar medium (atomic and molecular gas, ionized gas, dust) evolve during galaxy interactions and mergers at low and high redshift?
  •  How do environmental effects in galaxy groups affect the gas fraction and star formation efficiency of galaxies?
  •  Does radio luminosity permit the observer to infer the star-formation rate of galaxies equally well for starbursts and "normal" galaxies (i.e. spiral/disk galaxies with activity that persists on timescales of several billion years rather than occurring in a short, vigorous burst)?
  • How does the higher gas and dust content of high-redshift galaxies affect their internal structure (e.g. spiral arm properties, prevalence and appearance of dust lanes)?
  • How does the measured evolution of gas fractions and dust masses for real galaxies compare to the predictions from galaxy simulations?

Students will have ample opportunities to define her/his preferred research foci during the course of the PhD. The PhD project would typically be structured more or less as follows:

  • Year 1: Background reading/introduction to (radio) data reduction and analysis techniques; compile ancillary data sets necessary for project; test newly developed algorithms and apply this to test data for proof of concept; visit partner institutes and/or participate in an observing run. 
  • Year 2: Continued development of purpose-built algorithms and roll-out to full data set; subject first science results to robustness checks; work on first publication(s); write observing proposals to request new/complementary data; travel for conference participation and collaborative visits. 
  • Year 3: Extend analysis to different data sets; branch out into follow-up analyses motivated by findings during first 2 years of thesis; write thesis and publications; travel for conference participation and collaborative visits. 

Should the student so desire, there are experts of modelling and simulation of galaxy evolution at the Astronomy Centre who could support the student in linking observational findings with theoretical predictions.

Skills/career development

Radio astronomy is a booming sector of astronomy, with several observatories being upgraded (e.g. JVLAGMRTe-MERLINPdBI/NOEMA), newly completed/under construction (e.g. ALMAMeerKATASKAPLOFAR) or planned (e.g. SKA). The student embarking on this project will thus acquire skills that may be expected to be in high demand in the years ahead. She/he will become a full participating member of the international collaborations relevant to carrying out the thesis research and will be able to grow a network of international collaborators, e.g. by interacting with scientists at partner institutes and/or attending collaboration meetings and conferences. As the project progresses, the student is expected to become involved in follow-up proposals for observing time on science targets of particular interest. The student will acquire generic research skills (literature searching, oral presentations, document preparation, setting/meeting goals etc.) through on-the-job training and through the specialist skills courses available through the University. She/he will also develop skills specific to astronomy (programming, use of astronomy archives, use of astronomy tools, proposal writing, optical observing, complex statistic analysis and data modelling etc.) in courses offered at the department and by day-to-day work on the project.