Astrophysics Research

I study the evolution of galaxies using a mix of analytical and computational methods. My core interests are gas in galaxies, angular momentum, and cosmic large-scale structure. These are intimately related topics, because the state of the gas in galaxies and its capacity to form stars sensibly depend on angular momentum. In turn, the supply of gas and acquisition of angular momentum are set by interactions with large-scale structure. I endeavor to establish these mechanisms in quantitative detail to clarify how the remarkable diversity of galaxies emerges naturally from simple physical laws. To secure the impact of this research, I maintain an exceptionally strong link to observers and work in a data-driven way.

Cold gas in galaxies

Galaxies form stars by converting ionized gas (HII) into neutral atomic gas (HI), then into molecular gas (H2), and finally into stars. This chain is bidirectional and regulated by radiative feedback from stars, supernovae, black holes and the cosmic UV background. Understanding star formation requires modelling these complex processes. My goal is to uncover the cosmic supply chain from HII to stars, especially the HI-H2 transition, across cosmic times. To achieve this goal my collaborators and I run numerical simulations that can be compared against data from radio and millimetre telescopes. For instance, by adapting a semi-analytic model (SAM) for the Millennium simulation, we produced at a virtual universe with millions of galaxies with resolved HI and H2 masses (Obreschkow et al. 2009a). This model resulted in a virtual sky with atomic and molecular emission lines, the so-called S3-SAX sky (Obreschkow et al. 2009b), that can be downloaded here. This model is used to predict the observations of the Atacama Large Millimeter/submillimeter Array (ALMA), the Square Kilometre Array (SKA), and the SKA pathfinders (e.g., Obreschkow et al. 2011). The same model also revealed that the so-called velocity function of HI detected galaxies is consistent with CDM paradigm (Obreschkow et al. 2013a), despite contradicting earlier claims. My most important discovery, first published in 2009 (Obreschkow & Rawlings 2009c) and refined in subsequent years, is that the H2/HI ratio in galaxies evolves dramatically with cosmic time. This discovery is crucial, since it links the cosmic history of star formation to the evolving state of the cold gas. This discovery was a pure prediction. However, in recent years, other groups, including observers, managed to verify and confirm this prediction in much detail.


Small extract (3′ x 1′) of the S3-SAX sky (Obreschkow et al. 2009b) for ALMA and SKA.

Angular momentum in galaxies

Mass and angular momentum are arguably the two most fundamental properties in galaxies. It is well-established that most galaxy properties scale with the galaxy mass, but similar scaling relations for angular momentum are only just being discovered. Difficulties in simulating the growth of angular momentum, paralleled by observational challenges to obtain 2D kinematic maps, hindered the systematic study of angular momentum until the early 21st century. My objective is to make the concept of angular momentum an essential pillar and tool of mainstream astronomy, by combining observational data of modern integral field spectroscopy/interferometry with analytical models and high-performance computer simulations. My most important contribution so far is the discovery of a tight mass-spin-morphology relation in spiral galaxies (Obreschkow & Glazebrook 2014) based on an exquisite sample of galaxies with accurate kinematic maps from the THINGS survey (see figure).


LEFT: Combined HI intensity map and color-coded HI velocity map of 16 spiral galaxies of the THINGS sample. Colors range from red to blue for projected velocities from –vmaxsin(i) to +vmaxsin(i). The white bars represent 10 kpc scales. RIGHT: The 16 spiral galaxies turn out to be highly correlated in the 3D-space spanned by the (log) baryon mass, (log) specific baryon angular momentum jb, and the bulge-to-total ratio β. Points represent the positions of barred (open circles) and unbarred (filled circles) galaxies, while the blue plane represents the best fitting plane (see Obreschkow & Glazebrook 2014).

To expand this research to larger samples, I am leading the analysis of angular momentum in disk galaxies of the SAMI survey. In parallel, I also investigate galaxy properties and the large-scale distribution of galaxies as a function of angular momentum in semi-analytic models, N-body simulations, and data from major HI surveys.

Cosmic large-scale structure

Galaxies are not randomly placed in the Universe. They span a giant cosmic web, shaped by the combined action of dark matter and dark energy. Therefore, the precise arrangements of galaxies holds clues about the unknown nature of these dark substances. Most current research on the cosmic web focuses on the so-called galaxy-galaxy correlation (or its Fourier transform, the power spectrum). While easy to calculate, this correlation function only contains a fraction of the information contained in the cosmic web.

My aim, pursued in collaboration with scientists at Cambridge, is to develop higher-order spatial statistics for the cosmic web. Our most important contribution so far (Obreschkow et al., 2013), is the development of a statistical estimator that, when applied to galaxy positions, reveals information about the Universe not contained in the galaxy-galaxy correlation. This new estimator is particularly sensitive to filamentary structure and allows a more accurate determination of fundamental cosmic parameters than possible so far. For instance, the figure shows the cosmic web of two model Universes, one made of cold dark matter (blue) and one made of warm dark matter (red) with a particle mass-energy of 0.1keV. The two mathematical functions represent our new estimator, the so-called line correlation. This function can distinguish between the two Universes ten times more precisely and on larger scales than the standard galaxy-galaxy correlation.