... working on dwarf galaxies and near-field cosmology. My Alma Mater is the University of Basel, where I studied physics and astronomy under the supervision of Prof. Bruno Binggeli. I am especially interested in the search for and characterization of dwarf galaxies in nearby galaxy groups. For that purpose I use several different optical telescopes like the Dark Energy Camera, the Very Large Telescope, and the Sloan Digital Sky Survey telescope.
WLM galaxy. Captured by ESO’s OmegaCAM on the VLT.
Dwarf galaxies are the most abundant and most elusive type of galaxy. And more, they are thought to be the most dark matter-dominated objects in the Universe. They are also quite distinct in size, shape and mass from other galaxies, such as elliptical or spiral galaxies. The barely visible dwarf galaxies are a major puzzle to our understanding of structure formation in the Universe, described by the cosmological dark energy + cold dark matter model. The many discrepancies for dwarf galaxies between theory and observations in small-scale cosmology have now standing names like the Too-Big-Too-Fail problem, the cusp/core problem, or the planes-of-satellites problem.
I have published several papers on the discovery of new dwarf galaxies in our local galactic neighbourhood, the so-called Local Volume, which is a sphere of 10 Mpc (= 33 million light years) around our point of view on Earth. In total these discoveries add up to roughly 10 percent of the known nearby galaxies.
In this project we are carrying out a search for very faint, hitherto undetected dwarf galaxy members of the southern groups of galaxies in Sculptor and Centaurus. The search, done in collaboration with my PhD co-supervisor Prof. Helmut Jerjen of the Australian National University in Canberra, is based on dedicated observations with the wide-field CCD camera of the SkyMapper telescope in Siding Spring, Australia, and the DECam (Dark Energy Camera) at Cerro Tololo Observatory in Chile.
A sky area of totally 750 square degrees covering essential parts of the two groups is being imaged down to significantly fainter magnitudes than previously reached by photographic surveys. Drawing on direct inspection of the deep CCD frames, backed by the application of special filtering techniques which are currently being developed. Multicolor surface photometry and profile modelling for the new, as well as the previously known dwarf members will yield a complete set of structural parameters for the galaxies.
The main scientific questions addressed in collaboration with Dr. Marcel Pawlowski are the following: We will trace the luminosity function of galaxies to very faint magnitudes and use its faint end to put constraints on the processes of baryon settlement in the dark matter minihalos. Furthermore, we will search for anisotropies in the spatial distribution of the new and old dwarfs in the groups; such anisotropies are known to exist in the dwarf satellite members of the Local Group, constituting a major challenge to the standard ΛCDM scenario of structure formation. Last but not least, the newly discovered dwarfs will also provide important targets for follow-up studies. This project is almost done and lead to the six publications: Müller et al. (2015, 2016, 2017a, 2018a, 2018b, 2018c), of which one appeared in Science.
To study our newly discovered dwarf galaxies follow-up measurements are conducted with the Very Large Telescope (VLT) at Cerro Paranal, the largest optical telescope in the southern hemisphere. This work, in collaboration with Dr. Marina Rejkuba of the European Southern Observatory (ESO), will give better statistics on the existence of the discovered planes of satellites in the Centaurus Group. Are there two almost parallel planes of dwarf galaxies as suggested by Tully et al. (2015) or is one of the two planes just an artifact from low number statistics as we proposed (Müller et al. 2016)? Intruigingly, we showed that the satellites around Cen A move in a coherent fashion, similar to the planes-of-satellites in the Local Group (Müller et al. 2018a), posing a major challenge to our understanding of structure formation.
The Local Volume – our local galactic neighbourhood – is still mainly uncharted territory if it comes down to the low-surface brightness population, especially the gas-poor dSph which will remain undetected in large-scale radio surveys.
Together with the Master student Roberto Scalera we survey nearby group of galaxies, beginning with the M101 complex, using the public data from the Sloan Digital Sky Survey (SDSS) (Müller et al. 2017b) and whatever data we can exploit (DES, KiDS, ATLAS and more). Of special interest will be the Canes Venatici cloud with its many dwarf members. The Canes Venatici cloud, together with the Local Group, the Sculptor group and the Centaurus group, forms the socalled Local Sheet, the large-scale filament going through the Local Volume. What will we uncover in such a survey? Will we find ultra diffuse galaxies there? Such dwarf galaxy surveys will lay the foundation for future studies.
To support the efforts to detect such low-surface brightness objects we develop an open-source software, embracing both visual inspection of the image and automated pattern recognition algorithms.
An astronomical image with a hidden dwarf galaxy. Click on this image to apply a Gaussian filter to make the dwarf galaxy visible.
In collaboration with Dr. Behnam Javanmardi we study the puzzling correlation of the number of dwarf galaxies and the size of the bulge of the host galaxy. In the standard model of cosmology such correlations are not predicted as the number of dwarfs should only correlate with the dark matter mass of the host and not the size of the bulge (Javanmardi et al. 2018). The bulge itself is thought to be created in major mergers, so it shouldn't affect the census of dwarf galaxies. However, in an alternative cosmology (e.g. MOND) such correlations are expected, as the dwarf galaxies form in tidal interaction events, the same events that form bulges, i.e. the more past interaction a galaxy had, the more dwarf galaxies were created and the larger the bulge.
Apart from being a scientist, I enjoy doing sports, especially Martial Arts. I am a board member of the Swiss branch and instructor of the japanese Martial Arts Shinkendo. Additionally, I am an enthusiastic table top player and member of the Tabletop Verein Basilisk, a club dedicated to provide a friendly and lively environment for this hobby, which involves model building, painting, and of course playing.
I also like travel and see new places. Being a scientist certainly helps coming around. When I have free time, especially during my travels I photograph, with focus on landscape and street photography.
When at home, I also like to program (i.e. this and the two previous websites), read books, practice the e-guitar, play games, and write on my blog.
At a seminar with the founder of Shinkendo, Kaiso Toshishiro Obata.
A roadtrip through Iceland, with uncommonly good weather.
Three week hike through Sweden's wilds on the so-called Kungsleden.
Photography. True colour, black beach in Iceland.
If you are interested in inviting me as a speaker, please write me an e-mail about the occasion. I enjoy talking in front of a crowd, be this in a school or to interested people, and of course for academics as well.