Tobias Buck

Research group leader in computational astrophysics and machine learning

Hi, since August 2022 I am a junior research group leader of the AstroAI-Lab combining computational astrophysics and machine learning. My group is located at the Interdisciplinary Centre for Scientific Computing (IWR) and the Centre for Astronomy at Heidelberg University (ZAH) funded through the NEXUS program of the Carl-Zeiss foundation.

My research focuses on the formation of Milky Way type galaxies in high-resolution cosmological hydrodynamical simulations. My main interest lies in the satellite galaxies of Milky Way type galaxies as cosmological probes of structure formation and the build-up of the stellar disc and its structure in a cosmological context. In order to study galaxy formation I employ state-of-the-art numerical simulations and modern machine learning tools to model the relevant physical processes governing the dynamics of the inter-stellar gas content and stellar populations of galaxies. For more information on ongoing projects see also the AstroAI-Lab webpage. My research has recently been featured in a public outreach article by the Quanta magazine and a Podcast episode.

Research Interest

I am interested in the process of galaxy formation. It is fascinating how the Universe was able to evolve from an almost homogenous state at the time when the CMB was released to a highly structured state with dark matter filaments and halos, galaxy clusters and galaxies. Thus, I study galaxy formation in a cosmological context using hydrodynamical simulations. Some cool Movies can be found here.

The production and distribution of chemical elements in the Universe

Almost all chemical elements we know were produced inside stars like our sun. Only the ightest elements, like hydrogen or helium were produced at the Big Bang. Having enough heavy elements like carbon or silicon is a prerequisite for life in the Universe and therefore understanding the formation and distribution of chemical elements across cosmic time is a fundamental question in Astrophysics. I have recently developed a new chemical enrichment prescription for the simulation code Gasoline. With this new model we are able to follow the production and distribution of elements in Milky Way like galaxies. This helps us understand where the key elements for life on earth are coming from and enables a far better comparison of simulation data with observations. Those kind of siimulations will eventually allow us to reconstruct the formation history of our Milky Way. See this paper and this paper for more information.

The PICASSSO project: Painting Intrinsic Attributes onto SDSS Objects

Multi-band images of galaxies reveal a huge amount of information about their morphology and structure. However, inferring properties of the underlying stellar populations such as age, metallicity or kinematics from those images is notoriously difficult. Traditionally such information is best extracted from expensive spectroscopic observations. With the PICASSSO project I test the information content of photometric multi-band images of galaxies. Using modern machine learning techniques, I establish a connection between galaxy images and their underlying physical stellar and gaseous properties. I test the ML algorithm with SDSS ugriz mock images for which uncertainties and systematics are exactly known. This proof-of-concept work shows that multi-band galaxy images contain enough information to reconstruct 2d maps of stellar mass, metallicity, age and gas mass, gas metallicity as well as star formation rate. PICASSSO is able to recover the true stellar properties on a pixel by pixel basis with only little scatter, smaller than 10% compared to 20-30% statistical uncertainty from traditional mass-to-light-ratio based methods. Morphology alone constrains stellar properties of galaxies to better than 30%. See the CV section for talk slides.

The impact of different physical models on the formation of Milky Way like galaxies

The formation of galaxies like our own Milky Way are governed by a range of different physical processes such gas cooling and heating via radiation, supernova explosions or cosmic rays. The very details of those processes are able to change the morphology and properties of the galaxies as they evolve. In a series of recent papers I have been studying the impact of different model ingredients on the formation disk galaxies. I focussed thereby on the way star formation is modelled in the simulations or the impact of non-thermal components such as cosmic rays. I found significant differences in galaxy properties when applying different models. Connecting the simulation results to detailed observations of our own Milky Way can help to us constrain galaxy formation models in more detail. See this paper or this paper for more information.

High resolution hydrodynamical simulation of Milky Way type galaxies

As part of my PhD I was running high-resolution hydrodynamical simulations of analogues of our own Milky Way. These simulations are state of the art in terms of resolution and modelling technique. The resulting galaxies show surprisingly thin discs of stars and spiral arms of young stars. The simulation volume is furthermore filled with a wealth of smaller satellite galaxies well in agreement with the Milky Way's companions. More information can soon be found in my recent paper.

NIHAO XV: The environmental impact of the host galaxy on galactic satellite and field dwarf galaxies

The two major galaxies in the Local Group, the Milky Way and its companion the Andromeda galaxy, both host a large number of small satellite galaxies. The properties of these small galaxies are significantly effected by the environment of Milky Way and Andromeda. In this recent work I studied the effect that the Milky Way has on the properties of its satellite galaxies and what the observational possibilities are in finding strongly effected galaxies. The Milky Way environment leads to a significant loss of mass, both dark matter mass as well as gas mass. Using the simulations we are able to establish a relation between the mass of a satellite before falling onto the Milky Way and its stellar mass today, thus, being able to account for the mass loss and give estimates of the mass prior to infall for observed Milky Way satellites. Furthermore, this study nicely shows that modern high-resolution simulations are finally able to predict the right number, stellar mass and structure of small dwarf galaxies. See this paper for more information.

Stars behind Bars II: A cosmological formation scenario of the Milky Way's central stellar structure

In this second part of the study of the peanut shaped bulge I used a kinematic decomposition technique to separate the stars in the simulation into different orbit families. I am able to find 5 different components including two bulge components and two disc components as well as a stellar halo. The bulge stars are all coming from the stellar disc brought to the center via disc instabilities. The reason why the bulge stars separate into two different orbit families is their different birth properties. Peanut bulge stars have on average larger birth angular momentum. See this paper for more information.

Stars behind Bars I: The Milky Way's central stellar populations

The Milky Way is a barred spiral galaxy and thus its central region, the bulge, looks somewhat like a peanut. In a series of two recent papers I studied the properties and the formation of such a structure in a high-resolution cosmological hydrodynamical simulation of a Milky Way type galaxy. I find very good agreement between the properties of the simulated bulge and Milky Way's bulge. Especially the morphology and the kinematics of stars are in excellent agreement. I further test observational ways to distinguish between stars in the bar and the surrounding disc. See this paper for more information.

The edge of galaxy formation II: Milky Way satellite after accretion

The second part of my side project dealed with the evolution of Milky Way satellites after their accretion onto the Milky Way. Using idealised simulations of various different orbits for one satellite we find that all structural parameters are influenced by the accretion history and quantify the effect of the orbit. Satellites show severe dark matter mass loss after infall while the stellar mass stays nearly constant. Due to that the rotation curves and velocity dispersion profiles of the stars are altered significantly. See this paper for more information.

The edge of galaxy formation I: MW-satellites before accretion

In a side project I contributed to the exploration of the formation of the smallest galaxies we know of - the satellites of the Milky Way. In this paper we show that the global properties of Milky Way satellites such as total stellar mass are already in place before their accretion. Furthermore we find that galaxy formation on these mass scales is barely able to alter the host dark matter halo of the smallest galaxies. For more information see our paper.

Giant UV bright clumps at high redshift

Disc galaxies in the eraly universe are observed to have an irregular morphology opposed to the nice spiral structure we see in present day disc galaxies. These early galaxies show several giant kpc sized off-center clumps which mostly stick out in the UV images. Early simulation work suggested these are giant star cluster due to violent disc instabilities. In this paper, part of my first PhD project, I could show that these clumps are the result of a non-linear mapping of stellar mass into stellar light. Using the NIHAO simulations we find clumps in stellar light matching the observed features clumps but we do not see clumps in stellar mass. In NIHAO the clumps result from the strong contribution of young stars to the UV light. Since stars in the simulations form clustered also the light morphology is clumpy. See my paper, my poster or my talk slides if you are interested.

The stability of satellite planes

In a small project continuing the work of my master thesis I analysed the stability of planes of satellites. The observed seemingly alignment of satellites of our own Milky Way and of its companion Andromeda in a thin planar structure is sometimes interpreted as a challenge to the widely accepted cosmological concordance model of cold dark matter. In this work I used dark matter only simulations to study the stability of thin planar structures. Such thin planes are present in the concordance cosmology but not stable. More details can be found in this paper or on my Poster.

Thin Satellite planes from filamentary accretion

The satellite galaxies of the companion galaxy of our Milky Way - Andromeda (M31) - are observed to be aligned in a thin seemingly rotating planar structure. In my master thesis I ran dark matter only simulations of Milky Way mass dark matter haloes to search for such configurations. In this paper we find that filamentary accretion of satellites from coherent directions ca explain the planar alignment.

The formation of the Hyades

During bachelor studies I explored several formation scenarios of the Hyades open star cluster. In order to match the observed present day properties of the Hyades the model star cluster needed to be much more massive at formation time and loose a significant amount of stars to the galaxy. The observable feature of this model are pronounced tidal tails of stars leaving the star cluster via the lagrangian points. See my bachelor thesis for more details.

Publications

Link to ADS

Link to Benty Fields

ORCID iD icon https://orcid.org/0000-0003-2027-399X

Refereed Papers

  • The challenge of simultaneously matching the observed diversity of chemical abundance patterns in cosmological hydrodynamical simulations (Buck et al. 2021, MNRAS,): ADS, astro-ph
  • Universal properties of the high- and low-alpha disk: small intrinsic abundance scatter and migrating stars (Yuxi et al. 2021, AJ,): ADS, astro-ph
  • The GALAH+ Survey: Third Data Release (Buder et al. 2021, MNRAS,): ADS, astro-ph
  • Creating a galaxy lacking dark matter in a dark matter-dominated universe (Macciò et al. 2021, MNRAS,): ADS, astro-ph
  • Exploring the origin of low-metallicity stars in Milky-Way-like galaxies with the NIHAO-UHD simulations (Sestito et al. 2021, MNRAS,): ADS, astro-ph
  • NIHAO XXV: Convergence in the cusp-core transformation of cold dark matter haloes at high star formation thresholds (Dutton et al. 2020, MNRAS,): ADS, astro-ph
  • The effects of cosmic rays on the formation of Milky Way-mass galaxies in a cosmological context (Buck et al. 2020, MNRAS,): ADS, astro-ph
  • The Strength of the Dynamical Spiral Perturbation in the Galactic Disk (Eilers et al. 2020, ApJ,): ADS, astro-ph
  • On the origin of the chemical bimodality of disc stars: a tale of merger and migration (Buck 2020, MNRAS,): ADS, astro-ph
  • NIHAO-UHD: the properties of MW-like stellar discs in high-resolution cosmological simulations (Buck et al. 2020, MNRAS,): ADS, astro-ph
  • Local photoionization feedback effects on galaxies (Obreja et al. 2019, MNRAS,): ADS, astro-ph
  • Drivers of disc tilting I: correlations and possible drivers for Milky Way analogues (Earp et al. 2019, MNRAS,): ADS, astro-ph
  • An observational test for star formation prescriptions in cosmological hydrodynamical simulations (Buck et al. 2019, MNRAS,): ADS, astro-ph
  • The edge of galaxy formation III: The effects of warm dark matter on Milky Way satellites and field dwarfs (Macciò et al. 2019, MNRAS,): ADS, astro-ph
  • A deeper look into the structure of CDM haloes: correlation between halo parameters from Einasto fits (Udrescu et al. 2019, MNRAS,): ADS, astro-ph
  • NIHAO XVIII: Origin of the MOND phenomenology of galactic rotation curves in a LCDM universe (Dutton et al. 2019, MNRAS,): ADS, astro-ph
  • NIHAO XVI: The properties and evolution of kinematically selected discs, bulges and stellar haloes (Obreja et al. 2019, MNRAS,): ADS, astro-ph
  • NIHAO XX: The impact of the star formation threshold on the cusp-core transformation of cold dark matter haloes (Dutton et al. 2018, MNRAS,): ADS, astro-ph
  • Introducing galactic structure finder: the multiple stellar kinematic structures of a simulated Milky Way mass galaxy (Obreja et al. 2018, MNRAS,): ADS, astro-ph
  • NIHAO XV: The environmental impact of the host galaxy on galactic satellite and field dwarf galaxies (Buck et al. 2018, arXiv,): ADS, astro-ph
  • Stars behind bars II: A cosmological formation scenario of the Milky Way's central stellar structure (Buck et al. 2018, arXiv,): ADS, astro-ph
  • Stars behind bars I: The Milky Way's central stellar populations (Buck et al. 2018, ApJ,): ADS, astro-ph
  • Inspiraling Halo Accretion Mapped in Lyman-alpha Emission around a z~3 Quasar (Battaia et al. 2017, MNRAS,): astro-ph
  • The edge of galaxy formation II: evolution of Milky Way satellite analogues after infall (Frings et al. 2017, MNRAS,): ADS, astro-ph
  • The edge of galaxy formation I: formation and evolution of MW-satellites analogues before accretion (Macciò et al. 2017, MNRAS,): ADS, astro-ph
  • NIHAO XIII: Clumpy discs or clumpy light in high-redshift galaxies? (Buck et al. 2017, MNRAS, 468, 3628): ADS, astro-ph
  • NIHAO XII: galactic uniformity in a ΛCDM universe (Dutton et al. 2017, MNRAS, 467, 4937): ADS, astro-ph
  • NIHAO IX: the role of gas inflows and outflows in driving the contraction and expansion of cold dark matter haloes (Dutton et al. 2016, MNRAS, 461, 2658): ADS, astro-ph
  • Simulated ΛCDM analogues of the thin plane of satellites around the Andromeda galaxy are not kinematically coherent structures (Buck et al. 2016, MNRAS, 460, 4348): ADS, astro-ph
  • NIHAO - IV: core creation and destruction in dark matter density profiles across cosmic time (Tollet et al. 2016, MNRAS, 456, 3542): ADS, astro-ph
  • The response of dark matter haloes to elliptical galaxy formation: a new test for quenching scenarios (Dutton et al. 2015, MNRAS, 453, 2447): ADS, astro-ph
  • Evidence for Early Filamentary Accretion from the Andromeda Galaxy’s Thin Plane of Satellites (Buck et al. 2015, ApJ, 809, 49): ADS, astro-ph

Conferences and Workshops

Invited Talks

  • American museum for natural history, (March 1st 2022) Talk slides
  • Erwin Schrödinger Institut workshop: Tomographic Reconstructions and their Startling Applications, (March 2021) Conference Website, Talk slides
  • Max-Planck Institut für Intellgente Systeme, (November 2020) Talk slides
  • Price Colloquium Wilhelm und Else Heraeus-Dissertationspreis für Physik and Astronomie (24.1.2020), Heidelberg University
  • HGSFP Winterschool School (Jan. 2019), 2 invited lectures on Cosmology and Galaxy Formation, Heidelberg University
  • Columbia University Astrophysics Colloquium (24.7.2018), Columbia University, NYC
  • Königstuhl Colloquium (13.4.2018), MPIA, Heidelberg
  • MPA Colloquium (10.1.2018), MPA, Munich

Theses

Curriculum Vitae

Academic Education

2022 - now
Research group leader, Interdisciplinary Center fir Scientific Computing, Heidelberg, Germany
2018 - 2022
Postdoc, Leibniz-Institute for Astrophysics, Potsdam, Germany
2015 - 2018
Graduate student, Max-Planck-Institut für Astronomie, Heidelberg, Germany, Advisor: Prof. Dr. Hans-Walter Rix, Thesis title: "On the formation of the MW system in cosmological context" (grade: summa cum laude)
06/2015
Master's Degree, University of Heidelberg, Germany, Advisor: Prof. Dr. Andrea Macciò, Thesis title: "The Emergence of spatially and kinematically coherent Planes of Satellite Galaxies in high-resolution Dark Matter only simulations"
2013 - 2015
Master's Programme, University of Heidelberg, Germany
07/2013
Bachelors's Degree, University of Heidelberg, Germany, Advisor: Prof. Dr. Andreas Just, Thesis title: "Dynamical analysis of the Hyades star cluster using direct N-body simulations"
2010 - 2013
Bachelor's Programme, University of Heidelberg, Germany

Fellowships/Scholarships

Jan. 2019
Nominee for Wilhelm und Else Heraeus-Dissertationspreis für Physik and Astronomie, University Heidelberg
2017 - 2019
30M CPUh computing time, Leibniz SuperComputing Centre, SuperMUC, co-I
2015 - 2018
Member of the "International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg" (IMPRS-HD)
2010 - 2015
Studienstiftung des deutschen Volkes e.V.
Nov. 2014
Travel grant for DWIH Indo-German Winter School for Astrophysics, India, Mumbai

A more detailed pdf version of my CV can be found here

Coding

  • For my PhD project I intensively use the python package pynbody to which I also contribute own code. Scientific python code for simulation analysis can be found on my github repositories. Simulation data is publicly available here.
  • During my bachelors and masters studies at the University Heidelberg I participated a lot in the robotics lab. My first project was a xylophon robot, the second was a self driving car and the third was a 3D display using the Persistence Of Vision technique. Most projects can also be found on my github.

Outside Astronomy

Outside astronomy I love to do music. I am heavily involved in the Spielmannszug TSV Malente as one of the conductors and where I am teaching kids on how to play instruments. I am further involved in the drum line of Spielmannszug TSV Malente. I am also part of the Bundesorchester Spielleute a flute orchestra which consists of people from all over Germany. We meet twice a year for a week and give one concert a year.

Movies

I am interested in the process of galaxy formation. It is fascinating how the Universe was able to evolve from an almost homogenous state at the time when the CMB was released to a highly structured state with dark matter filaments and halos, galaxy clusters and galaxies. Thus, I study galaxy formation in a cosmological context using hydrodynamical simulations. Below you can find some cool movies.

Simulation Data

Below you find download links for some of the simulations I run during my PhD. Each link leads to the redshift zero snapshot in tipsy format and the AHF halo catalogue files. Higher redshift snapshots can be made available upon request. Please cite the introduction paper when using the simulations. The simulated galaxies have been further studied in a series of paper which you might find worth citing as well.

  • g8.26e11
  • g7.55e11
  • g7.08e11
  • g6.96e11
  • g2.79e12
  • g1.12e12

      • Further papers using the simulations

      • On the origin of the chemical bimodality of disc stars: a tale of merger and migration (Buck 2020, MNRAS,): ADS, astro-ph
      • NIHAO XV: The environmental impact of the host galaxy on galactic satellite and field dwarf galaxies (Buck et al. 2019, MNRAS,): ADS, astro-ph
      • NIHAO XVI: The properties and evolution of kinematically selected discs, bulges and stellar haloes (Obreja et al. 2019, MNRAS,): ADS, astro-ph
      • Stars behind bars II: A cosmological formation scenario of the Milky Way's central stellar structure (Buck et al. 2018, MNRAS,): ADS, astro-ph
      • Stars behind bars I: The Milky Way's central stellar populations (Buck et al. 2018, ApJ,): ADS, astro-ph

Contact Me

  • (+49/0) 331-7499-285
  • tobias 'dot' buck 'at' iwr 'dot' uni 'minus' heidelberg 'dot' de
  • Leibniz-Insitut für Astrophysik, An der Sternwarte 16, D-14482 Potsdam, Germany
  • tobias-buck