Highlights 2017

In observations of galaxy clusters, astronomers in collaboration with the MPA discovered a new class of cosmic radio sources. With the digital radio telescope Low Frequency Array (LOFAR) they received the longest radio waves that can be measured on Earth. They identified a remarkable "tail"behind a galaxy in the radio light, which must have been re-energized after it had faded away. In the journal Science Advances, the team describes this discovery, which either confirms a theoretical prediction on the interaction between shock waves and radio plasma or represents a novel phenomenon. more

A team of astrophysicists from Queen’s University Belfast, the Max Planck Institute for Astrophysics (MPA), and Monash University (Australia) has, for the first time, performed three-dimensional computer simulations that follow the evolution of a massive star from its final phase of nuclear burning, through the collapse of the stellar iron core, into the first seconds of the beginning explosion as a supernova. The simulations show that the large-scale violent convective motions that stir the oxygen burning layer at the onset of collapse can provide crucial support for the explosion of the star. more

Type Ia supernovae (SNe Ia) are spectacular explosions in white dwarf stars and play an essential role in astrophysics in general and in cosmological studies in particular. However, many puzzles about the nature and the inherent physical mechanisms in SNe Ia are still waiting to be answered. Robotic surveys of the next decade will provide an unprecedented wealth of observed Type Ia supernovae, detected shortly after explosion. Researchers at MPA examine here whether different explosion models are expected to leave clear imprints in such early observations that could be used in future photometric surveys to help shedding light on the progenitors and explosion mechanism of SNe Ia. more

The centre of the Milky Way is a very special place, harboring many exotic objects, such as the supermassive black hole Sagittarius A* and giant molecular clouds. Some of these clouds, despite being cold, are sources of high energy photons. It is believed that the clouds are not producing these photons themselves, but rather scatter the X-ray radiation coming from outside.  Even though Sgr A* is currently very faint in X-rays, it is considered as the main culprit of this radiation, in the form of short but intense flares, which happened over the past few hundred years. The time delay caused by light propagation from Sgr A* to the clouds and then to us, allows one to study Sgr A*’s past activity. At the same time, flares serve as an extremely powerful probe of molecular gas properties. In particular, the full 3D structure of molecular clouds and their density distribution on small scales can be reconstructed. more

Using three-dimensional general relativistic magnetohydrodynamic simulations, scientists at the Max Planck Institute for Astrophysics (MPA) have studied thick accretion disks orbiting around black holes. They find that weak magnetic fields can suppress the development of large-scale over-densities in the accretion flow. The onset of magnetic turbulence reshapes the disk's structure and could even quench the gravitational-wave signal produced by the accreting torus without magnetic fields. more

The magnetic fields of the Milky Way cause electrons with nearly the speed of light to rotate and to emit radio waves. As consequence, this radiation should also "rotate" slightly, it is circularly polarized. This very weak circular polarization of the Milky Way, however, has not been observed so far. Researchers at the Max Planck Institute for Astrophysics and colleagues have now predicted some properties of this polarization and created a "wanted poster" to allow targeted searches. A measurement of the circular polarization would provide important insights into the structure of the galactic magnetic fields and confirm that electrons - and not positrons - are the source of this radio emission in the Milky Way. more

Only a small fraction of the stars that form in the Milky Way are much more massive than our Sun and explode as supernovae type II at the end of their lifetimes. Still, these high-mass stars influence the surrounding interstellar medium (ISM) much more than their small number might suggest, both by their intense radiation and powerful winds (“pre-supernova feedback”) and through their violent supernova explosions (“supernova feedback”). Scientists at the Max Planck Institute for Astrophysics, in the framework of the SILCC collaboration, use complex supercomputer simulations to investigate the detailed impact of the different feedback processes on the ISM with conditions similar to our solar neighborhood. Ionizing radiation from young, massive stars dominates their energy output and can exceed the energy released during supernova explosions by an order of magnitude. Only if the simulation includes this radiative feedback and the momentum input from stellar winds are the results consistent with observations of the ISM and the star formation rate is reduced. more

It is widely known that our planet Earth and the Solar System itself are embedded in the Milky Way, and it is through this galaxy that we look out onto the Universe. As it turns out, this has a larger impact on astrophysical studies than previously thought. Our Galaxy’s gravitational field and its non-uniformity limit the accuracy of astrometric observations of distant – extragalactic – objects. An international group of astrophysicists including a researcher at the Max Planck Institute for Astrophysics tried to find out how strong this effect is. more

In the standard cosmological model, dark matter makes up roughly 25 % of the total energy budget of the Universe. However it cannot be observed directly, since it does not emit light. Understanding the way dark matter clusters together and forms structures is of crucial importance since it would help our understanding of the observed spatial distribution of galaxies (which should closely follow the dark matter distribution) and link this with early-Universe physics and the origin of initial perturbations. In this context, researchers at MPA and in other institutions worldwide came up with a new way of simulating the impact of large-scale primordial perturbations on the abundance of structures observed at late times, the so-called separate universe simulations. Using this technique, the MPA researchers recently obtained some of the most precise measurements of the local bias, confirming the known trend that more massive halos are more biased than smaller halos. more

In collaboration with researchers from the USA, MPA scientists have mounted a series of ambitious experiments that use a combination of quasar absorption-line spectra, neutral hydrogen line data, and state-of-the-art cosmological hydrodynamical simulations to probe the interface between galaxies and their surrounding gaseous environment. The researchers found that galaxies with gas-rich disks are embedded within gas-rich halos and that the gas in these halos is distributed smoothly and relatively isotropically. more

The origin of the current accelerated expansion of the Universe remains one of the major unsolved mysteries in physics today. While this could be a sign of the mysterious “Dark Energy”, this puzzling observation might also be evidence for the inadequacy of Einstein’s theory of General Relativity (GR) to describe the law of gravity on very large cosmological scales. These considerations would have strong implications on our understanding of fundamental physics, warranting dedicated studies such as the one undertaken recently by researchers at MPA and MPE. In this work, the authors created mock universes with non-GR theories of gravity to test the validity of current observational methods to determine the rate at which structures grow in the Universe. This allowed them to place bounds on how much the current data allows the Universe to depart from Einstein’s prediction. Reassuringly, current observational methods do not show evidence for a biased performance when tested on mock universes with modified gravity. more

Why do galaxies that live in the enormous structures known as galaxy clusters look different from normal, isolated galaxies, such as our Milky Way? To answer this question, an international research team led by MPA has created the Hydrangea simulations, a suite of 24 high-resolution cosmological hydrodynamic simulations of galaxy clusters. Containing over 20,000 cluster galaxies in unprecedented detail and accuracy, these simulations provide astrophysicists with a powerful tool to understand how galaxies have formed and evolved in one of the most extreme environments of our Universe. more