Contributions to MPG Yearbook by MPA members

Recent observations have revealed that the first quasars are often surrounded by bright, giant nebulae. These span up to several 100,000 light years, about ten times larger than the quasar host galaxy. According to new detailed computer simulations of galaxy evolution performed at MPA, the observed extended nebulae can be explained as quasar light that reflects off surrounding cool neutral hydrogen clouds. Crucially, this mechanism only works if the energy provided by the quasar is able to produce gigantic galactic winds that blow out large masses of gas from its immediate vicinity.
 

In 2020, a tantalizing hint of new physics violating “parity symmetry” was found in polarization data of the cosmic microwave background obtained with the Planck satellite at high frequencies. Based on the Planck data and a simplified assumption about the impact of the polarized dust emission in the Milky Way, the scientists reported a violation of the symmetry of the laws of physics under inversion of spatial coordinates with 99.2% confidence level.
 

Intermediate-mass black holes (IMBH) should be linking stellar black holes and supermassive black holes, but their formation mechanisms are uncertain. Young massive star clusters (SCs) are promising environments for the formation of such objects. An international team led by MPA researchers, has realized realistic simulations of SCs, where these missing links form by stars - black holes mergers.

13 billion years ago, the radiation of the first galaxies transformed the Universe, ionizing the hydrogen between galaxies in a process called cosmic reionization. Despite their intimate connection, the formation of the first galaxies and the reionization process are typically studied separately. A team led by an MPA researcher has now produced the first suite of simulations that simultaneously capture these processes, as well as their connection, called THESAN.

When gas falls into a supermassive black hole, it liberates amounts of energy so vast as to be capable of ejecting much of a galaxy’s gaseous reservoir. Such black holes may thus cause the end of their own growth and that of their host galaxies. A new model developed at MPA now makes it possible to simulate winds accelerated by accreting black holes in galaxy evolution simulations in a physically accurate and validated way. By blowing dense gas from galactic nuclei, and by halting inward flows from the galactic halo, winds play a vital role in shaping the galaxy evolution.

By examining the Auriga suite of simulations, which model the formation of galaxies from soon after the Big Bang to the present day, scientists at MPA have been able to place constraints on the history of our galaxy. By comparing these simulations to observations of the Milky Way – specifically to the motions of stars in its inner regions — they concluded that our galaxy has been quite isolated in the last 12 billion years, only swallowing small galaxies with less than 5 % of its mass since then.

The Information Field Theory Group at the Max Planck Institute for Astrophysics has released a new version of the NIFTy software for scientific imaging. NIFTy5 generates an optimal imaging algorithm from the complex probability model of a measured signal. Such algorithms have already proven themselves in a number of astronomical applications and can now be used in other areas as well.

At the current epoch, approximately a half of the baryonic matter budget of the Universe is contributed by the Warm-Hot Intergalactic Medium. Being extremely tenuous and highly ionised, this matter is very difficult to observe and stays only poorly studied. Researchers at MPA have shown how it can be explored using X-ray line emission from heavier elements as a tracer. Due to scattering of the cosmic X-ray background, emission of this matter in resonant lines can be boosted strongly and become accessible for the upcoming X-ray missions

Quantum vacuum fluctuations in spacetime in the very early Universe generate gravitational waves, whose probability distribution is close to a Gaussian. However, they can also be generated by other sources, and carry imprints of the energy content of the early Universe. Scientists at MPA showed that these gravitational waves can be highly non-Gaussian, with a skewness much larger than for those generated by vacuum fluctuations.

Previous studies of large AGN samples both a low and at high redshifts seemed to rule out galaxy mergers as the drivers for black hole growth. A new technique developed at MPA for selecting a rare type of active galactic nuclei now show that it is possible to identify a new class of AGN in which more than 80% of the galaxies turn out to be merging or interacting systems, with clear indications of an accreting black hole. A detailed statistical analysis then reveals that mergers drive black hole formation in the most massive galaxies in the local Universe.

By using galaxies as giant gravitational lenses, an international group of astronomers including researchers at the Max Planck Institute for Astrophysics have made an independent measurement of how fast the Universe is expanding. The newly measured expansion rate for the local Universe is consistent with earlier findings. These are, however, in intriguing disagreement with measurements of the early Universe. This hints at a fundamental problem at the very heart of our understanding of the cosmos.

On 17 August 2017, two merging neutron stars were seen for the first time by their gravitational wave si  gnal as well as high-energy gamma radiation. Follow-up observations revealed optical emission powered by the radioactive decay of r-process elements - a so-called kilonova.

Using recent, extensive cosmological simulations, researchers at the Max Planck Institute for Astrophysics have shown that the expected signal from the Sunyaev-Zeldovich (SZ) effect of galaxy clusters on the Cosmic Microwave Background agrees remarkably well with observations by the Planck satellite. However, only a small fraction of this predicted signal is currently observable. The scientists developed a simple analytical model to understand the SZ probability distribution function, which is also helpful in interpreting the observed distribution of galaxy clusters masses.

An international team of experts from Europe and China has performed the first simulations of globular clusters with a million stars on the high-performance GPU cluster of the Max Planck Computing and Data Facility. These – up to now – largest and most realistic simulations can reproduce observed properties of stars in globular clusters at unprecedented detail and shed light into the dark world of black holes. The computer models deliver high quality synthetic data and predict nuclear clusters of single and binary black holes.

Latest three-dimensional computer simulations are closing in on the solution of an decades-old problem: how do massive stars die in gigantic supernova explosions? Since the mid-1960s, astronomers thought that neutrinos, elementary particles that are radiated in huge numbers by the newly formed neutron star, could be the ones to energize the blast wave that disrupts the star. However, only now the power of modern supercomputers has made it possible to actually demonstrate the viability of this neutrino-driven mechanism.

By combining data for more than 250,000 individual objects, an MPA-based team has for the first time been able to measure X-ray emission in a uniform manner for objects with masses ranging from that of the Milky Way up to that of rich galaxy clusters. The results are surprisingly simple and give insight into how ordinary matter is distributed in today's universe, and how this distribution has been affected by energy input from galactic nuclei.

Neutron stars are born as extremely hot and dense objects at the centers of massive stars exploding as supernovae. They cool by intense emission of neutrinos. Three-dimensional supercomputer simulations at the very forefront of current modelling efforts reveal the stunning and unexpected possibility that this neutrino emission can develop a hemispheric (dipolar) asymmetry. If this new neutrino-hydrodynamical instability happens in nature, it will lead to a recoil acceleration of the neutron star and will have important consequences for the formation of chemical elements in stellar explosions.

Scientists at the Max Planck Institute for Astrophysics propose a crucially improved distance measurement. They use a strong gravitational lens system with a time-varying source (e. g. a quasar) to measure the angular diameter distance to the lens.

Seismic vibrations on Earth contain information about the structure of our planet, seismic vibrations on distant stellar remnants could shed light not only on the star itself but also on the basic constituents of all matter. The objects under study: neutron stars with strong magnetic fields. The method: a new model that combines both the elastic shear vibrations of the crust and pulsations caused by the magnetic field. Current X-ray observations can only be explained by the coupled vibrations and the model even predicts how high-energy radiation is modulated by these oscillations.

For decades, theorists have been faced with a problem: How can we explain the diverse chemical properties seen in different types of galaxies in the nearby Universe? Now, an international team of astrophysicists, led by members of the MPA, have found a single, self-consistent model that can indeed simultaneously reconcile these chemical properties. This model follows the standard hierarchical merging scenario of structure formation, and therefore shows that − at least in this respect − what we see in our Universe is what we expect.

The famous Millennium Run (MR) simulations now appear in a completely new light: The Millennium Run Observatory (MRObs) project combines detailed predictions from cosmological simulations with a virtual observatory in order to produce synthetic astronomical observations. These virtual observations allow theorists and observers to analyse the purely theoretical data in exactly the same way as they would purely observational data. The team expects that the advantages offered by this approach will lead to a richer collaboration between theoretical and observational astronomers.

In the collision of neutron stars, the extremely compact remnants of evolved and collapsed stars, two light stars merge to one massive star. The newly-born heavyweight vibrates, sending out characteristic waves in space-time. Model calculations at the Max Planck Institute for Astrophysics now show how such signals can be used to determine the size of neutron stars and how we can learn more about the interior of these exotic objects.

The Lambda CDM model of cosmological structure formation has very successfully matched many observational aspects of the Universe. However, the nature of the main ingredient of this model, the so-called Dark Energy, is currently still a mystery. Scientists at the Max Planck Institute for Astrophysics have recently performed the largest ever computer simulation of cosmic structure formation. Combined with new observational campaigns, this might help to constrain the properties of the Dark Energy and solve one of the most important puzzles in modern cosmology.

Astronomers have so far found more than five hundred "exoplanets", i. e. planets orbiting other stars. A group of these are large planets with orbits very close to their host stars, the so-called "hot Jupiters". Their mass is similar to our Jupiter but they are often much bigger, indicating that their interior is much hotter. Left to themselves, they should cool down and deflate fairly rapidly to a size similar to the Jupiter in our solar system.

The "Planck Surveyor" satellite mission to study the Big Bang, 14 billion years ago, via measuring the cosmic microwave background has produced impressive results during its first year of operation: a catalogue with 15,000 celestial objects, 25 scientific papers, as well as the most precise measurement of the far infrared background, revealing star formation in the early universe. The Max Planck Institute for Astrophysics developed software components for Planck and is heavily involved in the scientific interpretation of the mission data.

New models developed at the Max Planck Institute for Astrophysics (MPA) change our paradigms about the physics and evolution of the Milky Way Galaxy. Scientists at the MPA determine the parameters of about 16000 stars in the solar neighbourhood. The data confirms predictions of a model developed at the institute and provide insight into the physics of galactic discs, into the history of our Galaxy and the provenance of our Sun.

A team of astronomers, including members of the Max Planck Institute for Astrophysics, have combined the observational power provided by the Hubble Space Telescope with the predictive power of the Millennium Run cosmological simulations to investigate an intriguing cosmic puzzle. If luminous quasars in the early Universe mark the regions that were the first to collapse and form massive clusters of galaxies as predicted by theory, then why is the observational evidence for such cosmic "cities-under-construction" currently so scarce?

Scientists of the Max Planck Institute for Astrophysics and the Cluster of Excellence "Universe" at the TU Munich show by detailed computer simulations how the interaction of neutrinos may cause supernova explosions of stars with 11 to 15 solar masses.

How would the surface of our Sun look like, if we had telescopes which have a resolution ten times that one of present instruments? Would the Sun look any different further inside? In an international collaboration scientists at the University of Vienna, and the Max Planck Institute for Astrophysics have used numerical simulations on high performance computers to answer these questions. They found a highly turbulent flow showing ever more details hidden underneath the smooth looking surface which we know from images of our Sun in visual light.

Scientists at the Max Planck Institute for Astrophysics (MPA) have carried out the largest simulation thus far of the formation of a Milky Way-like dark matter halo. This allowed the first detailed theoretical predictions of the dark matter distribution in the vicinity of the Earth.

Scientists at Max Planck Institute for Astrophysics (MPA) have performed detailed computations of the highly redshifted radiation that is released during the epoch of cosmological hydrogen recombination. Progress in the development of radio detectors may render these small deviations of the Cosmic Microwave Background (CMB) spectrum from a perfect blackbody observable, thereby offering a complementary way to measure the exact value of the CMB temperature, the entropy of the Universe, and to provide direct evidence about how our Universe became transparent.

Three years since its completion, the Millennium Run remains the largest simulation of cosmological structure formation. Over 100 papers [1] have been written based on its numerical data. More than half of these are by authors who have accessed the data through a web service of the German Astrophysical Virtual Observatory (GAVO). This is the most complete application yet of Virtual Observatory techniques to the publication of theoretical data.

A team of X-ray astronomers at the Max Planck Institute for Astrophysics resolved the thirty-years-old puzzle of the origin of the Galactic X-ray background emission. Combining data from various space-based X-ray instruments (RXTE/PCA, INTEGRAL/IBIS, CHANDRA/ACIS, ROSAT/PSPC) and infrared instruments (COBE/DIRBE) they showed that the Galactic X-ray background predominantly consists of emission of a large number of point sources, mostly cataclysmic variables and coronally active stars.

A team of X-ray astronomers resolved the thirty-years-old puzzle of the origin of the Galactic X-ray background emission. Combining data from various space-based X-ray instruments (RXTE/PCA, INTEGRAL/IBIS, CHANDRA/ACIS, ROSAT/PSPC) and infrared instruments (COBE/DIRBE) they showed that the Galactic X-ray background predominantly consists of emission of a large number of point sources, mostly cataclysmic variables and coronally active stars.

Abstract Using a catalog of more than 80000 galaxies with active nuclei, drawn from the Sloan Digital Sky Survey, a team from the MPI for Astrophysics and from Johns Hopkins University has studied the connection between the growth of supermassive black holes at the centers of galaxies and of their host galaxies. Most of the black hole growth today is occurring in relatively low-mass black holes (comparable to the one in the center of our Milky Way), whereas the main epoch of growth of the most massive black holes dates back much earlier. They also find that those galaxies in which the central black hole is currently growing have recently formed stars -- a fact that highlights how the mass of the black hole is tightly linked with the stellar mass of its host.

Abstract In order to solve the puzzle of the origin of the Galactic ridge X-ray mission (GXRE), its spatial distribution was studied in detail with the RXTE observatory. The obtained X-ray map is very similar to the near-infrared map of the Galactic disk and bulge, which implies that the GRXE closely traces the stellar population of the Galaxy. In the second part of this study RXTE and ROSAT observations were used to evaluate the total volume emissivity of faint X-ray sources in the Solar neighborhood. Based on this estimate it was shown that the bulk of the GRXE is likely a superposition of emission from thousands of cataclysmic variables and millions of coronally active stars.

Researchers at the Max-Planck-Institute for Astrophysics have developed new relativistic models which allow predictions of so far unknown properties of short gamma-ray bursts. Their simulations will come under scrutiny by the Swift Gamma-Ray Burst Explorer, a NASA mission that was launched on November 20, 2004.

Scientists at the Max-Planck Institute for Astrophysics have carried out the worldwide largest cosmological simulation of structure formation and used it to make accurate theoretical predictions for the growth of galaxies and supermassive black holes. For the first time, the model allows a detailed comparison of the theory of hierarchical galaxy formation to observations in a volume comparable to that of the largest spectroscopic redshift surveys, including rare objects such as the first quasars or massive galaxy clusters.

If the dark matter in the universe consists of weakly interacting elementary particles that can annihilate each other, it should be possible to detect their annihilation radiation directly. High-resolution cosmological simulations of the distribution of dark matter in the Milky Way can be used to make detailed predictions for the expected annihilation radiation from the galactic center and the satellite galaxies of the Milky Way. If the dark matter particles are neutralinos, these predictions imply favourable detection possibilities for next generation gamma ray telescopes.

Scientists at the Max-Planck-Institute for Astrophysics in Garching and the University of Chicago have substantiated an explanation for the high space velocities of observed pulsars. Their computer models confirm the likely connection with asymmetries during supernova explosions.