Highlights 2016

Complex predictions such as election forecasts or the weather reports often have to be simplified before communication. But how should one best simplify these predictions without facing embarrassment? In astronomical data analysis, researchers are also confronted with the problem of simplifying probabilities. Two researchers at the Max Planck Institute for Astrophysics now show that there is only one mathematically correct way to measure how embarrassing a simplified prediction can be. According to this, the recipient of a prediction should be deprived of the smallest possible amount of information. more

The diffuse gas around galaxies is hard to detect, but shows properties which are quite different to the star-forming gas inside a galaxy. Scientists at MPA have used observations from the recent MaNGA survey to study how the ionized gas changes with distance from the center of the galaxy. They have demonstrated the usefulness of adding spectra from multiple galaxies in order to analyze the gas in the outskirts of galaxies. Their study shows that the brightness of the gas decreases, while its temperature increases the further the gas is located from the center of the galaxy. The differences between star-forming and circumgalactic gas also seem to correlate with the star-formation rate and stellar mass of the galaxies. more

The stellar discs of nearby spiral galaxies are generally not flat and often show waves and warps. Even our own Galactic disc seems to be corrugated. It is still not clear what causes these structures. A research team at MPA, together with external collaborators, have revisited this question by analyzing new simulations of spiral galaxy formation. Their study shows that close encounters with satellite galaxies and more distant flybys of massive companions are the most common drivers. However, in some cases, bending patterns in discs can also be driven by the accretion of cold gas. The vertical motions produced by these patterns can be as large as 60 km/s. Such perturbations should be easily detectable in line-of-sight velocity fields of nearly face-on galaxies. This provides a new way to study the structure of galactic stellar discs, allowing us to understand how and how often such corrugation patterns arise in the nearby universe. more

Observations are beginning to be sensitive enough to see the outskirts of galaxy clusters, where theory predicts interesting features in the dark matter and gas profiles: the so-called splashback and the accretion shock. Scientists at MPA use an analytical model to compute the locations of these features, and shed new light on the underlying physics. more

From X-ray and SZ observations we know all major characteristics of the hot intracluster medium (ICM) filling the entire volume of galaxy clusters - the largest virialized objects in our Universe. However, several important properties are still poorly known, including thermal conduction in the ICM, mediated by electrons. To explain the sharp temperature gradients in galaxy clusters, it is often proposed that thermal conduction is suppressed both by the topology of magnetic-field lines, which tangle electron trajectories, and by variations of the field strength that can trap electrons. The latter mechanism can be crucially important when the so-called mirror instability generates fluctuations of the magnetic field strength: this kinetic instability is triggered by pressure anisotropies in turbulent plasma. Even if such fluctuations are present on truly microscopic scales, they have the potential to completely shut down heat conduction. Scientists at MPA have investigated such a possibility by analysing the results of recent simulations and found that the suppression of thermal conductivity is in fact rather modest, a factor of ~5 compared to unmagnetized plasma. The effect operates in addition to other suppression mechanisms and independently of them, and depends only weakly on the macroscopic parameters of the intracluster medium. more

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. more

Dwarf galaxies form stars very inefficiently compared to spiral galaxies like our Milky-Way. To investigate the origin of this deficiency in star formation, scientists at MPA have used high-resolution numerical simulations to resolve the evolution of the interstellar medium (ISM) in dwarf galaxies. They find that supernova explosions have a significant impact on the structure of the ISM and regulate the star formation rates of the whole galaxy. The reservoir for star formation on scales comparable to molecular clouds in our Milky Way consists mainly of cold atomic hydrogen rather than molecular hydrogen. These findings might also shed light into the birth processes of most other galaxies. Within the current paradigm of hierarchical structure formation, low mass, chemically un-evolved dwarf galaxies are the building blocks of all, more massive galaxies. more

In cosmology, one of the major challenges in next decades will be probing the epoch of reionization in the early universe. Scientists at MPA, the University of Oslo, and INAF have now used cosmological hydrodynamical, radiative transfer simulations to understand the impact of the complex distribution of neutral gas in the intergalactic medium on distant galaxies. Combining the simulations with observations of so-called Lyman alpha emitting galaxies they find that despite the uncertainty, the current simulation-calibrated measurements favour a late and rapid reionization history. The study also emphasizes that both the large-scale distribution of ionised gas regions and the small-scale structures of the intergalactic gas around galaxies must be understood to derive more robust constraints on the reionization epoch. more

The nature of dark matter is still unknown, but one potential candidate is a theoretical particle known as the “sterile neutrino”. In 2014, two independent groups of astronomers detected an unknown X-ray emission line around an energy of 3.5 keV in stacked X-ray spectra of galaxy clusters and in the centre of the Andromeda galaxy. The properties of this emission line are consistent with many of the expectations for the decay of sterile neutrino dark matter. However, if this hypothesis is correct, all massive objects in the Universe should exhibit this spectral feature. To test this intriguing possibility, scientists at MPA and the University of Michigan examined two large samples of galaxies, finding no evidence for the line in their stacked galaxy spectra. This strongly suggests that the mysterious 3.5 keV emission line does not originate from decaying dark matter. The nature of dark matter, and the origin of this emission line, both remain unknown. more

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 not only reproduce observed properties of stars in globular clusters at unprecedented detail but also shed light into the dark world of black holes. The computer models produce high quality synthetic data comparable to Hubble Space Telescope observations. They also predict nuclear clusters of single and binary black holes. The recently detected gravitational wave signal might have originated from a binary black hole merger in the center of a globular cluster. more

The ultimate fate of low-mass stars, like our own Sun, is to exhaust the nuclear furnace in their cores, expel their extended atmospheres, and leave behind a hot remnant called a white dwarf. Left to their own devices, these objects will simply cool slowly over billions of years. However, if a white dwarf comes to accrete material from some stellar companion, it can become an incredibly luminous source of extreme UV and soft X-ray emission, a “supersoft X-ray source” or SSS. Such radiation is readily absorbed by any surrounding interstellar gas, producing emission line nebulae. Therefore, we would expect such nebulae to be found accompanying all supersoft X-ray sources. However, of all SSSs found in the past three decades, only one has been observed to have such a nebula. Clearly, something is amiss in our understanding of these incredible objects. Now, scientists at MPA and the Monash Centre for Astrophysics have pieced together the puzzle. more

The stellar halos of galaxies are diffuse and faint components which provide scientists with a window into the assembling history of galaxies. A research team at MPA has investigated the properties of stellar halos in large disk galaxies by using both observations and state-of-the-art simulations of galaxy formation. They find a great diversity in the halo properties for galaxies that are – otherwise – alike in terms of morphology, mass, and luminosity. Observed properties, such as a mean metallicity as a function of galactocentric distance, can be reproduced by the simulations if they are analyzed in the same way as the data. more