Seminars and Colloquia at ESO Garching and on the campus

In my talk I will first briefly review the application of galaxy clusters as tools to trace cosmic evolution. I will then discuss the recent advances in the this field, as driven by the increasing quality of observational data, and by the much improved description of clusters through detailed numerical simulations. In this context I will present recent results on the analysis of such simulations aimed at calibrating clusters as precision tools for cosmology. Finally, I will highlight the potential that future large surveys of galaxy clusters could have to trace the growth of cosmic structures and to shed light on the constituents of the Universe and the nature of primordial perturbations.

A decade of exoplanet search has led to surprising discoveries, from giant planets close to their star, to planets orbiting two stars, all the way to the first extremely hot, rocky worlds with potentially permanent lava on their surfaces due to the star's proximity. Observing mass and radius alone can not break the degeneracy of a planet's nature due to the effect of an extended atmosphere that can also block the stellar light and increase the observed planetary radius significantly. Even if a unique solution would exist, planets with similar density, like Earth and Venus, present very different planetary environments in terms of habitable conditions. Therefore the question refocuses on atmospheric features to characterize a planetary environment and what the spectra of an exoplanet can reveal.

I review some recent developments in attempting to reconcile the observed galaxy population with numerical models of structure formation in the 'LCDM' concordance cosmology. Focussing on behaviour of dwarf galaxies, I describe the infamous 'cusp-core' dichotomy - a long-standing challenge to the LCDM picture on small scales - and use toy models to show how it is resolved in recent numerical simulations (Pontzen & Governato 2012). I then discuss the current observational status of this picture (Teyssier, Pontzen & Read 2012; Penarrubia et al 2012). In the second half of the talk, I apply the analytic techniques developed for probing the effect of gas on dark matter dynamics to the question of how, in the absence of baryons, a universal "NFW" dark matter halo profile emerges (independent of scale or details of the initial conditions). Thus the generation of NFW halos on the one hand and the destruction of their central cusps on the other can be ascribed to surprisingly similar physical arguments.

I will talk about two different aspects of massive galaxy evolution. First, I will discuss the assembly of elliptical galaxies based on high signal to noise spectra beyond the effective radius taken with an integral-field spectrograph. The stars in the outskirts of these galaxies are enriched in alpha elements but have low metallicity, similar to thick disk stars in our galaxy. We suggest that these stars were accreted from small galaxies that formed early and had a truncated star formation history. Second, I will discuss supermassive black hole scaling relations, and what they may tell us about the coevolution of black holes and galaxies. I will end with tantalizing new clues about the lifetimes of sub-pc supermassive black hole binaries.

A major uncertainty in models for photoionised outflows in AGN is the distance of the gas to the central black hole. We present the results of a massive multiwavelength monitoring campaign on the bright Seyfert 1 galaxy Mrk 509 to constrain the location of the outflow components dominating the soft X-ray band. Mrk 509 was monitored by XMM-Newton, Integral, Chandra, HST/COS and Swift in 2009. We have studied the response of the photoionised gas to the changes in the ionising flux produced by the central regions. We were able to put tight constraints on the variability of the absorbers from day to year time scales. This allowed us to develop a model for the time-dependent photoionisation in this source. We find that the more highly ionised gas producing most X-ray line opacity is at least 5 pc away from the core; upper limits to the distance of various absorbing components range between 20 pc up to a few kpc. The more lowly ionised gas producing most UV line opacity is at least 100 pc away from the nucleus. These results point to an origin of the dominant, slow (v < 1000 km/s) outflow components in the NLR or torus-region of Mrk 509. We find that while the kinetic luminosity of the outflow is small, the mass carried away is likely larger than the 0.5 Solar mass per year accreting onto the black hole. We also determined the chemical composition of the outflow as well as valuable constraints on the different emission regions. We find for instance that the resolved component of the Fe-K line originates from a region 40-1000 gravitational radii from the black hole, and that the soft excess is produced by Comptonisation in a warm (0.2-1 keV), optically thick (tau~10-20) corona near the inner part of the disk.

The colonization of the continents by land plants over 450 million years was one of the most far-reaching chapters in the history of life on Earth. It fundamentally changed the ecology and climate of the planet. In this talk, I will discuss how new findings are supporting the exciting long-standing hypothesis that symbiotic relationships between early land plants and soil-dwelling fungi fuelled the ‘greening of the Earth’, and how by relentlessly burning fossil photosynthesis we are threatening to destabilize the climate system with potentially disastrous impacts for future generations and nature. Given that astronomers will eventually be able to diagnose the composition and state of the atmospheres of planets in the 'habitable zone', I will outline some of the atmospheric changes that might be expected to relate to plant evolution.

It is now almost certain that at the center of our Milky Way Galaxy lies a super massive black hole - 4 million times more massive than our Sun. Because of its proximity to Earth, this object, known as Sagittarius A*, presents astronomers with the best opportunity in the Universe to spatially resolve and image a black hole Event Horizon. To do this requires using Very Long Baseline Interferometry (VLBI), the technique whereby radio telescopes around the world are linked together in a Global phased array. Very short wavelength VLBI observations have now confirmed structure on ~4 Schwarzschild radius scales within SgrA*, and have revealed time variability in this source on the same spatial scales. For the much more massive (6 billion solar mass) black hole powering the relativistic jet in M87, similarly compact structures have been detected. I will describe the instrumentation efforts that enable these observations, discuss what current and future VLBI observations can tell us about these super-massive black holes, and describe plans for assembling a Global submm-VLBI Event Horizon Telescope.

Circumstellar discs in young stars provide the raw material - gas and dust - for planet formation; discs also effect the radial re-distribution of planets and thus play a major role in shaping planetary architecture. Understanding the interplay between discs and planet formation and migration therefore requires a good understanding of how discs evolve and how they are dispersed. Currently, however, it is still debated whether discs are self-consumed through planet formation or whether planet formation occurs in competition with some other agent for disc dispersal. Much of this talk is devoted to new work that shows how the photoevaporation of gas from such discs by Xrays from the central star is an important agent for disc dispersal. The talk gives an overview of the disc properties that can be deduced from multi-wavelength data, increasingly supplemented by imaging, showing that our now good statistical knowledge of various stages of disc clearing is largely thanks to large scale surveys conducted in recent years by the Spitzer Space Telescope. I will briefly discuss the several candidate disc clearance mechanisms and then describe the theory of Xray photoevaporation in more detail, including also some observational signatures and possible discriminants. I conclude with a description of the bewildering zoo of "transition discs" (i.e. discs with cleared inner regions) and discuss the extent to which they can be understood in terms of clearing by photoevaporation / planet formation.

Recent spectroscopic and photometric observations have revealed that massive stellar clusters are not made of a single stellar population but rather host multiple populations. Also, they should have been produced within very extensive episodes of star formation, which involved much more stars than currently present in the globular clusters. I will review the evidences for multiple populations in globular clusters and explore some of its implications on their formation and early history. I will also comment on the relation between the formation of the globular clusters and that of the galactic halo, and implicitly of all stellar populations characterized by a high relative frequency of globular clusters.

I will argue that observations of the diffuse gas in the outskirts of galaxies, the so called circumgalactic medium, are essential for constraining the 'initial conditions' for galaxy formation. Such observations provide a fruitful comparison to theory, because hydrodynamics at moderate overdensities is much easier to simulate than molecule or star-formation. A novel technique will be introduced, whereby a foreground quasar (and massive galaxy) can be studied in absorption against a background quasar, resolving scales as small as 30kpc. This experiment reveals a rich absorption spectrum which contains a wealth of information about the physical conditions of diffuse gas around massive proto-galaxies. I will summarize the implications of these new measurements in the context of galaxy formation models, and also discuss a sensitive search for emission from the same gas we study in absorption.

Intervening structures in the universe give rise to small distortions in the shapes of distant galaxies. By measuring this tiny coherent signal, we can study the mass distribution in the universe directly, without relying on baryonic tracers. This makes weak lensing by large-scale structures a powerful probe of cosmology. It allows us to differentiate between dark energy models or modifications of the law of gravity. I will briefly review the topic of cosmic shear and discuss how the signal is extracted from the data and present results from the recently completed analysis of the CFHT Legacy Survey.

Debris disks are dusty disks orbiting mature stars. They owe their origin to collisional replenishment by small bodies and as such represent a major piece in the unfolding story of the nature of planetary systems. After their discovery by IRAS in 1982, their rich and diverse diagnostics have been studied by major ground-based facilities (VLT, VLTI, and, recently, ALMA) and space missions (HST, ISO, Spitzer). The role of Herschel is to offer for the first time the possibility to study their properties in the main domain in which they radiate their thermal energy, at an appreciable spatial resolution. Building on the important censuses Spitzer has achieved, Herschel has uncovered the anticipated existence of cold debris disks. We will discuss in detail the major results Herschel has achieved on nearby debris disks that can be well resolved spatially, in particular those of Vega, Beta Pictoris and Fomalhaut.

The most characteristic property of active galaxies, including quasars, are prominent broad emission lines. These lines allowed to discover the cosmological nature of quasars, and at present they provide the most convenient method of weighting black holes residing in their nuclei. However, a question remains why such strong lines form there in the first place. I will discuss an interesting possibility that dust is responsible for this phenomenon. The dust is known to be present in quasars in the form of a dusty/molecular torus which results in complexity of the appearance of active galaxies. However, this dust is located further from the black hole than the Broad Line Region. We propose that the dust is present also closer in and it is actually responsible for formation of the broad emission lines. The argument is based on determination of the temperature of the disk atmosphere underlying the Broad Line Region: it is close to 1000 K independently from the black hole mass and accretion rate of the object. The mechanism is simple and universal but leads to a considerable complexity of the active nucleus surrounding. I will conclude with the discussion how this idea fits into the general scheme of the quasar structure.

In addition to the major planets, the solar system contains a large number of so-called small bodies in dynamically distinct reservoirs. These include the main-belt asteroids, main-belt comets, Trojans, Centaurs, irregular satellites, the comets of the Oort cloud and Kuiper belt, and others. These reservoirs are important both for the relics of solar system formation contained therein, and as sources for short-lived populations in the inner solar system. For the most part, the small body populations were discovered recently, their investigation is still firmly in the exploration phase and much of the excitement in planetary science stems directly from them. I will use new observations of three "freak" and very surprising objects as a vehicle to discuss the small body populations, to highlight our ignorance of even basic issues in the origin and evolution of the solar system, and to indicate potentially productive paths to future research.

Black holes, resident in the centers of galaxies, will be fed by accretion of ambient gas whenever gas reaches those central regions. This can be due to mergers, but even without mergers the evolution of the stellar populations of normal galaxies provides very large amounts of gas, as stars pass through the planetary nebula stage, the total mass release being greater than 10$^{11}$ Msolar for massive ellipticals. Much of that gas will cool and fall to the centers of the systems, where it will induce starbursts and accretion events onto the central black holes. Accretion then induces outbursts of UV from the BH discs, Xrays from coronal gas and polar, radiation driven winds, with the efficiencies in these three categories roughly 10$^{-1}$, 10$^{-2}$ and 10$^{-3}$. The mass, momentum and energy in these outbursts can have dramatic consequences for the growth of the BH and for the ambient galaxy. Most AGN feedback treatments do not include the mass and momentum components. We follow these events with 1D, 2D and 3D hydrodynamic codes. BH growth is similar to what has been found by others, but the momentum driving produces much more energetic winds than does thermal feedback. Observable consequences include the narrow line AGN absorption lines, shock accelerated synchrotron emitting particles and wind driven bubbles in the IGM. In addition, we find that the feedback strongly inhibits inflow, causing episodic accretion and a low "duty cycle". The simulations help us to understand many phenomena including the black hole stellar mass relation, the paucity of gas in ellipticals, the incidence of the ¡°E+A¡± phenomena and the observed fact that most of the black holes found in galactic centers are found in the "off" state.

Thermonuclear explosions of white dwarfs are thought to explain Type Ia supernovae (SNe Ia) -- but it is still unclear how these explosions come about and how they proceed in detail. This is a fundamental problem as SNe Ia are key players in many fields of astrophysics including galactic chemical evolution and observational cosmology. Since their progenitors elude observation as well as modeling, I will argue that simulations of explosions arising from different scenarios can help to overcome the problem. A new generation of three-dimensional hydrodynamic explosion simulations in combination with radiative transfer calculations allows us to predict observables that can be directly compared to astronomical data. Avoiding tunable parameters in the models, conclusions on progenitor properties are possible. Contradicting 'textbook wisdom', recent results demonstrated that a normal SN Ia does not necessarily require the explosion of a Chandrasekhar-mass white dwarf, but they also do not rule out this possibility. While the question of the progenitor channel responsible for the bulk of objects remains open, the synthesis of modeling and recent observations underpins the emerging picture of SNe Ia being much less homogeneous than previously thought. This could complicate their use as cosmic distance indicators.

Measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* at the Galactic Centre is a black hole four million times the mass of the Sun. With the exception of modest X-ray and infrared flares Sgr A* is surprisingly faint, suggesting that the accretion rate currently is very low. In 2011 we discovered a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations fully constrain the cloud's orbit. It is highly eccentric with a pericenter distance of only 3100 times the Schwarzschild radius. The pericenter passage will happen in summer 2013 and already now we can see that the cloud has begun to tidally disrupt due to the black hole's gravitational force. The cloud also is a probe for the properties of the accretion flow, and ultimately we might have a chance to see how a massive black hole is being fed.

This is a fairly short paper, but it is the first from the multi-cycle HST program to find the most distant supernovae. I will expand on that a little bit as well.

The CoRoT and Kepler space missions, while still ongoing, already revolutionised our view on stellar pulsation. After a very brief introduction in asteroseismology, we illustrate how the immense advantage of having long-term uninterrupted data from space with a factor 100 better precision compared to data from the ground implies large progress in the tuning of stellar physics. In particular, we focus on some cases where gravity-mode oscillations were detected in addition to acoustic modes and show how this allowed modelling of the near-core regions in stars. We end by highlighting future prospects based on the continuing data gathering by both missions while pointing out some challenging issues in the theory of stellar structure that need to be resolved to properly interpret the space data.

Galaxies grow with time through both discrete galaxy mergers and smooth gas accretion. When and how this growth occurs, and the role of mergers in defining the properties of today's galaxies, remain outstanding observational questions. Observational estimates of the galaxy merger rate and its evolution can vary by factors of 10, depending upon the method and assumptions used to count mergers. Using physical-motivated timescales from a large suite of galaxy merger simulations, I am able to reconcile the discrepancies between different measurements of the galaxy merger rate at z<1. The frequency of gas-rich mergers has increased strongly from z~0 to z~1, while the global galaxy merger rate evolved more modestly. Finally, I will discuss the prospects for identifying galaxy mergers at z~2 and beyond with the Cosmic Assembly Near-infrared Extragalactic Legacy Survey.