Metal poor stars as tracers of cosmic nucleosynthesis

• Sara Lucatello

Room: New seminar room The low end of the Galaxy halo stellar metallicity distribution is characterized by the presence of a large population of peculiar composition objects. In particular, a large fraction of very metal poor stars show large overabundances in C. It is estimated that at least 20% of stars with [Fe/H]<-2 have [C/Fe]>1, and such fraction likely increases with decreasing metallicity (the two most metal poor stars known have both extremely high C overabundance). Most of these stars are characterized by various degrees of n- capture elements enrichments (s and/or r), and some show also very large overabundances of alpha- elements. It is now clear that the investigation of such objects, aimed to explain the origin of these peculiar abundance pattern, and of their extremely high incidence is crucial to the understanding of the Early Galaxy formation and evolution. I will present the largest so far sample of metal poor, C-rich stars with high resolution analysis, describe our present knowledge of their formation mechanisms as well as discuss the constraints and the implications they provide our understanding of the mechanisms that enriched scenario the Early Galaxy, and in particular low Z AGB nucleosynthesis.

Spectroscopy: fingerprints of cosmic nuclesynthesis and evolution of the Galaxy

• Martin Asplund

Room: New seminar room I will describe how the chemical compositions of stars can be used to probe stellar, galactic and cosmic evolution. Besides tracing things like stellar nucleosynthesis and the history of the Milky Way Galaxy and its different stellar populations, the elemental information imprinted in stellar spectra can also shed light on for example the baryon density of the Universe and perhaps even the existence of supersymmetric particles. Unfortunately the stellar elemental abundances can not be directly read off from the observed spectra but require realistic modelling of the stellar atmospheres and the spectrum formation, something my group has worked very hard on improving. Finally I will describe our ongoing and planned efforts in discovering and understanding the very first stars born in the Universe.

Some applications of gauge/string duality to strong interactions

• Oleg Andreev

Room: New seminar room The thermodynamic properties of a gauge theory without quarks are derived using a string (gravity) formulation for a temperature window 1.2 T_c – 3 T_c. The results are in good agreement with available lattice simulations. As a cross-check, we calculate the spatial string tension. Finally, we make some predictions about SU(N) gauge theories.

Suzaku observations of NGC 4395

• Kazushi Iwasawa

Room: New seminar room NGC4395 harbors the lowest luminosity type-I AGN, known to exhibit strong X-ray variability, in a bulge- less dwarf host. The low black hole mass (~10^5 solar masses) makes this object of great interest for the formation of massive black holes and the mass- scaling of AGN phenomena. We observed NGC4395 with Suzaku for the net exposures of 100 ks and 83 ks with the XIS and the HXD-PIN, respectively. A significant signal above 10 keV was detected for the first time from this object with the HXD-PIN. The 2-10 keV mean flux remains similar to the previous values, suggesting no large power in variability on time- scales of years. This and the observed rapid, strong X- ray variability (the characteristic time-scale of a few 100 s and the rms amplitude as large as 85%) agree with the expected behaviour of a low mass black hole. The rms amplitude decreases towards higher energies but this energy dependence appears to be weaker than in the previous XMM-Newton observation. The 2-10 keV spectrum is very hard as usual, and can be approximated by a power-law with photon-index of ~1. This hard spectrum extends up to 20 keV and then rolls over. The hard spectrum is partly due to a thick warm absorber with n_H of 10^23 cm^-2, but the soft photons below 1.5 keV are more absorbed than previously observed. This is likely due to a decrease of the ionisation parameter, hence increasing the OVII opacity. A relatively weak Fe K line is present which peaks at 6.4 keV with a small red wing.

Where are the cosmic metals at redshift z ~ 3 ?

• Jesper Sommer-Larsen

Room: New seminar room From observations of the cosmic star-formation history, as probed by young galaxies at z>3 (that is during the first 15% of the age of the Universe), one can infer the cosmic density of heavy elements, like Oxygen and Iron, expected by z~3. However, searches for such metals at z~3 have only been able to account for 10-20% of the amount expected - this has been coined 'the Missing Metals Problem'. In the talk I will discuss possible reasons for this controversy, and also where the 80- 90% 'missing metals' might be located.

Understanding the origin of cosmic acceleration from the growth rate of large-scale structure

• Luigi Guzzo

Room: New seminar room I will review the current status of studies of the large- scale structure of the Universe, in the light of the surprising evidence for acceleration of the cosmic expansion rate. Deep redshift surveys with 8-m class telescopes are just starting to be large enough to estimate 'global' quantities that directly probe the parameters of the cosmological model at z~1, in particular in its dark sector. I will show how measurements of linear redshift-space distortions due to galaxy coherent motions can be used to measure the evolution of the growth rate of cosmic structures f(z) at different epochs. Combined to estimates of the cosmic expansion rate H(z), e.g. from Type Ia Sne or Baryonic Acoustic Oscillations, these observations can pinpoint the physical origin of the accelerated expansion, distinguishing true 'dark- energy' models from modifications of the gravitation theory.

SUSY and precision measurements

• Wolfgang Hollik

Room: New seminar room Precision measurements at high and low energies and theoretical predictions of supersymmetric models provide an indirect access to SUSY particles. Present data and global analyses are shown and their implications for forthcoming LHC experiments are discussed.

Massive Black hole binaries for the ages

• Piero Madau

Room: New seminar room I will review scenarios for the assembly of massive black holes (MBHs) that trace their hierarchical build- up far up in the dark halo merger tree. The first active galactic nuclei may have formed 200 Myr after the big bang in one million solar mass halos. I will discuss the conditions under which MBHs become incorporated through a series of mergers into larger and larger halos, sink to the center owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, form a binary system, and eventually coalesce. The spin distribution of MBHs is determined by gas accretion, and is predicted to be heavily skewed towards fast- rotating Kerr holes, already in place at early epochs, and not to change significantly below redshift 5. Decaying MBH binaries will eject hypervelocity stars, shape the innermost central parsec of galaxies, and should be detected in significant numbers by LISA.

Structure of topological strings

• Murad Alim

Room: New seminar room The realization of background independence in Topological String Theory leads to a set of equations which have to be satisfied by the topological string partition function. These so called holomorphic anomaly equations lead to interesting insights into the structure of the partition function and provide an efficient calculation tool. The so computed partition function carries information on mathematical invariants as well as on string dualities.

Universe Cluster

• Stephan Paul

Room: New seminar room