Astronomy

Studies of interstellar medium have continued with data from Planck and Herschel satellites and ground based near-infrared and radio telescopes. Planck satellite observations have been used to locate cold interstellar clouds within our Galaxy. The resulting Planck Catalogue of Cold Clumps (PGCC) consists of over 13000 objects and it was made public in early 2015 (Planck 2015 Results XVIII). Several researchers of the department have contributed to the project that was also co-coordinated by Helsinki. In the related Herschel satellite programme, investigations have concentrated on the evolution of interstellar dust in pre-stellar and star-forming clouds. Studies were also completed on interstellar extinction that is estimated from near-infrared observations. This resulted in papers on methodology and a new set of all-sky extinction maps.

857GHz

PGCC sources (blue symbols) plotted on top of the Planck all-sky image of 857GHz surface brightness. The plane of the Milky Way runs horizontally with the centre of the Galaxy in the middle.

Observational cosmology. In Finoguenov et al. (2015) we obtained a catalog of X-ray selected groups and clusters of galaxies in the deepest surveys to date, Chandra Deep Field South, which enabled the studies of galaxy evolution in the most common environments of the Universe during more than a half of the Universe age, most of these studies were performed either by us or in collaboration with us (Popesso et al. 2015, Gozaliasl et al. 2015; Gobat et al. 2015; Smolcic et al. 2015; Strazzulo et al. 2015; Khosroshahi et al. 2015). We have completed the survey of CFHTLS fields (Mirkazemi, Finoguenov et al 2015) aimed to obtain a calibration of clusters for their cosmological use, which was also conducted in Helsinki (Kettula et al. 2015). The first results of a targeted campaign to constrain cosmology with eBOSS have been presented in (Alam et al. 2015, including Finoguenov), which has already collected 125 citations.

The research on variable stars was done in cooperation with e.g. the ReSoLVE CoE, Uppsala University, Nordita and AIP (Potsdam). The research consisted of both observational and theoretical studies

Jetsu and Porceddu continued their research of the Ancient Egyptian Cairo Calendar (CC) dated to 1244-1163 B.C. They confirmed that CC is the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol.

Käpylä, Snellman and Cole continued their work on numerical magnetohydrodynamic modelling of solar and stellar convective dynamos. Lehtinen, Jetsu and Hackman made a comprehensive study on the magnetic activity of young solar analogues (Lehtinen et al. 2015). They confirmed previous results of three different branches of magnetic activity. As a new result, they identified a sharp division between the more active stars, showing persistent active longitudes, and less active stars with no long-term active longitudes. Hackman, Lehtinen and collaborators mapped the surface magnetic fields of three young solar-type stars utilising spectropolarimateric observations obtained through observations with the 3.6 m telescope at the European Southern Observatory (Hackman et al. 2015). Furthermore Cole, Hackman and collaborators completed a Doppler imaging study of the young active late-type star LQ Hya, revealing its spot activity.

In the theoretical extragalactic group we study the formation and evolution of galaxies using both numerical simulations performed on high-performance computing facilities and analytical calculations. In 2015 we continued our studies of the formation of the first supermassive black holes in the Universe. Our adaptive-mesh-refinement simulations included a radiation particle with a realistic first-galaxy spectrum situated near a collapsing gas cloud. Using these simulations which also include full radiative transfer we could for the first time demonstrate that the production of a supermassive direct collapse black hole in the early Universe requires a relatively nearby strong radiation source that is able to dissociate the hydrogen molecules that would otherwise form in the collapsing gas cloud.

Work also continued on developing a new simulation code that can accurately resolve the dynamics of supermassive black holes and their nearby stellar particles in large-scale galaxy simulations. This novel approach will allow much more realistic studies of the final stages of merging supermassive black holes and estimations of the resulting gravitational wave signal. Using numerical galaxy merger simulations we also began to study tidal dwarf galaxies, which are expected to form in the tidal arms of interacting gas-rich galaxies and have the peculiar property of being almost completely devoid of dark matter. Initial results show that the strength and type of the included supernova feedback has a large influence on the formation frequency and longevity of tidal dwarf galaxies.

In addition to numerical galaxy simulations, researchers in the group used analytic calculations to study in detail the physics involved in a supermassive black hole binary system. Specifically the calculations provided analytic predictions for the light curves expected when the orbiting secondary black hole impacts on the accretion disk of the primary black hole. The next step is to follow-up these pioneering analytic calculations with numerical simulations to test the validity of the approximations and physics used in the construction of the model.

Towards the end of the year we also took the first steps towards a new project to study the formation and evolution of the Local group of galaxies at an unprecedented level of detail. The aim of the "Simulations Beyond the Local Universe" (SIBELIUS) project is to perform a constrained high-resolution simulation of the Local Universe that includes in addition to the Local galaxy group structure also the Virgo cluster of galaxies with extended simulations potentially going as far as the Coma cluster. The full potential of this ambitious project will come to its fruition in the years to come. Finally, we continued our collaborations with observational groups, which used the outputs from our numerical merger simulations to aid in the interpretation of their observational data.

pericenter

The image depicts an output from a numerical simulation of a merger between two gas-rich disk galaxies right after the two galaxies have passed their first pericenter. The smoothed gas density is shown in red and the smoothed stellar component is shown in blue. In the simulation we can see the formation of tidal dwarf features as clump-like stellar concentrations both in the overlap region between the galaxies and in their respective tidal arms.