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Cosmology with Lyman alpha

Cosmology with Lyman alpha

Cosmology is a research field in astronomy that aims to unravel the structure and evolution of the universe as well as its contents. The modern surveys focus on specific parts in the evolution of the Universe (e.g. cosmological inflation, microwave background radiation (CMB), large-scale structures, intergalactic medium, galaxies, etc.) and try to understand them and their role in the evolution of the Universe.

At the turn of the century large surveys were primarily focused on the cosmological microwave background (WMAP, VSA, Boomerang, …) and galaxies (2dFGRS, SDSS, …) which answered many cosmological questions of the previous century. The new generation surveys are mostly oriented towards the physical phenomena that would put very tight constraints on various theoretical predictions (polarization of the CMB, weak gravitational lensing, absorption in the intergalactic medium).

Figure (top): Shows the distribution of the matter density in the numerical simulation Millenium The density field was computed with the TreeSPH code Gadget. The plot shows the slice of order 400 Mpc through the entire simulation. The absorption in the intergalactic medium is mostly due to the filamentary structures around the central density peak (a cluster of galaxies).

At the Faculty of Mathematics and Physics (FMF) we are studying the absorption in the intergalactic medium. Because the intergalactic medium (IGM) consists mostly of hydrogen (which is the most common element in the universe), the absorption is primarily due to the electronic transition in the hydrogen atom (from n=1 to n=2 state). Because the universe is expanding, the wavelengths of the photons from the distant quasar are stretching on their way to the Earth and at different distances from the Earth photons with different emitted (initial) wavelengths will be absorbed in the IGM, leaving a distinct feature in the spectrum of a quasar. This characteristic structure of absorption lines is called the Lyman alpha forest and is found bluewards of the Lyman alpha emission in the quasar spectrum.

By studying the statistical properties of those structures one can constrain the power spectrum of the matter distribution in the IGM. The matter power spectrum measurements can then be further used to put tighter constraints on many different physical parameters: light neutrino masses (light elemental particles without electric charge), cosmological inflation and re-ionization.

Research work at FMF on cosmology and Lyman alpha absorption systems is done by: doc. dr. Anže Slosar and Vid Iršič (MR – researcher)

Figure (bottom): Shows a typical quasar spectrum. Bluewards of the Lyman alpha emission line is the Lyman alpha forest colored in green. The data are from the large galaxy survey SDSS (Sloan Digital Sky Survey). The Lyman alpha absorption in the IGM is due to the neutral hydrogen between the Earth and the distant quasar.

Spekter kvazarja z gozdom Lyman alfa

Spectroscopy of the Galaxy

Spectroscopy of the Galaxy

Our Galaxy is a typical galaxy in the Universe, so it can be used to better understand the origin and evolution of galaxies in general. On the other hand a complete information on the position, motion and physical properties of a large number of individual stars can be obtained only for stars in our Galaxy. We are participating to international collaborations GaiaRAVEHermes, and ESO-Gaia. Study of properties and evolution of individual stars, binaries, exoplanets, as well as individual galactic components and the Galaxy as a whole are in the center of our research.

All these projects profit from new technological advances, for example they use optical fiber technology to obtain a simultaneous observation of a large number of stars and automated pipelines to reduce and extract the relevant information. The preferred technique of observation is optical and near infrared spectroscopy. In fact spectroscopy is the most useful technique to gather information on the physical processes in the Universe. It uses the light collected by the telescope to study its intensity variation in tiny colour or wavelength intervals. An important property studied by spectroscopy is the fact that every chemical element, which is present in the atmosphere of the star we observe, causes absorbs or emits light at some well defined characteristic wavelengths. So a pattern resembling a bar-code is formed which reveals the presence of a particular chemical element. Spectroscopy can thus be used to study “from a distance” what is the chemical composition of stars, as well as some other physical properties like temperature, gravity acceleration and rotation. It can also help to determine the position of the star in space and its velocity in all three directions. For the case of stars in physical pairs or binaries we can sometimes measure even their masses and sizes, which is of particular importance as we know that both stars were formed together and are therefore of the same age.

Esa space mission Gaia, to be launched in 2013, will give as a unique view of positions, kinematics and spectrophotometry for a billion stars in our Galaxy. This is expected to be a huge leap forward in our understanding of the Galaxy and its stellar populations. Individual galactic components could be studied in unprecedented detail and compare their properties with the predictions of models of galaxy formation within a cosmological context. A full 3-dimensional velocity vector for a large number of stars and their detailed chemical abundances shall allow to discuss the quantity and distribution of dark matter and to understand the formation and evolution of individual galactic components. We participate in a group which prepares for reduction of spectroscopic data. Our task is to optimize treatment of observations with very high noise levels and to contribute to measurements of radial and rotational velocities. Despite the fantastic potential of the Gaia mission our knowledge of our Galaxy will be limited by the fact that the spectrograph aboard Gaia will be able to determine only radial velocity for most faint stars. So we are participating to three of the largest complementary spectroscopy projects operating from the Earth’s surface. The goal is to complement the Gaia mission results with spectroscopic observation of stellar temperatures, gravity, detailed chemical composition, distances, ages, masses and temporal variability. These additional information shall increase the dimensionality of the parameter space, and so allow for better comparison and matching to the theoretical models of formation of the Galaxy. These projects are as follows:

  1. RAVE is the largest spectroscopic survey of stars in the Solar neighbourhood so far. It obtained over half-a-million spectra already. The goal is to continue with observation till early 2013 and to publish reliable values of physical parameters of observed stars (Steinmetz et al. 2006, Zwitter et al. 2008, Siebert et al. 2011) and their distances (Breddels et al. 2010, Zwitter et al. 2010, Burnett et al. 2011). The results allow for a very successful study of binary star populations (Matijevič et al. 2010, Matijevič et al. 2011) and of stars in rapid phases of stellar evolution (Munari et al. 2009). T.Z. is the project scientist of RAVE.
  2. Hermes is an Australian project, expected to become the largest survey of detailed chemical compositions of stars so far. Detailed abundances of over two dozen chemical elements will be measured for a million stars in our Galaxy. The purpose-built spectrograph, expected to be operational in 2013, shall profit from a huge 2-degree field of view of the AAT telescope in Australia. 400 optical fibers will allow for a simultaneous observation of that many stars at high resolution and in four wavelength ranges. We are coordinating the working group to study binary stars and the properties of interstellar space, including a 3-D mapping of interstellar reddening.
  3. ESO-Gaia is a public survey of the European Southern Observatory observing for 400 nights with the Flames spectrograph of the 8-m VLT telescope in Chile. The goal is to observe stars too faint for spectroscopic observation with the Gaia satellite. We are participating to the study of interstellar space.

Of particular importance for our group a very fruitful and continuing collaboration with dr. Ulisse Munari and his colleagues from theAstronomical observatory in Padova which started in 1993. The corrent projects include an observation of a large number of red clump stars and other stars of particular interest. The objects are being observed with the echelle and with a Boller&Chivens spectrograph inAsiago and partly also at our astronomical observatory at Golovec. These stars are bright but studying their variability requires a lot of observing time. So a continued access to facilities in Asiago, kindly provided by the Astronomical observatory in Padova and by University of Padova, as well as possibility to observe from Golovec, presents a valuable asset.

Members of the stellar spectroscopy group include dr. Tomaž Zwitterdr. Gal Matijevič, Maruša Žerjal and Janez Kos. Here is the list of all and refereed publications. Our interest in binary stars is shared by dr. Andrej Prša who obtained his PhD with us in 2005. The work described above is done in collaboration with our valued partner Center of excellence Space-SI.

Two results from a huge number of spectroscopic observations with the RAVE project are shown below. The image on the left plots the recession (red) and approach (blue) velocities of individual observed stars across the sky. A clear dipole pattern is a consequence of Solar motion with respect to the stars in our neighborhood. RAVE observes the stars south of equator (yellow line). An all-sky panorama of Axel Mellinger is added as a background. Graph on the right is a Hertzsprung-Russel diagram of RAVE stars. Measured positions on the diagram show a very good match with computed stellar isochrones for stars of different ages.

Rezultata projekta RAVE



Simulations of galaxy clusters

Simulations of galaxy clusters

Galaxy clusters represent an important link between the formation of large structures, since they formed after the collapse of first perturbations in the initial density field, and the astrophysical processes acting in their interior, such as supernova explosions or the feedback from active galactic nuclei.


Using cosmological hydrodynamical simulations of galaxy clusters and groups we can follow the evolution of the cluster as well as the physical processes occurring and affecting the properties of galaxies and the hot gas.

Left: X-ray brightness profile map of the intracluster medium for a simulated galaxy cluster (software Smac, K. Dolag)

Clusters of galaxies are currently used in cosmology as probes to constrain cosmological parameters. What we are trying to do is to understand the importance of baryon physics in modelling the clusters’ mass structure estimations, usually assessed through the scaling relations between cluster’s mass and X-ray observables, and through the Sunyaev-Zeldovich effect. Thanks to these relations we can independently extract cosmological parameters and compare them with observations from radio telescopes (e.g. APEX, SPT). This kind of research is of great importance for sky surveys to come in the X-ray band (for future X-ray satellites as WFXT).

Dr. Dunja Fabjan, member of the Center of Excellence Space-SI at FMF, is currently doing research in galaxy clusters simulations. She is involved in a collaboration with the numerical astrophysics group at the University of Trieste. Simulations with the Tree-SPH Gadgetcode will run on the local cluster system Camelot.

Gamma ray bursts

Gamma ray bursts

Izbruhi sevanja gama (angl. Gamma Ray Bursts – GRBs) so najmočnejše eksplozije v vesolju po velikem poku. So nenapovedljive in kratke (0.01-1000 s) in prihajajo iz naključnih smeri neba, točneje iz drugih galaksij.

Izbruhe sevanja gama detektirajo sateliti (SwiftFermiIntegral), ki podatke preko Gamma-ray bursts Coordinates Network sporočijo opazovalcem, da lahko lokacijo izbruhov čimprej opazujejo s teleskopi na Zemlji. Na mestu izbruha sevanja gama je lahko v vidni svetlobi nekaj ur do dni po izbruhu viden izvor, ki mu rečemo optični zasij (angl. afterglow) izbruha sevanja gama. Za opazovanje teh so najprimernejši hitri robotski teleskopi.

Izbruhe sevanja gama povzroči smrt zelo masivnih, hitro vrtečih se zvezd ali zlitje nevtronskih zvezd in/ali črnih lukenj. V njih se v nekaj sekundah sprosti ogromna količina energije – več kot je odda eksplozija supernove v nekaj mesecih ali kot je bo Sonce oddalo v 10 milijardah let. Izbruhe sevanja gama lahko zato opazimo tudi, če se zgodijo v več milijard svetlobnih let oddaljenih galaksijah.

Izbruhi sevanja gama so ena najbolj vročih tem moderne astrofizike. Proučevanje teh izbruhov je pomembno v okviru razumevanja razvoja zvezd in okolja v drugih galaksijah, še zlasti pomembni pa so za študij galaksij, ki so nastale kmalu po nastanku vesolja, saj nam zaradi izjemne moči služijo kot edinstvene kozmološke sonde, ki “osvetlijo” šibke oddaljene galaksije. Pomembni so tudi za širšo fiziko, saj so laboratoriji za proučevanje visoko-energijske fizike, širjenja ultra-relativističnih eksplozij in kot možni izvori visoko-energijskih delcev in gravitacijskih valov.

Na FMF se s proučevanjem izbruhov sevanja gama ukvarja dr. Drejc Kopač.