Tempel E.,Tartu Observatory |
Tempel E.,Estonian National Institute of Chemical Physics and Biophysics |
Libeskind N.I.,Leibniz Institute for Astrophysics Potsdam
Astrophysical Journal Letters | Year: 2013
The properties of galaxies are known to be affected by their environment. One important question is how their angular momentum reflects the surrounding cosmic web. We use the Sloan Digital Sky Survey to investigate the spin axes of spiral and elliptical galaxies relative to their surrounding filament/sheet orientations. To detect filaments, a marked point process with interactions (the "Bisous model") is used. Sheets are found by detecting "flattened" filaments. The minor axes of ellipticals are found to be preferentially perpendicular to hosting filaments. A weak correlation is found with sheets. These findings are consistent with the notion that elliptical galaxies formed via mergers, which predominantly occurred along the filaments. The spin axis of spiral galaxies is found to align with the host filament, with no correlation between spiral spin and sheet normal. When examined as a function of distance from the filament axis, a much stronger correlation is found in the outer parts, suggesting that the alignment is driven by the laminar infall of gas from sheets to filaments. When compared with numerical simulations, our results suggest that the connection between dark matter halo and galaxy spin is not straightforward. Our results provide an important input to the understanding of how galaxies acquire their angular momentum. © 2013. The American Astronomical Society. All rights reserved..
Veledina A.,University of Oulu |
Poutanen J.,University of Oulu |
Vurm I.,Hebrew University of Jerusalem |
Vurm I.,Tartu Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2013
Multiwavelength observations of Galactic black hole transients have opened a new path to understanding the physics of the innermost parts of the accretion flows. While the processes giving rise to their X-ray continuum have been studied extensively, the emission in the optical and infrared (OIR) energy bands was less investigated and remains poorly understood. The standard accretion disc, which may contribute to the flux at these wavelengths, is not capable of explaining a number of observables: the infrared excesses, fast OIR variability and a complicated correlation with the X-rays. It was suggested that these energy bands are dominated by the jet emission; however, this scenario does not work in a number of cases. We propose here an alternative, namely that most of the OIR emission is produced by the extended hot accretion flow. In this scenario, the OIR bands are dominated by the synchrotron radiation from the non-thermal electrons. An additional contribution is expected from the outer irradiated part of the accretion disc heated by the X-rays. We discuss the properties of the model and compare them to the data. We show that the hot-flow scenario is consistent with many of the observed spectral data, at the same time naturally explaining X-ray timing properties, fast OIR variability and its correlation with the X-rays. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
Vurm I.,Hebrew University of Jerusalem |
Vurm I.,Tartu Observatory |
Lyubarsky Y.,Ben - Gurion University of the Negev |
Piran T.,Hebrew University of Jerusalem
Astrophysical Journal | Year: 2013
The low-energy spectral slopes of the prompt emission of most gamma-ray bursts (GRBs) are difficult to reconcile with radiatively efficient optically thin emission models irrespective of the radiation mechanism. An alternative is to ascribe the radiation around the spectral peak to a thermalization process occurring well inside the Thomson photosphere. This quasi-thermal spectrum can evolve into the observed non-thermal shape by additional energy release at moderate to small Thomson optical depths, which can readily give rise to the hard spectral tail. The position of the spectral peak is determined by the temperature and Lorentz factor of the flow in the thermalization zone, where the total number of photons carried by the jet is established. To reach thermalization, dissipation alone is not sufficient and photon generation requires an efficient emission/absorption process in addition to scattering. We perform a systematic study of all relevant photon production mechanisms searching for possible conditions in which thermalization can take place. We find that a significant fraction of the available energy should be dissipated at intermediate radii, ≳1010 to a few ×1011 cm, and the flow there should be relatively slow: the bulk Lorentz factor could not exceed a few tens for all but the most luminous bursts with the highest E pk values. The least restrictive constraint for successful thermalization, Γ ≲ 20, is obtained if synchrotron emission acts as the photon source. This requires, however, a non-thermal acceleration deep below the Thomson photosphere transferring a significant fraction of the flow energy to relativistic electrons with Lorentz factors between 10 and 100. Other processes require bulk flow Lorentz factors of order of a few for typical bursts. We examine the implications of these results to different GRB photospheric emission models. © 2013. The American Astronomical Society. All rights reserved.
Hutsi G.,University College London |
Hutsi G.,Tartu Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2010
We use the direct Fourier method to calculate the redshift-space power spectrum of the maxBCG cluster catalogue - currently by far the largest existing galaxy cluster sample. The total number of clusters used in our analysis is 12 616. After accounting for the radial smearing effect caused by photometric redshift errors and also introducing a simple treatment for the non-linear effects, we show that currently favoured low matter density 'concordance' Λ cold dark matter cosmology provides a very good fit to the estimated power. Thanks to the large volume (∼0.4 h-3 Gpc3), high clustering amplitude [linear effective bias parameter beff ∼ 3 × (0.85/σ8)] and sufficiently high sampling density (∼3 × 10-5 h3 Mpc-3), the recovered power spectrum has a high enough signal-to-noise ratio to allow us to find weak evidence [∼2σ confidence level (CL)] for the baryonic acoustic oscillations (BAO). In case the clusters are additionally weighted by their richness, the resulting power spectrum has slightly higher large-scale amplitude and smaller damping on small scales. As a result, the CL for the BAO detection is somewhat increased: ∼2.5σ. The ability to detect BAO with a relatively small number of clusters is encouraging in the light of several proposed large cluster surveys. © 2009 RAS.
Haud U.,Tartu Observatory
Astronomy and Astrophysics | Year: 2010
Context. In the previous papers of this series, we decomposed all the Hi 21-cm line profiles of the Leiden-Argentina-Bonn (LAB) database into Gaussian components (GCs), and studied the statistical distributions of the obtained GCs. Aims.Now we are interested in separating the "clouds" of similar closely spaced GCs from the general database of the components. In this paper we examine the most complicated case for our new cloud-finding algorithm - the clouds of very narrow GCs. Methods.To separate the clouds of similar GCs, we started with the single-link hierarchical clustering procedure in five-dimensional (longitude, latitude, velocity, GC width, and height) space, but made some modifications to accommodate it to the large number of components. We also used the requirement that each cloud may be represented at any observed sky position by only one GC and take the similarity of global properties of the merging clouds into account. We demonstrate that the proposed algorithm enables us to find the features in gas distribution, which are described by similar GCs. As a test, we applied the algorithm for finding the clouds of the narrowest Hi 21-cm line components. Results.Using the full sky search for cold clouds, we easily detected the coldest known Hi clouds and demonstrate that actually they are a part of a long narrow ribbon of cold clouds. We modeled these clouds as a part of a planar gas ring, then deduce their spatial placement and discuss their relation to supernova shells in the solar neighborhood. Many other narrow-lined Hi structures are also found. We conclude that the proposed algorithm satisfactorily solves the posed task. In testing the algorithm, we found a long ribbon of very cold Hi clouds and demonstrated that all the observed properties of this band of clouds are described very well by the planar ring model. Conclusions.We also guess that the study of the narrowest Hi 21-cm line components may be a useful tool for finding the structure of neutral gas in solar neighborhood. © ESO 2010.
Malyuto V.,Tartu Observatory
Astronomische Nachrichten | Year: 2013
Homogeneous independent subsamples from selected catalogues and libraries of effective temperatures for metal deficient F-G stars are treated here by combining them in triples and pairs for the stars in common to determine their external errors from data intercomparisons. The effective temperatures are then averaged (with the weights inversely proportional to the squared errors) to produce mean homogenized catalogues which may be used for calibration of spectral and photometric data in large Galactic surveys. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Kallel A.,Tartu Observatory
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2010
Many physically based approaches for modeling the scattering properties of vegetation (i.e. methods based on radiative transfer models, RTM) suffer from significant shortcomings. In particular, the energy conservation problem has remained unsolved for a long time. This is particularly evident when introducing finite size scattering elements (leaves or shoots) into equations originally describing a turbid medium. This phenomenon, called the hot spot effect, is treated in classical RTM by increasing the reflectance value at the first collision of incident photons. To overcome this shortcoming, we propose in this paper a new model called the flux decomposition model (FDM) and based on the Kallel et al. approach (AddingSD) which propose a formulation showing that the hot spot could be viewed as an increase of the posterior gap probability. The formalism is based on a decrease of the vegetation density and is called 'the effective vegetation density'. Thus, inspired from this idea, in our study, energy conservation is achieved using the same effective density to estimate the upward diffuse flux provided by the first collision of the solar irradiance (E+1) as well as the diffuse fluxes created by E+1 scattering. Finally, to solve the RT equations, E+1 is divided into virtual subfluxes having simple expressions, allowing the division of the problem into a finite number of subproblems, each one corresponding to a given subflux easily solved based on SAIL++ formalism. Simulation tests show that the proposed model conserves energy with good accuracy. Compared to 3-D models in the ROMC/RAMI three database, our model performs similarly. Finally, compared to AddingSD, the running time is drastically reduced from about 15. min to a few milliseconds. © 2010 Elsevier Ltd.
Kallel A.,Tartu Observatory
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2012
As an approximation, the forest could be assumed a discrete media composed of three main components: trees, understory vegetation and soil background. To describe the reflectance of such a canopy in the optical wavelength domain, it is necessary to develop a radiative transfer model which considers two vegetation layers (understory and trees). In this article, we propose a new model, FDM-2, an extension of the flux decomposition model (FDM), to take into account such a canopy architecture. Like FDM, FDM-2 models the diffuse flux anisotropy and takes into account the hot spot effect as well as conserves energy. The hot spot which corresponds to an increase of the probability of photon escape after first collision close to the backscattering direction is modeled as a decrease of "the effective vegetation density" encountered by the diffuse flux (E + 1) and the radiance both created by first order scattering of the direct sun radiation. Compared to the turbid case (for which our model is equivalent to SAIL++ and therefore accurately conserving energy), such a density variation redistributes energy but does not affect the budget. Energy remains well conserved in the discrete case as well. To solve the RT problem, FDM-2 separates E + 1 from the high order diffuse flux. As E + 1 corresponding effective density is not constant function of the altitude (when traveling along the canopy) therefore it is decomposed into sub-fluxes of constant densities. The sub-flux RT problems are linear and simply solved based on SAIL++ formalism. The global RT solution is obtained summing the contribution of the sub-fluxes. Simulation tests confirm that FDM-2 conserves energy (i.e., radiative budget closes to zero in the purist corner case with an error due to the discretization less than 0.5%). Compared to the Rayspread model (among the best 3-D models of the RAMI Exercise third phase), our model provides similar performance. © 2012 Elsevier Ltd.
Kuusk A.,Tartu Observatory
Agricultural and Forest Meteorology | Year: 2016
In a vegetation canopy part of leaves reflect sky radiation specularly to the LAI-2000 sensor. This fraction is smaller in a planophile canopy and higher in an erectophile canopy. As the specularly reflected radiation does not enter the leaf nor it is absorbed by leaf pigments and depending on the incidence angle on the leaf the reflection coefficient may be very high. Depending on the leaf area index (LAI) and leaf angle distribution the scattered sky radiation may cause an overestimation of the gap fraction and consequently an underestimation of LAI up to 20-25% if the LAI measurements with plant canopy analyzer LAI-2000 are carried out under ideal perfectly overcast conditions. Similar biases are present in measurements of gap fraction and LAI with hemispherical photos if these measurement procedures are calibrated against LAI-2000. Considering the scattered sky radiation in optical techniques (LAI-2000, hemispherical photos) of measuring gap fraction in vegetation canopies explains at least part of the observed discrepancies in gap fraction estimated with terrestrial laser scanner and hemispherical photos. The scattered sky radiation in the LAI-2000 signal introduces bias both in the estimated LAI and foliage clumping index. © 2016 Elsevier B.V.
Kipper T.,Tartu Observatory
Baltic Astronomy | Year: 2011
The high resolution spectra of a post-AGB candidate, binary system 89 Her, were analysed for the chemical composition. The star was found to be metal deficient with [Fe/H] = -0.50 ± 0.20. No enhancement of s-process elements was found. The refractory elements are depleted but this is not the reason of metal deficiency. More than 320 narrow and weak emission lines from low levels of neutral metals were identified. Radial velocities of these lines coincide with the systemic velocity. We propose that the circum-binary dusty disk is observed face-on.