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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. Source

Guo Q.,Leibniz Institute for Astrophysics Potsdam | Tempel E.,Tartu Observatory | Tempel E.,Estonian National Institute of Chemical Physics and Biophysics | Libeskind N.I.,Leibniz Institute for Astrophysics Potsdam
Astrophysical Journal | Year: 2015

We investigate whether the satellite luminosity function (LF) of primary galaxies identified in the Sloan Digital Sky Survey (SDSS) depends on whether the host galaxy is in a filament or not. Isolated primary galaxies are identified in the SDSS spectroscopic sample, and potential satellites (that are up to four magnitudes fainter than their hosts) are searched for in the much deeper photometric sample. Filaments are constructed from the galaxy distribution by the Bisous process. Isolated primary galaxies are divided into two subsamples: those in filaments and those not in filaments. We examine the stacked mean satellite LF of both the filament and nonfilament samples and find that, on average, the satellite LF of galaxies in filaments is significantly higher than those of galaxies not in filaments. The filamentary environment can increase the abundance of the brightest satellites (M sat. < M prim. + 2.0) by a factor of ∼2 compared with nonfilament isolated galaxies. This result is independent of the primary galaxy magnitude, although the satellite LF of galaxies in the faintest magnitude bin is too noisy to determine if such a dependence exists. Because our filaments are extracted from a spectroscopic flux-limited sample, we consider the possibility that the difference in satellite LF is due to a redshift, color, or environmental bias, finding these to be insufficient to explain our result. The dependence of the satellite LF on the cosmic web suggests that the filamentary environment may have a strong effect on the efficiency of galaxy formation. © 2015. The American Astronomical Society. All rights reserved. Source

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. Source

Kalberla P.M.W.,University of Bonn | Haud U.,Tartu Observatory
Astronomy and Astrophysics | Year: 2015

Context. The Galactic All-Sky Survey (GASS) is a survey of Galactic atomic hydrogen (Hi) emission in the southern sky observed with the Parkes 64-m Radio Telescope. The first data release (GASS I) concerned survey goals and observing techniques, the second release (GASS II) focused on stray radiation and instrumental corrections. Aims. We seek to remove the remaining instrumental effects and present a third data release. Methods. We use the HEALPix tessellation concept to grid the data on the sphere. Individual telescope records are compared with averages on the nearest grid position for significant deviations. All averages are also decomposed into Gaussian components with the aim of segregating unacceptable solutions. Improved priors are used for an iterative baseline fitting and cleaning. In the last step we generate 3D FITS data cubes and examine them for remaining problems. Results. We have removed weak, but systematic baseline offsets with an improved baseline fitting algorithm. We have unraveled correlator failures that cause time dependent oscillations; errors cause stripes in the scanning direction. The remaining problems from radio frequency interference (RFI) are spotted. Classifying the severeness of instrumental errors for each individual telescope record (dump) allows us to exclude bad data from averages. We derive parameters that allow us to discard dumps without compromising the noise of the resulting data products too much. All steps are reiterated several times: in each case, we check the Gaussian parameters for remaining problems and inspect 3D FITS data cubes visually. We find that in total ∼1.5% of the telescope dumps need to be discarded in addition to ∼0.5% of the spectral channels that were excluded in GASS II. Conclusions. The new data release (GASS III) facilitates data products with improved quality. A new web interface, compatible with the previous version, is available for download of GASS III FITS cubes and spectra. © ESO, 2015. Source

He L.,University of Toronto | Chen J.M.,University of Toronto | Pisek J.,Tartu Observatory | Schaaf C.B.,Boston University | And 2 more authors.
Remote Sensing of Environment | Year: 2012

Clumping index (CI), quantifying the level of foliage grouping within distinct canopy structures relative to a random distribution, is a key structural parameter of plant canopies and very useful in ecological and meteorological models. Previously, a Normalized Difference between Hotspot and Darkspot (NDHD) index has been proposed to accurately retrieve the global CI using POLarization and Directionality of the Earth's Reflectances (POLDER) data at ~6km resolution (Chen et al., 2005). In this study, we show for the first time a global CI map at 500m resolution derived using the Bidirectional Reflectance Distribution Function (BRDF) product from Moderate Resolution Imaging Spectroradiometer (MODIS), which currently provides the finest pseudo multi-angular data for the global land surface. In computing the NDHD we found that the hotspot calculated from the MODIS BRDF product is underestimated in comparison with POLDER measurements very near the hotspot. Without correcting the bias in MODIS data, the MODIS-derived CI could be overestimated. We have developed an approach to correct the MODIS hotspot magnitude with co-registered POLDER-3 data acquired at about the same time. Field-measured element-CIs in zenith angles of 30-60° by the Tracing Radiation and Architecture of Canopies (TRAC) instrument from 38 sites together with corresponding needle-to-shoot area ratios were used to evaluate the performance of hotspot-corrected NDHD and to tune the parameters to get accurate CI. It is found that the MODIS-derived CI has a dependence on the solar zenith angle, and this dependence in the red band is smaller than that in the near infrared (NIR) band. The highest correlation between field measurements and MODIS-derived CI is achieved when hotspot at nadir is used. The correct estimation of forest species (and therefore the crown shapes) from land cover is very important to the accuracy of MODIS-derived CI. For the red band, the coefficient of determination (R 2) between MODIS-derived CI and field-measured CI is 0.76 for all sites and R 2 is 0.53 for only the needle-leaved forest sites after the hotspot correction, indicating that a strong correlation between MODIS-derived NDHD and CI indeed exists. The MODIS-derived CI is regularly smaller than the field measurement. The red band in the MODIS BRDF product is found to be better than the NIR band for deriving CI for the vegetation covers investigated (mostly dense forests). Therefore a global clumping index map has been produced using the corrected MODIS hot\spot in the red band and optimized parameters. However there were 10 broad-leaved forest sites used in the validation, which are too few for a reliable validation. For the broad-leaved forest area, where the same crown shape (ellipsoid) is used in the algorithm, the global CI spatial pattern is interestingly very similar to the pattern of the land cover distribution, suggesting that the MODIS-derived map has effectively captured the structural differences among cover types. © 2011 Elsevier Inc. Source

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