Dark Cosmology Center

Copenhagen, Denmark

Dark Cosmology Center

Copenhagen, Denmark
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De Ugarte Postigo A.,Institute Astrofisica Of Andalucia Iaa Csic | De Ugarte Postigo A.,Dark Cosmology Center
Proceedings of the International Astronomical Union | Year: 2015

Since 2009 X-shooter has collected spectroscopy of over 80 gamma-ray burst afterglows with redshifts ranging from 0.05 to 6.3. Thanks to its efficiency, broad wavelength coverage (3,000 to 24,800 A), and intermediate spectral resolution (R∼8,000) it has become the most efficient tool for gamma-ray burst afterglow spectroscopy. In this Focus Meeting we presented the sample and some preliminary results of the analysis of absorption systems. © International Astronomical Union 2016.

Scrimgeour M.I.,University of Western Australia | Davis T.,University of Queensland | Blake C.,Swinburne University of Technology | James J.B.,Dark Cosmology Center | And 26 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We have made the largest volume measurement to date of the transition to large-scale homogeneity in the distribution of galaxies. We use the WiggleZ survey, a spectroscopic survey of over 200000 blue galaxies in a cosmic volume of ∼1h -3Gpc 3. A new method of defining the 'homogeneity scale' is presented, which is more robust than methods previously used in the literature, and which can be easily compared between different surveys. Due to the large cosmic depth of WiggleZ (up to z = 1), we are able to make the first measurement of the transition to homogeneity over a range of cosmic epochs. The mean number of galaxies N(< r) in spheres of comoving radius r is proportional to r 3 within 1per cent, or equivalently the fractal dimension of the sample is within 1per cent of D 2 = 3, at radii larger than 71 ± 8h -1Mpc at z ∼ 0.2, 70 ± 5h -1Mpc at z ∼ 0.4, 81 ± 5h -1Mpc at z ∼ 0.6 and 75 ± 4h -1Mpc at z ∼ 0.8. We demonstrate the robustness of our results against selection function effects, using a Λ cold dark matter (ΛCDM) N-body simulation and a suite of inhomogeneous fractal distributions. The results are in excellent agreement with both the ΛCDM N-body simulation and an analytical ΛCDM prediction. We can exclude a fractal distribution with fractal dimension below D 2 = 2.97 on scales from ∼80h -1Mpc up to the largest scales probed by our measurement, ∼300 h -1Mpc, at 99.99per cent confidence. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Penner K.,University of Arizona | Pope A.,National Optical Astronomy Observatory | Chapin E.L.,University of British Columbia | Greve T.R.,Max Planck Institute for Astronomy | And 14 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present a study of the cosmic infrared background, which is a measure of the dust-obscured activity in all galaxies in the Universe. We venture to isolate the galaxies responsible for the background at 1 mm; with spectroscopic and photometric redshifts we constrain the redshift distribution of these galaxies. We create a deep 1.16 mm map (σ∼ 0.5 mJy) by combining the AzTEC 1.1 mm and MAMBO 1.2 mm data sets in the Great Observatories Origins Deep Survey North (GOODS-N) region. This combined map contains 41 secure detections, 13 of which are new. By averaging the 1.16 mm flux densities of individually undetected galaxies with 24 μm flux densities >25 μJy, we resolve 31-45 per cent of the 1.16 mm background. Repeating our analysis on the SCUBA 850 μm map, we resolve a higher percentage (40-64 per cent) of the 850 μm background. A majority of the background resolved (attributed to individual galaxies) at both wavelengths comes from galaxies at z > 1.3. If the ratio of the resolved submillimetre to millimetre background is applied to a reasonable scenario for the origins of the unresolved submillimetre background, 60-88 per cent of the total 1.16 mm background comes from galaxies at z > 1.3. © 2010 The Authors. Journal compilation © 2010 RAS.

Svensmark J.,Dark Cosmology Center | Wojtak R.,Dark Cosmology Center | Wojtak R.,Kavli Institute for Particle Astrophysics and Cosmology | Hansen S.H.,Dark Cosmology Center
Monthly Notices of the Royal Astronomical Society | Year: 2015

The caustic technique for measuring mass profiles of galaxy clusters relies on the assumption of spherical symmetry. When applied to aspherical galaxy clusters, the method yields mass estimates affected by the cluster orientation. Here, we employ mock redshift catalogues generated from cosmological simulations to study the effect of clusters intrinsic shape and surrounding filamentary structures on the caustic mass estimates. To this end, we develop a new method for removing perturbations from large-scale structures, modelled as the two-halo term, in a caustic analysis of stacked cluster data.We find that the cluster masses inferred from kinematical data of 1014M⊙ clusters observed along the major axis are larger than masses from those observed along the minor axis by a factor of 1.7 within the virial radius, increasing to 1.8 within three virial radii. This discrepancy increases by 20 per cent for the most massive clusters. In addition, a smaller but still significant mass discrepancy arises when filamentary structures are present near a galaxy cluster. We find that the mean cluster mass from random sight lines is unbiased at all radii and their scatter ranges from 0.14 to 0.17 within one and three virial radii, with a 40 per cent increase for the most massive clusters. We provide tables which estimate the caustic mass bias given observational constraints on the cluster orientation. © 2015 The Authors.

Kankare E.,University of Turku | Ergon M.,The Oskar Klein Center | Bufano F.,National institute for astrophysics | Spyromilio J.,ESO | And 20 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present an optical and near-infrared photometric and spectroscopic study of supernova (SN) 2009kn spanning ∼1.5yr from the discovery. The optical spectra are dominated by the narrow (full width at half-maximum ∼1000kms -1) Balmer lines distinctive of a Type IIn SN with P Cygni profiles. Contrarily, the photometric evolution resembles more that of a Type IIP SN with a large drop in luminosity at the end of the plateau phase. These characteristics are similar to those of SN 1994W, whose nature has been explained with two different models with different approaches. The well-sampled data set on SN 2009kn offers the possibility to test these models, in the case of both SN 2009kn and SN 1994W. We associate the narrow P Cygni lines with a swept-up shell composed of circumstellar matter and SN ejecta. The broad emission line wings, seen during the plateau phase, arise from internal electron scattering in this shell. The slope of the light curve after the post-plateau drop is fairly consistent with that expected from the radioactive decay of 56Co, suggesting an SN origin for SN 2009kn. Assuming radioactivity to be the main source powering the light curve of SN 2009kn in the tail phase, we infer an upper limit for 56Ni mass of 0.023M ⊙. This is significantly higher than that estimated for SN 1994W, which also showed a much steeper decline of the light curve after the post-plateau drop. We also observe late-time near-infrared emission which most likely arises from newly formed dust produced by SN 2009kn. As with SN 1994W, no broad lines are observed in the spectra of SN 2009kn, not even in the late-time tail phase. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Thone C.C.,IAA CSIC | Fynbo J.,Dark Cosmology Center | Goldoni P.,CNRS Astroparticle and Cosmology Lab | Goldoni P.,SAP | And 3 more authors.
Astronomische Nachrichten | Year: 2011

GRB 100219A at z = 4.667 has been the highest redshift gamma-ray burst observed with the X-shooter spectrograph up to now. The spectrum covering the range from 5000 to 24000 Å and a large number of absorption lines allows to make a detailed study of the interstellar medium in a high redshift galaxy. The ISM in the low ionisation state and the kinematics of the absorption line components reveal a complex velocity field. The metallicity measured from different absorption lines is around 0.1 solar. Other GRB hosts at redshift beyond ∼3 have similar metallicities albeit with a large scatter in the metallicity distribution. X-shooter will allow us to determine metallicities of a larger number of GRB hosts beyond redshift 5, to probe the early chemical enrichment of the Universe and to study its evolution from redshift 2 to beyond 10. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Konig S.,Dark Cosmology Center | Konig S.,University of Cologne | Garcia-Marin M.,University of Cologne | Eckart A.,University of Cologne | And 3 more authors.
Astrophysical Journal | Year: 2012

For the first time, we study the eastern nucleus in greater detail and search for the more extended emission in the molecular gas in different CO line transitions of the famous ultraluminous infrared galaxy (ULIRG) Arp 220. Furthermore, we present a model of the merger in Arp 220 on large scales with the help of the CO data and an optical and near-infrared composite Hubble Space Telescope image of the prototypical ULIRG. Using the Plateau de Bure interferometer (PdBI), we obtained CO(2-1) and (1-0) data at wavelengths of 1 and 3mm in 1994, 1996, 1997, and 2006 at different beam sizes and spatial resolutions. The simulations of the merger in Arp 220 were performed with the Identikit modeling tool. The model parameters that describe the galaxy merger best give a mass ratio of 1:2 and result in a merger of 6 × 10 8yr. The low-resolution CO(1-0) PdBI observations suggest that there are indications for emission 10″ toward the south, as well as to the north and to the west of the two nuclei. © 2012. The American Astronomical Society. All rights reserved..

Richards J.W.,University of California at Berkeley | Starr D.L.,University of California at Berkeley | Brink H.,Dark Cosmology Center | Miller A.A.,University of California at Berkeley | And 6 more authors.
Astrophysical Journal | Year: 2012

Despite the great promise of machine-learning algorithms to classify and predict astrophysical parameters for the vast numbers of astrophysical sources and transients observed in large-scale surveys, the peculiarities of the training data often manifest as strongly biased predictions on the data of interest. Typically, training sets are derived from historical surveys of brighter, more nearby objects than those from more extensive, deeper surveys (testing data). This sample selection bias can cause catastrophic errors in predictions on the testing data because (1) standard assumptions for machine-learned model selection procedures break down and (2) dense regions of testing space might be completely devoid of training data. We explore possible remedies to sample selection bias, including importance weighting, co-training, and active learning (AL). We argue that AL - where the data whose inclusion in the training set would most improve predictions on the testing set are queried for manual follow-up - is an effective approach and is appropriate for many astronomical applications. For a variable star classification problem on a well-studied set of stars from Hipparcos and Optical Gravitational Lensing Experiment, AL is the optimal method in terms of error rate on the testing data, beating the off-the-shelf classifier by 3.4% and the other proposed methods by at least 3.0%. To aid with manual labeling of variable stars, we developed a Web interface which allows for easy light curve visualization and querying of external databases. Finally, we apply AL to classify variable stars in the All Sky Automated Survey, finding dramatic improvement in our agreement with the ASAS Catalog of Variable Stars, from 65.5% to 79.5%, and a significant increase in the classifier's average confidence for the testing set, from 14.6% to 42.9%, after a few AL iterations. © 2012 The American Astronomical Society. All rights reserved.

Simpson F.,University of Edinburgh | James J.B.,Dark Cosmology Center | James J.B.,University of California at Berkeley | Heavens A.F.,University of Edinburgh | Heymans C.,University of Edinburgh
Physical Review Letters | Year: 2011

A large fraction of the information collected by cosmological surveys is simply discarded to avoid length scales which are difficult to model theoretically. We introduce a new technique which enables the extraction of useful information from the bispectrum of galaxies well beyond the conventional limits of perturbation theory. Our results strongly suggest that this method increases the range of scales where the relation between the bispectrum and power spectrum in tree-level perturbation theory may be applied, from k max∼0.1 to ∼0.7hMpc -1. This leads to correspondingly large improvements in the determination of galaxy bias. Since the clipped matter power spectrum closely follows the linear power spectrum, there is the potential to use this technique to probe the growth rate of linear perturbations and confront theories of modified gravity with observation. © 2011 American Physical Society.

Berian James J.,University of California at Berkeley | Berian James J.,Dark Cosmology Center
Information Systems Development: Reflections, Challenges and New Directions | Year: 2013

We suggest a technique to calculate the impact of distinct physical processes inducing non-gaussianity on the cosmological density field. The decomposition of the cosmic genus statistic with an orthogonal polynomial sequence allows expression of the scale-dependent evolution of the morphology of large-scale structure, in which effects including galaxy bias, non-linear gravitational evolution and primordial non-gaussianity might be delineated. © Springer Science+Business Media New York 2013.

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