Showing papers by "Hendrik Hildebrandt published in 2013"
University of Edinburgh1, University of British Columbia2, University of Bonn3, Leiden University4, University of Victoria5, Institut d'Astrophysique de Paris6, University of Oxford7, Academia Sinica8, Shanghai Normal University9, University of Toronto10, Perimeter Institute for Theoretical Physics11, University of Waterloo12, University College London13, California Institute of Technology14, Stanford University15
TL;DR: In this paper, the authors present a finely-binned tomographic weak lensing analysis of the Canada-FranceHawaii Telescope Lensing Survey, CFHTLenS, mitigating contamination to the signal from the presence of intrinsic galaxy alignments via the simultaneous fit of a cosmological model and an intrinsic alignment model.
Abstract: We present a finely-binned tomographic weak lensing analysis of the Canada-FranceHawaii Telescope Lensing Survey, CFHTLenS, mitigating contamination to the signal from the presence of intrinsic galaxy alignments via the simultaneous fit of a cosmological model and an intrinsic alignment model. CFHTLenS spans 154 square degrees in five optical bands, with accurate shear and photometric redshifts for a galaxy sample with a median redshift of zm = 0:70. We estimate the 21 sets of cosmic shear correlation functions associated with six redshift bins, each spanning the angular range of 1:5 < < 35 arcmin. We combine this CFHTLenS data with auxiliary cosmological probes: the cosmic microwave background with data from WMAP7, baryon acoustic oscillations with data from BOSS, and a prior on the Hubble constant from the HST distance ladder. This leads to constraints on the normalisation of the matter power spectrum 8 = 0:799 0:015 and the matter density parameter m = 0:271 0:010 for a flat CDM cosmology. For a flat wCDM cosmology we constrain the dark energy equation of state parameter w = 1:02 0:09. We also provide constraints for curved CDM and wCDM cosmologies. We find the intrinsic alignment contamination to be galaxy-type dependent with a significant intrinsic alignment signal found for early-type galaxies, in contrast to the late-type galaxy sample for which the intrinsic alignment signal is found to be consistent with zero.
688 citations
University of Bonn1, University of British Columbia2, University of Oxford3, University of Edinburgh4, University of Victoria5, Leiden University6, Institut d'Astrophysique de Paris7, Swinburne University of Technology8, Spanish National Research Council9, Academia Sinica10, Shanghai Normal University11, University of Waterloo12, Perimeter Institute for Theoretical Physics13, Herzberg Institute of Astrophysics14, University College London15, California Institute of Technology16, Stanford University17
TL;DR: In this paper, the authors present data products from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) data set and demonstrate that their data meet necessary requirements to fully exploit the survey for weak gravitational lensing analyses in connection with photometric redshift studies.
Abstract: We present data products from the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS). CFHTLenS is based on the Wide component of the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS). It encompasses 154 deg^2 of deep, optical, high-quality, sub-arcsecond imaging data in the five optical filters u*g′r′i′z′. The scientific aims of the CFHTLenS team are weak gravitational lensing studies supported by photometric redshift estimates for the galaxies. This paper presents our data processing of the complete CFHTLenS data set. We were able to obtain a data set with very good image quality and high-quality astrometric and photometric calibration. Our external astrometric accuracy is between 60 and 70 mas with respect to Sloan Digital Sky Survey (SDSS) data, and the internal alignment in all filters is around 30 mas. Our average photometric calibration shows a dispersion of the order of 0.01–0.03 mag for g′r′i′z′ and about 0.04 mag for u* with respect to SDSS sources down to i_(SDSS) ≤ 21. We demonstrate in accompanying papers that our data meet necessary requirements to fully exploit the survey for weak gravitational lensing analyses in connection with photometric redshift studies. In the spirit of the CFHTLS, all our data products are released to the astronomical community via the Canadian Astronomy Data Centre at http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/community/CFHTLens/query.html. We give a description and how-to manuals of the public products which include image pixel data, source catalogues with photometric redshift estimates and all relevant quantities to perform weak lensing studies.
443 citations
University of Oxford1, University of British Columbia2, University of Bonn3, Leiden University4, University of Victoria5, Pierre-and-Marie-Curie University6, University College London7, California Institute of Technology8, Academia Sinica9, University of Michigan10, Shanghai Normal University11, University of Toronto12, University of Waterloo13, Perimeter Institute for Theoretical Physics14, Stanford University15
TL;DR: In this article, a likelihood-based method for measuring weak gravitational lensing shear in deep galaxy surveys is described and applied to the Canada-France-Hawaii Telescope (CFHT) Lensing Survey.
Abstract: A likelihood-based method for measuring weak gravitational lensing shear in deep galaxy surveys is described and applied to the Canada–France–Hawaii Telescope (CFHT) Lensing Survey (CFHTLenS) CFHTLenS comprises 154 deg^2 of multi-colour optical data from the CFHT Legacy Survey, with lensing measurements being made in the i′ band to a depth i′_(AB) < 247, for galaxies with signal-to-noise ratio ν_(SN) ≳ 10 The method is based on the lensfit algorithm described in earlier papers, but here we describe a full analysis pipeline that takes into account the properties of real surveys The method creates pixel-based models of the varying point spread function (PSF) in individual image exposures It fits PSF-convolved two-component (disc plus bulge) models to measure the ellipticity of each galaxy, with Bayesian marginalization over model nuisance parameters of galaxy position, size, brightness and bulge fraction The method allows optimal joint measurement of multiple, dithered image exposures, taking into account imaging distortion and the alignment of the multiple measurements We discuss the effects of noise bias on the likelihood distribution of galaxy ellipticity Two sets of image simulations that mirror the observed properties of CFHTLenS have been created to establish the method's accuracy and to derive an empirical correction for the effects of noise bias
430 citations
Shanghai Normal University1, University of Edinburgh2, University of British Columbia3, University of Bonn4, University of Toronto5, University of Victoria6, Leiden University7, Institut d'Astrophysique de Paris8, University of Oxford9, Spanish National Research Council10, Academia Sinica11, University of Waterloo12, Perimeter Institute for Theoretical Physics13, University College London14, California Institute of Technology15, Stanford University16
TL;DR: In this paper, the cosmological constraints from 2D weak gravitational lensing by the large-scale structure in the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) are presented.
Abstract: We present cosmological constraints from 2D weak gravitational lensing by the large-scale structure in the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS) which spans 154 deg^2 in five optical bands. Using accurate photometric redshifts and measured shapes for 4.2 million galaxies between redshifts of 0.2 and 1.3, we compute the 2D cosmic shear correlation function over angular scales ranging between 0.8 and 350 arcmin. Using non-linear models of the dark-matter power spectrum, we constrain cosmological parameters by exploring the parameter space with Population Monte Carlo sampling. The best constraints from lensing alone are obtained for the small-scale density-fluctuations amplitude σ_8 scaled with the total matter density Ωm. For a flat Λcold dark matter (ΛCDM) model we obtain σ_8(Ω_m/0.27)0.6 = 0.79 ± 0.03.
We combine the CFHTLenS data with 7-year Wilkinson Microwave Anisotropy Probe (WMAP7), baryonic acoustic oscillations (BAO): SDSS-III (BOSS) and a Hubble Space Telescope distance-ladder prior on the Hubble constant to get joint constraints. For a flat ΛCDM model, we find Ω_m = 0.283 ± 0.010 and σ_8 = 0.813 ± 0.014. In the case of a curved wCDM universe, we obtain Ω_m = 0.27 ± 0.03, σ_8 = 0.83 ± 0.04, w0 = −1.10 ± 0.15 and Ω_K = 0.006^(+0.006)_(− 0.004).
We calculate the Bayesian evidence to compare flat and curved ΛCDM and dark-energy CDM models. From the combination of all four probes, we find models with curvature to be at moderately disfavoured with respect to the flat case. A simple dark-energy model is indistinguishable from ΛCDM. Our results therefore do not necessitate any deviations from the standard cosmological model.
405 citations
University of Edinburgh1, University of Queensland2, Swinburne University of Technology3, Ludwig Maximilian University of Munich4, University of British Columbia5, University of Bonn6, Leiden University7, University of Victoria8, Institut d'Astrophysique de Paris9, University of Oxford10, Academia Sinica11, Shanghai Normal University12, University of Toronto13, Perimeter Institute for Theoretical Physics14, University of Waterloo15, University College London16, California Institute of Technology17, Stanford University18
TL;DR: In this paper, the authors combine measurements of cosmic shear tomography from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) with the growth of structure from the WiggleZ Dark Energy Survey and the Six-degree-Field Galaxy Survey, producing the strongest existing joint constraints on the metric potentials that describe general theories of gravity.
Abstract: Dark energy may be the first sign of new fundamental physics in the Universe, taking either a physical form or revealing a correction to Einsteinian gravity. Weak gravitational lensing and galaxy peculiar velocities provide complementary probes of general relativity, and in combination allow us to test modified theories of gravity in a unique way. We perform such an analysis by combining measurements of cosmic shear tomography from the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS) with the growth of structure from the WiggleZ Dark Energy Survey and the Six-degree-Field Galaxy Survey, producing the strongest existing joint constraints on the metric potentials that describe general theories of gravity. For scale-independent modifications to the metric potentials which evolve linearly with the effective dark energy density, we find present-day cosmological deviations in the Newtonian potential and curvature potential from the prediction of general relativity to be ΔΨ/Ψ = 0.05 ± 0.25 and ΔΦ/Φ = −0.05 ± 0.3, respectively (68 per cent confidence limits).
193 citations
University of British Columbia1, University of Edinburgh2, University of Victoria3, Leiden University4, University College London5, Institut d'Astrophysique de Paris6, University of Oxford7, Academia Sinica8, University of Toronto9, Shanghai Normal University10, Perimeter Institute for Theoretical Physics11, University of Waterloo12, California Institute of Technology13, Stanford University14
TL;DR: In this paper, the authors presented a quantitative analysis of the largest contiguous maps of projected mass density obtained from gravitational lensing shear using data from the 154'deg^2 covered by the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS).
Abstract: We present a quantitative analysis of the largest contiguous maps of projected mass density obtained from gravitational lensing shear. We use data from the 154 deg^2 covered by the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS). Our study is the first attempt to quantitatively characterize the scientific value of lensing maps, which could serve in the future as a complementary approach to the study of the dark universe with gravitational lensing. We show that mass maps contain unique cosmological information beyond that of traditional two-point statistical analysis techniques.
Using a series of numerical simulations, we first show how, reproducing the CFHTLenS observing conditions, gravitational lensing inversion provides a reliable estimate of the projected matter distribution of large-scale structure. We validate our analysis by quantifying the robustness of the maps with various statistical estimators. We then apply the same process to the CFHTLenS data. We find that the two-point correlation function of the projected mass is consistent with the cosmological analysis performed on the shear correlation function discussed in the CFHTLenS companion papers. The maps also lead to a significant measurement of the third-order moment of the projected mass, which is in agreement with analytic predictions, and to a marginal detection of the fourth-order moment. Tests for residual systematics are found to be consistent with zero for the statistical estimators we used. A new approach for the comparison of the reconstructed mass map to that predicted from the galaxy distribution reveals the existence of giant voids in the dark matter maps as large as 3° on the sky. Our analysis shows that lensing mass maps are not only consistent with the results obtained by the traditional shear approach, but they also appear promising for new techniques such as peak statistics and the morphological analysis of the projected dark matter distribution.
142 citations
University of British Columbia1, University of Edinburgh2, University of Bonn3, University of Victoria4, Leiden University5, Institut d'Astrophysique de Paris6, University of Oxford7, University College London8, California Institute of Technology9, Stanford University10, Academia Sinica11, Shanghai Normal University12, University of Toronto13, Perimeter Institute for Theoretical Physics14, University of Waterloo15
TL;DR: In this paper, the authors present cosmological constraints using tomographic weak gravitational lensing by large-scale structure, and demonstrate that the redshift probability distribution function summed over galaxies provides an accurate representation of the galaxy redshift distribution accounting for random and catastrophic errors.
Abstract: The Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS) comprises deep multicolour (u^*g′r′i′z′) photometry spanning 154 deg^2, with accurate photometric redshifts and shape measurements. We demonstrate that the redshift probability distribution function summed over galaxies provides an accurate representation of the galaxy redshift distribution accounting for random and catastrophic errors for galaxies with best-fitting photometric redshifts z_p < 1.3.
We present cosmological constraints using tomographic weak gravitational lensing by large-scale structure. We use two broad redshift bins 0.5 < z_p ≤ 0.85 and 0.85 < z_p ≤ 1.3 free of intrinsic alignment contamination, and measure the shear correlation function on angular scales in the range ∼1–40 arcmin. We show that the problematic redshift scaling of the shear signal, found in previous Canada–France–Hawaii Telescope Legacy Survey data analyses, does not affect the CFHTLenS data. For a flat Λ cold dark matter model and a fixed matter density Ω_m = 0.27, we find the normalization of the matter power spectrum σ_8 = 0.771 ± 0.041. When combined with cosmic microwave background data (Wilkinson Microwave Anisotropy Probe 7-year results), baryon acoustic oscillation data (BOSS) and a prior on the Hubble constant from the Hubble Space Telescope distance ladder, we find that CFHTLenS improves the precision of the fully marginalized parameter estimates by an average factor of 1.5-2. Combining our results with the above cosmological probes, we find Ω_m = 0.2762 ± 0.0074 and σ_8 = 0.802 ± 0.013.
142 citations
TL;DR: In this paper, the stellar mass functions (SMFs) of star-forming and quiescent galaxies from observations of ten rich, red-sequence selected, clusters in the Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS) in the redshift range 0.86 < z < 1.34 were presented.
Abstract: Aims. We present the stellar mass functions (SMFs) of star-forming and quiescent galaxies from observations of ten rich, red-sequence selected, clusters in the Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS) in the redshift range 0.86 < z < 1.34. We compare our results with field measurements at similar redshifts using data from a K_s-band selected catalogue of the COSMOS/UltraVISTA field.
Methods. We construct a K_s-band selected multi-colour catalogue for the clusters in eleven photometric bands covering u-8 μm, and estimate photometric redshifts and stellar masses using spectral energy distribution fitting techniques. To correct for interlopers in our cluster sample, we use the deep spectroscopic component of GCLASS, which contains spectra for 1282 identified cluster and field galaxies taken with Gemini/GMOS. This allowed us to correct cluster number counts from a photometric selection for false positive and false negative identifications. Both the photometric and spectroscopic samples are sufficiently deep that we can probe the SMF down to masses of 10^10 M_⊙.
Results. We distinguish between star-forming and quiescent galaxies using the rest-frame U − V versus V − J diagram, and find that the best-fitting Schechter parameters α and M∗ are similar within the uncertainties for these galaxy types within the different environments. However, there is a significant difference in the shape and normalisation of the total SMF between the clusters and the field sample. This difference in the total SMF is primarily a reflection of the increased fraction of quiescent galaxies in high-density environments. We apply a simple quenching model that includes components of mass- and environment-driven quenching, and find that in this picture 45^(+4)_(-3)% of the star-forming galaxies, which normally would be forming stars in the field, are quenched by the cluster.
Conclusions. If galaxies in clusters and the field quench their star formation via different mechanisms, these processes have to conspire in such a way that the shapes of the quiescent and star-forming SMF remain similar in these different environments.
135 citations
TL;DR: In this paper, the stellar mass functions (SMFs) of star-forming and quiescent galaxies from observations of 10 rich clusters in the Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS) in the redshift range 0.86
Abstract: We present the stellar mass functions (SMFs) of star-forming and quiescent galaxies from observations of 10 rich clusters in the Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS) in the redshift range 0.86
121 citations
Journal Article•
TL;DR: The Kilo-Degree Survey (KiDS) as discussed by the authors is a large scale optical imaging survey with the recently commissioned OmegaCAM wide-field imager on the VLT Survey Telescope (VST).
Abstract: The Kilo-Degree Survey (KiDS), a 1500-square-degree optical imaging survey with the recently commissioned OmegaCAM wide-field imager on the VLT Survey Telescope (VST), is described. KiDS will image two fields in u-,g-,r- and i-bands and, together with the VIKING survey, produce nine-band (u- to K-band) coverage over two fields. For the foreseeable future the KiDS/VIKING combination of superb image quality with wide wavelength coverage will be unique for surveys of its size and depth. The survey has been designed to tackle some of the most fundamental questions of cosmology and galaxy formation of today. The main science driver is mapping the dark matter distribution in the Universe and putting constraints on the expansion of the Universe and the equation of state of dark energy, all through weak gravitational lensing. However, the deep and wide imaging data will facilitate a wide variety of science cases.
96 citations
University of British Columbia1, Paris Diderot University2, University of Paris-Sud3, California Institute of Technology4, Jet Propulsion Laboratory5, Imperial College London6, University of Colorado Boulder7, University of California, Irvine8, University of Edinburgh9, Cardiff University10, Virginia Tech11, University of Padua12, Johns Hopkins University13, UK Astronomy Technology Centre14, University of Sussex15, University College London16, Spanish National Research Council17, University of La Laguna18, European Space Agency19, Leiden University20
TL;DR: In this paper, the average mass and dust content of sub-millimetre galaxies with 250μm flux densities of S_250 > 15μm were determined using data from the Herschel Multi-tiered Extragalactic Survey.
Abstract: Dust emission at submillimetre wavelengths allows us to trace the early phases of star formation in the Universe. In order to understand the physical processes involved in this mode of star formation, it is essential to gain knowledge about the dark matter structures – most importantly their masses – that submillimetre galaxies live in. Here we use the magnification effect of gravitational lensing to determine the average mass and dust content of submillimetre galaxies with 250 μm flux densities of S_250 > 15 mJy selected using data from the Herschel Multi-tiered Extragalactic Survey. The positions of hundreds of submillimetre foreground lenses are cross-correlated with the positions of background Lyman-break galaxies at z ∼ 3–5 selected using optical data from the Canada–France–Hawaii Telescope Legacy Survey. We detect a cross-correlation signal at the 7σ level over a sky area of 1 deg^2, with ∼80 per cent of this signal being due to magnification, whereas the remaining ∼20 per cent comes from dust extinction. Adopting some simple assumptions for the dark matter and dust profiles and the redshift distribution enables us to estimate the average mass of the haloes hosting the submillimetre galaxies to be log _(10)[M_200/M_⊙] =
13.17^(+0.05)_(−0.08)(stat.) and their average dust mass fraction (at radii of >10 kpc) to be M_dust/M_200 ≈ 6 × 10^(−5). This supports the picture that submillimetre galaxies are dusty, forming stars at a high rate, reside in massive group-sized haloes and are a crucial phase in the assembly and evolution of structure in the Universe.
University of Waterloo1, Perimeter Institute for Theoretical Physics2, University of Bonn3, University of Edinburgh4, University of British Columbia5, University of Victoria6, Leiden University7, Institut d'Astrophysique de Paris8, University of Oxford9, Spanish National Research Council10, Academia Sinica11, Shanghai Normal University12, SLAC National Accelerator Laboratory13, Stanford University14, California Institute of Technology15, University College London16
TL;DR: In this article, weak gravitational lensing was used to analyze the dark matter haloes around satellite galaxies in galaxy groups in the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) data set.
Abstract: We use weak gravitational lensing to analyse the dark matter haloes around satellite galaxies in galaxy groups in the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS) data set. This data set is derived from the Canada–France–Hawaii Telescope Legacy Survey Wide survey, and encompasses 154 deg^2 of high-quality shape data. Using the photometric redshifts, we divide the sample of lens galaxies with stellar masses in the range 10^(9)–10^(10.5) M_⊙ into those likely to lie in high-density environments (HDE) and those likely to lie in low-density environments (LDE). Through comparison with galaxy catalogues extracted from the Millennium Simulation, we show that the sample of HDE galaxies should primarily (∼61 per cent) consist of satellite galaxies in groups, while the sample of LDE galaxies should consist of mostly (∼87 per cent) non-satellite (field and central) galaxies. Comparing the lensing signals around samples of HDE and LDE galaxies matched in stellar mass, the lensing signal around HDE galaxies clearly shows a positive contribution from their host groups on their lensing signals at radii of ∼500–1000 kpc, the typical separation between satellites and group centres. More importantly, the subhaloes of HDE galaxies are less massive than those around LDE galaxies by a factor of 0.65 ± 0.12, significant at the 2.9σ level. A natural explanation is that the haloes of satellite galaxies are stripped through tidal effects in the group environment. Our results are consistent with a typical tidal truncation radius of ∼40 kpc.
University of Bonn1, University of Edinburgh2, University of British Columbia3, Leiden University4, University of Victoria5, Institut d'Astrophysique de Paris6, University of Oxford7, Spanish National Research Council8, Academia Sinica9, Shanghai Normal University10, Perimeter Institute for Theoretical Physics11, University of Waterloo12, University College London13, California Institute of Technology14, Stanford University15
TL;DR: In this paper, the authors present the first direct measurement of the galaxy-matter bispectrum as a function of galaxy luminosity, stellar mass and type of spectral energy distribution.
Abstract: We present the first direct measurement of the galaxy–matter bispectrum as a function of galaxy luminosity, stellar mass and type of spectral energy distribution (SED). Our analysis uses a galaxy–galaxy–galaxy lensing technique (G3L), on angular scales between 9 arcsec and 50 arcmin, to quantify (i) the excess surface mass density around galaxy pairs (excess mass hereafter) and (ii) the excess shear–shear correlations around single galaxies, both of which yield a measure of two types of galaxy–matter bispectra. We apply our method to the state-of-the-art Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS), spanning 154 square degrees. This survey allows us to detect a significant change of the bispectra with lens properties. Measurements for lens populations with distinct redshift distributions become comparable by a newly devised normalization technique. That will also aid future comparisons to other surveys or simulations. A significant dependence of the normalized G3L statistics on luminosity within −23 ≤ Mr ≤ −18 and stellar mass within 5 × 10^9 M_⊙ ≤ M* ≤ 2 × 10^(11) M_⊙ is found (h = 0.73). Both bispectra exhibit a stronger signal for more luminous lenses or those with higher stellar mass (up to a factor of 2–3). This is accompanied by a steeper equilateral bispectrum for more luminous or higher stellar mass lenses for the excess mass. Importantly, we find the excess mass to be very sensitive to galaxy type as recently predicted with semi-analytic galaxy models: luminous (M_r < −21) late-type galaxies show no detectable signal, while all excess mass detected for luminous galaxies seems to be associated with early-type galaxies. We also present the first observational constraints on third-order stochastic galaxy biasing parameters.
Posted Content•
TL;DR: In this article, the authors focus on the period when groups and clusters assembled (0.5
Abstract: Major astrophysical questions related to the formation and evolution of structures, and more specifically of galaxy groups and clusters, will still be open in the coming decade and beyond: what is the interplay of galaxy, supermassive black hole, and intergalactic gas evolution in the most massive objects in the Universe - galaxy groups and clusters? What are the processes driving the evolution of chemical enrichment of the hot diffuse gas in large-scale structures? How and when did the first galaxy groups in the Universe, massive enough to bind more than 10^7 K gas, form? Focussing on the period when groups and clusters assembled (0.5
10 Jun 2013
TL;DR: In this article, it was shown that the universe is extremely violent and in constant motion and that the whole universe is in accelerating expansion and that humans are searching urgently for its origin.
Abstract: 1 The Universe was always the final frontier of the Human quest for knowledge Through all its history, humanity has observed the sky trying to understand the Cosmos outside the limits of our planet Today, this effort has yielded significant results. Now we know that our sun is a typical star, which does not differ significantly from the other stars of the starry sky. We have discovered the planets of our Solar System and we have studied the conditions prevailing in them. We studied asteroids and comets and found their important role in the formation of planets. We understand the basic principles of the formation, the life and the death of stars. We have also discovered thousands of exoplanets orbiting other stars. We studied giant star clusters. We have discovered dense clouds of interstellar dust and gas where new stars are born continuously. We have managed to describe the gigantic complex of stars to which we belong. Our Galaxy. We realized that our Galaxy is not alone in the universe and that there are hundreds of billions of galaxies. We discovered that the universe of galaxies is extremely violent and in constant motion. Finally we found that the whole universe is in accelerating expansion and we are searching urgently for its origin. This quest is an epic journey towards knowledge, which abolish superstitions and defines human existence.