1,720,959 research outputs found
Searching for High-redshift Galaxies in Hubble Space Telescope Deep Data
Full text linkThe history of our Universe spans 13.7 billions of years and could be divided into several stages from the Big Bang up to now.
Around 370 000 years after the Big Bang (z~1100) the temperature of the Universe low- ered enough for the first simple atoms to form. Matter and radiation decoupled, and the Universe became transparent to radiation. Cosmic microwave background (CMB) photons that we detect nowadays were last scattered at z~1100 and, since then, have been traveling in straight line. This is the reason why CMB is usually defined as the picture of the Universe at that redshift. Right after the CMB was emitted, the Universe entered the so called Dark Ages when no sources of light exist. Studying the early Universe, one of the most important phases is the subsequent phase-transition named reionization, i.e. the process that reionized the matter in the Universe after the formation of the first sources of light, namely the first stars and galaxies. Consequently, the detection and study of these objects are the key to unveil the early stages of the history of the Universe.
Imaging plays a more important role than spectroscopy in searching for high-redshift galaxies because it permits to observe more objects at the same time, better managing the telescope time. Deep surveys, obtained by observing the same sky area for several days, are the answer to the need for detections of high-redshift galaxies.
This thesis is focused on the study of the galaxy population existing when the Universe was less than 1.5 Gyr old. When studying the early Universe, the detection of high-redshift sources depends strongly on the detection limit of the survey and the surface brightness of the objects themselves. Taking this into account, we made use of the deepest datasets currently available obtained with the Hubble Space Telescope (HST) in both the optical and near-infrared (NIR) domain to carefully study how these two issues affect the identification and photometry of high-redshift galaxies.
The important role played by high-redshift galaxies in cosmic reionization is no longer debated and, lately, most of studies agreed on the key relevance of galaxies that are below the current detection limit. While we are waiting for the James Webb Space Telescope (JWST) to directly observe these faint galaxies, finding an alternative way to estimate their overall light contribution is mandatory. To this aim we developed a technique based on the power spectrum to analyze background fluctuations. Relying on a Lyman break-like approach we compared the power spectra of background signal derived from observations obtained in two adjacent bands to identify the light contribution from a population of galaxies lying within a specific redshift range. Then, Monte Carlo simulations permitted us to disentangle the information embedded in the light excess identified via power spectra, in particular deriving a constraint on the faint-end slope of the luminosity function.
The UDF05 dataset, follow-up of the original Hubble Ultra Deep Field (HUDF), consists in observations in the optical bands obtained with the Advanced Camera for Survey (ACS). It permitted us to constrain the slope of the luminosity function at z~6 (0.95 Gyr after the Big Bang), which turned out the be steep enough to allow bright and faint galaxies at that redshift to account for the ionizing photon budget required for cosmic reionization.
The subsequent analysis aimed at deriving similar constraints on the faint-end slope of the luminosity function at z~7-8 (between 0.64 and 0.77 Gyr after the Big Bang) using deep observations in the near-infrared obtained with the infrared channel of the Wide Field Camera 3 (WFC3-IR) during the HUDF09 program.
Regarding z ∼ 8, the quality of the NIR dataset did not permit to disentangle any light produced by the faint galaxy population from the background noise and spurious signals. On the basis of the drop in the star formation rate density from z~6 to z~7 and beyond, there should be a more relevant contribution in terms of photoionizing photons at z~7 than at z~8 and we expected to be able to detect it. Unfortunately, the analysis at z~7 implied dealing with different detectors that are characterized by systematics that can not be erased by simply considering the ratio of the power spectra. Up to now the understanding of all WFC3/IR related problems is not as good as for ACS and a fur- ther analysis is needed before being able to use the IR dataset for the analysis of surface brightness fluctuations.
Since a perfect reduction procedure of the images turned out to be an essential requirement to study any background signal, we performed an advanced data reduction to get an improved version of the deepest image of the Universe currently available, the so called eXtreme Deep Field (XDF). The goal was to create an image that allows to verify our findings on the faint-end slope of the luminosity function at z~6 since the XDF did not permit us to get any constraint on background fluctuations. We started from raw frames obtained from several proposals over 10 years and created hyperbiases and hyperdarks taking into account all the issues affecting ACS data, including the minor ones such as the herringbone effect. Then, we masked the satellite trails, aligned all the frames, and corrected for the chip-to-chip jump. We are still working on the dataset, in particular we are focused on modelling and correcting for the electronic ghost. Anyway, the preliminary check on photometry suggests a promising, even though small, achievement in term of signal to noise of the sources.
The effect of surface brightness on the detection of primordial galaxies in deep surveys is directly depending on the cosmological surface brightness dimming that can be express in the form (1 + z)−4 and that affects all the sources. The strong dependence of surface brightness dimming with increasing redshift suggests the presence of a selection bias when searching for high-redshift galaxies, i.e. we tend to detect only those galaxies with a high surface brightness. Unresolved knots of emission are not affected by surface brightness dimming, thus allowing, in principle, to test clumpiness within high-redshift galaxies. We followed an empirical approach based on HST legacy datasets characterized by different depth to study the surface brightness dimming of galaxies. We selected a sample of Lyman-break galaxies at z~4 (1.5 Gyr after the Big Bang) detected in the XDF, HUDF, and the Great Observatories Origins Deep Survey (GOODS) datasets and found no significant trend when comparing the total magnitudes measured from images with different depth. Then, we compared our results to the prediction for mock sources derived from Monte Carlo simulations. In particular, considering different surface brightness profiles for the mock galaxies we were able to rule out all the extended profiles as fit for our data, getting a confirmation on the clumpy distribution of the light in high-redshift galaxies.
The study of cosmological surface brightness dimming is also important since it could affect our prediction of what the upcoming JWST can observe at higher redshifts, where younger galaxies may exhibit a larger fraction of clumpiness. Our direct comparison showing that galaxies detected in GOODS do not become significantly brighter in the HUDF suggests that most of their light is compact and hints to the fact that JWST will likely not find diffuse star forming components.
Finally, to complete the study on high-redshift galaxies we also focused on lower-redshift galaxies that could enter the high-redshift sample due to photometric scatter. In general
interlopers are galaxies at z~1-2 showing colors similar to those of real dropout galaxies due to the 4000 A break. Even though their colors are likely to include them in the dropout sample, contaminants have a non negligible detection in the bands blueward of the Lyman-break.
The preliminary study we performed using the multi-wavelength catalog obtained from CANDELS GOODS-South shows that the number counts of contaminants are significantly different from those of dropout galaxies at z~5-6 suggesting a clear difference in the luminosity functions of the two populations and little or no evolution in the population of interlopers entering the sample at different redshifts. Finally, we used the 3D-HST catalogs for the GOODS-South field that provided us with photometric data in ground based, HST, and Spitzer/IRAC bands as well as with photometric redshfits. This catalog allowed a study on the interlopers at z~4-5
THE EFFECT OF SURFACE BRIGHTNESS DIMMING IN THE SELECTION OF HIGH-zGALAXIES
No full textCosmological surface brightness (SB) dimming of the form (1 + z)−4 affects all sources. The strong dependence
of SB dimming on redshift z suggests the presence of a selection bias when searching for high-z galaxies, i.e., we
tend to detect only those galaxies with a high SB. However, unresolved knots of emission are not affected by SB
dimming, thus providing a way to test the clumpiness of high-z galaxies. Our strategy relies on the comparison
of the total flux detected for the same source in surveys characterized by different depth. For all galaxies, deeper
images permit the better investigation of low-SB features. Cosmological SB dimming makes these low-SB features
hard to detect when going to higher and higher redshifts.We used the GOODS and HUDF Hubble Space Telescope
legacy data sets to study the effect of SB dimming on low-SB features of high-z galaxies and compare it to the
prediction for smooth sources. We selected a sample of Lyman-break galaxies at z ∼ 4 (i.e., B435-band dropouts)
detected in all of the data sets and found no significant trend when comparing the total magnitudes measured
from images with different depth. Through Monte Carlo simulations we derived the expected trend for galaxies
with different SB profiles. The comparison to the data hints at a compact distribution for most of the rest-frame
ultraviolet light emitted from high-z galaxies
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Stellar population properties for a sample of hard X-ray AGNs
No full textAims: The aim of this paper is to study the stellar population of galaxies hosting an active galactic nucleus (AGN). We studied a subsample of hard X-ray emitting AGNs from the INTEGRAL and Swift catalogs, which were previously identified and characterized through optical spectroscopy. Our analysis provides complementary information, namely age and metallicity, which is necessary to complete the panoramic view of these interesting objects.
Methods: We selected hard X-ray emitting objects, identified as AGNs, by checking their optical spectra in search of absorption lines suitable for the stellar population analysis. We obtained a final sample consisting of 20 objects with a redshift lower than 0.3. We used the full-spectrum fitting method; particularly, we use penalized pixel method and apply the PPXF code. After masking all the regions affected by emission lines, we fitted the spectra with the MILES single stellar population templates, and we derived mass-weighted ages and metallicities.
Results: Most of the objects in our sample show an old stellar population; however, three of them are characterized by a bimodal distribution with a non-negligible contribution from young stars. The values of the mass-weighted metallicity span a wide range with most of them slightly above the solar value. No relations between the stellar population properties and the morphological ones have been found
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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