Thèses sur le sujet « Foresta Lyman-alpha »

2006/2007
The aim of this PhD thesis is the study of the properties of the Inter-Galactic Medium (IGM) through Quasi Stellar Objects (QSOs) absorption lines at redshifts z~2-4. The study of the IGM provides a unique picture of the Universe in the early phases of structure formation and is crucial to test cosmological models. Furthermore, the metal content of the IGM can be used to constrain the nature of the enrichment mechanisms at high redshifts. Two main mechanisms have been proposed predicting different distribution of metals in the IGM: a late enrichment, due to the observed galaxies at redshift ~3, and an early enrichment, due to a population of very massive first stars (the so called popIII stars), not yet observed, at higher redshift. The existing observations are not sufficient to discriminate between the two mechanisms. QSO spectra show interesting features to study the IGM: 1. The Lyman-alpha (Lya) forest : the forest of absorption lines blue-ward of the Lya emission arising in large scale neutral hydrogen density fluctuations of moderate amplitude in the warm photo-ionized IGM. Since the dynamical state of the low density IGM is governed mainly by the Hubble expansion, gravitational instabilities and photo-ionization, the involved physics is quite simple and mildly non-linear. The Lyman forest is then a fair tracer of the underlying matter density field. 2. Metal absorptions: many absorption features due to ionic transitions in chemical elements heavier than He (``metals'') are clearly present red-ward of the Lya emission. Some of them are associated with the QSO itself but the majority are tracers of intervening metals belonging to structures of different nature, from diffuse gas to galaxies. Chapter 1 of the thesis presents in detail how the physics of the IGM can be understood studying QSO absorption lines, and the classical results obtained in this field. Then the thesis focuses on the work done during the PhD, which has addressed three research areas: 1. Study of lines of sight (LOSs) to isolated QSOs (Chapter 2,3): (i) a sample of 22 high-resolution QSO spectra has been studied. Classical statistics have been applied: all the absorption features in the spectra have been fit with Voigt profiles to extract physical parameters associated with the absorbing material; quantities like the evolution in redshift of the number density of the lines and the two-point correlation function of the line distribution along the spectra have been studied and compared with measurements available in the literature; (ii) a new method to analyze the Lya forest has been implemented. Traditionally, absorption spectra are resolved in a collection of discrete absorption systems. This method instead reconstructs the underlying density field processing the lines on the basis of the physical properties of the IGM. Therefore, a continuous density field is built and the main drawbacks of the Voigt fitting approach are overcome. (iii) the new algorithm has been tested with N-body hydrodynamical simulations of the IGM; (iiii) the new algorithm has been applied to the observational data sample to study the hydrogen density field ( its evolution with redshift and its clustering properties ) and the so called proximity effect of QSO, estimating the overdensity around the object. 2. Study of lines of sight to multiple QSOs (Chapter 4,5): The thesis presents also the work done studying multiple QSO LOSs, i.e. studying the properties of the IGM not only with the spectrum of a single object, by trying to get transversal informations comparing different LOSs, close both in angular separation and in the emission redshift of the source. A sample of 15 QSO forming 21 pairs have been studied to get the transverse correlation function, using the statistics of the transmitted flux of the objects. Furthermore, we have observed a pair of close QSO during two nights (7-8 august 2007) with UVES, the high resolution spectrograph at the ESO Very Large Telescope (VLT) located at Cerro Paranal, in Chile. These two objects have been observed to study the correspondence between metal absorptions and galaxies in the field; preliminary results and the description of the work in progress are presented in detail in the thesis (Chapter 5). 3. New instrumentation (Chapter 6): Looking ahead to new possibilities of advance in this field of research, an important role will be played by the high sensitivity and medium resolution spectrograph X-shooter. It will receive first light at the ESO VLT in July 2008 and will start operating in early 2009. When in operation, its wide spectral-range observing capability will be unique at very large telescopes and extremely relevant for the study of QSO spectra. The thesis describes the Science Case "Tomography of the IGM" for the instrument, and the work done at ESO within the X-shooter project. For the operation of this instrument we have carried out laboratory measurements of calibration sources for the Near-InfraRed arm and I participated to an observational project to build a spectro-photometric flux catalogue of standard stars for the instrument.
Scopo di questa tesi di dottorato e` lo studio delle proprieta` del mezzo intergalattico (IGM) attraverso le righe in assorbimento negli spettri di quasar (QSO) osservate a redshift z~2-4. Lo studio dell'IGM e` di fondamentale importanza per verificare modelli cosmologici in quanto rende possibile l'osservazione dell'universo nelle prime fasi del processo di formazione delle strutture cosmiche. Inoltre, le tracce di elementi chimici piu` pesanti dell'He (detti ``metalli'') contenuti nell'IGM possono essere utilizzate per comprendere i meccanismi di arricchimento metallico dell'universo ad alti redshift. La distribuzione osservata di questi metalli e` al momento oggetto di un intenso dibattito all'interno della comunita` scientifica; recentemente sono stati proposti due scenari per spiegare questa distribuzione: il cosiddetto di ``late enrichment'', per cui i metalli osservati nell'IGM sarebbero dovuti principalmente alle galassie osservate a redshift ~3, e l'``early enrichement'', per cui una popolazione di stelle massicce (``popIII stars'', non ancora osservate) a redshift ancora maggiore, sarebbe responsabile della maggior parte di metalli presenti nell'IGM. Al momento le osservazioni non sono sufficienti per falsificare o confermare i meccanismi proposti.L'IGM viene studiato attraverso l'analisi di spettri di QSOs ed in particolare: 1. La foresta Lyman-alpha (Lya): l'insieme delle righe in assorbimento che si osservano a lunghezze d'onda minori rispetto all'emissione Lya negli spettri di QSOs; questi assorbimenti sono dovuti a piccole fluttuazioni su larga scala del campo di idrogeno neutro presente nell'IGM. La fisica che descrive l'IGM e` relativamente semplice. Il gas si trova in regime lineare o moderatamente non-lineare ed in equilibrio di fotoionizzazione. La sua dinamica, alla densita` media, e` dominata dall'espansione di Hubble e dall'instabilita` gravitazionale, percio` la foresta Lya risulta essere un buon candidato per la descrizione del campo di densita` presente a questi redshift. 2. Gli assorbimenti metallici: a lunghezze d'onda maggiori dell'emissione Lya sono chiaramente presenti molti assorbimenti dovuti a transizioni di ioni metallici. Alcuni di essi hanno origine in gas associato al QSO stesso, ma la maggior parte di questi assorbimenti riflette la presenza di sistemi metallici lungo la linea di vista tra l'oggetto e l'osservatore. Questi sistemi non associati sembrano essere dovuti ad una grande varieta` di strutture, da componenti di gas diffuso fino alle galassie di campo. Il capitolo 1 della presente tesi descrive nel dettaglio come si possa comprendere la fisica dell'IGM studiando i sistemi in assorbimento degli spettri di QSO, presentando i principali risultati ottenuti tradizionalmente in questo campo. Sucessivamente la tesi si concentra sul lavoro svolto durante il periodo di dottorato, ed in particolare sulle tre principali aree di ricerca affrontate: 1. Studio di linee di vista (LOSs) verso QSO singoli (Capitoli 2,3): (i) e` stato analizzato un campione di 22 spettri di QSO ad alta risoluzione, al quale sono state applicate le statistiche tradizionali: tutti gli assorbimenti Lya negli spettri sono stati fittati con profili di Voigt, al fine di ottenere parametri fisici associati ai sistemi di assorbimento; in particolare sono state studiate, e confrontate con risultati ottenuti in passato, quantita` come l'evoluzione in redshift della densita` in numero di righe e la funzione di correlazione a due punti; (ii) e` stato sviluppato un nuovo metodo per analizzare la foresta Lya. A differenza dell'approccio tradizionale, dove da uno spettro di QSO si ottiene semplicemente un insieme discreto di assorbitori, questo metodo ricostruisce il campo di densita` di idrogeno responsabile degli assorbimenti partendo da assunzioni sulla fisica dell'IGM. Il risultato e` un campo di densita` continuo la cui analisi permette di risolvere alcuni problemi introdotti invece dalla classica analisi di profili di Voigt. (iii) il nuovo algoritmo e` stato testato con simulazioni idrodinamiche ``N-body'' dell'IGM; (iiii) il nuovo algoritmo e` stato applicato ai dati osservativi citati in precedenza per studiare il campo di densita` di idrogeno (la sua evoluzione con il redshift e la funzione di correlazione a due punti) ed il cosiddetto ``effetto di prossimita`'', portando ad una stima della sovradensita` del campo nella regione dove si trova un QSO. 2. Studio di linee di vista verso sistemi di QSOs (Capitoli 4,5): la tesi presenta anche uno studio di linee di vista multiple; il confronto di diverse linee di vista, vicine in separazione angolare e in redshift di emissione, permette di avere informazioni non solo sulle regioni di universo tracciate dalle linee di vista, ma anche sulle regioni tra le linee di vista stesse. A questo riguardo e` stata studiata la ``funzione di correlazione trasversale'' ottenuta dall'analisi del flusso trasmesso da un campione di 15 QSO che formano 21 coppie. Per lo studio di linee di vista multiple e` stata osservata una coppia di QSO per due notti (7-8 agosto 2007); le osservazioni sono state fatte con lo spettrografo ad alta risoluzione UVES, al ``Very Large Telescope'' (VLT), situato al Cerro Paranal, in Cile. Questi oggetti sono stati osservati per studiare le corrispondenze tra assorbimenti metallici e galassie di campo. Il lavoro fatto ed i risultati preliminari ottenuti fino ad ora sono presentati nel capitolo 5. 3. Nuova strumentazione (Capitolo 6): Un ruolo importante per il progresso della scienza dell'IGM verra` svolto dal nuovo spettrografo a media risoluzione ed alta` sensibilita`, X-shooter. X-shooter ricevera` la prima luce all'osservatorio ESO VLT in luglio 2008 per poter essere in pieno funzionamento agli inizi del 2009. L'ampio intervallo spettrale coperto e le caratteristiche di questo strumento sono particolarmente vantaggiose per lo studio di spettri di QSOs. Nella tesi viene descritto il caso scientifico ``Tomography of the IGM'' per lo strumento, ed il lavoro svolto all'ESO nel contesto del progetto X-shooter. Questo lavoro include le misure di laboratorio di sorgenti di calibrazione per il braccio nel vicino infrarosso dello strumento e la partecipazione al progetto osservativo per la costruzione di un catalogo spettro-fotometrico di stelle standard per la calibrazione assoluta in flusso degli oggetti che si osserveranno con X-shooter.
XX Ciclo
1980

In this thesis we investigate the large-scale distribution of Ly alpha forest absorption, the effect of ionizing radiation from QSOs on their surrounding intergalactic medium and the primordial abundance of deuterium. We develop a new technique for detecting structure on Mpc scales in the Ly alpha forest. This technique does not rely on identifying individual absorption lines but is rather based on the statistics of the transmitted flux. We demonstrate that the new method is significantly more sensitive to the presence of large-scale structure in the Ly alpha forest than a two-point correlation function analysis. We apply this method to 2 A resolution spectra of ten QSOs which cover the redshift range 2.2 < z < 3.4. The QSOs form a closely spaced group on the sky and are concentrated within a 1-deg^2 field. We find evidence for large-scale structure in the distribution of Ly alpha forest absorption at the > 99 per cent confidence level. Along the line of sight we find overdense Ly alpha absorption on scales of up to 1200 km s^-1. There is also strong evidence for correlated absorption across line of sight pairs separated by < 3 h^-1 Mpc. For larger separations the cross-correlation signal becomes progressively less significant. Using the same technique and dataset we confirm the existence of the proximity effect. We derive a value for the mean intensity of the extragalactic background radiation at the Lyman limit of J = (3.6^+3.5_-1.3) x 10^-22 ergs s^-1 cm^-2 Hz^-1 sr^-1. This value assumes that QSO redshifts measured from high ionization lines differ from the true systemic redshifts by Delta v = 800 km s^-1. Allowing for known QSO variability we find evidence at a level of 2.1 sigma that the significance of the proximity effect is correlated with QSO Lyman limit luminosity. From the complete sample we find no evidence for the existence of a foreground proximity effect, implying either that J > 20 x 10^-22 ergs s^-1 cm^-2 Hz^-1 sr^-1 or that QSOs emit at least a factor of 1.4 less ionizing radiation in the plane of the sky than along the line of sight to Earth. We do, however, find one counter-example where a foreground QSO apparently depletes the absorbing gas in four surrounding lines of sight. We discuss the feasibility of pre-selecting absorption systems from low resolution data for a measurement of the primordial deuterium abundance. We present a new, low resolution spectroscopic survey of 101 high redshift QSOs aimed at identifying candidate D/H systems. We further present an echelle spectrum of a Lyman limit system at z = 2.917. We find that this system is most likely heavily contaminated and does not yield an interesting limit on D/H.

We have observed two high -redshift quasars with the echelle spectrograph and 2D- Frutti Photon Counter at Las Campanas, in order to investigate the statistical properties of the Lyman -a forest. The two-point correlation function for the Lyman -a forest lines at z .^s 3 is consistent with zero, for all velocity splittings A > 50 km /sec. When Lyman -a lines and other metal lines from known metalline systems are included, the correlation function shows a weak non -zero signal at small A . We suggest that the weak clustering of the Lyman -a forest detected by other workers may be the result of contamination by a small number of metal -line systems and their associated Lyman -a lines.

The Lyα forest is a series of absorption lines seen in quasar spectra and is a powerful tool for probing the thermal state of the intergalactic medium (IGM) across a wide redshift range. At intermediate redshifts (2< z <5), the statistical properties of the Lyα forest predicted by recent hydrodynamical simulations are in good agreement with a range of spectroscopic data. However, at lower and higher redshifts this is still not the case. Some of the key questions still challenging our understanding at low redshifts are the nature of absorbers, the evolution of the ultraviolet background and the impact of feedback from supernovae and active galactic nuclei (AGN). Furthermore, as a range of reionsation models remain unconstrained and the precise timing of reionsation remains elusive, high redshift Lyα forest data can provide valuable insight due to its sensitivity to the post-reionsation thermal properties of the IGM. At low redshift, this investigation focuses on understanding the effect of different feed-back prescriptions on the properties of the Lyα forest using simulations from the Sherwood simulation suite. The simulations incorporate three different prescriptions for treating cold dense gas and galactic feedback from supernovae and AGN. These implementations have only a small effect on the properties of the Lyα column density distribution function (CDDF) and Lyα line velocity width distribution. Therefore, feedback does not solve the discrepancy between the Cosmic Origins Spectrograph (COS) observations of the CDDF at z≃0.1 for high column density systems (NHI > 1014cm−2), as well as the line width distribution, which has lines broader then the simulation data. Some of the possible solutions may be feedback that ejects more overdense gas into the IGM, an increase in the temperature of the overdense gas (which is rather fine-tunedso that the gas is able to still contribute to the Lyα forest), or an unresolved turbulentin the IGM. The low redshift Lyα forest investigation is concluded by performing a series of numerical convergence tests on the quantities most widely employed in absorption line studies at low redshift. The mass resolution of the simulations can significantly impact on the estimated line velocity widths, by overestimating line widths for low mass resolution runs. By contrast, the Lyα CDDF is quite well converged for low column density absorbers. At higher redshifts, a feasibility analysis to constrain the thermal history of the IGM using cosmological hydrodynamical simulations of the Lyα forest is performed. This problem is approached by utilising the Lyα forest transmitted flux power spectrum at z∼5. The integrated heating during reionsation has a measurable impact on the power spectrum. The integrated heating is parameterised using the cumulative energy per proton deposited into a gas parcel at the mean background density. A Markov Chain Monte Carlo approach is used to recover the cumulative energy per proton with a statistical uncertainty of ∼ 20 per cent (at 68 per cent confidence interval), by making assumptions consistent with current observational data sets. However, systematics may increase the uncertainty to ∼ 30 per cent at these redshifts. This method can distinguish between early (z = 12) and late (z= 7) reionisation in the simulations. Finally, to expand on this investigation, the first constraints on the cumulative energy per proton using recent Lyα flux power spectrum measurements at high redshift are obtained. A consistent picture of galaxy driven reionsation with reionsation occurring at z∼9 is found.

We present evidence for redshift quantization in the Lyman -a forest of several QSOs. The Ly -a data are at redshifts z from 1.89 to 3.74, and the theory of redshift quantization proposed by Cocke (1983, 1085) is used to scale the quantization interval (24.15 km s -') to these high redshift. The sealing depends on the deceleration parameter qo, and the quantization is present at a statistical significance of greater than 99% for qo = 1/2. This may be taken as confirming the inflationary model of the early history of the universe. The significance of the quantization is highest at go rs 0.48, and the width of the peak is about 0.03 . The result can also be seen as providing confirmatory evidence for both the theory of the redshift quantization and the above value of qo, but at a significance of only 03 %. The scenario proposed for the relativistic generalization of the theory is that of fermion wavefunctione and quantum operators in a background Riemannian spacetime satisfying Einstein's field equations.

This dissertation addresses two cosmological applications of the Lyman-alpha (Ly ɑ) forest observed in QSO pairs separated by several arcminutes or less. The Ly ɑ flux autocorrelation and cross-correlation provide a measurement of cosmic geometry at z > 2, via a variant of the Alcock-Paczyński test. I present the results of an observing campaign to obtain moderate resolution spectroscopy of the Ly ɑ forest in QSO pairs with small redshift differences (Δz < 0.25) and arcminute separations (θ < 5'). This new sample includes 29 pairs and one triplet suitable for measuring the cross-correlation and 78 individual QSO spectra for determining the autocorrelation. Continuum fits are provided, as are seven revisions for previously published QSO identifications and/or redshifts. Using a suite of hydrodynamic simulations, anisotropies in the Ly ɑ flux correlation function due to redshift-space distortions and spectral smoothing are investigated for 1:8 ≤ z ≤ 3, further enabling future applications of the Alcock-Paczyński test with Ly ɑ correlation measurements. Sources of systematic error including limitations in mass-resolution and simulation volume, prescriptions for galactic outflow, and the observationally uncertain mean flux decrement are considered. The latter is found to be dominant. An approximate solution for obtaining the zero-lag cross-correlation for arbitrary spectral resolution is presented, as is a method for implementing the resulting anisotropy corrections while mitigating systematic uncertainty.

El Medi Intergalàctic permet una caracterització de propietats bàsiques de l'univers tals com paràmetres fonamentals del model cosmològic, el creixement de pertorbacions primordials de densitat i formació de galàxies. Aquesta caracterització s'assoleix mitjançant l'absorció de la llum per part del Medi Intergalàctic a l'espectre d'espectres d'objectes llunyans i lluminosos. Tot això ens motiva considerar dos aspectes del Medi Intergalàctic: primerament utilitzarem simulacions numèriques i models teòric per comprendre les complexitats de la evolució no lineal del medi intergalàctic. D'altra banda l'ús d'un extens catàleg de sistemes d'absorció en espectres quasars proveïdes pel sondeig astronòmic anomenat Bariònic Oscillation Spectroscopic Survey (BOSS). Per una part cal entendre tots els processos complexos que ocorren al medi intergalàctic, per això és necessari l'ús de simulacions hidrodinàmiques que permeten el seu modelat. S'ha procedit a extraure l'espectre de potències de la fracció transmesa a través de l'hidrogen atòmic d'un gran nombre de simulacions cosmològiques. Aquest espectre de potencies s'ha descrit acuradament amb una model teòric, el qual amb pocs paràmetres és capaç de ajustar les dades i permet fer prediccions i comparacions. En particular paràmetre de biaix que determina l'amplitud de l'espectre de potencies del bosc Lyα i distorsió deguda al redshift han segut fetes per a nombroses simulacions amb diferents cosmologies i propietats físiques. Per l'altra part el catàleg de dades de BOSS que disposa de més de trenta-mil sistemes d'alta absorció, o sistemes Lyα esmorteïts (DLAs, sigles en anglès), permet l'estudi detallat d'aquests. Els DLAs són denses reserves de gas clau per a la formació de galàxies i permeten traçar propietats cosmològiques. Aleshores s'ha procedit a fer un nou estudi en que es mesuren diverses amplades equivalents dels les línies d'absorció d'elements pesats associats al DLA. S'han combinat diverses línies a un nou paràmetre que s'anomenat “força metàl·lica”, la qual cosa permet obtenir informació d'una combinació de la dispersió de velocitats i metal·licitat per DLAs. Amb aquest mètode s'ha realitzat una classificació d'aquestos segons el nou paràmetre, que permet vore l'espectre conjunt per diferents poblacions, i així les seues propietats mitjanes. S'ha estudiat la evolució de la ”força metàl·lica” amb el redshift, que permetria una comparació amb models i simulacions. Finalment es podrà utilitzar per estudiar la variació del factor de biaix dels DLAs depenent de la nova parametrització.
The Intergalactic Medium (IGM), by means the Lyα absorption in quasar spectra ( this is called Lyα forest), is a unique tool allowing the study of the large scales of the universe, which in this way constrains several cosmological, the growth factor of the structure. Moreover the IGM also plays primordial role in galaxy formation. Understanding the physics of the IGM and the way non-linear processes affect the observed Lyα forest, hydrodynamical simulations are necessary because the non linear evolution of the physics cannot be treated in any analytic framework. At the same time, a large number of observations, which have been obtained from the Barionic Oscilation Spectrographic survey (BOSS) to be used in thesis allow studding in unprecedented large number of Lyα forest from quasar spectra, allowing to achieve major breakthroughs. In the 2nd chapter of this thesis we describe the methodology that we developed to extract the power spectrum from the transmission flux of the Hydrodynamical simulations. Using a set of simulations provided by various collaborators we study the effects of various simulation characteristics, such as resolution, box size, particles, and cell division, to see how they might affect the extracted power spectrum. Then we proceed to develop a model to fit the power spectrum of each of the simulations and in this way see the effects of different simulation and physical properties on the predictions from the power spectrum. Once the model fit has been obtained, the non linear power spectrum of the transmission can be related on the matter power spectrum, which on the form of its biased relation with the matter power spectrum contains the cosmological information. This relation is measured by the bias and redshift distortions that can be directly measured from simulations, making predictions for these values that could be later tested with the observations. In the 3rd chapter we use the data from the large catalogue of Damped Lyα systems (DLAs) provided by BOSS. These DLAs, broad regions of completely absorbed flux in the spectra of about 10% of the high redshift objects, the quasars. The DLAs are a large repository of neutral hydrogen, that is why they absorb all the flux that corresponds to their redshift. Since there is so much Neutral Hydrogen the absorption is damped, meaning that wavelength close to the one of the DLA also get absorbed, forming characteristic Lorenz wings that are sensitive to the column density of Hydrogen of the DLA. DLAs are interesting objects that are expected to be very relevant for galaxy formation, and also because they trace the large structure, therefore also being relevant for cosmology. They are, similarly to IGM of which they form part, a bridge between the large cosmological framework and the galaxies that populate the universe. The BOSS survey has allowed us for the first time have a set of thousands of DLAs. Given this amount of data and the interest of the DLAs, we developed a new method and a new parameterization to study the DLAs in the framework of large surveys such as BOSS. First we constructed a method to measure the equivalent width of several metal absorption lines associated with each DLA, even if individually they are not detected because they can not be observed due to the noise and resolution of the spectrum. The equivalent width are combined in a newly defined parameter called Metal Strength, which is a an optimal combination of the equivalent width of the metals measured for a DLA. This classification can then have many applications to measure the mean stack, mean bias and mean redshift evolution with respect this new parameter. The Metal Strength is therefore used to analyse the evolution and characteristics of the various populations of DLAs.

En els pròxims anys, la quantitat de dades observacionals disponibles per l’astrofísica en conjunt, i particularment la cosmologia, hi augmentarà en ordres de magnitud degut als nous programes observacionales i als avançaments tecnològics. Per aprofitar totalment aquestes observacions venideres, nous mètodes estadistics per a obtenir resultats científics han de ser desenvolupats i aplicats. Intensity Mapping és una d’aquests nous mètodes, que consisteix a traçar una línia d’emissió en coordenades angulars i redshift, sense considerar objectes resolts. Aquesta tesi hi està enfocada a la simulació i avaluació d’Intensity Mapping amb la línea Lyα fent la correlació creuada de dos tipus de surveys òptiques: la survey fotomètrica de banda estreta PAUS, i les surveys espectroscòpiques eBOSS i DESI. La primera hauria de contenir emissió Lyα estesa provinent del mitjà intergalàctic al fons de les imatges, mentres que les segones contenen el bosc Lyα (és a dir, el conjunt de línies d’absorció generades pel mitjà intergalàctic) en el seus espectres de quàsars. Al fer la correlació creuada d’ambdues, hauriem d’obtenir una senyal de l’emissió de Lyα difusa, a pesar dels objectes no correlacionats i el soroll a les imatges de PAUS. Per simular aquesta correlació creuada, una simulació hidrodinàmica dissenyada específicament per a l’estudi de Lyα al mitjà intergalàctic ha sigut emprada per simular tant l’emissió Lyα com l’absorció, mentres que els objectes no correlacionats a les imatges de PAUS han sigut modelats utilitzant un catàleg simulat profund al que s’han ajustat plantilles d’emissió espectral als seus objectes. El soroll instrumental i atmosfèric s’ha afegit a les imatges de PAUS simulades mesurant directament el soroll de conjunts d’imatges apilades i emmascarades, i s’ha introduït a la simulació com una distribució gaussiana. A més a més, un codi de correlació creuada ha estat desenvolupat des de zero, optimitzat i validat dins d’aquesta tesi per calcular les correlacions creuades. Els resultats es presenten de forma probabilística: per casos diferents (PAUS- eBOSS, PAUS-DESI i dues extensions hipotètiques de PAUS) han sigut calculades 1,000 correlacions creuades amb realitzacions diferents del soroll instrumental i atmosfèric de les imatges de PAUS, a més de les posicions dels quàsars. Amb l’aproximació optimista de soroll instrumental no correlacionat (el que requeriria més treball reduint les dades), fins l’extensió més llarga de PAUS que hem considerat té tan sols una probabilitat de detecció del 15%; si emprem el soroll correlacionat que s’observa a les imatges, totes les probabilitats passen a ser negligibles. A pesar d’aquests resultats negatius, podem extreure algunes conclusions valuoses. Els diferents tipus de funcions de correlació de dos punts que hem considerat (monopol, paral·lela i perpendicular a la línia de visió) mostren comportaments complementaris: el monopol i la paral·lela tenen un millor rendiment a escales més grans, mentres que la perpendicular sondeja millor les escales menors de 10 Mpc/h. A més a més, el suavitzat en redshift a la senyal de Ly-alpha degut als filtres fotomètrics ha sigut implementat de forma adecuada al model teòric, i fins i tot pot ser emprada per a predir les escales amb millor ràtio entre soroll i senyal per a cada funció de correlació. Per tant, aquests descobriments indiquen que aquesta metodologia pot ser més adequada per les surveys de banda ampla; concloem la tesi donant algunes directrius bàsiques sobre com aquest estudi es pot replicar per aquest tipus de dades.
En los próximos años, la cantidad de datos observacionales disponibles para la astrofísica en conjunto, y particularmente la cosmología, se incrementará en órdenes de magnitud. Para aprovechar totalmente estas próximas observaciones, se han de desarrollar nuevos métodos estadísticos para extraer resultados científicos. Uno de estos nuevos métodos es el Intensity Mapping, que consiste en trazar una línea de emisión en coordenadas angulares y redshift, sin considerar objetos resueltos. Esta tesis está enfocada en la simulación y evaluación del Intensity Mapping con la línea Lyman-alpha haciendo la correlación cruzada de dos tipos de surveys ópticas: la survey fotométrica de banda estrecha PAUS y las surveys espectroscópicas eBOSS y DESI. La primera debe contener emisión Lyman-alpha extensa proveniente del medio intergaláctico del fondo de las imágenes, mientras que las segundas contienen el bosque Lyman-alpha (el conjunto de líneas de absorción generadas por el medio intergaláctico) en los espectros de sus quásares. Al hacer la correlación cruzada de ambas, deberíamos obtener una señal de la emisión de Lyman-alpha difusa, a pesar de los objetos no correlacionados y el ruido en las imágenes de PAUS. Para simular esta correlación cruzada, se ha usado una simulación hidrodinámica especialmente diseñada para el estudio de Lyman-alpha en el medio intergaláctico para simular tanto la emisión como absorción Lyman-alpha, mientras que los objetos no correlacionados de las imágenes de PAUS han sido modelados usando un catálogo simulado profundo al que se le han ajustado plantillas de emisión espectrales a sus objetos. El ruido instrumental y atmosférico se ha añadido a las imágenes de PAUS simuladas midiendo directamente el ruido de conjuntos de imágenes apiladas y enmascaradas, y se ha introducido en la simulación como una distribución gaussiana. Además, se ha desarrollado, optimizado y validado un código de correlación cruzada desde cero dentro del marco de esta tesis, para calcular las correlaciones. Los resultados se presentan de forma probabilística: para los diferentes casos (PAUS-eBOSS, PAUS-DESI y dos extensiones hipotéticas de PAUS) se han calculado 1.000 correlaciones cruzadas con diferentes realizaciones del ruido instrumental y atmosférico, así como las posiciones de los quásares. Con la aproximación optimista del ruido instrumental no correlacionado (lo que requeriría más trabajo reduciendo los datos), incluso la extensión más larga de PAUS considerada tiene tan solo una probabilidad de detección del 15%; si empleamos el ruido correlacionado que se observa en las imágenes, todas las probabilidades pasan a ser despreciables. A pesar de estos resultados negativos, podemos extraer algunas conclusiones valiosas. Los diferentes tipos de de funciones de correlación de dos puntos (monopolo, paralela y perpendicular a la línea de visión) muestran comportamientos complementarios: el monopolo y la paralela tienen un mejor rendimiento a escalas más grandes, mientras que la perpendicular sondea mejor las escalas menores de 10 Mpc/h. Además, el suavizado en redshift de la señal Lyman-alpha debido a los filtros fotométricos se ha implementado de forma adecuada en el modelo teórico, e incluso puede ser utilizada para predecir las escalas con mejor ratio entre señal y ruido para cada función de correlación. Por tanto, estos descubrimientos indican que esta metodología puede ser más adecuada para las surveys de banda ancha; concluimos la tesis dando algunas directrices básicas de cómo este estudio se puede replicar para este tipo de datos.
In the upcoming years, the amount of observational data available for astrophysics as a whole, and particularly cosmology, is expected to increase by ordersof magnitude due to the new observational programs and technological advancements. To fully capitalise on these upcoming observations, new statistical methodsto draw scientific results have to be developed and applied. Intensity Mapping isone of such new methods, which consists on the tracing of a sharp spectral feature(e.g., a emission line) in angular coordinates and redshift, without considering resolved objects.This thesis is focused on the simulation and evaluation of Intensity Mappingwith the Lyα ; line by cross-correlating two kinds of optical surveys: the narrowband photometric survey PAUS, and the spectroscopic surveys eBOSS and DESI.The former should contain extended Lyα ; emission from the intergalactic medium inthe background of the images, while the latter contains the Lyα ; forest (i.e., the setof absorption lines generated by the intergalactic medium) in their quasar spectra.Cross-correlating both then should yield a signal of the diffuse Lyα ; emission, despitethe foregrounds and noise in PAUS images.In order to simulate this, a hydrodynamic simulation specifically designed forthe study of Lyα ; in the intergalactic medium has been used to simulate both Lyα ;emission and absorption, while the foregrounds in PAUS images have been modeled using a deep lightcone mock catalogue and fitting spectral energy distributions to its objects. Instrumental/atmospheric noise has been added to the simulated PAUS images by directly measuring the noise from sets of masked and stackedimages, and introducing it on the simulation as a Gaussian distribution. Besides, across-correlation code in Python has been developed from scratch, optimised andvalidated in the framework of the thesis to compute the cross-correlations.The results are presented in a probabilistic manner: for different cases (PAUSeBOSS, PAUS-DESI and two hypothetical PAUS extensions with DESI) 1,000 crosscorrelations are computed with different realisations of the instrumental/atmosphericnoise of PAUS images, as well as quasar positions. With the optimistic approximation of uncorrelated instrumental noise (which would require further work in datareduction), even the largest PAUS extension considered only yields a probability of detection of - 15%; using the actual correlated noise all the probabilities becomenegligible.In spite of these negative results, some valuable conclusions are extracted. Thedifferent kinds of two-point correlation functions (monopole, parallel and perpendicular to the line of sight) show complementary behaviours: monopole and parallel perform better at larger scales, while perpendicular samples better the scalessmaller than 10 Mpc/h. Besides, the redshift smoothing in the Lyα ; signal due to the photometric filters has been properly implemented in the theoretical model, andcan even be used to predict the scales with better SNR for each correlation function.Therefore, these findings indicate that this methodology may be more suitable forbroad-band surveys; we conclude the thesis giving some basic guidelines on howthis study could be replicated for broad-band data.
Universitat Autònoma de Barcelona. Programa de Doctorat en Física

Clues about the timing of reionization and the nature of the ionizing sources responsible are imprinted in the ionization and thermal state of the IGM. In this thesis, I use high-resolution quasar spectra in conjunction with state-of-the-art hydrodynamical simulations to probe the IGM at high redshift, focusing on the ionization and thermal state of the gas. After reionization, the ionization state of the IGM is set by the intensity of the ultraviolet background(UVB), quantified by the hydrogen photoionization rate, Γ_bkg. At high redshifts this has been estimated by measuring the mean flux in the Lyα forest, and scaling Γ_bkg in simulations such that the simulated mean flux matches the observed value. In Chapter 3 I investigate whether the precision of these estimates can be improved by using the entire flux probability distribution function (PDF) instead of only the mean flux. Although I find it cannot improve the precision directly, the flux PDF can potentially be used to constrain other sources of error in observational estimates of Γ_bkg, and so may increase the precision indirectly. The ionizing output of a quasar will locally dominate over the UVB, and this leads to enhanced transmission bluewards of the quasar Lyα line, known as the proximity effect. In Chapter 4 I present the first measurements of Γ_bkg at z > 5 from the proximity effect. The UVB intensity declines smoothly with redshift over 4.6 < z < 6.4, implying a smooth evolution in the mean free path of ionizing photons. This suggests that reionization ends at z > 6.4. There is a drop in Γ_bkg by roughly a factor of five, which corresponds to a drop in the ionizing emissivity by about a factor of two. Such a redshift evolution in the emissivity cannot continue to much higher redshift without reionization failing to complete, which suggests that reionization cannot have ended much higher than z = 6.4. Estimates of Γ_bkg from the proximity effect and the mean flux are generally discrepant at z ~ 2-4, with those from the proximity effect systematically higher. This is generally attributed to effects of the quasar environment. I investigate the significance of several environmental biases on proximity effect measurements at z ~ 5-6 in Chapter 5. The biases are found to be small, and so the proximity effect is expected to give relatively unbiased estimates of Γ_bkg at z > 5, in contrast to lower redshifts. Photoionization heats the gas in the IGM, and so the thermal history of the IGM provides important constraints on reionization. The thermal state of the IGM is reflected in the level of small-scale structure in the Lyα forest. In Chapter 6 I quantify the small-scale structure using two independent statistics, the curvature and the peakiness, and convert these into a temperature by comparing with simulations. These are the first measurements of the temperature in the general IGM at z > 5. Both statistics show an increase in the temperature by a factor of roughly two from z = 4.4 to 5.6. This rise is sensitive, however, to any smoothing of the gas density distribution due to the thermal history spanning reionization. I find that this should only be a small effect, as otherwise the corrected temperatures at z ~ 4-5 are implausibly low. The temperature evolution therefore suggests a late reionization. The temperatures at z ≥ 4.8 are well fit by an adiabatic cooling curve, for which reasonable peak temperatures at the end of reionization are reached at 6 ≲ z ≲ 7. The temperatures at z ~ 4-5 are consistent with reionization being carried out by Pop II stars. In conclusion, the ionization and thermal state of the IGM at z ~ 5-6 suggest a late hydrogen reionization, driven by star-forming galaxies and ending around 6.5 ≲ z ≲ 7. This is consistent with other recent lines of observational evidence, and supports theoretical models that infer a late reionization from the observed star formation rate history.

Bien que le modèle cosmologique standard LCDM offre un cadre remarquablement en accord avec de nombreuses observations indépendantes, beaucoup de mystères persistent. En particulier, la masse des neutrinos est toujours inconnue. De par leur masse non nulle, ils laissent une empreinte sur les grandes structures de l'univers à travers l'échelle à laquelle ils diffusent, qui se manifeste comme un déficit de fluctuations de densité de matière aux petites échelles. Par ailleurs, le modèle de matière noire froide montre des tensions persistantes avec les observations sur des échelles inférieures au Mpc. Dans ce travail de thèse, j'utilise le spectre de puissance de la transmission de flux dans la forêt Lyman-α dans des spectres de quasars lointains afin de contraindre la somme des masses des neutrinos et étudier la plausibilité d'un modèle de matière noire tiède. Dans un premier temps, j'ai mesuré le spectre de puissance 1D de la forêt Lyα à 13 redshifts tel que 2 ≤ z ≤ 4.25 en utilisant 43,751 spectres de quasars de haute qualité des programmes BOSS et eBOSS du relevé spectroscopique SDSS. Afin d'obtenir des résultats robustes sachant que les incertitudes statistiques ont été réduites d'un facteur 3 par rapport à la mesure précédente, j'ai identifié et contrôlé plusieurs effets systématiques dans l'analyse de données. Modéliser le flux Lyα implique de recourir à des simulations hydrodynamiques cosmologiques sachant qu'il émerge de la combinaison complexe entre l'évolution des grandes structures et de la physique baryonique des petites échelles. En effet, les processus astrophysiques tels que la formation stellaire et les phénomènes de retro-action de supernovae et des trous noirs rejettent une quantité considérable d'énergie dans le milieu environnant et modifient l'état thermique et la distribution du gaz dans le milieu intergalactique. Afin d'améliorer les prédictions théoriques de la forêt Lyα à un niveau comparable des données, je contrains l'impact des mécanismes de retro-action des trous noirs sur le spectre de puissance Lyα grâce à un jeu de 8 simulations que j'ai produit en me basant sur des observations astrophysiques et qui couvre l'ensemble des modèles de feedback plausibles. Je fournis une correction analytique de cet effet ainsi qu'une limite supérieure et inférieure tel que 2 ≤ z ≤ 4.25 et je montre qu'ignorer un tel mécanisme induit un biais de 2σ sur n_s et 1σ sur σ_8. Finalement, je combine les mesure des spectres de puissance Lyα avec des données CMB et BAO afin de les comparer statistiquement aux prédictions théoriques des simulations hydrodynamiques pour améliorer la contrainte sur la somme des masses des neutrinos de Σ mν < 0.12 eV établie précédemment à Σ mν < 0.09 eV dans le cas le plus extrême à 95% de vraisemblance. Ce résultat indique une préférence des données pour un modèle normal de hiérarchie de masse. La combinaison des données Lyα eBOSS avec les données Lyα XQ-100, contraint la masse des reliques thermiques à m_X > 5.3 keV à 95% de vraisemblance dans un cas de modèle de matière noire entièrement constitué de matière noire tiède, ce qui se traduit par une limite supérieure sur les neutrinos stériles produits de manière non-résonante à mν_s > 34 kev. Aussi, les données Lyα-eBOSS confirme une tension existante avec les données CMB sur l'indice spectral n_s et indique une préférence pour une dépendance d'échelle non nulle de n_s à 3σ
Even if the standard cosmological LCDM model provides a remarkably successful framework to explain many independent observations, it still faces many challenges. In particular, the masses of neutrinos are still unknown and significantly alter structure formation because of their free-streaming that suppresses density fluctuations below a typical length scale inversely proportional to their rest mass. In addition, the cold dark matter (CDM) scenario is in tension with observations on scales smaller than the Mpc. In this thesis work, I use the power spectrum of the transmitted flux in the Lyman-α forest of distant quasar spectra to constrain the sum of neutrino masses, Σ mν, and determine the plausibility of a warm dark matter model, which is conveniently consistent with cold dark matter predictions on large scales while circumventing its issues at small scales because of its non-negligible velocity dispersion. First I measure the 1D power spectrum of the Lyα forest of 43,751 high quality quasar spectra between 2 ≤ z ≤ 4.6 from the BOSS and eBOSS programs of the SDSS spectroscopic survey. To obtain robust results given the unprecedented statistical power of the data I perform a careful investigation of observational systematic sources and their sources. Modeling the Lyα flux power spectrum requires to run hydrodynamical cosmological simulations because it arises from the complex interplay between large-scale structure evolution and small-scale baryonic physics. Indeed, astrophysical processes such as star formation or AGN feedback inject energy in the ambient medium and strongly impact the thermal state and gas distribution in the intergalactic medium. Including such processes in hydrodynamical simulations requires to rely on arbitrary parameters calibrated on astrophysical observations leading to discrepancies between different state-of-the-art simulations. In order to improve theoretical predictions of the Lyα forest, I constrain the impact of AGN feedback using a series of 8 hydro-cosmological simulations covering the whole plausible range of feedback models. I provide upper and lower limit for this signature for 2 ≤ z ≤ 4.25 and also show that ignoring this effect leads to 2σ shift on n_s and 1σ shift on σ_8. Finally, I combine the Lyα flux power spectrum measurements with CMB data, BAO data and theoretical predictions from hydrodynamical simulations to enhance the previously established constraints on the sum of neutrino masses from Σ mν < 0.12 eV to the most stringent constraints to date Σ mν < 0.09 eV in the most extreme case with 95% confidence, which tends to favor the normal hierarchy neutrino mass scenario. Combining eBOSS with XQ-100 Lyα data the mass m_X of hypothetical thermal relics is constrained to m_X > 5.3 keV at the 95% confidence level in the case of a pure warm dark matter scenario, which translates into mν_s > 34 kev for non-resonantly produced sterile neutrinos. Also, a mild-tension is found on n_s between eBOSS Lyα and CMB data, which translates into a preference for a non-zero running of n_s at the level of about 3σ

Cette thèse s’inscrit dans le cadre des projets eBOSS et DESI. Ces projets utilisent, entreautres, l’absorption Lyman-α (Lyα) afin de sonder la répartition de matière dans l’univers et ainsi mesurer l’échelle des oscillations acoustiques de baryon (BAO). La mesure du rapport de la taille de l’échelle BAO et de la taille de l’horizon acoustique lors du découplage des baryons et des photons permet de contraindre l’expansion de l’univers, et donc les paramètres de l’équation d’état de l’énergie noire. Cette thèse présente le développement de simulations (ou pseudo-données, ou mocks) à partir de champs aléatoires gaussiens (GRF) utilisées afin de tester les analyses BAO des groupes Lyα de eBOSS et DESI. Les GRF permettent de générer un champ de densité δ. A partir de ce champ de densité, les positions des quasars (QSO) sont tirées, puis à partir de chaque quasar, les lignes de visées sont constituées. Le champ de densité δ est interpolé le long de ces lignes de visées. Enfin, à l’aide de l’approximation FGPA (Fluctuating Gunn Peterson Approximation), la densité interpolée est transformée en profondeur optique τ , puis en absorption Lyα. Grâce à un programme développé par la communauté de DESI, un continuum est ajouté aux forêts Lyα afin de créer des spectres de quasars synthétiques. Les mocks présentées dans ce manuscrit fournissent donc un relevé de quasars dont les forêts Lyα présentes dans les spectres possèdent les bonnes fonctions d’auto-corrélation Lyα×Lyα, de corrélation croisée Lyα×QSO, ainsi que d’auto-corrélationQSO×QSO et HCD×HCD (High Column Density systems).L’étude de ces mocks permet de montrer quel’analyse BAO menée sur l’ensemble des données Lyα du relevé eBOSS produit une mesure non biaisée des paramètres BAO αk et α⊥. Par ailleurs, une étude approfondie du modèle utilisé pour ajuster les fonctions de corrélation montre que la forme de la fonction d’auto-corrélation Lyα×Lyα, c’est à dire les mesures du biais bLyα et du paramètre RSD (Redshift Space Distorsions) βLyα, est comprise à environ 20 % près. Les systématiques qui affectent les mesures des paramètres Lyα (bLyα et βLyα) sont issues de deux effets. Le premier effet provient de la matrice de distorsion qui ne capture pas l’intégralité des distorsions produites par l’ajustement du continuum des quasars. Le second effet est lié à la modélisation des HCD. La modélisation de ces absorbeurs denses n’est pas parfaite et affecte la mesure des paramètres Lyα, en particulier le paramètre RSD βLyα. L’analyse de ces mocks permet donc de valider un bon contrôle des systématiques pour les analyses BAO avec le Lyα. Cependant, une meilleure compréhension des mesures des paramètres Lyα est nécessaire afin d’envisager une analyse RSD à l’aide de la combinaison de l’auto-corrélation Lyα×Lyα et de la corrélation croisée Lyα×QSO
This PhD thesis is part of eBOSS and DESI projects. These projects, among other tracers, use the Lyman-α (Lyα) absorption to probe the matter distribution in the universe and measure thebaryon acoustic oscillations (BAO) scale. The measurement of the BAO scale to the sound horizon ratio allows to constrain the universe expansion and so the ΛCDM model, the standard model of cosmology. This thesis presents the development of mock data sets used in order to check the BAO analyses carried out by the Lyα group within the eBOSS and DESI collaborations. These mocks make use of gaussian random fields (GRF). GRF allow to generate a density field δ. From this density field, quasar (QSO) positions are drawn. From each quasar, a line of sight is constructed. Then, the density field δ is interpolated along each line of sight. Finally, the fluctuating Gunn Peterson approximation (FGPA) is used to convert the interpolated density into the optical depth τ , and then into the Lyα absorption. Thanks to a program developed by the DESI community, a continuum is added to each Lyα forest in order to produce quasar synthetic spectra. The mocks presented in the manuscript provide a survey of quasars whose Lyα forests in the quasar spectra have the correct Lyα×Lyα auto-correlation, Lyα×QSO cross-correlation, as well as the correct QSO×QSO and HCD×HCD (High Column Density systems) auto-correlation functions. The study of these mocks shows that the BAO analysis run on the whole Lyα eBOSS data set produces a non-biaised measurement of the BAO parameters αk et α⊥. In addition, the analysis of the model used to fit the correlation functions shows that the shape of the Lyα×Lyα auto-correlation, which is linked to the bias bLyα and redshift space distorsions (RSD) parameter βLyα, are understood up to 80 %. The systematics affecting the measurement of the Lyα parameters (bLyα et βLyα) come from two different effects. The first one originates from thedistortion matrix which does not capture all the distortions produced by the quasar continuum fittingprocedure. The second one is linked to the HCD modelling. The modelling of these strong absorbers is not perfect and affects the measurement of the Lyα parameters, especially the RSD parameter βLyα. Thus, the analysis of these mocks allows to validate the systematic control of the BAO analyses done with the Lyα. However, a better understanding of the measurement of the Lyα parameters is required in order to consider using the Lyα, which means combining the Lyα×Lyα autocorrelation and Lyα×QSO cross-correlation, to do a RSD analysis

Due to the finite speed of light and a vast, expanding universe, telescopes are just now receiving the light emitted by galaxies as they were forming in the very early universe. The light from these galaxies has been redshifted (stretched to longer, redder wavelengths) as a result of its journey through expanding space. Using sophisticated techniques and exceptional multi-wavelength optical and infrared data, we isolate a population of 378 galaxies in the process of formation when the Universe was only two billion years old. By matching the distinctive properties of the light spectra of these galaxies to models, the redshift, age, dust content, star formation rate and total stellar mass of each galaxy are determined. Comparing our results to similar surveys of galaxy populations at other redshifts, a picture emerges of the growth and evolution of massive, star-forming galaxies over the course of billions of years.

La propagation des oscillations acoustiques baryoniques a laissé une empreinte dans la répartition de la matière de l’univers, visible sous la forme d’un excès de probabilité pour deux objets d’être séparés par une distance égale à l’échelle acoustique. La mesure de l’échelle acoustique dans la fonction de corrélation à deux points de la matière au redshift z, parallèlement et perpendiculairement à la ligne de visée donne accès, respectivement, aux rapports DH(z)/rd et DM(z)/rd où DH est la distance de Hubble, DM la distance angulaire comobile et rd l’horizon acoustique. On peut suivre la répartition de la matière en utilisant les absorptions Lyman-alpha visibles, sous la forme de forêts, dans les spectres des quasars à haut redshift. Puisque chaque spectre donne accès une centaine d’absorptions, cela permet de mesurer l’échelle acoustique même quand la densité de quasars observés est faible. Dans cette thèse, je décris le processus d’analyse qui, en utilisant environ 200 000 spectres de quasars du relevé eBOSS-SDSS IV, aboutit aux mesures DH(2.34)/rd = 8.86+/-0.29 et DM(2.34)/rd = 37.41+/-1.86. En combinant ces résultats avec les mesures de l’échelle acoustique à d’autres redshifts, j’obtiens la plus forte contrainte actuelle, à bas redshift, sur les paramètres Omega-m et Omega-Lambda dans le cadre du modèle Lambda-CDM
The propagation of the baryonic acoustic oscillations has been unprinted in the matter distribution in the Universe as a probability excess for two objets to be separated by the acoustic scale. Measuring the acoustic scale in the matter 2 point correlation function at redshift z, along and transversally to the line-of-sight, gives access to the DH(z)/rd et DM(z)/rd ratios, with DH the Hubble distance, DM the comoving angular distance and rd the acoustic horizon. We are able to trace the matter in the Universe by using the Lyman-alpha absorptions which shape the spectra of the high redshift quasars. Since each spectrum contain hundreds of absorption, this allow us to measure the acoustic scale even if the observed quasar density is low. In this thesis, I describe the analysis of about 200,000 spectra from the eBOSS-SDSS IV survey which conducts to the measurements DH(2.34)/rd = 8.86 0.29 et DM(2.34)/rd = 37.41 1.86. By combining these results with measurements of the acoustic scale at other redshifts, I obtain the strongest current constraints at low redshift on the Omega-m and Omega-Lambda Lambda-CDM parameters

This thesis presents work around 3 themes: dark energy, gravitational waves and Bayesian inference. Both dark energy and gravitational wave physics are not yet well constrained. They present interesting challenges for Bayesian inference, which attempts to quantify our knowledge of the universe given our astrophysical data. A dark energy equation of state reconstruction analysis finds that the data favours the vacuum dark energy equation of state $w {=} -1$ model. Deviations from vacuum dark energy are shown to favour the super-negative ‘phantom’ dark energy regime of $w {< } -1$, but at low statistical significance. The constraining power of various datasets is quantified, finding that data constraints peak around redshift $z = 0.2$ due to baryonic acoustic oscillation and supernovae data constraints, whilst cosmic microwave background radiation and Lyman-$\alpha$ forest constraints are less significant. Specific models with a conformal time symmetry in the Friedmann equation and with an additional dark energy component are tested and shown to be competitive to the vacuum dark energy model by Bayesian model selection analysis: that they are not ruled out is believed to be largely due to poor data quality for deciding between existing models. Recent detections of gravitational waves by the LIGO collaboration enable the first gravitational wave tests of general relativity. An existing test in the literature is used and sped up significantly by a novel method developed in this thesis. The test computes posterior odds ratios, and the new method is shown to compute these accurately and efficiently. Compared to computing evidences, the method presented provides an approximate 100 times reduction in the number of likelihood calculations required to compute evidences at a given accuracy. Further testing may identify a significant advance in Bayesian model selection using nested sampling, as the method is completely general and straightforward to implement. We note that efficiency gains are not guaranteed and may be problem specific: further research is needed.

碩士
國立清華大學
物理學系
106
Fuzzy dark matter is one of the dark matter candidates, which can not only have the same large-scale success of cold dark matter but also alleviates the ”small scale crisis” of it when the fuzzy dark matter mass is smaller than 10^{−22} eV. With recent Lyman-alpha forest data from BOSS and XQ-100, some studies suggested that the lower mass limit on the fuzzy dark matter particles is lifted up to 10^{−21} eV. However, such a limit was obtained by cold dark matter simulations with the fuzzy dark matter initial condition and the quantum pressure of fuzzy dark matter was not taken into account which could have generated non-trivial effects on small scales. After checking the validity of our methodology using one-dimensional simulation, we investigate the effects of quantum pressure in cosmological simulations systematically, and find that quantum pressure leads to further suppression on the matter power spectrum on small scales, as well as the halo mass function in the low mass end. Furthermore, we estimate the one-dimensional flux power spectrum of Lyman-alpha forest, and compare it with the data from BOSS and XQ-100. We carefully estimate the uncertainty in the calculation of one-dimensional flux power spectrum due to the temperature of hydrogen gas. We conclude that if one properly takes into account the effect of quantum pressure and the temperature of the hydrogen gas, one cannot exclude the fuzzy dark matter of mass smaller than 10^{−22} eV, which is the interesting mass range for solving the small scale crisis, at statistically significant levels.