Littérature scientifique 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.