Large scale structure with Lyman-alpha absorption surveys

[cat] Gran part dels estudis presentats en aquesta tesi doctoral estan vinculats als esforços per detectar les oscil·lacions acústiques dels barions (BAO, de l’anglès Baryon Acoustic Oscillations) en la funció de correlació de l’absorció en l’espectre de quasars llunyans, relacionada amb la transició de Lyman-α (Lyα) en els atoms d’hidrogen intergalàctic. Aquesta absorció és coneguda com a “bosc de Lyman-α”. El Baryon Oscillation Spectroscopic Survey (BOSS) és un dels quatre projectes que composen la tercera fase de l’Sloan Digital Sky Survey (SDSS-III) i té com a principal objectiu la detecció del senyal de BAO en la correlació de galàxies i en la del bosc de Lyα. Per fer-ho, des de la tardor de 2009 s’estan obtenint espectres electromagnètics de cen- tenars de milers de galàxies i quàsars utilitzant un telescopi de 2.5 metres de diàmetre a Apache Point (Nou Mèxic, Estat Units d’Amèrica). La meva vinculació amb BOSS començà el gener de 2009, i des d’un bon principi m’he centrat en generar simulacres d’espectres amb absorció de Lyα. Aquests simulacres han tingut una funció essencial en la primera publicació de la col·laboració (Slosar et al., 2011). L’estructura de la tesi es divideix en quatre capítols, que resumeixo a continuació. Simulant la Mesura de l’Espectre de Potències del Bosc de Lyα en un Catàleg Espectroscòpic de Quàsars a Grans Escales En el capítol 2 presento un mètode per simular l’absorció de Lyα en l’espectre de quàsars. El mètode, desenvolupat en col·laboració amb el Dr. Patrick McDonald i el Dr. Jordi Miralda-Escudé, permet generar espectres amb qualsevol distribució de flux i amb qualsevol espectre de potències. El Bosc de Lyα en Tres Dimensions: Mesura de la Correlació del Flux a Grans Escales en les Dades del Primer Any de BOSS La primera detecció de la correlació a grans escales del bosc de Lyα va ser presentada a mitjans 2011 per la col·laboració SDSS-III, utilitzant les dades obtingudes durant el primer any del projecte BOSS (Slosar et al., 2011). El capítol 3 conté un resum de l’estudi, fent èmfasi en la meva aportació i en el paper dels simulacres explicats en el capítol 2. L’Efecte dels Sistemes d’Alta Densitat de Columna en la Mesura de la Funció de Correlació del Bosc de Lyα En el Capítol 4, presento un estudi analític de l’efecte que els sistemes d’alta densitat de columna tenen en la mesura de la funció de correlació del bosc de Lyα. A continuació presento un mètode per introduir aquests sistemes en els espectres simulats, desenvolupat també amb la col·laboració de Miralda-Escudé i McDonald. Correlacions creuades del Bosc de Lyα En el darrer capítol d’aquesta tesi, 5, presento un estudi sobre la possibilitat de detectar la correlació creuada entre una població de galàxies i l’absorció de Lyα. Presento també un mètode senzill per mesurar la correlació creuada en un catàleg espectroscòpic com ara BOSS, i estudio en concret la possibilitat de mesurar el biaix dels sistemes Lyα esmorteits (“Damped Lyα systems” en anglès)

How to estimate the 3D power spectrum of the Lyman-α\alpha forest

We derive and numerically implement an algorithm for estimating the 3D power spectrum of the Lyman-$\alpha$ (Ly-$\alpha$) forest flux fluctuations. The algorithm exploits the unique geometry of Ly-$\alpha$ forest data to efficiently measure the cross-spectrum between lines of sight as a function of parallel wavenumber, transverse separation and redshift. The key to fast evaluation is to approximate the global covariance matrix as block-diagonal, where only pixels from the same spectrum are correlated. We then compute the eigenvectors of the derivative of the signal covariance with respect to cross-spectrum parameters, and project the inverse-covariance-weighted spectra onto them. This acts much like a radial Fourier transform over redshift windows. The resulting cross-spectrum inference is then converted into our final product, an approximation of the likelihood for the 3D power spectrum expressed as second order Taylor expansion around a fiducial model. We demonstrate the accuracy and scalability of the algorithm and comment on possible extensions. Our algorithm will allow efficient analysis of the upcoming Dark Energy Spectroscopic Instrument dataset.Comment: 29 pages, many figures. Minor changes in v2, accepted in JCA

Optimal strategies for identifying quasars in DESI

As spectroscopic surveys continue to grow in size, the problem of classifying spectra targeted as quasars (QSOs) will need to move beyond its historical reliance on human experts. Instead, automatic classifiers will increasingly become the dominant classification method, leaving only small fractions of spectra to be visually inspected in ambiguous cases. In order to maximise classification accuracy, making best use of available classifiers will be of great importance, particularly when looking to identify and eliminate distinctive failure modes. In this work, we demonstrate that the machine learning-based classifier QuasarNET will be of use for future surveys such as the Dark Energy Spectroscopic Instrument (DESI), comparing its performance to the DESI pipeline classifier redrock. During the first of four passes across its footprint DESI will need to select high-$z$ ($z\geq2.1$) QSOs for reobservation, and so we first assess the classifiers' performance at identifying high-$z$ QSOs from single-exposure spectra. We then quantify the classifiers' abilities to construct QSO catalogues in both low- and high-$z$ bins, using coadded spectra to simulate end-of-survey data. For such tasks, QuasarNET is able to outperform redrock in its current form, identifying approximately 99% of high-$z$ QSOs from single exposures and producing QSO catalogues with sub-percent levels of contamination. By combining QuasarNET and redrock's outputs, we can further improve the classification strategies to identify up to 99.5% of high-$z$ QSOs from single exposures and reduce final QSO catalogue contamination to below 0.5%. These combined strategies address DESI's QSO classification needs effectively.Comment: 26 pages, 5 figures, published in JCA

Compressing the cosmological information in one-dimensional correlations of the Lyman-α\alpha forest

Observations of the Lyman-$\alpha$ (Ly$\alpha$) forest from spectroscopic surveys such as BOSS/eBOSS, or the ongoing DESI, offer a unique window to study the growth of structure on megaparsec scales. Interpretation of these measurements is a complicated task, requiring hydrodynamical simulations to model and marginalise over the thermal and ionisation state of the intergalactic medium. This complexity has limited the use of Ly$\alpha$ clustering measurements in joint cosmological analyses. In this work we show that the cosmological information content of the 1D power spectrum ($P_\mathrm{1D}$) of the Ly$\alpha$ forest can be compressed into a simple two-parameter likelihood without any significant loss of constraining power. We simulate $P_\mathrm{1D}$ measurements from DESI using hydrodynamical simulations and show that the compressed likelihood is model independent and lossless, recovering unbiased results even in the presence of massive neutrinos or running of the primordial power spectrum.Comment: Submitted to ApJ. 17 pages, 7 figure

Baryon Acoustic Oscillations and the Hubble Constant: Past, Present and Future

We investigate constraints on the Hubble constant ($H_0$) using Baryon Acoustic Oscillations (BAO) and baryon density measurements from Big Bang Nucleosynthesis (BBN). We start by investigating the tension between galaxy BAO measurements and those using the Lyman-$\alpha$ forest, within a Bayesian framework. Using the latest results from eBOSS DR14 we find that the probability of this tension being statistical is $\simeq6.3\%$ assuming flat $\Lambda$CDM. We measure $H_0 = 67.6\pm1.1$ km s$^{-1}$ Mpc$^{-1}$, with a weak dependence on the BBN prior used, in agreement with results from Planck Cosmic Microwave Background (CMB) results and in strong tension with distance ladder results. Finally, we forecast the future of BAO $+$ BBN measurements of $H_0$, using the Dark Energy Spectroscopic Instrument (DESI). We find that the choice of BBN prior will have a significant impact when considering future BAO measurements from DESI.Comment: 14 pages, 3 figure

Correlations in the three-dimensional Lyman-alpha forest contaminated by high column density absorbers

Correlations measured in three dimensions in the Lyman-alpha forest are contaminated by the presence of the damping wings of high column density (HCD) absorbing systems of neutral hydrogen (HI; having column densities $N(\mathrm{HI}) > 1.6\times10^{17}\,\mathrm{atoms}\,\mathrm{cm}^{-2}$), which extend significantly beyond the redshift-space location of the absorber. We measure this effect as a function of the column density of the HCD absorbers and redshift by measuring 3D flux power spectra in cosmological hydrodynamical simulations from the Illustris project. Survey pipelines exclude regions containing the largest damping wings. We find that, even after this procedure, there is a scale-dependent correction to the 3D Lyman-alpha forest flux power spectrum from residual contamination. We model this residual using a simple physical model of the HCD absorbers as linearly biased tracers of the matter density distribution, convolved with their Voigt profiles and integrated over the column density distribution function. We recommend the use of this model over existing models used in data analysis, which approximate the damping wings as top-hats and so miss shape information in the extended wings. The simple 'linear Voigt model' is statistically consistent with our simulation results for a mock residual contamination up to small scales ($|k| < 1\,h\,\mathrm{Mpc}^{-1}$). It does not account for the effect of the highest column density absorbers on the smallest scales (e.g., $|k| > 0.4\,h\,\mathrm{Mpc}^{-1}$ for small damped Lyman-alpha absorbers; HCD absorbers with $N(\mathrm{HI}) \sim 10^{21}\,\mathrm{atoms}\,\mathrm{cm}^{-2}$). However, these systems are in any case preferentially removed from survey data. Our model is appropriate for an accurate analysis of the baryon acoustic oscillations feature. It is additionally essential for reconstructing the full shape of the 3D flux power spectrum.Comment: 13 pages, 11 figures. Minor changes to match version published in MNRA