177 research outputs found

    Radiative Transfer Distortions of Lyman Α Emitters: A New Fingers-Of-God Damping in the Clustering in Redshift Space

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    Complex radiative transfer (RT) of the Lyman α photons poses a theoretical challenge to galaxy surveys that infer the large-scale structure with Lyman α emitters (LAEs). Guided by RT simulations, prior studies investigated the impact of RT on the large-scale LAE clustering, and claimed that RT induces a selection effect which results in an anisotropic distortion even in real space but in an otherwise negligible effect in redshift space. However, our previous study, which relies on a full RT code run on the Illustris simulations, shows that the anisotropic selection effect was drastically reduced with higher spatial resolution. Adopting the same simulation framework, we further study the impact of RT on the LAE clustering in redshift space. Since we measure LAE\u27s radial position through a spectral peak of Lyman α emission, the frequency shift due to RT contaminates the redshift measurement and hence the inferred radial position in redshift space. We demonstrate that this additional RT offset suppresses the LAE clustering along the line of sight, which can be interpreted as a novel Fingers-of- God (FoG) effect. To assess the FoG effect, we develop a theoretical framework modelling the impact of the RT similar to that of the small-scale peculiar velocity which is commonly studied in the context of the redshift space distortion (RSD). Although our findings strongly encourage a more careful RSD modelling in LAE surveys, we also seek a method to mitigate the additional FoG effect due to RT by making use of other information in a Lyman α spectrum

    The impact of Lyman-α\alpha radiative transfer on large-scale clustering in the Illustris simulation

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    Lyman-α\alpha emitters (LAEs) are a promising probe of the large-scale structure at high redshift, z2z\gtrsim 2. In particular, the Hobby-Eberly Telescope Dark Energy Experiment aims at observing LAEs at 1.9 <z<<z< 3.5 to measure the Baryon Acoustic Oscillation (BAO) scale and the Redshift-Space Distortion (RSD). However, Zheng et al. (2011) pointed out that the complicated radiative transfer (RT) of the resonant Lyman-α\alpha emission line generates an anisotropic selection bias in the LAE clustering on large scales, s10s\gtrsim 10 Mpc. This effect could potentially induce a systematic error in the BAO and RSD measurements. Also, Croft et al. (2016) claims an observational evidence of the effect in the Lyman-α\alpha intensity map, albeit statistically insignificant. We aim at quantifying the impact of the Lyman-α\alpha RT on the large-scale galaxy clustering in detail. For this purpose, we study the correlations between the large-scale environment and the ratio of an apparent Lyman-α\alpha luminosity to an intrinsic one, which we call the `observed fraction', at 2<z<62<z<6. We apply our Lyman-α\alpha RT code by post-processing the full Illustris simulations. We simply assume that the intrinsic luminosity of the Lyman-α\alpha emission is proportional to the star formation rate of galaxies in Illustris, yielding a sufficiently large sample of LAEs to measure the anisotropic selection bias. We find little correlations between large-scale environment and the observed fraction induced by the RT, and hence a smaller anisotropic selection bias than what was claimed by Zheng et al. (2011). We argue that the anisotropy was overestimated in the previous work due to the insufficient spatial resolution: it is important to keep the resolution such that it resolves the high density region down to the scale of the interstellar medium, 1\sim1 physical kpc. (abridged)Comment: 11 pages, published in A&

    Radiative transfer simulations of Lyman-alpha photons in the universe

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    The Lyman-alpha emission line is one of the brightest lines in the high-redshift Universe. With new instruments and surveys mapping larger areas of the sky with higher spectral resolving power, spatial resolution, and depth, Lyman-alpha observations are going to providing competitive constraints of astrophysical theories and cosmological models. Even though the Lyman-alpha line has huge potential, the interpretation of its observation is difficult given the complex radiative transfer (RT) Lyman-alpha photons experience. The Lyman-alpha line is resonant and even small amounts of neutral hydrogen lead to large optical depths. As a consequence, Lyman-alpha photons will scatter numerous times until they either escape from its source or destruction by dust. Hence, spectral and spatial information of Lyman-alpha emission is significantly altered and this modification needs to be accounted for in order to obtain the encoded physical information in its observation. In this thesis, I present a framework for Lyman-alpha RT simulations to understand the complex RT involved in recent and upcoming Lyman-alpha observations. Primarily applied to cosmological hydrodynamic galaxy formation simulations, I present three applications of such framework in an astrophysical and cosmological context. First, I analyze the effects of Lyman-alpha RT on the clustering of Lyman-alpha emitting galaxies in cosmological redshift surveys. Current and future blind spectroscopic surveys map out the distribution of such galaxies in the high-redshift Universe. The spatial clustering can be used to constrain cosmological models as planned with the HETDEX survey currently under way. However, the observed clustering signal can be distorted due to the RT Lyman-alpha photons experience before reaching the observer. I present a new phenomenological effect distorting the clustering signal due to the spectral shape of Lyman-alpha emitters. I demonstrate that such effect can be modelled analogously to the well-studied Fingers-of-God effect and discuss different methods to correct for this distortion. Second, I statistically analyze and disentangle the shaping factors of the Lyman-alpha spectra. Lyman-alpha spectra are shaped by both the small-scale structure within the galaxy and the structure of the intergalactic medium along the line of sight. The imprinted spectral features from those two different spatial scales can appear degenerate. Properly distinguishing the features to arise on either scale allows us to infer important insights, such as the kinematic structure of the originating halo and the escape of ionizing photons. I sketch out different simple statistical measures to break the scale degeneracy, which might enable inferences on the underlying physical processes on both scales. By providing a public data set of Lyman-alpha transmission curves in the intergalactic medium, more dedicated studies by the astronomy community are made possible. Third, I investigate the nature of so-called Lyman-alpha halos (LAHs). In observations, LAHs describe the diffuse Lyman-alpha glow around star-forming galaxies. Modelling the commonly considered emission mechanisms and using the state-of-the-art TNG50 galaxy formation simulation, I provide predictions for LAHs with an unprecedented combination of statistical sample and resolution. I find that scattering of Lyman-alpha photons from inner star-forming regions within the galaxies dominate the faint glow in the circumgalactic medium, but substantial contributions from diffuse emission can be present. The predicted surface brightness radial profiles show a promising agreement with latest observational constraints at z=3.Die Lyman-alpha Emissionslinie ist eine der hellsten Linien im Universum. Mit neuen Instrumenten werden zunehmend größere Bereiche des Himmels mit höherer spektraler und räumlicher Auflösung und Tiefe durchmustert. Lyman-alpha Beobachtungen werden dadurch zunehmend bedeutsame Erkenntnisse für unser astrophysikalisches und kosmologisches Verständnis liefern. Somit hat die Lyman-alpha Emissionslinie ein enormes Potential, jedoch ist die Interpretation jener Beobachtungen aufgrund des komplexen Strahlungstransports der Lyman-alpha Photonen schwierig. Die Lyman-alpha Emissionslinie ist eine resonante Emissionslinie und zugleich reichen kleine Mengen neutralen Wasserstoffs für hohe optische Tiefen aus. Lyman-alpha Photonen werden somit vielfach gestreut bevor sie ihrer Umgebung entkommen oder durch Staub absorbiert werden. Diese Streuprozesse modifizieren die beobachteten räumlichen und spektralen Eigenschaften der Emissionslinie in Beobachtungen grundlegend, was den Rückschluss auf die zugrunde liegenden physikalischen Eigenschaften des abstrahlenden Gases erschwert. In dieser Disseration präsentiere ich einen Code für Lyman-alpha Strahlungstransport, um die komplexen Streuvorgänge nachvollziehbar zu machen, die die Interpretation heutiger und zukünftiger Lyman-alpha Beobachtungen erschweren. Ich präsentiere drei Anwendungen jenes Codes in der Astrophysik und Kosmologie unter Zuhilfenahme existierender numerischer Simulationen der Galaxieentstehung. Zuerst präsentiere ich den Effekt des Lyman-alpha Strahlungstransports auf das beobachtete Clustering von Lyman-alpha emittierenden Galaxien. Heutige und zukünftige spektroskopische Durchmusterungen des Himmels kartieren solche Galaxien im jungen Universum. Die räumliche Häufung solcher Galaxien kann genutzt werden um die Parameter des kosmologischen Modells genauer zu bestimmen, was etwa mit der HETDEX Durchmusterung derzeit vorangetrieben wird. Die räumliche Häufung jener Galaxien wird jedoch durch die Interaktion der Lyman-alpha Photonen mit dem neutralen Wasserstoff auf dem Weg zum Beobachter verfälscht. Ich präsentiere in dieser Arbeit wie die spektrale Form der beobachteten Lyman-alpha Emissionslinie die Statistik der räumlichen Häufung modifiziert und wie dieser Störungseffekt korrigiert werden kann. Weiterhin zeige ich auf wie der Einfluss des Lyman-alpha Strahlungstransports auf die Form der Lyman-alpha Spektren durch verschiedene räumlicher Skalen auseinander gehalten werden kann. Die Form der Lyman-alpha Spektren wird sowohl durch den Strahlungstransport innerhalb des Galaxie als auch durch Streuungen im intergalaktischen Medium geprägt. Die Effekte von diesen verschiedenen räumlichen Skalen auf die Spektren ist schwierig auseinanderzuhalten. Die korrekte Interpretation der Stärke jener Aufprägung ermöglicht es wichtige Rückschlüsse etwa bezüglich der Kinematik innerhalb der Galaxien zu ziehen. Ich zeige verschiedene einfache statistische Methoden auf mit welchen die Aufprägung der verschiedenen räumlichen Skalen auseinandergehalten werden kann. Weiterhin mache ich einen Katalog der verwendeten Daten öffentlich, um weitere Studien zu ermöglichen. Zum Ende untersuche ich mit dem Lyman-alpha Strahlungstransportcode die Natur sogenannter Lyman-alpha Halos (LAHs). Beobachtungen zeigen LAHs als diffuses Leuchten der Lyman-alpha Linie um Galaxien mit aktiver Sternentstehung. Durch Modellierung der gewöhnlich betrachteten Emissionsmechanismen und Nutzung der hochmodernen TNG50 Galaxieentstehungssimulationen stelle ich Voraussagen für die erwarteten LAHs um die simulierten Galaxien auf. In den Simulationen zeigt sich, dass sich die beobachteten Radialprofile von LAHs mit Streuungen von Photonen, welche den sternentstehenden Regionen innerhalb der Galaxien entkommen, erklären lassen. Dennoch kann die diffuse Emission im zirkumgalaktischen Medium für individuelle Galaxien erheblich sein. Weiterhin zeigen die simulierten Radialprofile der Lyman-alpha Halos eine vielversprechende Übereinstimmung mit neuesten Beobachtungsdaten bei Rotverschiebungen um z=3 auf

    The impact of feedback on galactic and extra-galactic scales

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    Recent cosmological hydrodynamical simulations were for the first time able to produce galaxy populations with realistic sizes and morphologies. This success can be attributed to the inclusion of subgrid models for supernovae winds and active galactic nuclei (AGN) feedback. In this thesis, we investigate the impact of feedback driven galactic outflows. First, the expulsion of gas proves to be crucial for the rotational support of haloes hosting realistic galaxies. We employ the state-of-the-art hydrodynamical simulation suites Illustris and IllustrisTNG to characterise the amount of specific angular momentum in the baryonic component of haloes. We find the baryonic spin at z = 0 to be a factor of ∼ 2 higher than the dark matter spin, which is due to the transfer of a constant cumulative spin of Δλ = 0.0013 by z = 0 from dark matter to the gas during mergers, and to the preferential expulsion of low angular momentum gas by mostly AGN feedback. Second, galactic outflows impact the state of the diffuse gas on large scales. We employ the Lyman-α forest to examine the feedback induced changes in the inter-galactic medium (IGM) that serves as gas reservoir for accretion onto galaxies. For a clean comparison, we have run a suite of simulations with both galaxy formation physics and with the Quick Lyman-α (QLA) technique yielding an unperturbed IGM. We find the Lyman-α flux power spectrum to exhibit increasingly more power at large scales and correspondingly less power at small scales in the presence of outflows, as well as the IGM to be generally hotter. Employing IllustrisTNG we investigate the excess Lyman-α absorption as a function of impact parameter for haloes exhibiting strong and weak feedback and find significant differences that can largely be explained by the higher temperature of the perturbed gas

    Observational Diagnostics of Gas Flows: Insights from Cosmological Simulations

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    Galactic accretion interacts in complex ways with gaseous halos, including galactic winds. As a result, observational diagnostics typically probe a range of intertwined physical phenomena. Because of this complexity, cosmological hydrodynamic simulations have played a key role in developing observational diagnostics of galactic accretion. In this chapter, we review the status of different observational diagnostics of circumgalactic gas flows, in both absorption (galaxy pair and down-the-barrel observations in neutral hydrogen and metals; kinematic and azimuthal angle diagnostics; the cosmological column density distribution; and metallicity) and emission (Lya; UV metal lines; and diffuse X-rays). We conclude that there is no simple and robust way to identify galactic accretion in individual measurements. Rather, progress in testing galactic accretion models is likely to come from systematic, statistical comparisons of simulation predictions with observations. We discuss specific areas where progress is likely to be particularly fruitful over the next few years.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics and Space Science Library, eds. A. J. Fox & R. Dave, to be published by Springer. Typos correcte

    Simulating the effect of high column density absorbers on the one-dimensional Lyman-alpha forest flux power spectrum

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    We measure the effect of high column density absorbing systems of neutral hydrogen (HI) on the one-dimensional (1D) Lyman-alpha forest flux power spectrum using cosmological hydrodynamical simulations from the Illustris project. High column density absorbers (which we define to be those with HI column densities N(HI)>1.6×1017atomscm2N(\mathrm{HI}) > 1.6 \times 10^{17}\,\mathrm{atoms}\,\mathrm{cm}^{-2}) cause broadened absorption lines with characteristic damping wings. These damping wings bias the 1D Lyman-alpha forest flux power spectrum by causing absorption in quasar spectra away from the location of the absorber itself. We investigate the effect of high column density absorbers on the Lyman-alpha forest using hydrodynamical simulations for the first time. We provide templates as a function of column density and redshift, allowing the flexibility to accurately model residual contamination, i.e., if an analysis selectively clips out the largest damping wings. This flexibility will improve cosmological parameter estimation, e.g., allowing more accurate measurement of the shape of the power spectrum, with implications for cosmological models containing massive neutrinos or a running of the spectral index. We provide fitting functions to reproduce these results so that they can be incorporated straightforwardly into a data analysis pipeline.Comment: 11 pages, 6 figures. Minor changes to match version published in MNRA

    Emisores de Lyman-alpha en volúmenes cosmológicos

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    Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Tierra y Astrofísica, leída el 09-07-2019Esta tesis estudia cómo los complejos procesos de transferencia radiativa que sufren los fotones Lyman-α dentro de nubes de hidrogeno neutro determinan la function de selección de muestras de galaxies detectadas por su flujo en Lyman-α.Primero, hacemos una introducción teórica a los procesos radiativos de Lyman-α. Es de gran importancia la descripción cuántica del átomo de hidrogeno. Este formalismo predice que el átomo de hidrogeno presenta una estructura discreta de nivele de energía, el cual explica las diferentes lineas de emisión encontradas en el espectro del hidrógeno, y en particular Lyman-α. La mecánica cuántica también predice que la probabilidad de interacción entre un fotón Lyman-α y un átomo de hidrógeno es muy elevada. Esto causa que cuando uno de estos fotones entra en una nube de hidrogeno neutro, este es absorbido y reemitido constantemente, cambiando su frecuencia y dirección en cada interacción. De esta manera, la longitud de la trayectoria de los fotones Lyman-α se incrementa, lo cual hace esta radiación muy sensible a mecanismos de destrucción,como el polvo...Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEunpu

    Intensity mapping with neutral hydrogen and the Hidden Valley simulations

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    This paper introduces the Hidden Valley simulations, a set of trillion-particle N-body simulations in gigaparsec volumes aimed at intensity mapping science. We present details of the simulations and their convergence, then specialize to the study of 21-cm fluctuations between redshifts 2 and 6. Neutral hydrogen is assigned to halos using three prescriptions, and we investigate the clustering in real and redshift-space at the 2-point level. In common with earlier work we find the bias of HI increases from near 2 at z = 2 to 4 at z = 6, becoming more scale dependent at high z. The level of scale-dependence and decorrelation with the matter field are as predicted by perturbation theory. Due to the low mass of the hosting halos, the impact of fingers of god is small on the range relevant for proposed 21-cm instruments. We show that baryon acoustic oscillations and redshift-space distortions could be well measured by such instruments. Taking advantage of the large simulation volume, we assess the impact of fluctuations in the ultraviolet background, which change HI clustering primarily at large scales.Comment: 36 pages, 21 figures. Simulations available at http://cyril.astro.berkeley.edu/HiddenValley/ Minor changes in HI normalization described in footnote of section
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