Radiative transfer simulations of Lyman-alpha photons in the universe

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

scida: scalable analysis for scientific big data

scida is a Python package for reading and analyzing large scientific data sets with support for various cosmological and galaxy formation simulations out-of-the-box. Data access is provided through a hierarchical dictionary-like data structure after a simple load() function. Using the dask library for scalable, parallel and out-of-core computation, all computation requests from a user session are first collected in a task graph. Arbitrary custom analysis, as well as all available dask (array) operations, can be performed. The subsequent computation is executed only upon request, on a target resource (e.g. a HPC cluster).Comment: recommended for acceptance in the Journal of Open Source Software; open-source development at https://github.com/cbyrohl/scid

The cosmic web in Lyman-alpha emission

We develop a comprehensive theoretical model for Lyman-alpha emission, from the scale of individual Lyman-alpha emitters (LAEs) to Lyman-alpha halos (LAHs), Lyman-alpha blobs (LABs), and Lyman-alpha filaments (LAFs) of the diffuse cosmic web itself. To do so, we post-process the high-resolution TNG50 cosmological magnetohydrodynamical simulation with a Monte Carlo radiative transfer method to capture the resonant scattering process of Lyman-alpha photons. We build an emission model incorporating recombinations and collisions in diffuse gas, including radiative effects from nearby AGN, as well as emission sourced by stellar populations. Our treatment includes a physically motivated dust model, which we empirically calibrate to the observed LAE luminosity function. We then focus on the observability, and physical origin, of the $z=2$ Lyman-alpha cosmic web, studying the dominant emission mechanisms and spatial origins. We find that diffuse Lyman-alpha filaments are, in fact, illuminated by photons which originate, not from the intergalactic medium itself, but from within galaxies and their gaseous halos. In our model, this emission is primarily sourced by intermediate mass halos ($10^{10} - 10^{11}\,$M$_{\odot}$), principally due to collisional excitations in their circumgalactic media as well as central, young stellar populations. Observationally, we make predictions for the abundance, area, linear size, and embedded halo/emitter populations within filaments. Adopting an isophotal surface brightness threshold of $10^{-20}\,$erg$\,$s$^{-1}\,$cm$^{-2}\,$arcsec$^{-2}$, we predict a volume abundance of Lyman-alpha filaments of ${\sim}10^{-3}$ cMpc$^{-3}\,$ for lengths above $400\,$pkpc. Given sufficiently large survey footprints, detection of the Lyman-alpha cosmic web is within reach of modern integral field spectrographs, including MUSE, VIRUS, and KCWI.Comment: Submitted to MNRA

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

Lyman-$\alpha$ emitters (LAEs) are a promising probe of the large-scale structure at high redshift, $z\gtrsim 2$. In particular, the Hobby-Eberly Telescope Dark Energy Experiment aims at observing LAEs at 1.9 $<z<$ 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-$\alpha$ emission line generates an anisotropic selection bias in the LAE clustering on large scales, $s\gtrsim 10$ 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-$\alpha$ intensity map, albeit statistically insignificant. We aim at quantifying the impact of the Lyman-$\alpha$ 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-$\alpha$ luminosity to an intrinsic one, which we call the `observed fraction', at $2<z<6$. We apply our Lyman-$\alpha$ RT code by post-processing the full Illustris simulations. We simply assume that the intrinsic luminosity of the Lyman-$\alpha$ 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, $\sim1$ physical kpc. (abridged)Comment: 11 pages, published in A&

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

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

Resonant scattering of the OVII X-ray emission line in the circumgalactic medium of TNG50 galaxies

We study the impact of resonantly scattered X-ray line emission on the observability of the hot circumgalactic medium (CGM) of galaxies. We apply a Monte Carlo radiative transfer post-processing analysis to the high-resolution TNG50 cosmological magnetohydrodynamical galaxy formation simulation. This allows us to model the resonant scattering of OVII(r) X-ray photons within the complex, multi-phase, multi-scale CGM. The resonant transition of the OVII He-like triplet is one of the brightest, and most promising, X-ray emission lines for detecting the hot CGM and measuring its physical properties. We focus on galaxies with stellar masses 10 < log(M*/Msun) < 11 at z ~ 0. After constructing a model for OVII(r) emission from the central galaxy as well as from CGM gas, we forward model these intrinsic photons to derive observable surface brightness maps. We find that scattering significantly boosts the observable OVII(r) surface brightness of the extended and diffuse CGM. This enhancement can be large -- an order of magnitude on average at a distance of 200 projected kpc for high-mass M* = 10^10.7 Msun galaxies. The enhancement is larger for lower mass galaxies, and can even reach a factor of 100, across the extended CGM. Galaxies with higher star formation rates, AGN luminosities, and central OVII(r) luminosities all have larger scattering enhancements, at fixed stellar mass. Our results suggest that next-generation X-ray spectroscopic missions including XRISM, LEM, ATHENA, and HUBS -- which aim to detect the hot CGM in emission -- could specifically target halos with significant enhancements due to resonant scattering.Comment: Published in MNRAS. See https://www.lem-observatory.org/ and https://www.tng-project.org/ for more details; 2023MNRAS.522.3665