Searcharxiv⌕ Search

arXiv subjects

Laurent Legrand

Publications and source records attributed to Laurent Legrand.

16 recordsLinked to original sources
arXiv2023

Astraeus VIII: A new framework for Lyman-$α$ emitters applied to different reionisation scenarios

We use the {\sc astraeus} framework to investigate how the visibility and spatial distribution of Lyman-$α$ (Ly$α$) emitters (LAEs) during reionisation is sensitive to a halo mass-dependent fraction of ionising radiation escaping from the galactic environment ($f_\mathrm{esc}$) and the ionisation topology. To this end, we consider the two physically plausible bracketing scenarios of $f_\mathrm{esc}$ increasing and decreasing with rising halo mass. We derive the corresponding observed Ly$α$ luminosities of galaxies for three different analytic Ly$α$ line profiles and associated Ly$α$ escape fraction ($f_\mathrm{esc}^\mathrm{Lyα}$) models: importantly, we introduce two novel analytic Ly$α$ line profile models that describe the surrounding interstellar medium (ISM) as dusty gas clumps. They are based on parameterising results from radiative transfer simulations, with one of them relating $f_\mathrm{esc}^\mathrm{Lyα}$ to $f_\mathrm{esc}$ by assuming the ISM of being interspersed with low-density tunnels. Our key findings are: (i) for dusty gas clumps, the Ly$α$ line profile develops from a central to double peak profile as a galaxy's halo mass increases; (ii) LAEs are galaxies with $M_h\gtrsim10^{10}M_\odot$ located in overdense and highly ionised regions; (iii) for this reason, the spatial distribution of LAEs is primarily sensitive to the global ionisation fraction and only weakly in second-order to the ionisation topology or a halo mass-dependent $f_\mathrm{esc}$; (iv) furthermore, as the observed Ly$α$ luminosity functions reflect the Ly$α$ emission from more massive galaxies, there is a degeneracy between the $f_\mathrm{esc}$-dependent intrinsic Ly$α$ luminosity and the Ly$α$ attenuation by dust in the ISM if $f_\mathrm{esc}$ does not exceed $\sim50\%$.

astro-ph.GA↗
arXiv2022

Astraeus VII: The environmental-dependent assembly of galaxies in the Epoch of Reionization

Using the ASTRAEUS (semi-numerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dark matter simUlationS) framework, we explore the impact of environmental density and radiative feedback on the assembly of galaxies and their host halos during the Epoch of Reionization. The ASTRAEUS framework allows us to study the evolution of galaxies with masses ($\rm 10^{8.2}M_\odot < M_{\rm h} < 10^{13}M_\odot$) in wide variety of environment ($-0.5 < {\rm log}(1+δ) < 1.3$ averaged over $(2~{\rm cMpc})^3$). We find that : (i) there exists a mass- and redshift- dependent "characteristic" environment (${\rm log} (1+δ_a(M_{\rm h}, z)) = 0.021\times (M_{\rm h}/M_\odot)^{0.16} + 0.07 z -1.12$, up to $z\sim 10$) at which galaxies are most efficient at accreting dark matter, e.g at a rate of $0.2\%$ of their mass every Myr at $z=5$; (ii) the number of minor and major mergers and their contributions to the dark matter assembly increases with halo mass at all redshifts and is mostly independent of the environment; (iii) at $z=5$ minor mergers contribute slightly more (by up to $\sim 10\%$) to the dark matter assembly while for the stellar assembly, major mergers dominate the contribution from minor mergers for $M_{\rm h}\lesssim 10^{11.5}M_\odot$ galaxies; (iv) radiative feedback quenches star formation more in low-mass galaxies ($M_{\rm h} \lesssim 10^{9.5}M_\odot$) in over-dense environments (${\rm log}(1+δ) > 0.5$); dominated by their major branch, this yields star formation histories biased towards older ages with a slower redshift evolution.

astro-ph.GA↗
arXiv2022

Astraeus V: The emergence and evolution of metallicity scaling relations during the Epoch of Reionization

In this work, we have implemented a detailed physical model of galaxy chemical enrichment into the ${\it Astraeus}$ (semi-numerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dark matter simUlationS) framework which couples galaxy formation and reionization in the first billion years. Simulating galaxies spanning over 2.5 orders of magnitude in halo mass with $M_h \sim 10^{8.9-11.5} M_\odot$ ($M_h \sim 10^{8.9-12.8} M_\odot$) at $z \sim 10 ~ (5)$, we find: (i) smooth-accretion of metal-poor gas from the intergalactic medium (IGM) plays a key role in diluting the interstellar medium (ISM) metallicity which is effectively restored due to self-enrichment from star formation; (ii) a redshift averaged gas-mass loading factor that depends on the stellar mass as $η_g \approx 1.38 ({M_*}/{10^{10} M_\odot})^{-0.43}$; (iii) the mass-metallicity relation is already in place at $z \sim 10$ and shows effectively no redshift evolution down to $z \sim 5$; (iv) for a given stellar mass, the metallicity decreases with an increase in the star formation rate (SFR); (v) the key properties of the gas-phase metallicity (in units of 12+log(O/H), stellar mass, SFR and redshift are linked through a high-redshift fundamental plane of metallicity (HFPZ) for which we provide a functional form; (vi) the mass-metallicity-SFR relations are effectively independent of the reionization radiative feedback model for $M_* \geq 10^{6.5} M_\odot$ galaxies; (vii) while low-mass galaxies ($M_h \leq 10^9 M_\odot$) are the key contributors to the metal budget of the IGM at early times, higher mass halos provide about 50% of the metal budget at lower-redshifts.

astro-ph.GA↗
arXiv2022

Astraeus VI: Hierarchical assembly of AGN and their large-scale effect during the Epoch of Reionization

In this work, the sixth of a series, we use the ASTRAEUS (semi-numerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dark matter simUlationS) framework to investigate the nature of the sources that reionized the Universe. We extend ASTRAEUS, which already couples a galaxy formation semi-analytical model with a detailed semi-numerical reionization scheme, to include a model for black hole formation, growth, and the production of ionizing radiation from associated AGN (active galactic nuclei). We calibrate our fiducial AGN model to reproduce the bolometric luminosity function at z ~ 5, and explore the role of the resulting AGN population in reionizing the Universe. We find that in all the models yielding a reasonable AGN luminosity function, galaxies dominate overwhelmingly the ionizing budget during the Epoch of Reionization, with AGN accounting for 1-10% of the ionizing budget at z=6 and starting to play a role only below z < 5.

astro-ph.GA↗
arXiv2021

AstraeusIV : Quantifying the star formation histories of galaxies in the Epoch of Reionization

We use the \textsc{astraeus} framework, that couples an N-body simulation with a semi-analytic model for galaxy formation and a semi-numerical model for reionization, to quantify the star formation histories (SFHs) of galaxies in the first billion years. Exploring four models of radiative feedback, we fit the SFH of each galaxy at $z>5$ as $\mathrm{log}(\mathrm{SFR}(z))=-α(1 + z)+β$; star formation is deemed stochastic if it deviates from this fit by more than $Δ_\mathrm{SFR}=0.6\,$dex. Our key findings are: (i) The fraction of stellar mass formed and time spent in the stochastic phase decrease with increasing stellar mass and redshift $z$. While galaxies with stellar masses of $M_\star\sim10^7M_\odot$ at $z\sim5~(10)$ form $\sim70\%~(20\%)$ of their stellar mass in the stochastic phase, this reduces to $<10\%$ at all redshifts for galaxies with $M_\star > 10^{10}M_\odot$; (ii) the fractional mass assembled and lifetime spent in the stochastic phase do not significantly change with the radiative feedback model used; (iii) at all redshifts, $α$ increases (decreases for the strongest radiative feedback model) with stellar mass for galaxies with $M_\star\lesssim 10^{8.5}M_\odot$ and converges to $\sim0.18$ for more massive galaxies; $β$ always increases with stellar mass. Our proposed fits can reliably recover the stellar masses and mass-to-light ratios for galaxies with $M_\star\sim10^{8-10.5}M_\odot$ and $M_{UV}\sim-17~{\rm to}~-23$ at $z\sim 5-9$. This physical model can therefore be used to derive the SFHs for galaxies observed by a number of forthcoming instruments.

astro-ph.GA↗
arXiv2021

Astraeus II: Quantifying the impact of cosmic variance during the Epoch of Reionization

Next generation telescopes such as the James Webb Space Telescope (JWST) and the Nancy Grace Roman Space Telescope (NGRST) will enable us to study the first billion years of our Universe in unprecedented detail. In this work we use the ASTRAEUS (semi-numerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dArk mattEr simUlationS) framework, that couples galaxy formation and reionization (for a wide range of reionization feedback models), to estimate the cosmic variance expected in the UV Luminosity Function (UV LF) and the Stellar Mass Function (SMF) in JWST surveys. We find that different reionization scenarios play a minor role in the cosmic variance. Most of the cosmic variance is completely driven by the underlying density field and increases above $100\%$ for ${\rm M}_{\rm UV} \sim -17.5 (-20)$ at $z =12 (6)$ for the JADES-deep survey (the deep JWST Advanced Extragalactic Survey with an area of 46 arcmin$^2$); the cosmic variance decreases with an increasing survey area roughly independently of redshift. We find that the faint-end (${\rm M}_{\rm UV} > -17$) slope of the Lyman Break Galaxies (LBGs) UV LF becomes increasingly shallower with increasing reionization feedback and show how JWST observations will be able to distinguish between different models of reionization feedback at $z>9$, even accounting for cosmic variance. We also show the environments (in terms of density and ionization fields)of Lyman Break Galaxies during the EoR, finding that the underlying over-density and ionization fraction scale positively with the UV luminosity. Finally, we also provide a public software tool to allow interested readers to compute cosmic variance for different redshifts and survey areas.

astro-ph.GA↗
arXiv2021

Astraeus III: The environment and physical properties of reionization sources

In this work, we use the {\sc astraeus} (seminumerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dArk mattEr simUlationS) framework which couples galaxy formation and reionization in the first billion years. Exploring a number of models for reionization feedback and the escape fraction of ionizing radiation from the galactic environment ($f_\mathrm{esc}$), we quantify how the contribution of star-forming galaxies {(with halo masses $M_h>10^{8.2}$M$_\odot$)} to reionization depends on the radiative feedback model, $f_\mathrm{esc}$, and the environmental over-density. Our key findings are: (i) for constant $f_\mathrm{esc}$ models, intermediate-mass galaxies (with halo masses of $M_h\simeq10^{9-11}$M$_\odot$ and absolute UV magnitudes of $M_{UV} \sim -15$ to $-20$) in intermediate-density regions drive reionization; (ii) scenarios where $f_\mathrm{esc}$ increases with decreasing halo mass shift the galaxy population driving reionization to lower-mass galaxies ($M_h\lesssim10^{9.5}$M$_\odot$) with lower luminosities ($M_{UV} \gtrsim-16$) and over-densities; (iii) reionization imprints its topology on the ionizing emissivity of low-mass galaxies ($M_h\lesssim10^{9}$M$_\odot$) through radiative feedback. Low-mass galaxies experience a stronger suppression of star formation by radiative feedback and show lower ionizing emissivities in over-dense regions; (iv) a change in $f_\mathrm{esc}$ with galaxy properties has the largest impact on the sources of reionization and their detectability, with the radiative feedback strength and environmental over-density playing a sub-dominant role; (v) JWST-surveys (with a limiting magnitude of $M_{UV} = -16$) will be able to detect the galaxies providing $\sim 60-70\%$ ($\sim 10\%$) of reionization photons at $z=7$ for constant $f_\mathrm{esc}$ models (scenarios where $f_\mathrm{esc}$ increases with decreasing halo mass).

astro-ph.GA↗
arXiv2021

Astraeus I: The interplay between galaxy formation and reionization

We introduce a new self-consistent model of galaxy evolution and reionization, ASTRAEUS (semi-numerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dArk mattEr simUlationS), which couples a state-of-the-art N-body simulation with the semi-analytical galaxy evolution DELPHI and the semi-numerical reionization scheme CIFOG. ASTRAEUS includes all the key processes of galaxy formation and evolution (including accretion, mergers, supernova and radiative feedback) and follows the time and spatial evolution of the ionized regions in the intergalactic medium (IGM). Importantly, it explores different radiative feedback models that cover the physically plausible parameter space, ranging from a weak and delayed to a strong and immediate reduction of gas mass available for star formation. From our simulation suite that covers the different radiative feedback prescriptions and ionization topologies, we find that radiative feedback continuously reduces star formation in galaxies with $M_h<10^{9.5}M_{\odot}$ upon local reionization; larger mass halos are unaffected even for the strongest and immediate radiative feedback cases during reionization. For this reason, the ionization topologies of different radiative feedback scenarios differ only on scales smaller than $1-2$Mpc, and significant deviations are only found when physical parameters (e.g. the escape fraction of ionizing photons) are altered based on galactic properties. Finally, we find observables (the ultra-violet luminosity function, stellar mass function, reionization histories and ionization topologies) are hardly affected by the choice of the used stellar population synthesis models that either model single stars or binaries.

astro-ph.GA↗
arXiv2021

Optical projection and spatial separation of spin entangled triplet-pairs from the S1 (21Ag-) state of pi-conjugated systems

The S1 (21Ag-) state is an optically dark state of natural and synthetic pi-conjugated materials that can play a critical role in optoelectronic processes such as, energy harvesting, photoprotection and singlet fission. Despite this widespread importance, direct experimental characterisations of the electronic structure of the S1 (21Ag-) wavefunction have remained scarce and uncertain, although advanced theory predicts it to have a rich multi-excitonic character. Here, studying an archetypal polymer, polydiacetylene, and carotenoids, we experimentally demonstrate that S1 (21Ag-) is a superposition state with strong contributions from spin-entangled pairs of triplet excitons (1(TT)). We further show that optical manipulation of the S1 (21Ag-) wavefunction using triplet absorption transitions allows selective projection of the 1(TT) component into a manifold of spatially separated triplet-pairs with lifetimes enhanced by up to one order of magnitude and whose yield is strongly dependent on the level of inter-chromophore coupling. Our results provide a unified picture of 21Ag-states in pi-conjugated materials and open new routes to exploit their dynamics in singlet fission, photobiology and for the generation of entangled (spin-1) particles for molecular quantum technologies.

physics.chem-ph↗
arXiv2021

Optical and electronic properties of colloidal CdSe Quantum Rings

Luminescent colloidal CdSe nanorings are a new type of semiconductor structure that have attracted interest due to the potential for unique physics arising from their non-trivial toroidal shape. However, the exciton properties and dynamics of these materials with complex topology are not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and single particle measurements to study these materials. We find that on transformation of CdSe nanoplatelets to nanorings, by perforating the center of platelets, the emission lifetime decreases and the emission spectrum broadens due to ensemble variations in the ring size and thickness. The reduced PL quantum yield of nanorings (~10%) compared to platelets (~30%) is attributed to an enhanced coupling between: (i) excitons and CdSe LO-phonons at 200 cm-1 and (ii) negatively charged selenium-rich traps which give nanorings a high surface charge (~-50 mV). Population of these weakly emissive trap sites dominates the emission properties with an increased trap emission at low temperatures relative to excitonic emission. Our results provide a detailed picture of the nature of excitons in nanorings and the influence of phonons and surface charge in explaining the broad shape of the PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that the excitonic properties of nanorings are not solely a consequence of the toroidal shape but are also a result of traps introduced by puncturing the platelet center.

physics.app-ph↗
arXiv2020

How Does the Symmetry of S1 Influence Exciton Transport in Conjugated Polymers?

Many optoelectronic devices based on organic materials require rapid and long-range singlet exciton transport. Key factors that control the transport of singlet excitons includes the electronic structure of the material, disorder and exciton-phonon coupling. An important parameter whose influence on exciton transport has not been explored is the symmetry of the singlet electronic state (S1). Here, we employ femtosecond transient absorption spectroscopy and microscopy to reveal the relationship between the symmetry of S1 and exciton transport in highly aligned, near-disorder free, one-dimensional conjugated polymers based on polydiacetylene.

cond-mat.mes-hall↗
arXiv2020

First principles modeling of exciton-polaritons in polydiacetylene chains

Exciton-polaritons in organic materials are hybrid states that result from the strong interaction of photons and the bound excitons that these materials host. Organic polaritons hold great interest for optoelectronic applications, however progress towards this end has been impeded by the lack of a first principles approach that quantifies light-matter interactions in these systems, and which would allow the formulation of molecular design rules. Here we develop such a first principles approach, quantifying light-matter interactions. We exemplify our approach by studying variants of the conjugated polymer polydiacetylene, and we show that a large polymer conjugation length is critical towards strong exciton-photon coupling, hence underlying the importance of pure structures without static disorder. By comparing to our experimental reflectivity measurements, we show that the coupling of excitons to vibrations, manifested by phonon side bands in the absorption, has a strong impact on the magnitude of light-matter coupling over a range of frequencies. Our approach opens the way towards a deeper understanding of polaritons in organic materials, and we highlight that a quantitatively accurate calculation of the exciton-photon interaction would require accounting for all sources of disorder self-consistently.

cond-mat.mtrl-sci↗
arXiv2019

Euclid preparation: II. The EuclidEmulator -- A tool to compute the cosmology dependence of the nonlinear matter power spectrum

We present a new power spectrum emulator named EuclidEmulator that estimates the nonlinear correction to the linear dark matter power spectrum. It is based on a spectral decomposition method called polynomial chaos expansion. All steps in the construction of the emulator have been tested and optimized: the large high-resolution N-body simulations carried out with PKDGRAV3 were validated using a simulation from the Euclid Flagship campaign and demonstrated to have converged up to wavenumbers $k\approx 5\,h\,{\rm Mpc}^{-1}$ for redshifts $z\leq 5$. The emulator is constructed using the uncertainty quantification software UQLab and it has been optimized first by creating mock emulators based on Takahashi's HALOFIT. We show that it is possible to successfully predict the performance of the final emulator in this way prior to performing any N-body simulations. We provide a C-code to calculate the nonlinear correction at a relative accuracy of $\sim0.3\%$ with respect to N-body simulations within 50 ms. The absolute accuracy of the final nonlinear power spectrum is comparable to one obtained with N-body simulations, i.e. $\sim 1\%$ for $k\lesssim 1\,h\,{\rm Mpc}^{-1}$ and $z\lesssim 3.5$. This enables efficient forward modeling in the nonlinear regime allowing for maximum likelihood estimation of cosmological parameters. EuclidEmulator has been compared to HALOFIT and CosmicEmu, an alternative emulator based on the Mira-Titan Universe, and shown to be more accurate than these other approaches. This work paves a new way for optimal construction of future emulators that also consider other cosmological observables, use higher resolution input simulations and investigate higher dimensional cosmological parameter spaces.

astro-ph.CO↗
arXiv2009

A twisted polydiacetylene quantum wire: Influence of conformation on excitons in polymeric quasi-1D systems

Highly luminescent isolated polydiacetylene (PDA) chains in a new diacetylene crystal are studied. The diacetylene crystal structure and theoretical calculations suggest that the chains are not planar and more strongly twisted than any known PDA. Yet the chains behave as quasi perfect quantum wires, showing that this behaviour is generic among PDA, provided crystal order is high enough. The exciton energy, E0, is higher than in any other well ordered PDA providing a relationship between E0 and twist angle. On the other hand the exciton binding energy and Bohr radius are significantly affected as demonstrated in electroabsorption experiments (and developed in a companion paper).

cond-mat.other↗