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Cosmic Web 2023 @KITP

Connecting Galaxies to Cosmology at High and Low Redshift

Date: Jan 3, 2023 - Mar 17, 2023

Coordinators: Joanne Cohn, Nick Kaiser, Christophe Pichon, and Dmitri Pogosyan

Application deadline is: Nov 19, 2021.

The matter distribution of the Universe follows a web-like structure at all overdensities[14], from the intergalactic medium to clusters of galaxies. It consists of filaments, sheets, knots and voids[9,18,69]. This cosmic web[53,115], the focus of this KITP programme, provides the framework for the formation and evolution of galaxies in the Universe[4,118], at high and low redshift, and is fundamentally connected to cosmology. Our goal is to bring together cosmic web experts in observations, simulations and theory, to address the following two intertwined questions:
* How may the cosmic web be used to constrain cosmological models and the cosmic re-ionisation history of the Universe?

  • What is the relationship between the formation and evolution of galaxies and their cosmic web environment?

During this programme, through talks and focussed discussion sessions, we expect to understand the web itself and its connections to galaxy formation, re-ionisation and cosmology. We also anticipate fostering new collaborations and idea cross-pollination beyond these fields.



Galaxies are not islands randomly distributed in the Universe[36]. They form a complex network – the so-called cosmic web – made of filaments and walls that border huge regions of low density, called voids[9,131], and intersect at clusters of galaxies[14,68,70,114].The prominent filamentary channels of the cosmic web might contain up to 40% of the matter of the Universe[31], and most of the gas at high redshift. At low redshift, around half of the warm gas, presumably accounting for the missing baryons, is also believed to be hiding in these filaments. One striking aspect of the cosmic web is its multiscale character, manifesting itself by web-like structures over many decades. It provides us with a unique opportunity to best estimate cosmological parameters on the one hand, which in turn optimally requires that we understand how its structure impacts galaxy formation[134,151] on the other hand.

This network indeed plays a paramount role in our quest to answer the fundamental questions at the heart of Cosmology: what are the constituents of our Universe and what laws dictate its behaviour?[12,100,154] More specifically, what is the nature of the dark matter that is believed to be the dominant form of matter in the Universe? Which physical processes drive the observed accelerating expansion of the cosmos?[47] Is there any observational evidence that new theories of gravity are needed, beyond Einstein’s theory of General Relativity? Since the cosmic web is certainly the most informative large and intermediate scale feature at low and high redshift, understanding its dynamics is essential to answering these questions. With the advent of very large galaxy surveys, astronomers have now ventured into the era of big data[1,47,57,60,65,87,92]. Hence there exists a dire need for theorists to build computational tools that can efficiently process complex multi-wavelength catalogues to help maximise the scientific return on quantitative cosmic parameter estimation. In particular, this means being able to probe both the high redshift ionised bubbly web, but also the nonlinear small-scale regime of structure formation[6] at low redshift (from the gas and the galaxy distribution), while tackling systematic effects like redshift space distortions, biasing[5x,29,32,53] or intrinsic alignments[22,25,31]. The timing for the present programme at KITP in Jan-March 2023 is therefore ideal, given the wealth of two and three dimensional existing (SDSS, Vipers, GAMA, MUSE, SAMI, MANGA, COSMOS, CLAMATO, CALIFA, DES, WEAVE), and upcoming (Euclid, LSST, PFS, WFIRST, SDSS-V, SPHEREx, DESI, Hector, 4MOST, MOON, SKA, CONCERTO, CDIM, COMAP, JPASS, WAVE, TIME, HERA, MOSAIC, MSE, HARMONI, SpecTel) surveys, which have been designed to probe the cosmic web from gas and galaxies, across an extensive range of epochs and wavelengths.

In anticipation, our community has developed sets of sophisticated multi-scale algorithms based on recent advances in the field of computational topology and geometry (MST, Bisous, FINE, T-web, V-web, T-Rex, CLASSIC, NEXUS+, MultiscaleMMF, Semita, Spineweb, watershed, adhesion, skeleton, DisPerSE, ORIGAMI, MSWA, Felix, skeleton-tree). Together with the corresponding statistical theory for the geometry and dynamical evolution[16,62,94] of the critical sets that trace structural elements (ridge length, curvature and torsion, wall area or coalescence rate, redshift space distortion, percolation threshold, bifurcation statistics, shell crossing counts, Alcock-Paczynski tests, etc.), these algorithms complement classical topological estimators based on Minkowski functionals[23,25,80] (Genus, Betti numbers, etc) or deep learning techniques (web-z, reconstruction, baryon painting etc),  and can readily be implemented on sparse or noisy datasets such as galaxy catalogues, intensity maps or Lyman-alpha tomography observations. More precisely, these void, ridge and wall tracers[35,36] establish a rigorous mathematical framework to study the formation of structures on all scales, encapsulating the main morphological properties of cold streams, percolating bubbles and the cosmic web[6,30,88].

High resolution hydro-dynamical simulations[33,55,74,76] have indeed revealed that structures originated in the cosmic web can be found down to the Virial radius of haloes [34,35,36], and that they play a prominent role in the accretion of cold gas onto protogalaxies in the early Universe. This ties in with results from a range of studies that have indicated substantial environmental influence[34,35,137,146,149] on the formation of galaxies across cosmic time. It has therefore become of key importance to gain more insight into the structure and dynamics of the cosmic web, to understand how it couples down to galactic scales.

Conceptually, the science of the cosmic web lies within the broader framework of large-scale structure science, but with a relevant emphasis on anisotropic features beyond angularly-averaged field statistics[75]. Immense cosmological information has already been gleaned from the very large (quasi-linear) scales on the one hand, or from properties of the nodes of the web[3,61], and, more generally, collapsed dark matter halos[27,59,60] and their immediate vicinity on the other hand[11,14,18,28,97,99,152]. Our understanding of the properties and dynamics of the other low density components of the web, i.e. the filaments, walls, and voids and their interconnections, was until recently lagging behind[16,132], limiting their usefulness to describe galaxy formation and evolution[118], as well as our understanding of the anisotropic web itself as an evolving physical system to probe cosmology. Indeed, since the cosmic web evolves across cosmic time in a bottom-up hierarchical way[12,74,100,151], the largest structures form the latest, and this growth is directly sensitive to the properties of the Universe as a whole (e.g the amount and nature of dark energy).

It has been known since the seminal morphology-density relation of Dressler[55], that certain fundamental galaxy properties correlate with the environment. Yet its often invoked quantification was typically too simple - usually a measure of the projected angular distance to the nth nearest neighbour. Recently, a variety of heuristic measures have been developed to analyse the specific spatial patterns of the Cosmic Web[88],. These different approaches address the complexity of individual structures in the matter distribution, as well as its connectivity[23], its lack of structural symmetries, intrinsic multiscale nature and the wide range of densities involved. They have been used to show that the cosmic web plays a significant role during galaxy formation, e.g. by funnelling the accretion of cold gas[79]. This impacts many observed galactic properties, such as spin orientation[63,83], specific star formation rate, disc size and thickness (i.e. virtually all parameters subject to the so-called assembly bias[7,14,31,51,60]). In turn, all these parameters govern the observed diversity of galactic morphologies[36,44] and spectrophotometric properties throughout cosmic history[108,122,127] and impacts cosmic estimators.

Achieving a detailed statistical comparison between theory, observations and numerical simulations of the cosmic web therefore provides our community with a unique opportunity to now

  • measure the geometry and re-ionisation history of the Universe,
  • unveil the processes at stake in the emergence of the first high redshift galaxies[133], and
  • explain the settling of galactic discs[109] and ellipticals at low redshift. These investigations complement each other.

An improved understanding of the impact of the anisotropic environment on galaxy formation at early and late time allows us to better model their mutual influence, which in turn allows us to probe smaller cosmic scales and extended cosmic epochs, providing us with unprecedented cosmic parameter estimation accuracy.

Implication for cosmology and the dawn of structure formation

The structure of the universe emerges out of a smooth, nearly perfectly homogenous distribution of matter, seeded with small primordial density fluctuations. The evolution of these overdensities in an expanding universe drives the formation of a cosmic foam[12,46], and sets the stage upon which baryons can cool and galaxies can form[9,133]

On the largest scales, these structures are believed to be the result of the gravitational growth of primordial perturbations originating from quantum fluctuations[152]. In this initially Gaussian Universe, the matter density field is therefore fully described by its two-point correlation function[70,75]. However, deviations from Gaussianity inevitably arise due to the non-linear dynamics of growing structures, generating information that is recorded in part in higher correlations. N-point correlation functions are notoriously difficult to measure when N increases, and can typically only be modelled accurately on very large scales. However, many more modes will be measured by future surveys on smaller scales[9,46,70,75,86]. It is therefore crucial to go beyond the correlation function approach and rely on complementary geometric and topologic observables, to extract as much cosmological information as possible.

The cosmic web defines the fundamental spatial organisation of matter and galaxies on intermediate scales and densities, at the present time of one to tens of mega-parsecs, at the boundary between the linear and the non-linear regime of gravitational clustering [125,150,154]. Structures at these scales contains most of the cosmological information, as the evolution and early dynamics of the cosmic web are to a large extent dependent on the nature of dark matter and dark energy. At intermediate redshifts, tomographic spectroscopy traces with higher sensitivity the neutral gas in Ly-alpha absorption, convolved by its thermal and turbulent state. At high redshifts, re-ionisation observables provide an inside-out reverse image of the early cosmic web: ionizing UV radiation emanates from dense regions and easily fills voids, before penetrating source-free walls and filaments. As the evolution of the cosmic web is directly dependent on the laws of gravity, each of the relevant cosmological variables leaves its imprint on its geometry and topology at each epoch[23,80], and on the relative importance of its structural elements, i.e. of filaments, walls, cluster nodes and voids. 

This arguably constitutes the main challenge faced by cosmologists nowadays: to establish a rigorous mathematical framework to study the web-like structures formed by matter on most scales, and capture the main morphological properties of these large structures on the widest range of scales. During our KITP programme, we apply our novel geometric estimators to state-of-the-art numerical simulations, reconstructions and recent large scale surveys to

  • constrain the equation of state of dark energy and primordial non-Gaussianity through a thorough complementary analysis of the cosmic evolution of the cosmic web down to the smallest possible scales;
  • build convincing error estimates for the corresponding likelihood analyses;
  • study the influence of modified gravity on the geometry of the large-scale structures[17];
  • probe the percolation process of re-ionisation fronts at high redshift which map the negative image of the cosmic web;
  • quantify the persistence of the position-velocity phase space structure of the cosmic web and its impact on non local biasing.

Implication for galaxies as cosmic tracers

On galactic scales, the characterisation of the web is crucial for elucidating environmental influences on the process of galaxy formation since it will impact their observational properties hence impact bias[81,82,84]. A typical example of such influence is the origin of the spin of galaxies[57,78,83,125]: the same tidal forces responsible for the torquing of collapsing protogalactic halos are also driving the anisotropic contraction of matter in the surroundings. Currently, on the basis of large and extensive galaxy redshift surveys, we are finding evidence for a range of other environmental influences. An archetypical influence is the history of our own Milky Way, set by the tides of the Local Group. Whether we live in a local filament or super-galactic sheet may shed light on important aspects of our own cosmic history, including the anisotropic distribution of satellite galaxies around our Milky Way, and the alignment of nearby disc galaxies within the super-galactic plane[139,140,152].

Nowadays, the vast majority of galaxy formation studies routinely embrace the impact of filamentary accretion, acknowledging that most galaxies are born via matter accreted onto a forming core via cold accretion of gas along filaments[125,139,140,153]. Their relative orientation impacts intrinsic alignments and the build up of the observed morphological diversity. The role of filaments as channels along which baryons accumulate and migrate also lies at the core of the “missing baryon” problem, one of the great challenges of our current cosmological paradigm. It is likely that most baryons in the Universe reside in a hot diffuse state, known as the warm/hot intergalactic medium, specifically concentrated along the filamentary  geometry of the cosmic web. Numerical simulations show that the morphology and shape of galaxies must also be driven in part by the environment, which is best qualified by the properties of the cosmic web. Its importance is amplified by the fact that most of the gas that is ultimately accreted by galaxies also closely follows the larger cosmic web, which best accounts for the anisotropic sourcing of the tidal torques spinning up galaxies[86,106,150]. This has now undisputedly been quantified in simulations, theory, and, to some extent, measured in data, albeit indirectly. Specifically, we now have a novel ab initio Lagrangian theory for galactic connectivity[23] and spin acquisition within the cosmic web[65,87,142], and the corresponding theory for accretion and merger rate using excursion set and critical event theory. For primordial Non-Gaussianity, scale dependent bias and higher order statistics probes deviation of the initial conditions and may help to bring down the constraints on fNL to O(1). This requires a better understanding and careful modelling of the large-scale bias, and in particular, its modulation across the cosmic web[48,51]. This is crucial to turn what is currently a nuisance parameter into an additional source of precise (sub percent) cosmological information.

Given the outstanding wealth of present and upcoming data and novel methodologies, it is timely to bring together experts of the cosmic web, in order to coalesce different approaches and ideas. To harvest all the cosmological information available on such intermediate scales, we must integrate in our models the pivotal role played by the anisotropic cosmic web in setting up the spine of the large scale structure on the one hand, and impacting the physical properties of dynamically coupled tracers on the other hand.

The KITP programme therefore aims to contribute to

  • constraining the dynamics and morphology of galaxies within the filamentary flow (spin, velocity, colour); self-consistently modelling galaxy intrinsic alignments for lensing;
  • identifying dynamical signatures of cold flows[141]; investigating when the cumulative effect of coherence of large-scale inflows dominates over the stochastic effect of feedback; following gas flows and angular momentum transport[15,19,33,34,41] from the very large scales down to the circumgalactic medium and their interaction with feedback driven outflows;
  • resolving disc scale-height numerically and observationally with IFUs, and jointly modelling the kinematics of flows within filaments; understanding cosmic disc settling through the evolution of open galactic systems within known anisotropic fluctuating tides[28,90] and accretion rates;
  • connecting theoretical secular work on galaxy evolution to its anisotropic (cosmic) framework on multiple scales;
  • reframe our improved understanding of galaxy formation across cosmic time into an effective biasing model across standard and exotic cosmic probes.

All in all, this KITP programme offers a perfect synergy between cosmic web experts working in typically distinct fields, but brought together via common tools, inspiration and methods.

Since the techniques transcend the remit of cosmic web science, the programme is of interest to scientists beyond extragalactic astrophysics, e.g. those interested in the structure of ISM turbulence, but also colleagues in solid state physics, graph theory etc.

We are carrying a three months programme. Its duration is motivated by the wealth of topics to cover. We also have a 4 days conference entitled “Co-evolution of the cosmic web and galaxies across cosmic time”. Its specific goal of the conference is to connect to the broader community of the Large-Scale Structures on the one hand, and to Galaxy Formation on the other hand, geared towards cosmological parameter estimation. It is also an opportunity to invite a couple of experts beyond the scope of astrophysics (e.g. computational topology, network physics, etc), to enlighten our community with state-of-the-art achievements in their respective fields. We finally aim to organise a hackathon week on ridge/wall/void extraction before the conference, as part of the programme, and present its results during the conference.

This programme will also be running in parallel with the “Building a Physical Understanding of Galaxy Evolution with Data-driven Astronomy” programme, enabling an additional synergy.

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Page last modified on April 26, 2024, at 12:05 PM