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Galaxies and their dark-matter halos are commonly presupposed to spin. But it is an open question how this spin manifests in halos and soliton cores made of scalar dark matter (SDM, including fuzzy/wave/ultralight-axion dark matter). One way spin could manifest in a necessarily irrotational SDM velocity field is with a vortex. But recent results have cast doubt on this scenario, finding that vortices are generally unstable except with substantial repulsive self-interaction. In this paper, we introduce an alternative route to stability: in both (non-relativistic) analytic calculations and simulations, a black hole or other central mass at least as massive as a soliton can stabilize a vortex within it. This conclusion may also apply to AU-scale halos bound to the sun and stellar-mass-scale Bose stars., Comment: Accepted by JCAP. 22 pages, 5 figures. Supplementary animations at https://doi.org/10.5281/zenodo.7675830 or https://www.youtube.com/playlist?list=PLHrf0iQS5SY7Xt2sjqskF3kmHd00HrdfX

According to the standard model of cosmology, the arrangement of matter in the cosmos on scales much larger than galaxies is entirely specified by the initial conditions laid down during inflation. But zooming in by dozens of orders of magnitude to microscopic (and human?) scales, quantum randomness reigns, independent of the initial conditions. Where is the boundary of determinism, and how does that interplay with chaos? Here, we make a first attempt at answering this question in an astronomical context, including currently understood processes. The boundary is a function, at least, of length scale, position, and matter type (dark matter being more simply predictable). In intergalactic voids, the primordial pattern of density fluctuations is largely preserved. But we argue that within galaxies, the conditions are at minimum chaotic, and may even be influenced by non-primordial information, or randomness independent of the initial conditions. Randomness could be supplied by events such as supernovae and jets from active galactic nuclei (AGN) and other accretion disks, which, with the help of chaotic dynamics, could broadcast any possible microscopic randomness to larger scales, eventually throughout a galaxy. This may be generated or amplified by a recently investigated process called spontaneous stochasticity, or effective randomness in turbulent systems arising from arbitrarily small perturbations., Comment: Submitted to Philosophy of Astrophysics volume, ed. Boyd et al., Springer. 17pp, 6 figs

Are there parallels between the furthest reaches of our universe, and the foundations of thought, awareness, perception, and emotion? What are the connections between the webs and structures that define both? What are the differences? "As Above As Below" was an exhibition that examined these questions. It consisted of six artworks, each of them the product of a collaboration that included at least one artist, astrophysicist, and neuroscientist. The installations explored new parallels between intergalactic and neuronal networks through media such as digital projection, virtual reality, and interactive multimedia, and served to illustrate diverse collaboration practices and ways to communicate across very different fields., Comment: In SciArt Magazine, https://www.sciartmagazine.com/collaboration-as-above-as-below.html ; exhibition video https://youtu.be/RM3gDjb3phU ; panel discussion https://youtu.be/qNhtyvPz8yw ; project website https://asaboveasbelow.com/

SPHEREx is a proposed NASA MIDEX mission selected for Phase A study. SPHEREx would carry out the first all-sky spectral survey in the near infrared. At the end of its two-year mission, SPHEREx would obtain 0.75-to-5$��$m spectra of every 6.2 arcsec pixel on the sky, with spectral resolution R>35 and a 5-$��$ sensitivity AB$>$19 per spectral/spatial resolution element. More details concerning SPHEREx are available at http://spherex.caltech.edu. The SPHEREx team has proposed three specific science investigations to be carried out with this unique data set: cosmic inflation, interstellar and circumstellar ices, and the extra-galactic background light. Though these three themes are undoubtedly compelling, they are far from exhausting the scientific output of SPHEREx. Indeed, SPHEREx would create a unique all-sky spectral database including spectra of very large numbers of astronomical and solar system targets, including both extended and diffuse sources. These spectra would enable a wide variety of investigations, and the SPHEREx team is dedicated to making the data available to the community to enable these investigations, which we refer to as Legacy Science. To that end, we have sponsored two workshops for the general scientific community to identify the most interesting Legacy Science themes and to ensure that the SPHEREx data products are responsive to their needs. In February of 2016, some 50 scientists from all fields met in Pasadena to develop these themes and to understand their implications for the SPHEREx mission. The 2016 workshop highlighted many synergies between SPHEREx and other contemporaneous astronomical missions, facilities, and databases. Consequently, in January 2018 we convened a second workshop at the Center for Astrophysics in Cambridge to focus specifically on these synergies. This white paper reports on the results of the 2018 SPHEREx workshop., 50 pages, 24 figures, more details at http://spherex.caltech.edu

Cosmological $N$-body simulations play a vital role in studying models for the evolution of the Universe. To compare to observations and make a scientific inference, statistic analysis on large simulation datasets, e.g., finding halos, obtaining multi-point correlation functions, is crucial. However, traditional in-memory methods for these tasks do not scale to the datasets that are forbiddingly large in modern simulations. Our prior paper proposes memory-efficient streaming algorithms that can find the largest halos in a simulation with up to $10^9$ particles on a small server or desktop. However, this approach fails when directly scaling to larger datasets. This paper presents a robust streaming tool that leverages state-of-the-art techniques on GPU boosting, sampling, and parallel I/O, to significantly improve performance and scalability. Our rigorous analysis of the sketch parameters improves the previous results from finding the centers of the $10^3$ largest halos to $\sim 10^4-10^5$, and reveals the trade-offs between memory, running time and number of halos. Our experiments show that our tool can scale to datasets with up to $\sim 10^{12}$ particles while using less than an hour of running time on a single GPU Nvidia GTX 1080., Comment: preprint

The cosmic web (the arrangement of matter in the universe), spider's webs, and origami tessellations are linked by their geometry (specifically, of sectional-Voronoi tessellations). This motivates origami and textile artistic representations of the cosmic web. It also relates to the scientific insights origami can bring to the cosmic web; we show results of some cosmological computer simulations, with some origami-tessellation properties. We also adapt software developed for cosmic-web research to provide an interactive tool for general origami-tessellation design., Comment: Accepted to Origami$^7$, proceedings of 7OSME, the 7th meeting of Origami, Science, Mathematics and Education, Oxford, Sep 2018. Software at https://github.com/neyrinck/sectional-tess

We propose a common terminology for use in describing both temporal merger trees and spatial structure trees for dark-matter halos. We specify a unified data format in HDF5 and provide example I/O routines in C, FORTRAN and PYTHON., Comment: Technical report: a proposed merger tree data format. Not submitted to any journal. Source code available at https://bitbucket.org/ProfPAThomas/mergertree

The star formation history of galaxies is a complex process usually considered to be stochastic in nature, for which we can only give average descriptions such as the color-density relation. In this work we follow star-forming gas particles in a hydrodynamical N-body simulation back in time in order to study their initial spatial configuration. By keeping record of the time when a gas particle started forming stars we can produce gas-star isochrone surfaces delineating the surfaces of accreting gas that begin producing stars at different times. These accretion surfaces are closely packed inside dense regions, intersecting each other, and as a result galaxies inside proto-clusters stop accreting gas early, naturally explaining the color dependence on density. The process described here has a purely gravitational / geometrical origin, arguably operating at a more fundamental level than complex processes such as AGN and supernovae, and providing a conceptual origin for the color-density relation.

We discuss the Kolmogorov complexity of primordial patches that collapse to form galaxies like the Milky Way; this complexity quantifies the amount of initial data available to form the structure. We also speculate on how the quantity changes with time. Because of dark-matter and baryonic collapse processes, it likely decreases with time, i.e.\ information sinks dominate sources. But sources of new random information do exist; e.g., a central black hole with an accretion disk and jets could in principle broadcast small-scale quantum fluctuations over a substantial portion of a galaxy. A speculative example of how this concept might be useful is in differentiating between warm (WDM) and cold (CDM) dark matter. With WDM, the initial patch that formed the Milky Way would have had few features, making the present high degree of structure a curiosity. The primordial patch would have had only several billion independent information-carrying `pixels' if the WDM particle had a mass of 1 keV. This number of `pixels' is much less than even the number of stars in the Milky Way. If the dark matter is proven to be warm, the high degree of structure in the Milky Way could have arisen in two ways: (1) from a high sensitivity to initial conditions, like an intricate fractal arising from a relatively simple computer code; or (2) from random information generated after the galaxy formed, i.e.\ not entirely deterministically from the initial conditions., Comment: Now matches version accepted to MNRAS Letters. 6 pages, 1 figure. Comments welcome

In the current cosmological paradigm, an initially flat three-dimensional manifold that pervades space (the `dark-matter sheet') folds up to build concentrations of mass (galaxies), and a cosmic web between them. Galaxies are nodes, connected by a network of filaments and walls. The folding is in six-dimensional (3D position, plus 3D velocity) phase space. The positions of creases, or caustics, mark the edges of structures. Here, I introduce an origami approximation to cosmological structure formation, in which the dark-matter sheet is not allowed to stretch. But it still produces an idealized cosmic web, with nodes, filaments, walls and voids. In 2D, nodes form in `polygonal collapse' (a twist-fold in origami), necessarily generating filaments simultaneously. In 3D, nodes form in `polyhedral collapse,' simultaneously generating filaments and walls. The masses, spatial arrangement, and angular momenta of nodes and filaments are related in the model. I describe some `tetrahedral collapse', or tetrahedral twist-fold, models., Comment: Accepted, after refereeing, to Origami^6: Proceedings of the 6th International Meeting on Origami in Science, Mathematics, and Education. 10 pages, 6 figs. Interactive "tetrahedral collapse" model at http://skysrv.pha.jhu.edu/~neyrinck/TetCollapse/

Tessellations are valuable both conceptually and for analysis in the study of the large-scale structure of the universe. They provide a conceptual model for the 'cosmic web,' and are of great use to analyze cosmological data. Here we describe tessellations in another set of coordinates, of the initially flat sheet of dark matter that gravity folds up in rough analogy to origami. The folds that develop are called caustics, and they tessellate space into stream regions. Tessellations of the dark-matter sheet are also useful in simulation analysis, for instance for density measurement, and to identify structures where streams overlap., To appear in the proceedings of "The World a Jigsaw: Tessellations in the Sciences." 15 pages, 11 figures. Origami-galaxy crease patterns available at http://skysrv.pha.jhu.edu/~neyrinck/origalaxies.html

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2012 RAS © The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved, We present a detailed comparison of the substructure properties of a single Milky Way sized dark matter halo from the Aquarius suite at five different resolutions, as identified by a variety of different (sub)halo finders for simulations of cosmic structure formation. These finders span a wide range of techniques and methodologies to extract and quantify substructures within a larger non-homogeneous background density (e.g. a host halo). This includes real-space-, phase-space-, velocity-space- and time-space-based finders, as well as finders employing a Voronoi tessellation, Friends-of-Friends techniques or refined meshes as the starting point for locating substructure. A common post-processing pipeline was used to uniformly analyse the particle lists provided by each finder. We extract quantitative and comparable measures for the subhaloes, primarily focusing on mass and the peak of the rotation curve for this particular study. We find that all of the finders agree extremely well in the presence and location of substructure and even for properties relating to the inner part of the subhalo (e.g. the maximum value of the rotation curve). For properties that rely on particles near the outer edge of the subhalo the agreement is at around the 20per cent level. We find that the basic properties (mass and maximum circular velocity) of a subhalo can be reliably recovered if the subhalo contains more than 100 particles although its presence can be reliably inferred for a lower particle number limit of 20. We finally note that the logarithmic slope of the subhalo cumulative number count is remarkably consistent and, AK is supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) in Spain through the Ramon y Cajal programme as well as the grants AYA 2009-13875-C03-02, AYA2009-12792-C03-03, CSD2009-00064 and CAM S2009/ESP-1496. HL acknowledges a fellowship from the European Commission’s Framework Programme 7, through the Marie Curie Initial Training Network CosmoComp (PITN-GA-2009-238356). MNacknowledges support through Alex Szalay from the Gordon and Betty Moore Foundation. YA is also supported by the Ramon y Cajal programme as well as grant AYA 2010-21887-C04-03. JXH is supported by the European Commissions Framework Programme 7, through the Marie Curie Initial Training Network Cosmo-Comp (PITNGA-2009-238356) and partially supported by NSFC (10878001,11033006, 11121062) and the CAS/SAFEA International Partnership Programme for Creative Research Teams (KJCX2-YW-T23). VS acknowledges partial support by SFB 881 ‘The Milky Way System’ of the DFG. PE acknowledges financial support from the Chinese Academy of Sciences (CAS), from NSFC grants (No. 11121062, 10878001, 11033006), and by the CAS/SAFEA International Partnership Programme for Creative Research Teams (KJCX2-YW-T23).

In a cold-dark-matter universe, cosmological structure formation proceeds in rough analogy to origami folding. Dark matter occupies a three-dimensional 'sheet' of free- fall observers, non-intersecting in six-dimensional velocity-position phase space. At early times, the sheet was flat like an origami sheet, i.e. velocities were essentially zero, but as time passes, the sheet folds up to form cosmic structure. The present paper further illustrates this analogy, and clarifies a Lagrangian definition of caustics and streams: caustics are two-dimensional surfaces in this initial sheet along which it folds, tessellating Lagrangian space into a set of three-dimensional regions, i.e. streams. The main scientific result of the paper is that streams may be colored by only two colors, with no two neighbouring streams (i.e. streams on either side of a caustic surface) colored the same. The two colors correspond to positive and negative parities of local Lagrangian volumes. This is a severe restriction on the connectivity and therefore arrangement of streams in Lagrangian space, since arbitrarily many colors can be necessary to color a general arrangement of three-dimensional regions. This stream two-colorability has consequences from graph theory, which we explain. Then, using N-body simulations, we test how these caustics correspond in Lagrangian space to the boundaries of haloes, filaments and walls. We also test how well outer caustics correspond to a Zel'dovich-approximation prediction., Comment: Clarifications and slight changes to match version accepted to MNRAS. 9 pages, 5 figures

Large-scale bulk flows in the Universe distort the initial density field, broadening the baryon-acoustic-oscillation (BAO) feature that was imprinted when baryons were strongly coupled to photons. Additionally, there is a small shift inward in the peak of the conventional overdensity correlation function, a mass-weighted statistic. This shift occurs when high density peaks move toward each other. We explore whether this shift can be removed by applying to the density field a transform (such as a logarithm) that gives fairer statistical weight to fluctuations in underdense regions. Using configuration-space perturbation theory in the Zel'dovich approximation, we find that the log-density correlation function shows a much smaller inward shift in the position of the BAO peak at low redshift than is seen in the overdensity correlation function. We also show that if the initial, Lagrangian density of matter parcels could be estimated at their Eulerian positions, giving a displaced-initial-density field, its peak shift would be even smaller. In fact, a transformed field that accentuates underdensities, such as the reciprocal of the density, pushes the peak the other way, outward. In our model, these shifts in the peak position can be attributed to shift terms, involving the derivative of the linear correlation function, that entirely vanish in this displaced-initial-density field., Comment: 5 pages, 3 figures, Accepted by ApjL

The power spectrum is widely used in astronomy, to analyze temporal or spatial structure. In cosmology, it is used to quantify large-scale structure (LSS) and the cosmic microwave background (CMB). This is because the power spectrum completely quantifies Gaussian random fields, which the CMB and LSS fields seem to be at early epochs. However, at late epochs and small scales, cosmological density fields become highly non-Gaussian. The power spectrum loses power to describe LSS and CMB fields on small scales, most obviously through high covariance in the power spectrum as a function of scale. Practically, this significantly degrades constraints that observations can place on cosmological parameters. However, if a nonlinear transformation that produces a (more) Gaussian 1-point distribution is applied to a field, the extra covariance in the field's power spectrum can be dramatically reduced. In the case of the roughly lognormal low-redshift matter density field, a log transform accomplishes this. Applying a log transform to the density field before measuring the power spectrum also tightens cosmological parameter constraints by a factor of several., 2 pages, 2 new figures. To appear in Statistical Challenges in Modern Astronomy V (June 2011) conference proceedings

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2011 RAS © 2011 The authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved, We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including friends-of-friends, spherical-overdensity and phase-space-based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allow halo finder developers to compare the performance of their codes against those presented here. This set includes mock haloes containing various levels and distributions of substructure at a range of resolutions as well as a cosmological simulation of the large-scale structure of the universe. All the halo-finding codes tested could successfully recover the spatial location of our mock haloes. They further returned lists of particles (potentially) belonging to the object that led to coinciding values for the maximum of the circular velocity profile and the radius where it is reached. All the finders based in configuration space struggled to recover substructure that was located close to the centre of the host halo, and the radial dependence of the mass recovered varies from finder to finder. Those finders based in phase space could resolve central substructure although they found difficulties in accurately recovering its properties. Through a resolution study we found that most of the finders could not reliably recover substructure containing fewer than 30-40 particles. However, also here the phase-space finders excelled by resolving substructure down to 10-20 particles. By comparing the halo finders using a high-resolution cosmological volume, we found that they agree remarkably well on fundamental properties of astrophysical significance (e.g. mass, position, velocity and peak of the rotation curve). We further suggest to utilize the peak of the rotation curve, vmax, as a proxy for mass, given the arbitrariness in defining a proper halo edge, We are greatly indebted to the ASTROSIM network of the European Science Foundation (Science Meeting 2910) for financially supporting the workshop ‘Haloes going MAD’ held in Miraflores de la Sierra near Madrid in 2010 May where all of this work was initiated.AK is supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) in Spain through the Ramón y Cajal programme as well as the grants AYA 2009-13875-C03-02, AYA2009- 12792-C03-03, CSD2009-00064 and CAM S2009/ESP-1496. SRK acknowledges the support by the MICINN under the Consolider-Ingenio, SyeC project CSD-2007-00050. SP and VQ have also been supported by the MICINN (grants AYA2010-21322-C03-01 and CONSOLIDER2007-00050) and the Generalitat Valenciana (grant PROMETEO-2009-103). MZ is supported by NSF grant AST-0708087. GY acknowledges financial support from MICINN (Spain) under project AYA 2009-13875-C03-02 and the ASTROMADRID project S2009/ESP-1496 financed by Comunidad de Madrid

We show that redshift-space distortions of galaxy correlations have a strong effect on correlation functions with distinct, localized features, like the signature of the baryon acoustic oscillations (BAO). Near the line of sight, the features become sharper as a result of redshift-space distortions. We demonstrate this effect by measuring the correlation function in Gaussian simulations and the Millennium Simulation. We also analyze the SDSS DR7 main-galaxy sample (MGS), splitting the sample into slices 2.5 degrees on the sky in various rotations. Measuring 2D correlation functions in each slice, we do see a sharp bump along the line of sight. Using Mexican-hat wavelets, we localize it to (110 +/- 10) Mpc/h. Averaging only along the line of sight, we estimate its significance at a particular wavelet scale and location at 2.2 sigma. In a flat angular weighting in the (pi,r_p) coordinate system, the noise level is suppressed, pushing the bump's significance to 4 sigma. We estimate that there is about a 0.2% chance of getting such a signal anywhere in the vicinity of the BAO scale from a power spectrum lacking a BAO feature. However, these estimates of the significances make some use of idealized Gaussian simulations, and thus are likely a bit optimistic., 17 pages, 27 figures. Minor changes to match final version accepted to ApJ

The Cold Spot on the Cosmic Microwave Background could arise due to a supervoid at low redshift through the integrated Sachs-Wolfe effect. We imaged the region with MegaCam on the Canada-France-Hawai'i Telescope and present galaxy counts in photometric redshift bins. We rule out the existence of a 100Mpc radius spherical supervoid with underdensity delta=-0.3 at 0.5, Comment: 9 pages. Minor updates to match the article accepted to ApJ.

We present a study of the substructure finder dependence of subhalo clustering in the Aquarius Simulation. We run 11 different subhalo finders on the haloes of the Aquarius Simulation and we study their differences in the density profile, mass fraction and 2-point correlation function of subhaloes in haloes. We also study the mass and vmax dependence of subhalo clustering. As the Aquarius Simulation has been run at different resolutions, we study the convergence with higher resolutions. We find that the agreement between finders is at around the 10% level inside R200 and at intermediate resolutions when a mass threshold is applied, and better than 5% when vmax is restricted instead of mass. However, some discrepancies appear in the highest resolution, underlined by an observed resolution dependence of subhalo clustering. This dependence is stronger for the smallest subhaloes, which are more clustered in the highest resolution, due to the detection of subhaloes within subhaloes (the sub-subhalo term). This effect modifies the mass dependence of clustering in the highest resolutions. We discuss implications of our results for models of subhalo clustering and their relation with galaxy clustering., Comment: 19 pages, 15 figures