The baryon mass content (i.e. stellar and gas mass) of dark matter halos in the early Universe depends on both global factors – for example, ionising ultraviolet (UV) radiation background – and local factors – for example, star formation efficiency and assembly history. We use a lightweight semi-analytical model to investigate how both local and global factors impact the halo baryon mass content at redshifts of $z\geq 5$. Our model incorporates a time delay between when stars form and when they produce feedback of $0\leq t^d/\mathrm{Myr} \leq 30$, which can drive bursts of star formation, and a mass and redshift-dependent UV background, which captures the influence of cosmological reionisation on gas accretion onto halos. We use statistically representative halo assembly histories and assume that the cosmological gas accretion rate is proportional to the halo mass accretion rate. Delayed ($t^d$>0) feedback leads to oscillations in gas mass with cosmic time, behaviour that cannot be captured with instantaneous feedback ($t^d$=0). Highly efficient star formation drives stronger oscillations, while strong feedback impacts when oscillations occur; in contrast, inefficient star formation and weak feedback produce similar long-term behaviour to that observed in instantaneous feedback models. If the delayed feedback timescale is too long, a halo retains its gas reservoir but the feedback suppresses star formation. Our model predicts that lower mass systems (halo masses $m_\mathrm{h} \leq 10^7 \mathrm{M}_\odot$) at $z \leq 10$ should be strongly gas deficient ($m_\mathrm{g}\rightarrow 0$), whereas higher mass systems retain their gas reservoirs because they are sufficiently massive to continue accreting gas through cosmological reionisation. Interestingly, in higher mass halos, the median $m_\star/(m_\star+m_\mathrm{g}) \simeq 0.01-0.05$, but is a factor of 3–5 smaller when feedback is delayed. Our model does not include seed supermassive black hole feedback, which is necessary to explain massive quenched galaxies in the early Universe.

We present a new method – called HINORA (HIgh-NOise RANdom SAmple Consensus) – for the identification of regular structures in 3D point distributions. Motivated by the possible existence of the so-called Council of Giants, that is, a ring of 12 massive galaxies surrounding the Local Group in the Local Sheet with a radius of 3.75 Mpc, we apply HINORA to the Local Volume Galaxy catalogue confirming its existence. When varying the lower limit of K-band luminosity of the galaxy entering the catalogue, we further report on the existence of another ring-like structure in the Local Volume that now contains the Milky Way and M31. However, this newly found structure is dominated by low-mass (satellite) galaxies. While we here simply present the novel method as well as its first application to observational data, follow-up work using numerical simulations of cosmic structure formation shall shed light into the origin of such regular patterns in the galaxy distribution. Further, the method is equally suited to identify similar (or even different) structures in various kinds of astrophysical data (e.g. locating the actual ‘baryonic-acoustic oscillation spheres’ in galaxy redshift surveys).

The amount and complexity of data delivered by modern galaxy surveys has been steadily increasing over the past years. New facilities will soon provide imaging and spectra of hundreds of millions of galaxies. Extracting coherent scientific information from these large and multi-modal data sets remains an open issue for the community and data-driven approaches such as deep learning have rapidly emerged as a potentially powerful solution to some long lasting challenges. This enthusiasm is reflected in an unprecedented exponential growth of publications using neural networks, which have gone from a handful of works in 2015 to an average of one paper per week in 2021 in the area of galaxy surveys. Half a decade after the first published work in astronomy mentioning deep learning, and shortly before new big data sets such as Euclid and LSST start becoming available, we believe it is timely to review what has been the real impact of this new technology in the field and its potential to solve key challenges raised by the size and complexity of the new datasets. The purpose of this review is thus two-fold. We first aim at summarising, in a common document, the main applications of deep learning for galaxy surveys that have emerged so far. We then extract the major achievements and lessons learned and highlight key open questions and limitations, which in our opinion, will require particular attention in the coming years. Overall, state-of-the-art deep learning methods are rapidly adopted by the astronomical community, reflecting a democratisation of these methods. This review shows that the majority of works using deep learning up to date are oriented to computer vision tasks (e.g. classification, segmentation). This is also the domain of application where deep learning has brought the most important breakthroughs so far. However, we also report that the applications are becoming more diverse and deep learning is used for estimating galaxy properties, identifying outliers or constraining the cosmological model. Most of these works remain at the exploratory level though which could partially explain the limited impact in terms of citations. Some common challenges will most likely need to be addressed before moving to the next phase of massive deployment of deep learning in the processing of future surveys; for example, uncertainty quantification, interpretability, data labelling and domain shift issues from training with simulations, which constitutes a common practice in astronomy.

We examine the redshifts of a comprehensive set of published Type Ia supernovae, and provide a combined, improved catalogue with updated redshifts. We improve on the original catalogues by using the most up-to-date heliocentric redshift data available; ensuring all redshifts have uncertainty estimates; using the exact formulae to convert heliocentric redshifts into the Cosmic Microwave Background (CMB) frame; and utilising an improved peculiar velocity model that calculates local motions in redshift-space and more realistically accounts for the external bulk flow at high-redshifts. We review 2607 supernova redshifts; 2285 are from unique supernovae and 322 are from repeat-observations of the same supernova. In total, we updated 990 unique heliocentric redshifts, and found 5 cases of missing or incorrect heliocentric corrections, 44 incorrect or missing supernova coordinates, 230 missing heliocentric or CMB frame redshifts, and 1200 missing redshift uncertainties. The absolute corrections range between $10^{-8} \leq \Delta z \leq 0.038$ , and RMS $(\Delta z) \sim 3{\times 10^{-3}}$ . The sign of the correction was essentially random, so the mean and median corrections are small: $4{\times 10^{-4}}$ and $4{\times 10^{-6}}$ respectively. We examine the impact of these improvements for $H_0$ and the dark energy equation of state w and find that the cosmological results change by $\Delta H_0 = -0.12\,\mathrm{km\,s}^{-1}\mathrm{Mpc}^{-1}$ and $\Delta w = 0.003$ , both significantly smaller than previously reported uncertainties for $H_0$ of 1.0 $\mathrm{km\,s}^{-1}\mathrm{Mpc}^{-1}$ and w of 0.04 respectively.

Feedback effects by supernovae (SNe) and active galactic nuclei (AGNs) are believed to be essential for galaxy evolution and shaping present-day galaxies, but their exact mechanisms on galactic scales and their impact on CGM/IGM are not well understood yet. In galaxy formation simulations, it is still challenging to resolve sub-parsec scales, and we need to implement subgrid models to account for the physics on small scales. In this article, we summarize some of the efforts to build more physically based feedback models, discuss about pushing the resolution to its limits in galaxy simulations, testing galaxy formation codes under the AGORA code comparison project, and how to probe the impact of feedback using cosmological hydrodynamic simulations via Lyα absorption and CGM/IGM tomography technique. We also discuss our future directions of research in this field and how we make progress by comparing our simulations with observations.

In Einstein’s special theory of relativity, all observers measure the speed of light, c, to be the same. However, this refers to the round-trip speed, where a clock at the origin times the outward and return trip of light reflecting off a distant mirror. Measuring the one-way speed of light is fraught with issues of clock synchronisation, and, as long as the average speed of light remains c, the speeds on the outward and return legs could be different. One objection to this anisotropic speed of light is that views of the distant universe would be different in different directions, especially with regard to the ages of observed objects and the smoothness of the Cosmic Microwave Background. In this paper, we explore this in the Milne universe, the limiting case of a Friedmann–Robertson–Walker universe containing no matter, radiation, or dark energy. Given that this universe is empty, it can be mapped onto flat Minkowski space-time and so can be explored in terms of the one-way speed of light. The conclusion is that the presence of an anisotropic speed of light leads to anisotropic time dilation effects, and hence observers in the Milne universe would be presented with an isotropic view of the distant cosmos.

Does space stretch its contents as the universe expands? Usually, we say the answer is no—the stretching of space is not like the stretching of a rubber sheet that might drag things with it. In this paper, we explore a potential counterexample—namely, we show that it is impossible to make an arbitrarily long object in an expanding universe, because it is impossible to hold the distant end of the object ‘stationary’ with respect to us (as defined in the Friedmann–Lemaître–Robertson–Walker metric). We show that this does not mean that expanding space has a force associated with it, rather, some fictitious forces arise due to our choice of reference frame. By choosing our usual time slice (where all comoving observers agree on the age of the universe), we choose a global frame that does not correspond to the frame of any inertial observer. As a result, simple relativistic velocity transforms generate an apparent acceleration, even where no force exists. This effect is similar to the fictitious forces that arise in describing objects in rotating reference frames, as in the case of the Coriolis effect.

During the recombination of the universe, supersonic relative motion between baryons and dark matter (DM) generally existed. In the presence of such streaming motions, gas clumps can collapse outside of virial radii of their closest dark matter halos. Such baryon dominant objects are thought to be self-gravitating and are called supersonically induced gas objects; SIGOs. We perform three-dimensional hydrodynamical simulations by including H2 chemical reactions and stream velocity and follow SIGO’s formation from z = 200 to z = 25. SIGOs can be formed under the influence of stream velocity, and cooling is effective in contracting gas clouds. We follow its further evolution with higher resolution. We find that there are SIGOs which become Jeans unstable outside of the virial radius of the closest DM halos. Those SIGOs are gravitationally unstable and trigger star formation.

We present results from high-resolution, zoom-in cosmological simulations to study the effect of feedback from galactic outflows on the physical and Lyα properties of high-redshift galaxies in highly overdense and normal environments at z >∼6. The Lyα properties have been obtained by post-processing the simulations with a Monte-Carlo radiative transfer (RT) code. Our results demonstrate that galactic outflows play an important role in regulating the growth of massive galaxies in overdense regions as well as the temperature and metallicity of the intergalactic medium. In particular, we find that galactic outflows are necessary to reproduce the observed Lyα luminosity functions as well as the apparent Lyα luminosity, line width and equivalent width distributions of luminous Lyα emitters at z ∼ 6.

Theories that attempt to unify the four fundamental interactions and alternative theories of gravity predict time and/or spatial variation of the fundamental constants of nature. Different versions of these theories predict different behaviours for these variations. As a consequence, experimental and observational bounds are an important tool to check the validity of such proposals. In this paper, we review constraints on the possible variation of the fundamental constants from astronomical observations and geophysical experiments designed to test the constancy of the fundamental constants of nature over different timescales. We also focus on the limits that can be obtained from white dwarfs, which can constrain the variation of the constants with the gravitational potential.

How special (or not) is the epoch we are living in? What is the appropriate reference class for embedding the observations made at the present time? How probable – or else – is anything we observe in the fulness of time? Contemporary cosmology and astrobiology bring those seemingly old-fashioned philosophical issues back into focus. There are several examples of contemporary research which use the assumption of typicality in time (or temporal Copernicanism) explicitly or implicitly, while not truly elaborating upon the meaning of this assumption. The present paper brings attention to the underlying and often uncritically accepted assumptions in these cases. It also aims to defend a more radical position that typicality in time is not – and cannot ever be – well-defined, in contrast to the typicality in space, and the typicality in various specific parameter spaces. This, of course, does not mean that we are atypical in time; instead, the notion of typicality in time is necessarily somewhat vague and restricted. In principle, it could be strengthened by further defining the relevant context, e.g. by referring to typicality within the Solar lifetime, or some similar restricting clause.

We present a suite of high-resolution cosmological zoom-in simulations of galaxies at z⩾ 5using the state-of-the-art models for the multi-phase ISM, star formation, and stellar feedback from the FIRE project. We present a series of key results from these simulations, including the stellar mass–halo mass relation, the ultraviolet luminosity functions, dust attenuation and dust temperatures, the ubiquitous formation of bound star clusters, morphology and clumpiness, and the escape fractions of ionizing photons from high-redshift galaxies. We discuss how different simulations in the literature agree and disagree and what observations are most useful for testing the models in the era of ALMA and JWST.

We present cosmological hydrodynamical simulations including atomic and molecular non-equilibrium chemistry, multi-frequency radiative transfer (0.7–100 eV sampled over 150 frequency bins) and stellar population evolution to investigate the host candidates of the seeds of supermassive black holes coming from direct collapse of gas in primordial haloes direct-collapse black holes, DCBHs. We consistently address the role played by atomic and molecular cooling, stellar radiation and metal spreading of C, N, O, Ne, Mg, Si, S, Ca, Fe, etc. from primordial sources, as well as their implications for nearby quiescent proto-galaxies under different assumptions for early source emissivity, initial mass function, and metal yields. We find that putative DCBH (direct-collapse black holes) host candidates need powerful primordial stellar generations, since common solar-like stars and hot OB-type stars are neither able to determine the conditions for direct collapse nor capable of building up a dissociating Lyman–Werner background radiation field. Thermal and molecular features of the identified DCBH host candidates in the scenario with very massive primordial stars seem favourable, with illuminating Lyman–Werner intensities featuring values of 1 – 50J21. Nevertheless, additional nonlinear processes, such as merger events, substructure formation, rotational motions, and photo-evaporation, should inhibit pure direct-collapse black hole formation in two-third of the cases. Local turbulence may delay gas direct collapse almost irrespectively from other environmental conditions. The impact of large Lyman–Werner fluxes at distances smaller than ~5 kpc is severely limited by metal pollution.

The 6th Focus Meeting (FM6) at the XXXth IAU GA 2018 aimed at overviewing the rise in angular momentum (AM) science seen in the last 10 years and debating new emerging views on galaxy evolution. The foundational works on galaxy formation of the 1970s and 80s clearly exposed the fundamental role of AM, suspected since the time of Kant. However, quantitative progress on galactic AM remained hampered by observational and theoretical obstacles. Only in the last 10 years, numerical simulations began to produce galactic disks with realistic AM. Simultaneously, the fast rise of Integral Field Spectroscopy (IFS) and millimetre/radio interferometry have opened the door for systematic AM measurements, across representative samples and cosmic volumes. The FM bridged between cutting-edge observational programs and leading simulations in order to review, debate and resolve core issues on AM science, ranging from galactic substructure (e.g. gas fraction, turbulence, clumps) to global properties (e.g. size evolution, morphologies) and cosmology (spin alignment, cosmic origin of AM). The co-chairs and SOC members strived to assemble a representative selection of leading scientists in the field, while adhering to principles of equal opportunity and inclusivity.

Reproducing the planes of co-orbiting satellites observed in the MW and M31 so far has represented a challenge for cosmological simulations. We have developed a new method to search for kinematically-coherent groups of satellites and applied it to 2 different cosmological hydro-simulations of disc galaxies. In each simulation we have found such a group, that represents roughly half of the total satellite population and is distributed on a fairly thin plane that persists in time. These results are compatible with the MW and M31 observed planes.

Dwarf galaxies make ideal laboratories to test galaxy evolution paradigms and cosmological models. Detailed studies of dwarfs across the spectrum allow us to gauge the efficacy of astrophysical processes at play in the lowest mass halos such as gas accretion, feedback, turbulence and chemical enrichment. Future observational studies will deliver unprecedented insights on the orbits of dwarf companions around the Milky Way, on their star formation histories and on the 3-D internal motions of their stars. Over large volumes, we will assess the impact of local environment on baryon cycling and star formation laws, leading to a full picture of the evolution of dwarfs across cosmic time. In combination, future discoveries promise to trace the history of assembly within the Local Group and beyond, probe how stars form under pristine conditions, and test models of structure formation on small scales.

A common thought in the 1950s was that galaxies rotate because they are remnants of primeval currents, as in turbulence. But this idea is quite unacceptable in an expanding universe described by general relativity theory. Since we are no smarter now than in the 1950s the lesson I draw is that we do well on occasion to pause to consider whether we might be missing something. An example is the pure disk galaxies that are so common nearby and so rare in simulations. We have something to learn from this.

The 21cm signal at epoch of reionization (EoR) should be observed within next decade. We expect that cosmic 21cm signal at the EoR provides us both cosmological and astrophysical information. In order to extract fruitful information from observation data, we need to develop inversion method. For such a method, we introduce artificial neural network (ANN) which is one of the machine learning techniques. We apply the ANN to inversion problem to constrain astrophysical parameters from 21cm power spectrum. We train the architecture of the neural network with 70 training datasets and apply it to 54 test datasets with different value of parameters. We find that the quality of the parameter reconstruction depends on the sensitivity of the power spectrum to the different parameter sets at a given redshift and also find that the accuracy of reconstruction is improved by increasing the number of given redshifts. We conclude that the ANN is viable inversion method whose main strength is that they require a sparse extrapolation of the parameter space and thus should be usable with full simulation.

There seems to be magnetic fields at all scales and epochs in our Universe, but their origin at large scales remains an important open question of cosmology. In this work we focus on the generation of magnetic fields in the intergalactic medium due to the photoionizations by the first galaxies, all along the Epoch of Reionization. Based on previous studies which considered only isolated sources, we develop an analytical model to estimate the mean magnetic energy density accumulated in the Universe by this process. In our model, without considering any amplification process, the Universe is globally magnetized by this mechanism to the order of, at least, several 10−18 G during the Epoch of Reionization (i.e. a few 10−20 G comoving).

We highlight recent progress in the sophistication and diversification of the simulations of cosmic dawn and reionization. The application of these modeling tools to recent observations has allowed us narrow down the timing of reionization. The midpoint of reionization is constrained to z = 7.6−0.7+0.8 (1 σ), with the strongest constraints coming from the optical depth to the CMB measured with the Planck satellite and the first detection of ongoing reionization from the spectra of the z = 7.1 QSOs ULASJ1120+0641. However, we still know virtually nothing about the astrophysical sources during the first billion years. The revolution in our understanding will be led by upcoming interferometric observations of the cosmic 21-cm signal. The properties of the sources and sinks of UV and X-ray photons are encoded in the 3D patterns of the signal. The development of Bayesian parameter recovery techniques, which tap into the wealth of the 21-cm signal, will soon usher in an era of precision astrophysical cosmology.