The Astronomy and Astrophysics Review最新文献

With their jet pointing towards us, blazars are ideal tools to study the physics and structure of extragalactic jets. Their powerful jets are cosmic particle accelerators and are alleged to be one of the production sites of the high-energy neutrinos detected by the IceCube Observatory. Doppler beaming of the jet nonthermal radiation increases blazar brightness, blue-shifts their emission, and shortens their variability time scales, which are observed to range from years down to minutes. This review will focus on blazar flux, spectral, and polarization variability across the electromagnetic spectrum. After introducing blazars and their peculiarities, we will consider the statistical tools that are used to characterize the variability and to reveal correlations and time delays between flux variations at different frequencies. Then we will outline the main observed properties of the blazar multiwavelength behaviour. Interpretation of blazar variability calls into question both intrinsic and extrinsic mechanisms. Shock waves, magnetic reconnection, and turbulence can accelerate particles inside the jet, while jet precession, rotation, and twisting can produce variations in Doppler beaming. Changes in the broad-band spectral energy distribution have commonly been explained by variations in the jet physical parameters in one- or two-zone models. However, microvariability observed at all wavelengths puts strong constraints on the size of the emitting regions, suggesting a multizone emitting jet. Twisting jets have been proposed to explain the long-term multiwavelength variability. They are supported by radio observations of bent or helical jets, and by results of relativistic magnetohydrodynamics simulations of plasma jets. Detection of (quasi)periodic behaviour at all frequencies and on all time scales has been ascribed to orbital motion in black hole binary systems, jet precession, kink instabilities developing inside the jet, or perturbations in the accretion disc. Gravitational microlensing has been suggested to explain blazar behaviour in some cases. Polarization is another important ingredient in blazar variability studies, providing information on the structure and behaviour of the magnetic field in the emission zones. Both the degree and angle of polarization can show strong and fast variability, which is sometimes correlated with flux. Overall, polarimetric observations indicate that turbulence must play an important role in the emitting regions. Recent results obtained by the Imaging X-ray Polarimetric Explorer (IXPE) satellite have revealed some unexpected behaviour favouring a multizone emitting jet model. The interpretation of flux, spectral, and polarization variability within a consistent picture challenges current models of blazar variability and tells us that we may still miss some tiles of the puzzle.

We review our current understanding of nuclear stellar discs (NSDs), rotating, and flattened stellar structures found in the central regions of both early and late-type galaxies. We examine their demographics, kinematics, stellar populations, metallicity gradients and star formation histories. We derive scaling relations linking NSDs to properties of their host galaxies, and compare them with analogous relations for nuclear star clusters. The relationship between NSDs and other central galactic components, including nuclear rings, nuclear bars, and nuclear star clusters, is explored. The role of NSDs as tracers of the secular evolution of barred galaxies is highlighted, emphasising how they can be used to constrain properties of galactic bars such as their ages. Special attention is given to the Milky Way’s NSD, which serves as a unique case study thanks to its proximity and the ability to resolve individual stars. The review covers the Milky Way’s NSD structure, kinematics, dynamics and stellar content, addressing ongoing debates about the presence of a nuclear bar and implications of the latter for central gas dynamics. We argue that NSDs form by in-situ star formation, most of them because of bar-driven gas inflow, but possibly in some cases because of external acquisition of gas during a gas-rich merger. The review concludes by outlining open questions, future research directions and the exciting prospects provided by upcoming observational facilities.

There is a long-standing discussion in the astrophysical/astrochemical community as to the structure and morphology of dust grains in various astrophysical environments (e.g., interstellar clouds, protostellar envelopes, protoplanetary and debris disks, and the atmospheres of exoplanets). Typical grain models assume a compact dust core which becomes covered in a thick ice mantle in cold dense environments. In contrast, less compact cores are likely to exhibit porosity, leading to a pronounced increase in surface area with concomitant much thinner ice films and higher accessibility to the bare grain surface. Several laboratory experimental and theoretical studies have shown that this type of dust structure can have a marked effect on several physico-chemical processes, including adsorption, desorption, mobility, and reactivity of chemical species. Porous grains are thus thought to likely play a particularly important and wide-ranging astrochemical role. Herein, we clarify what is meant by porosity in relation to grains and grain agglomerates, assess the likely astrochemical effects of porosity and ask whether a fractal/porous structural/morphological description of dust grains is appropriate from an astronomical perspective. We provide evidence for high porosity from laboratory experiments and computational simulations of grains and their growth in various astrophysical environments, and assess the observational constraints and perspectives on cosmic dust porosity. Overall, our paper discusses the effects of including porosity in dust models and the need to use such models for future astrophysical, astrochemical and astrobiological studies involving surface or solid-state processes.

The alignments of galaxies across the large-scale structure of the Universe are known to be a source of contamination for gravitational lensing, but they can also probe cosmology and the physics of galaxy evolution in many ways. In this review, I cover developments in our understanding of intrinsic alignments over the past 25 years on: (1) different approaches to model intrinsic alignments across a range of scales, (2) existing observational constraints, (3) predictions from cosmological numerical N-body and hydrodynamical simulations, (4) mitigation strategies to account for their contamination to lensing observables and (5) cosmological and astrophysical applications. While the review focuses mostly on two-point statistics of intrinsic alignments, I also give a summary of other statistics beyond two-point. Finally, I point out some of the open problems hindering the understanding or application of intrinsic alignments and how they might be overcome in the future.

Infrared (IR) fine-structure line (FSL) emission arises from the radiative de-excitation of collisionally-excited electrons in atoms and ions. Simple elements such as carbon (C), nitrogen (N), and oxygen (O) are widespread in the interstellar medium (ISM) as a result of metal enrichment. Thanks to their high luminosities and relatively simple physics, IR FSLs have quickly become the workhorse for studying the formation and evolution of galaxies in the nearby and distant Universe. In this review, we introduce the physics of FSL emission and the diagnostics of the ISM that we can derive from them via first principle arguments. We summarize the history of FSL observations with a focus on the far-IR wavelengths and a particular emphasis on the on-going efforts aimed at characterizing galaxies at cosmic noon and beyond. We explore the dependence of emission line trends, such as those observed in ‘line deficits’ or [C ii]–SFR relations, as a function of redshift and galaxy types. Once selection biases are controlled for, IR FSLs are a powerful tool to constrain the physics of galaxies. The precise redshift information inferred from fine-structure line observations have enabled tracing their ISM properties across cosmic reionization. FSL observations have also led to estimates of the mass of different ISM phases, and of the SFR of distant galaxies. It is thanks to IR FSL observations that we have been able to measure the internal dynamics of high-z galaxies, which in turns has allowed us to test, e.g., the onset of black hole–host galaxy relations in the first billion years of the Universe and the presence of gas outflows associated with the baryon cycle in galaxies. Finally, FSLs have provided important clues on the physics of the ISM in the most distant galaxies known to date. We demonstrate the strength and limitations of using IR FSLs to advance our understanding of galaxy formation and evolution in the early universe, and we outline future perspective for the field.

The ongoing discussion about the atomic chemical composition of the Sun is commented on. The main focus in this review is on the deviation of the solar composition from that of most other solar-type stars in that its ratio of volatiles (like the elements C, N, O, S, P and Zn) to the refractories (most metals, like Ba, Ca, Ti, Y, Al, Sc and Zr) tends to be higher in the Sun by 10 to 20%. What does this tell about the formation and evolution of the Solar System? Scenarios in terms of galactic evolution, formation of the pre-solar nebula, of the evolution of the protoplanetary disk, of the engulfing of planets, and of other processes within the Solar System are considered, as well as the evolution of binary stars with similarly different chemical composition. Finally, implications, if any, on the habitability of the Solar System will be commented on.

The most metal-poor stars found in the Galaxy and in nearby galaxies are witnesses of the early evolution of the Universe. In a general picture in which we expect the metallicity to increase monotonically with time, as a result of the metal production in stars, we also expect the most metal-poor stars to be the most primitive objects accessible to our observations. The abundance ratios in these stars provide us important information on the first generations of stars that synthesised the nuclei that we observe in these stars. Because they are so primitive, the modelling of their chemical inventory can be often satisfactorily achieved by assuming that all the metals were produced in a single Supernova, or just a few. This is simpler than modelling the full chemical evolution, using different sources, that is necessary at higher metallicity. The price to pay for this relative ease of interpretation is that these stars are extremely rare and require specifically tailored observational strategies in order to assemble statistically significant samples of stars. In this review, we try to summarise the main observational results that have been obtained in the last ten years.

Type Ia supernovae (SNe Ia) are runaway thermonuclear explosions in white dwarfs that result in the disruption of the white dwarf star, and possibly its nearby stellar companion. SNe Ia occur over an immense range of stellar population age and host galaxy environments, and play a critical role in the nucleosynthesis of intermediate-mass and iron-group elements, primarily the production of nickel, iron, cobalt, chromium, and manganese. Though the nature of their progenitors is still not well-understood, SNe Ia are unique among stellar explosions in that the majority of them exhibit a systematic lightcurve relation: more luminous supernovae dim more slowly over time than less luminous supernovae in optical light (intrinsically brighter SNe Ia have broader lightcurves). This feature, unique to SNe Ia, is rather remarkable and allows their peak luminosities to be determined with fairly high accuracy out to cosmological distances via measurement of their lightcurve decline. Further, studying SNe Ia gives us important insights into binary star evolution physics, since it is widely agreed that the progenitors of SNe Ia are binary (possibly multiple) star systems. In this review, we give a current update on the different proposed Type Ia supernova progenitors, including descriptions of possible binary star configurations, and their explosion mechanisms, from a theoretical perspective. We additionally give a brief overview of the historical (focussing on the more recent) observational work that has helped the astronomical community to understand the nature of the most important distance indicators in cosmology.

The study of the polarisation of light is a powerful tool for probing the physical and compositional properties of astrophysical sources, including Solar System objects. In this article, we provide a comprehensive overview of the state-of-the-art in polarimetric studies of various celestial bodies within our Solar System: planets, moons, asteroids, and comets. Additionally, we review relevant laboratory measurements and summarise the fundamental principles of polarimetric observational techniques.

Stellar occultations provide a powerful tool to explore objects of the outer solar system. The Gaia mission now provides milli-arcsec accuracy on the predictions of these events and makes possible observations that were previously unthinkable. Occultations return kilometric accuracies on the three-dimensional shape of bodies irrespective of their geocentric distances, with the potential of detecting topographic features along the limb. From the shape, accurate values of albedo can be derived, and if the mass is known, the bulk density is pinned down, thus constraining the internal structure and equilibrium state of the object. Occultations are also extremely sensitive to tenuous atmospheres, down to the nanobar level. They allowed the monitoring of Pluto’s and Triton’s atmospheres in the last three decades, constraining their seasonal evolution. They may unveil in the near future atmospheres around other remote bodies of the solar system. Since 2013, occultations have led to the surprising discovery of ring systems around the Centaur object Chariklo, the dwarf planet Haumea and the large trans-Neptunian object Quaoar, while revealing dense material around the Centaur Chiron. This suggests that rings are probably much more common features than previously thought. Meanwhile, they have raised new dynamical questions concerning the confining effect of resonances forced by irregular objects on ring particles. Serendipitous occultations by km-sized trans-Neptunian or Oort objects have the potential to provide the size distribution of a population that suffered few collisions until now, thus constraining the history of primordial planetesimals in the 1–100 km range.