The Astronomy and Astrophysics Review最新文献

Multiply imaged time-variable sources can be used to measure absolute distances as a function of redshifts and thus determine cosmological parameters, chiefly the Hubble Constant H(_0). In the two decades up to 2020, through a number of observational and conceptual breakthroughs, this so-called time-delay cosmography has reached a precision sufficient to be an important independent voice in the current “Hubble tension” debate between early- and late-universe determinations of H(_0). The 2020s promise to deliver major advances in time-delay cosmography, owing to the large number of lenses to be discovered by new and upcoming surveys and the vastly improved capabilities for follow-up and analysis. In this review, after a brief summary of the foundations of the method and recent advances, we outline the opportunities for the decade and the challenges that will need to be overcome in order to meet the goal of the determination of H(_0) from time-delay cosmography with 1% precision and accuracy.

After hydrogen and helium, oxygen, carbon, and nitrogen—hereinafter, the CNO elements—are the most abundant species in the universe. They are observed in all kinds of astrophysical environments, from the smallest to the largest scales, and are at the basis of all known forms of life, hence, the constituents of any biomarker. As such, their study proves crucial in several areas of contemporary astrophysics, extending to astrobiology. In this review, I will summarize current knowledge about CNO element evolution in galaxies, starting from our home, the Milky Way. After a brief recap of CNO synthesis in stars, I will present the comparison between chemical evolution model predictions and observations of CNO isotopic abundances and abundance ratios in stars and in the gaseous matter. Such a comparison permits to constrain the modes and time scales of the assembly of galaxies and their stellar populations, as well as stellar evolution and nucleosynthesis theories. I will stress that chemical evolution models must be carefully calibrated against the wealth of abundance data available for the Milky Way before they can be applied to the interpretation of observational datasets for other systems. In this vein, I will also discuss the usefulness of some key CNO isotopic ratios as probes of the prevailing, galaxy-wide stellar initial mass function in galaxies where more direct estimates from the starlight are unfeasible.

Despite their relative sparseness, during the recent years it has become more and more clear that extragalactic radio sources (both AGN and star-forming galaxies) constitute an extremely interesting mix of populations, not only because of their intrinsic value, but also for their fundamental role in shaping our universe the way we see it today. Indeed, radio-active AGN are now thought to be the main players involved in the evolution of massive galaxies and clusters. At the same time, thanks to the possibility of being observed up to very high redshifts, radio galaxies can also provide crucial information on both the star-formation history of our universe and on its large-scale structure properties and their evolution. In the light of present and forthcoming facilities such as LOFAR, MeerKAT and SKA that will probe the radio sky to unprecedented depths and widths, this review aims at providing the current state of the art on our knowledge of extragalactic radio sources in connection with their hosts, large-scale environments and cosmological context.

Line-intensity mapping (LIM) is an emerging approach to survey the Universe, using relatively low-aperture instruments to scan large portions of the sky and collect the total spectral-line emission from galaxies and the intergalactic medium. Mapping the intensity fluctuations of an array of lines offers a unique opportunity to probe redshifts well beyond the reach of other cosmological observations, access regimes that cannot be explored otherwise, and exploit the enormous potential of cross-correlations with other measurements. This promises to deepen our understanding of various questions related to galaxy formation and evolution, cosmology, and fundamental physics. Here, we focus on lines ranging from microwave to optical frequencies, the emission of which is related to star formation in galaxies across cosmic history. Over the next decade, LIM will transition from a pathfinder era of first detections to an early-science era where data from more than a dozen missions will be harvested to yield new insights and discoveries. This review discusses the primary target lines for these missions, describes the different approaches to modeling their intensities and fluctuations, surveys the scientific prospects of their measurement, presents the formalism behind the statistical methods to analyze the data, and motivates the opportunities for synergy with other observables. Our goal is to provide a pedagogical introduction to the field for non-experts, as well as to serve as a comprehensive reference for specialists.

Low-energy cosmic rays (up to the GeV energy domain) play a crucial role in the physics and chemistry of the densest phase of the interstellar medium. Unlike interstellar ionising radiation, they can penetrate large column densities of gas, and reach molecular cloud cores. By maintaining there a small but not negligible gas ionisation fraction, they dictate the coupling between the plasma and the magnetic field, which in turn affects the dynamical evolution of clouds and impacts on the process of star and planet formation. The cosmic-ray ionisation of molecular hydrogen in interstellar clouds also drives the rich interstellar chemistry revealed by observations of spectral lines in a broad region of the electromagnetic spectrum, spanning from the submillimetre to the visual band. Some recent developments in various branches of astrophysics provide us with an unprecedented view on low-energy cosmic rays. Accurate measurements and constraints on the intensity of such particles are now available both for the very local interstellar medium and for distant interstellar clouds. The interpretation of these recent data is currently debated, and the emerging picture calls for a reassessment of the scenario invoked to describe the origin and/or the transport of low-energy cosmic rays in the Galaxy.

Galaxies living in rich environments are suffering different perturbations able to drastically affect their evolution. Among these, ram pressure stripping, i.e. the pressure exerted by the hot and dense intracluster medium (ICM) on galaxies moving at high velocity within the cluster gravitational potential well, is a key process able to remove their interstellar medium (ISM) and quench their activity of star formation. This review is aimed at describing this physical mechanism in different environments, from rich clusters of galaxies to loose and compact groups. We summarise the effects of this perturbing process on the baryonic components of galaxies, from the different gas phases (cold atomic and molecular, ionised, hot) to magnetic fields and cosmic rays, and describe their induced effects on the different stellar populations, with a particular attention to its role in the quenching episode generally observed in high-density environments. We also discuss on the possible fate of the stripped material once removed from the perturbed galaxies and mixed with the ICM, and we try to estimate its contribution to the pollution of the surrounding environment. Finally, combining the results of local and high-redshift observations with the prediction of tuned models and simulations, we try to quantify the importance of this process on the evolution of galaxies of different mass, from dwarfs to giants, in various environments and at different epochs.

Since the discovery of the first fast radio burst (FRB) in 2007, and their confirmation as an abundant extragalactic population in 2013, the study of these sources has expanded at an incredible rate. In our 2019 review on the subject, we presented a growing, but still mysterious, population of FRBs—60 unique sources, 2 repeating FRBs, and only 1 identified host galaxy. However, in only a few short years, new observations and discoveries have given us a wealth of information about these sources. The total FRB population now stands at over 600 published sources, 24 repeaters, and 19 host galaxies. Higher time resolution data, sustained monitoring, and precision localisations have given us insight into repeaters, host galaxies, burst morphology, source activity, progenitor models, and the use of FRBs as cosmological probes. The recent detection of a bright FRB-like burst from the Galactic magnetar SGR 1935 + 2154 provides an important link between FRBs and magnetars. There also continue to be surprising discoveries, like periodic modulation of activity from repeaters and the localisation of one FRB source to a relatively nearby globular cluster associated with the M81 galaxy. In this review, we summarise the exciting observational results from the past few years. We also highlight their impact on our understanding of the FRB population and proposed progenitor models. We build on the introduction to FRBs in our earlier review, update our readers on recent results, and discuss interesting avenues for exploration as the field enters a new regime where hundreds to thousands of new FRBs will be discovered and reported each year.

Human activity is rapidly increasing the negative impact of artificial skyglow at even the most remote professional observatory sites. Assessment of the actual impact requires an understanding of the propagation as a function of source spectral energy distribution. The higher blue content of light-emitting diodes being widely used as replacement for sodium discharge lamps has greater impact closer to the source, and less impact for more distant mountain-top sites. All-sky cameras with moderate angular resolution provide data and metrics sufficient to model and remove celestial contributions and provide measures of artificial light contribution. The natural skyglow is significantly affected by solar activity, which must be accounted for in determining secular trends in the artificial component. With the availability of the New World Atlas of the Artificial Sky Brightness, a direct comparison is made of the modeled artificial contribution to the sites with the largest aperture telescopes, noting the possible systematic errors in individual cases. Population growth of the nearest urban centers allows a prediction of the change in that brightness over a decade. All site protections are effected primarily by national or regional regulation. A collection of worldwide regulations shows that most are leveraged off environmental protection statutes, while in the U.S., they are largely based on land-use zones. Particular examples are presented in more detail for Flagstaff, Arizona, and the Island of Hawai’i. The latest rapidly growing threat is that of reflected sunlight from large constellations of satellites in low-earth orbit. A snapshot is provided of that rapidly changing situation. In all cases, astronomers must become very proactive in educating the public about the cultural value of visual or naked eye astronomy as well as the science and the need for access to a dark night sky for astronomical research.