Annual Review of Astronomy and Astrophysics

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-aa-55-080817-100001

Freeman, Kenneth C.

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-091916-055249

This is an autobiographical account of my scientific career. My main research interest is the structure and assembly of galaxies. The assembly narrative has evolved from the monolithic and baryonic collapse picture of the early 1960s to the current hierarchical scenario underpinned by dark matter, and is still evolving. Technology has changed: CCDs replaced photographic plates and image tubes, large optical telescopes are much larger and instruments are much better, Galactic archaeology is supported by vast stellar surveys, and we have space astronomy and radio synthesis telescopes. The article describes the scientific areas in which I have worked and the colleagues who have influenced my progress. I have much to be grateful for: the people who have mentored and supported me over the years, the privilege of long-term collaborations, and the pleasure of advising many Ph.D. students and postdocs.

Alexander, Tal

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-091916-055306

Most galactic nuclei harbor a massive black hole (MBH), whose birth and evolution are closely linked to those of its host galaxy. The unique conditions near the MBHhigh velocity and density in the steep potential of a massive singular relativistic objectlead to unusual modes of stellar birth, evolution, dynamics, and death. A complex network of dynamical mechanisms, operating on multiple timescales, deflects stars to orbits that intercept the MBH. Such close encounters lead to energetic interactions with observable signatures and consequences for the evolution of the MBH and its stellar environment. Galactic nuclei are astrophysical laboratories that test and challenge our understanding of MBH formation, strong gravity, stellar dynamics, and stellar physics. I review from a theoretical perspective the wide range of stellar phenomena that occur near MBHs, focusing on the role of stellar dynamics near an isolated MBH in a relaxed stellar cusp.

Naab, Thorsten; Ostriker, Jeremiah P.

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-081913-040019

Numerical simulations have become a major tool for understanding galaxy formation and evolution. Over the decades the field has made significant progress. It is now possible to simulate the formation of individual galaxies and galaxy populations from well-defined initial conditions with realistic abundances and global properties. An essential component of the calculation is to correctly estimate the inflow to and outflow from forming galaxies because observations indicating low formation efficiency and strong circumgalactic presence of gas are persuasive. Energetic feedback from massive stars and accreting supermassive black holesgenerally unresolved in cosmological simulationsplays a major role in driving galactic outflows, which have been shown to regulate many aspects of galaxy evolution. A surprisingly large variety of plausible subresolution models succeeds in this exercise. They capture the essential characteristics of the problem, i.e., outflows regulating galactic gas flows, but their predictive power is limited. In this review, we focus on one major challenge for galaxy formation theory: to understand the underlying physical processes that regulate the structure of the interstellar medium, star formation, and the driving of galactic outflows. This requires accurate physical models and numerical simulations, which can precisely describe the multiphase structure of the interstellar medium on the currently unresolved few hundred parsec scales of large-scale cosmological simulations. Such models ultimately require the full accounting for the dominant cooling and heating processes, the radiation and winds from massive stars and accreting black holes, and an accurate treatment of supernova explosions as well as the nonthermal components of the interstellar medium like magnetic fields and cosmic rays.

Han, J.L.

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-091916-055221

Observational results of interstellar and intergalactic magnetic fields are reviewed, including the fields in supernova remnants and loops, interstellar filaments and clouds, Hii regions and bubbles, the Milky Way and nearby galaxies, galaxy clusters, and the cosmic web. A variety of approaches are used to investigate these fields. The orientations of magnetic fields in interstellar filaments and molecular clouds are traced by polarized thermal dust emission and starlight polarization. The field strengths and directions along the line of sight in dense clouds and cores are measured by Zeeman splitting of emission or absorption lines. The large-scale magnetic fields in the Milky Way have been best probed by Faraday rotation measures of a large number of pulsars and extragalactic radio sources. The coherent Galactic magnetic fields are found to follow the spiral arms and have their direction reversals in arms and interarm regions in the disk. The azimuthal fields in the halo reverse their directions below and above the Galactic plane. The orientations of organized magnetic fields in nearby galaxies have been observed through polarized synchrotron emission. Magnetic fields in the intracluster medium have been indicated by diffuse radio halos, polarized radio relics, and Faraday rotations of embedded radio galaxies and background sources. Sparse evidence for very weak magnetic fields in the cosmic web is the detection of the faint radio bridge between the Coma cluster and A1367. Future observations should aim at the 3D tomography of the large-scale coherent magnetic fields in our Galaxy and nearby galaxies, a better description of intracluster field properties, and firm detections of intergalactic magnetic fields in the cosmic web.

Linsky, Jeffrey L.

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-091916-055327

The discovery of exoplanets and the desire to understand their atmospheric chemical composition and habitability provides a new rationale for understanding the radiation from X-rays to radio wavelengths emitted by their host stars. Semiempirical models of stellar atmospheres that include accurate treatment of radiative transfer of all important atoms, ions, and molecules provide the essential basis for understanding a star's emitted radiation that is our main data source for characterizing a star and the radiation environment of its exoplanets. In Solar-type and cooler stars, the ultraviolet and extreme ultraviolet radiation formed in their chromospheres and transition regions drive the photochemistry in exoplanet atmospheres. In this review, I describe and critique the development of semiempirical static and time-dependent models of the chromospheres and transition regions of the Sun and cooler stars as well as the spectroscopic diagnostics upon which these models are based. The related topics of stellar coronae and winds and their theoretical bases are beyond the scope of this review.

Sharma, Sanjib

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-082214-122339

Markov chain Monte Carlobased Bayesian data analysis has now become the method of choice for analyzing and interpreting data in almost all disciplines of science. In astronomy, over the past decade, we have also seen a steady increase in the number of papers that employ Monte Carlobased Bayesian analysis. New, efficient Monte Carlobased methods are continuously being developed and explored. In this review, we first explain the basics of Bayesian theory and discuss how to set up data analysis problems within this framework. Next, we provide an overview of various Monte Carlobased methods for performing Bayesian data analysis. Finally, we discuss advanced ideas that enable us to tackle complex problems and thus hold great promise for the future. We also distribute downloadable computer software (https:github.comsanjibsbmcmc) Python that implements some of the algorithms and examples discussed here.

Kaspi, Victoria M.; Beloborodov, Andrei M.

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-081915-023329

Magnetars are young and highly magnetized neutron stars that display a wide array of X-ray activity including short bursts, large outbursts, giant flares, and quasi-periodic oscillations, often coupled with interesting timing behavior including enhanced spin-down, glitches, and antiglitches. The bulk of this activity is explained by the evolution and decay of an ultrastrong magnetic field, stressing and breaking the neutron-star crust, which in turn drives twists of the external magnetosphere and powerful magnetospheric currents. The population of detected magnetars has grown to about 30 objects and shows unambiguous phenomenological connection with highly magnetized radio pulsars. Recent progress in magnetar theory includes explanation of the hard X-ray component in the magnetar spectrum and development of surface heating models, explaining the sources remarkable radiative output.

Kaaret, Philip; Feng, Hua; Roberts, Timothy P.

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-091916-055259

We review observations of ultraluminous X-ray sources (ULXs). X-ray spectroscopic and timing studies of ULXs suggest a new accretion state distinct from those seen in Galactic stellar-mass black hole binaries. The detection of coherent pulsations indicates the presence of neutron-star accretors in three ULXs and therefore apparently super-Eddington luminosities. Optical and X-ray line profiles of ULXs and the properties of associated radio and optical nebulae suggest that ULXs produce powerful outflows, also indicative of super-Eddington accretion. We discuss models of super-Eddington accretion and their relationship to the observed behaviors of ULXs. We review the evidence for intermediate-mass black holes (IMBHs) in ULXs. We consider the implications of ULXs for super-Eddington accretion in active galactic nuclei, heating of the early Universe, and the origin of the black hole binary recently detected via gravitational waves.

Bullock, James S.; Boylan-Kolchin, Michael

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-091916-055313

The dark energy plus cold dark matter (CDM) cosmological model has been a demonstrably successful framework for predicting and explaining the large-scale structure of the Universe and its evolution with time. Yet on length scales smaller than 1 Mpc and mass scales smaller than 1011M, the theory faces a number of challenges. For example, the observed cores of many dark matterdominated galaxies are both less dense and less cuspy than navely predicted in CDM. The number of small galaxies and dwarf satellites in the Local Group is also far below the predicted count of low-mass dark matter halos and subhalos within similar volumes. These issues underlie the most well-documented problems with CDM: cusp/core, missing satellites, and too-big-to-fail. The key question is whether a better understanding of baryon physics, dark matter physics, or both is required to meet these challenges. Other anomalies, including the observed planar and orbital configurations of Local Group satellites and the tight baryonic/dark matter scaling relations obeyed by the galaxy population, have been less thoroughly explored in the context of CDM theory. Future surveys to discover faint, distant dwarf galaxies and to precisely measure their masses and density structure hold promising avenues for testing possible solutions to the small-scale challenges going forward. Observational programs to constrain or discover and characterize the number of truly dark low-mass halos are among the most important, and achievable, goals in this field over the next decade. These efforts will either further verify the CDM paradigm or demand a substantial revision in our understanding of the nature of dark matter.

Tumlinson, Jason; Peeples, Molly S.; Werk, Jessica K.

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-091916-055240

The gas surrounding galaxies outside their disks or interstellar medium and inside their virial radii is known as the circumgalactic medium (CGM). In recent years this component of galaxies has assumed an important role in our understanding of galaxy evolution owing to rapid advances in observational access to this diffuse, nearly invisible material. Observations and simulations of this component of galaxies suggest that it is a multiphase medium characterized by rich dynamics and complex ionization states. The CGM is a source for a galaxy's star-forming fuel, the venue for galactic feedback and recycling, and perhaps the key regulator of the galactic gas supply. We review our evolving knowledge of the CGM with emphasis on its mass, dynamical state, and coevolution with galaxies. Observations from all redshifts and from across the electromagnetic spectrum indicate that CGM gas has a key role in galaxy evolution. We summarize the state of this field and pose unanswered questions for future research.

Kaltenegger, Lisa

2017 Annual Review of Astronomy and Astrophysics

doi: 10.1146/annurev-astro-082214-122238

The detection of exoplanets orbiting other stars has revolutionized our view of the cosmos. First results suggest that it is teeming with a fascinating diversity of rocky planets, including those in the habitable zone. Even our closest star, Proxima Centauri, harbors a small planet in its habitable zone, Proxima b. With the next generation of telescopes, we will be able to peer into the atmospheres of rocky planets and get a glimpse into other worlds. Using our own planet and its wide range of biota as a Rosetta stone, we explore how we could detect habitability and signs of life on exoplanets over interstellar distances. Current telescopes are not yet powerful enough to characterize habitable exoplanets, but the next generation of telescopes that is already being built will have the capabilities to characterize close-by habitable worlds. The discussion on what makes a planet a habitat and how to detect signs of life is lively. This review will show the latest results, the challenges of how to identify and characterize such habitable worlds, and how near-future telescopes will revolutionize the field. For the first time in human history, we have developed the technology to detect potential habitable worlds. Finding thousands of exoplanets has taken the field of comparative planetology beyond the Solar System.