Annual Review of Astronomy and Astrophysics - Volume 52, 2014
Volume 52, 2014
-
-
Wondering About Things
Vol. 52 (2014), pp. 1–42More LessHere you will find facts about and the opinions of an American astrophysicist who practiced in the second half of the twentieth century. The title explains why I did it. I invented some new ideas, I applied them to some astro objects, I computed things with pen and paper; I ended up thinking that I had succeeded in pushing the field ahead a bit.
Attracted by Newtonian theory, I did some experiments too. I love hydrodynamics and magnetic fields in space. The math is beautiful, and the objects are stupendous in their brilliant displays. For some reason I meditated on gases between the stars, their pressures and motions. I left the stars to others, believing that their physics was under control.
As I grew older, I had to decide whether to direct others rather than just myself and ended up at the Harvard–Smithsonian Center for Astrophysics doing both. It was thrilling because I had never had management experience and was flying by the seat of my pants, as I guess other astrodirectors do. In the process, I advised the US government on future directions in astronomy, chairing a number of committees. It is astonishing that the government is interested in astronomy, and it is exciting to interact with the people in the National Academy of Sciences (NAS), the Congress, and the Executive branch who have dedicated their lives to enable the expansion of our knowledge of astronomy.
Along the way I studied more abstract concepts in physics, including magnetic helicity and its relation to the winding numbers of nonabelian particle physics. These are topological concepts that I should have learned in grad school but did not.
This review has two parts. The first part is for scientists, and covers my life in chronological order. The second part is for laymen who are interested in science. It gives a flavor of my scientific work with no math and a minimum of jargon.
-
-
-
Short-Duration Gamma-Ray Bursts
Vol. 52 (2014), pp. 43–105More LessGamma-ray bursts (GRBs) display a bimodal duration distribution with a separation between the short- and long-duration bursts at about 2 s. The progenitors of long GRBs have been identified as massive stars based on their association with Type Ic core-collapse supernovae (SNe), their exclusive location in star-forming galaxies, and their strong correlation with bright UV regions within their host galaxies. Short GRBs have long been suspected on theoretical grounds to arise from compact object binary mergers (neutron star–neutron star or neutron star–black hole). The discovery of short GRB afterglows in 2005 provided the first insight into their energy scale and environments, as well as established a cosmological origin, a mix of host-galaxy types, and an absence of associated SNe. In this review, I summarize nearly a decade of short GRB afterglow and host-galaxy observations and use this information to shed light on the nature and properties of their progenitors, the energy scale and collimation of the relativistic outflow, and the properties of the circumburst environments. The preponderance of the evidence points to compact object binary progenitors, although some open questions remain. On the basis of this association, observations of short GRBs and their afterglows can shed light on the on- and off-axis electromagnetic counterparts of gravitational wave sources from the Advanced LIGO/Virgo experiments.
-
-
-
Observational Clues to the Progenitors of Type Ia Supernovae
Vol. 52 (2014), pp. 107–170More LessType Ia supernovae (SNe Ia) are important distance indicators, element factories, cosmic-ray accelerators, kinetic-energy sources in galaxy evolution, and end points of stellar binary evolution. It has long been clear that a SN Ia must be the runaway thermonuclear explosion of a degenerate carbon-oxygen stellar core, most likely a white dwarf (WD). However, the specific progenitor systems of SNe Ia, and the processes that lead to their ignition, have not been identified. Two broad classes of progenitor binary systems have long been considered: single-degenerate (SD), in which a WD gains mass from a nondegenerate star; and double-degenerate (DD), involving the merger of two WDs. New theoretical work has enriched these possibilities with some interesting updates and variants. We review the significant recent observational progress in addressing the progenitor problem. We consider clues that have emerged from the observed properties of the various proposed progenitor populations, from studies of SN Ia sites—pre- and postexplosion—from analysis of the explosions themselves and from the measurement of event rates. The recent nearby and well-studied event, SN 2011fe, has been particularly revealing. The observational results are not yet conclusive and sometimes prone to competing theoretical interpretations. Nevertheless, it appears that DD progenitors, long considered the underdog option, could be behind some, if not all, SNe Ia. We point to some directions that may lead to future progress.
-
-
-
Tidal Dissipation in Stars and Giant Planets
Vol. 52 (2014), pp. 171–210More LessAstrophysical fluid bodies that orbit close to one another induce tidal distortions and flows that are subject to dissipative processes. The spin and orbital motions undergo a coupled evolution over astronomical timescales, which is relevant for many types of binary star, short-period extrasolar planetary systems, and the satellites of the giant planets in the Solar System. I review the principal mechanisms that have been discussed for tidal dissipation in stars and giant planets in both linear and nonlinear regimes. I also compare the expectations based on theoretical models with recent observational findings.
-
-
-
Gamma-Ray Pulsar Revolution
Vol. 52 (2014), pp. 211–250More LessIsolated neutron stars (INSs) were the first sources identified in the field of high-energy gamma-ray astronomy. In the 1970s, only two sources had been identified, the Crab and Vela pulsars. However, although few in number, these objects were crucial in establishing the very concept of a gamma-ray source. Moreover, they opened up significant discovery space in both the theoretical and phenomenological fronts. The need to explain the copious gamma-ray emission of these pulsars led to breakthrough developments in understanding the structure and physics of neutron star (NS) magnetospheres. In parallel, the 20-year-long chase to understand the nature of Geminga unveiled the existence of a radio-quiet, gamma-ray-emitting INS, adding a new dimension to the INS family.
We are living through an extraordinary time of discovery. The current generation of gamma-ray detectors has vastly increased the population of known gamma-ray-emitting NSs. The 100 mark was crossed in 2011, and we are now over 150. The gamma-ray-emitting NS population exhibits roughly equal numbers of radio-loud and radio-quiet young INSs, plus an astonishing, and unexpected, group of isolated and binary millisecond pulsars (MSPs). The number of MSPs is growing so rapidly that they are on their way to becoming the most numerous members of the family of gamma-ray-emitting NSs. Even as these findings have set the stage for a revolution in our understanding of gamma-ray-emitting NSs, long-term monitoring of the gamma-ray sky has revealed evidence of flux variability in the Crab Nebula as well as in the pulsed emission from PSR J2021+4026, challenging a four-decades-old, constant-emission paradigm. Now we know that both pulsars and their nebulae can, indeed, display variable emission.
-
-
-
Solar Dynamo Theory
Vol. 52 (2014), pp. 251–290More LessThe Sun's magnetic field is the engine and energy source driving all phenomena collectively defining solar activity, which in turn structures the whole heliosphere and significantly impacts Earth's atmosphere down at least to the stratosphere. The solar magnetic field is believed to originate through the action of a hydromagnetic dynamo process operating in the Sun's interior, where the strongly turbulent environment of the convection zone leads to flow-field interactions taking place on an extremely wide range of spatial and temporal scales. Following a necessarily brief observational overview of the solar magnetic field and its cycle, this review on solar dynamo theory is structured around three areas in which significant advances have been made in recent years: (a) global magnetohydrodynamical simulations of convection and magnetic cycles, (b) the turbulent electromotive force and the dynamo saturation problem, and (c) flux transport dynamos, and their application to model cycle fluctuations and grand minima and to carry out cycle prediction.
-
-
-
The Evolution of Galaxy Structure Over Cosmic Time
Vol. 52 (2014), pp. 291–337More LessI present a comprehensive review of the evolution of galaxy structure in the Universe from the first galaxies currently observable at z ∼ 6 down to galaxies observable in the local Universe. Observed changes in galaxy structures reveal formation processes that only galaxy structural analyses can provide. This pedagogical review provides a detailed discussion of the major methods used to study galaxies morphologically and structurally, including the well-established visual method for morphology; Sérsic fitting to measure galaxy sizes and surface brightness profile shapes; and nonparametric structural methods [such as the concentration (C), asymmetry (A), clumpiness (S) (CAS) method and the Gini/M20 parameters, as well as newer structural indices]. These structural indices measure fundamental properties of galaxies, such as their scale, star-formation rate, and ongoing merger activity. Extensive observational results demonstrate how broad galaxy morphologies and structures change with time up to z ∼ 3, from small, compact and peculiar systems in the distant Universe to the formation of the Hubble sequence, dominated by spirals and ellipticals. Structural methods accurately identify galaxies in mergers and allow measurements of the merger history out to z ∼ 3. I depict properties and evolution of internal structures of galaxies, such as bulges, disks, bars, and at z>1 large star-forming clumps. I describe the structure and morphologies of host galaxies of active galactic nuclei and starbursts/submillimeter galaxies, along with how morphological galaxy quenching occurs. The role of environment in producing structural changes in galaxies over cosmic time is also discussed. Galaxy sizes can also change with time, with measured sizes up to a factor of 2–5 smaller at high redshift at a given stellar mass. I conclude with a discussion of how the evolving trends, in sizes, structures, and morphologies, reveal the formation mechanisms behind galaxies and provides a new and unique way to test theories of galaxy formation.
-
-
-
Microarcsecond Radio Astrometry
Vol. 52 (2014), pp. 339–372More LessAstrometry provides the foundation for astrophysics. Accurate positions are required for the association of sources detected at different times or wavelengths, and distances are essential to estimate the size, luminosity, mass, and ages of most objects. Very long baseline interferometry at radio wavelengths, with diffraction-limited imaging at submilliarcsecond resolution, has long held the promise of microarcsecond astrometry. However, only in the past decade has this been routinely achieved. Currently, parallaxes for sources across the Milky Way are being measured with ∼10 μas accuracy, and proper motions of galaxies are being determined with accuracies of ∼1 μas year−1. The astrophysical applications of these measurements cover many fields, including star formation, evolved stars, stellar and supermassive black holes, Galactic structure, the history and fate of the Local Group, the Hubble constant, and tests of general relativity. This review summarizes the methods used and the astrophysical applications of microarcsecond radio astrometry.
-
-
-
Far-Infrared Surveys of Galaxy Evolution
Vol. 52 (2014), pp. 373–414More LessRoughly half of the radiation from evolving galaxies in the early Universe reaches us in the far-infrared and submillimeter wavelength ranges. Recent major advances in observing capabilities, in particular the launch of the Herschel Space Observatory in 2009, have dramatically enhanced our ability to use this information in the context of multiwavelength studies of galaxy evolution. Near its peak, three-quarters of the cosmic infrared background is now resolved into individually detected sources. The use of far-infrared diagnostics of dust-obscured star formation and of interstellar medium conditions has expanded from extreme and rare extreme high-redshift galaxies to more typical main-sequence galaxies and hosts of active galactic nuclei out to z≳2. These studies shed light on the evolving role of steady equilibrium processes and of brief starbursts at and since the peak of cosmic star formation and black hole accretion. This review presents a selection of recent far-infrared studies of galaxy evolution with an emphasis on Herschel results.
-
-
-
Cosmic Star-Formation History
Vol. 52 (2014), pp. 415–486More LessOver the past two decades, an avalanche of new data from multiwavelength imaging and spectroscopic surveys has revolutionized our view of galaxy formation and evolution. Here we review the range of complementary techniques and theoretical tools that allow astronomers to map the cosmic history of star formation, heavy element production, and reionization of the Universe from the cosmic “dark ages” to the present epoch. A consistent picture is emerging, whereby the star-formation rate density peaked approximately 3.5 Gyr after the Big Bang, at z≈1.9, and declined exponentially at later times, with an e-folding timescale of 3.9 Gyr. Half of the stellar mass observed today was formed before a redshift z = 1.3. About 25% formed before the peak of the cosmic star-formation rate density, and another 25% formed after z = 0.7. Less than ∼1% of today's stars formed during the epoch of reionization. Under the assumption of a universal initial mass function, the global stellar mass density inferred at any epoch matches reasonably well the time integral of all the preceding star-formation activity. The comoving rates of star formation and central black hole accretion follow a similar rise and fall, offering evidence for coevolution of black holes and their host galaxies. The rise of the mean metallicity of the Universe to about 0.001 solar by z = 6, one Gyr after the Big Bang, appears to have been accompanied by the production of fewer than ten hydrogen Lyman-continuum photons per baryon, a rather tight budget for cosmological reionization.
-
-
-
Mass Loss: Its Effect on the Evolution and Fate of High-Mass Stars
Vol. 52 (2014), pp. 487–528More LessOur understanding of massive star evolution is in flux due to recent upheavals in our view of mass loss and observations of a high binary fraction among O-type stars. Mass-loss rates for standard metallicity-dependent winds of hot stars are lower by a factor of 2–3 compared with rates adopted in modern stellar evolution codes, due to the influence of clumping on observed diagnostics. Weaker hot star winds shift the burden of H-envelope removal to the winds, pulsations, and eruptions of evolved supergiants, as well as binary mass transfer. Studies of stripped-envelope supernovae, in particular, require binary mass transfer. Dramatic examples of eruptive mass loss are seen in Type IIn supernovae, which have massive shells ejected just a few years earlier. These eruptions are a prelude to core collapse, and may signify severe instabilities in the latest nuclear burning phases. We encounter the predicament that the most important modes of mass loss are also the most uncertain, undermining the predictive power of single-star evolution models. Moreover, the influence of winds and rotation has been evaluated by testing single-star models against observed statistics that, it turns out, are heavily influenced by binary evolution. Altogether, this may alter our view about the most basic outcomes of massive-star mass loss—are Wolf-Rayet stars and Type Ibc supernovae the products of winds, or are they mostly the result of binary evolution and eruptive mass loss? This is not fully settled, but mounting evidence points toward the latter. This paradigm shift impacts other areas of astronomy, because it changes predictions for ionizing radiation and wind feedback from stellar populations, it may alter conclusions about star-formation rates and initial mass functions, it affects the origin of compact stellar remnants, and it influences how we use supernovae as probes of stellar evolution across cosmic time.
-
-
-
Hot Accretion Flows Around Black Holes
Feng Yuan, and Ramesh NarayanVol. 52 (2014), pp. 529–588More LessBlack hole accretion flows can be divided into two broad classes: cold and hot. Whereas cold accretion flows consist of cool optically thick gas and are found at relatively high mass accretion rates, hot accretion flows, the topic of this review, are virially hot and optically thin, and occur at lower mass accretion rates. They are described by accretion solutions such as the advection-dominated accretion flow and luminous hot accretion flow. Because of energy advection, the radiative efficiency of these flows is in general lower than that of a standard thin accretion disk. Moreover, the efficiency decreases with decreasing mass accretion rate. Observations show that hot accretion flows are associated with jets. In addition, theoretical arguments suggest that hot flows should produce strong winds. Hot accretion flows are believed to be present in low-luminosity active galactic nuclei and in black hole X-ray binaries in the hard and quiescent states. The prototype is Sgr A*, the ultralow-luminosity supermassive black hole at our Galactic center. The jet, wind, and radiation from a supermassive black hole with a hot accretion flow can interact with the external interstellar medium and modify the evolution of the host galaxy.
-
-
-
The Coevolution of Galaxies and Supermassive Black Holes: Insights from Surveys of the Contemporary Universe
Vol. 52 (2014), pp. 589–660More LessWe summarize what large surveys of the contemporary Universe have taught us about the physics and phenomenology of the processes that link the formation and evolution of galaxies with their central supermassive black holes. We present a picture in which the population of active galactic nuclei (AGNs) can be divided into two distinct populations. The radiative-mode AGNs are associated with black holes (BHs) that produce radiant energy powered by accretion at rates in excess of ∼1% of the Eddington limit. They are primarily associated with less massive BHs growing in high-density pseudobulges at a rate sufficient to produce the total mass budget in these BHs in ∼10 Gyr. The circumnuclear environment contains high-density cold gas and associated star formation. Major mergers are not the primary mechanism for transporting this gas inward; secular processes appear dominant. Stellar feedback is generic in these objects, and strong AGN feedback is seen only in the most powerful AGNs. In jet-mode AGNs the bulk of energetic output takes the form of collimated outflows (jets). These AGNs are associated with the more massive BHs in more massive (classical) bulges and elliptical galaxies. Neither the accretion onto these BHs nor star formation in their host bulge is significant today. These AGNs are probably fueled by the accretion of slowly cooling hot gas that is limited by the feedback/heating provided by AGN radio sources. Surveys of the high-redshift Universe paint a similar picture. Noting that the volume-averaged ratio of star formation to BH growth has remained broadly constant over the past 10 Gyrs, we argue that the processes that linked the cosmic evolution of galaxies and BHs are still at play today.
-
-
-
Numerical Relativity and Astrophysics
Vol. 52 (2014), pp. 661–694More LessThroughout the Universe many powerful events are driven by strong gravitational effects that require general relativity to fully describe them. These include compact binary mergers, black hole accretion, and stellar collapse, where velocities can approach the speed of light and extreme gravitational fields (ΦNewt/c2≃1) mediate the interactions. Many of these processes trigger emission across a broad range of the electromagnetic spectrum. Compact binaries further source strong gravitational wave emission that could directly be detected in the near future. This feat will open up a gravitational wave window into our Universe and revolutionize our understanding of it. Describing these phenomena requires general relativity, and—where dynamical effects strongly modify gravitational fields—the full Einstein equations coupled to matter sources. Numerical relativity is a field within general relativity concerned with studying such scenarios that cannot be accurately modeled via perturbative or analytical calculations. In this review, we examine results obtained within this discipline, with a focus on its impact in astrophysics.
-
Previous Volumes
-
Volume 61 (2023)
-
Volume 60 (2022)
-
Volume 59 (2021)
-
Volume 58 (2020)
-
Volume 57 (2019)
-
Volume 56 (2018)
-
Volume 55 (2017)
-
Volume 54 (2016)
-
Volume 53 (2015)
-
Volume 52 (2014)
-
Volume 51 (2013)
-
Volume 50 (2012)
-
Volume 49 (2011)
-
Volume 48 (2010)
-
Volume 47 (2009)
-
Volume 46 (2008)
-
Volume 45 (2007)
-
Volume 44 (2006)
-
Volume 43 (2005)
-
Volume 42 (2004)
-
Volume 41 (2003)
-
Volume 40 (2002)
-
Volume 39 (2001)
-
Volume 38 (2000)
-
Volume 37 (1999)
-
Volume 36 (1998)
-
Volume 35 (1997)
-
Volume 34 (1996)
-
Volume 33 (1995)
-
Volume 32 (1994)
-
Volume 31 (1993)
-
Volume 30 (1992)
-
Volume 29 (1991)
-
Volume 28 (1990)
-
Volume 27 (1989)
-
Volume 26 (1988)
-
Volume 25 (1987)
-
Volume 24 (1986)
-
Volume 23 (1985)
-
Volume 22 (1984)
-
Volume 21 (1983)
-
Volume 20 (1982)
-
Volume 19 (1981)
-
Volume 18 (1980)
-
Volume 17 (1979)
-
Volume 16 (1978)
-
Volume 15 (1977)
-
Volume 14 (1976)
-
Volume 13 (1975)
-
Volume 12 (1974)
-
Volume 11 (1973)
-
Volume 10 (1972)
-
Volume 9 (1971)
-
Volume 8 (1970)
-
Volume 7 (1969)
-
Volume 6 (1968)
-
Volume 5 (1967)
-
Volume 4 (1966)
-
Volume 3 (1965)
-
Volume 2 (1964)
-
Volume 1 (1963)
-
Volume 0 (1932)