Annual Review of Astronomy and Astrophysics - Volume 43, 2005

The scientific career of Riccardo Giacconi is summarized from an autobiographical point of view. The narrative moves from the discovery of Sco X-1, to the observations with Uhuru, Skylab, Einstein, Rosat, and the Chandra Observatory. His direction of the Space Telescope Science Institute (STScI) and of the European Southern Observatory are described. His recent involvement in the Atacama Large Millimeter Array (ALMA) project is briefly summarized.

Astrobiology is the study of the living universe. Astronomy provides the context for the origin and evolution of life on Earth. Conversely, discoveries about the terrestrial biosphere—from extremophilic microbes to the evolution of intelligence—inform our thinking about prospects for life elsewhere. Astrobiology includes the search for extraterrestrial life via in situ exploration, spectroscopy of solar and extrasolar planetary atmospheres, and the search for extraterrestrial intelligence. This review situates astrobiology within philosophical issues of the definition of life and the biological compatibility of the universe. It reviews the habitability of the Galaxy in general and of planets and moons in particular, and summarizes current controversies in origins-of-life research and in evidence for the earliest life on Earth. It critiques certain “rare Earth” and “anthropic” arguments, and considers four approaches to deciding whether intelligent life exists elsewhere in the Galaxy. It concludes that astrobiology must also speak to the future of human civilization.

After centuries of speculation that the early records of observations of bright comets near the sun involved repeated returns of a single sungrazing object, there came in the 1880s the realization, first by Kirkwood, and then most notably by Kreutz, that the observations could be explained by the existence of a group of related comets that had separated from each other at some earlier perihelion passage. After the appearance of further bright sungrazers in the mid-twentieth century, attempts were made to understand the specific manner in which the various members of this Kreutz Group of comets had evolved from each other. Beginning in 1979, coronagraphic observations from space allowed the detection of numerous additional Kreutz members that were completely vaporized as they grazed the sun. Since 1996, the Solar and Heliospheric Observatory (SOHO) coronagraphs have revealed some 700 of these comets, giving rise to the thought that there is a constant stream of small members, much of the break-up having occurred instead near aphelion and even quite recently. Whereas consideration of break-up only near perihelion requires that the observed dispersion of the orbital parameters would take many millennia, recent calculations have shown that the evolution can be substantially sped up by allowing fragments to be rotationally spun off at heliocentric distances of many tens of AU. The SOHO observations have also allowed the recognition of three other substantial groups of sungrazing comets, as well as some pairs arriving only hours apart.

Solar coronal mass ejections (CMEs) are a major form of activity on the Sun. A CME takes 10^15-16 g of plasma from the low corona into the solar wind, to disturb the near-Earth space if the CME direction is favorable. We summarize current observations and ideas of CME physics to provide a hydromagnetic view of the CMEs as the products of continual magnetic flux emergence and an interplay between magnetic reconnection and approximate magnetic-helicity conservation in the corona. Each flux emergence brings helicity to accumulate additively in a coronal structure while excess magnetic energy is flared away by reconnection. Self-confinement eventually fails with a CME shedding the accumulated helicity out of the low corona to enable the field to reach the minimum-energy state. Similar evolutionary processes may occur in other magnetic stars and galaxies.

Digital image reconstruction is a robust means by which the underlying images hidden in blurry and noisy data can be revealed. The main challenge is sensitivity to measurement noise in the input data, which can be magnified strongly, resulting in large artifacts in the reconstructed image. The cure is to restrict the permitted images. This review summarizes image reconstruction methods in current use. Progressively more sophisticated image restrictions have been developed, including (a) filtering the input data, (b) regularization by global penalty functions, and (c) spatially adaptive methods that impose a variable degree of restriction across the image. The most reliable reconstruction is the most conservative one, which seeks the simplest underlying image consistent with the input data. Simplicity is context-dependent, but for most imaging applications, the simplest reconstructed image is the smoothest one. Imposing the maximum, spatially adaptive smoothing permitted by the data results in the best image reconstruction.

The establishment of new spectral classes cooler than type M has had a brief, yet already rich, history. Prototypes of the new “L dwarf” and “T dwarf” classes were first found in the late 1980s to mid-1990s, with a flood of new discoveries occurring in the late 1990s with the advent of deep, large-area, digital sky surveys. Over four hundred and fifty L and T dwarfs are now cataloged. This review concentrates on the spectroscopic properties of these objects, beginning with the establishment of classification schemes rooted in the MK Process. The resulting grid of spectral types is then used as a tool to ferret out the underlying physics. The temperature ranges covered by these spectral types, the complex chemical processes responsible for the shape of their emergent spectra, their nature as either true stars or brown dwarfs, and their number density in the Galaxy are discussed. Two promising avenues for future research are also explored: the extension of the classification system to three dimensions to account for gravity- and metallicity-dependent features, and the capability of newer large-area surveys to uncover brown dwarfs cooler than those now recognized.

A number of recent investigations suggest that cool white dwarfs are more numerous than predicted by conventional Galactic models and that those stars make a significant contribution to the mass budget of the Milky Way. In particular, there is speculation that cool white dwarfs are linked with the dark-matter halos. This review examines those recent results and matches them against current understanding of the properties of the stellar populations that make up the Milky Way, taking due account of the relative star formation histories of the disk and the thick disk. The new white dwarf observations do not require any additions to the conventional stellar populations of the Milky Way. There is no credible evidence of either an underlying population of ancient white dwarfs or a link between high-velocity degenerates and dark matter. Nonetheless, placed in the proper context, these high-velocity white dwarfs provide interesting insight on the likely history of the Milky Way. We show that the thick disk is likely to contribute ∼20% of Solar Neighborhood white dwarfs. If the thick disk is an old, single-burst population, as favored by most investigations, then those white dwarfs dominate at faint absolute magnitudes. As a result, analyses of the low-luminosity cutoff in the local white dwarf–luminosity function actually set limits on the age of the thick disk, rather than the thin disk. In absolute terms, these results imply that the thick disk is no older than 10 Gyrs, whereas major star formation in the thin disk may not have started until 7 to 8 Gyrs ago. Moreover, the enhanced [α/Fe] ratios in thick-disk and halo main-sequence stars suggest that those two populations have similar ages, implying a relatively young age for the field halo.

Standard star photometry dominated the latter half of the twentieth century reaching its zenith in the 1980s. It was introduced to take advantage of the high sensitivity and large dynamic range of photomultiplier tubes compared to photographic plates. As the quantum efficiency of photodetectors improved and the wavelength range extended further to the red, standard systems were modified and refined, and deviations from the original systems proliferated. The revolutionary shift to area detectors for all optical and IR observations forced further changes to standard systems, and the precision and accuracy of much broad- and intermediate-band photometry suffered until more suitable observational techniques and standard reduction procedures were adopted. But the biggest revolution occurred with the production of all-sky photometric surveys. Hipparcos/Tycho was space based, but most, like 2MASS, were ground-based, dedicated survey telescopes. It is very likely that in the future, rather than making a measurement of an object in some standard photometric system, one will simply look up the magnitudes and colors of most objects in catalogs accessed from the Virtual Observatory. In this review the history of standard star photometry will be outlined, and the calibration and realization of standard systems will be examined. Finally, model atmosphere fluxes are now very realistic, and synthetic photometry offers the best prospects for calibrating all photometric systems. Synthetic photometry from observed spectrophotometry should also be used as a matter of course to provide colors within standard systems and to gain insights into the spectra and colors of unusual stars, star clusters and distant galaxies.

The interstellar medium in the vicinity of the Sun is arranged in large-scale structures of bubble walls, sheets, and filaments of warm gas, within which close to the midplane there are subsheets and filaments of cold dense material; the whole occupies roughly half the available volume and extends with decreasing mean density to at least a kiloparsec off the plane. The remainder of the volume is in bubble interiors, cavities, and tunnels of much lower density, with some but not all of those lower density regions hot enough to be observable via their X-ray emission. This entire system is pervaded by a rather strong and irregular magnetic field and cosmic rays, the pressures of which are confined by the weight of the interstellar gas, particularly that far from the plane where gravity is strong. Observations suggest that the cosmic rays and magnetic field have an even more extended vertical distribution than the warm gas, requiring either the weight of additional coronal material or magnetic tension to confine it to the disk. Adjusting one's perception of this medium to embrace the known aspects is difficult. After this adjustment, there are many problems to solve and prejudices to overcome—the weak role of thermal instability, the suppression of certain gravitational instabilities, the problem of determining the state in the low-density regions, the twin difficulties of not having too much OVI (O⁺⁵) and getting enough diffuse 3/4 keV X-ray emission, the possible importance of large old-barrel–shaped supernova remnants in clarifying matters, the possible role of dust evolution in adjusting the heating to make clouds stable, the factors influencing the magnitudes of the interstellar pressure and scale height—things that global models of the medium might examine to clarify some of these matters; attention to these details and more constitute the challenge of this subject.

Most of what we know about the stellar population of nearby, resolved galaxies comes from the interpretation of their color-magnitude diagrams, by comparison with stellar evolutionary models. We review how well current stellar evolution models reproduce the properties of simple stellar populations. Emphasis is given to the regions of the color-magnitude diagram which are most useful for deriving age, metallicity, or distance of a population. Extensive comparison is made between the predictions of the most-used stellar evolution libraries, in order to estimate how model dependent the results are. The present review, written from a user perspective, aims at emphasizing the strengths and weaknesses of the models, and is intended both for observers and theoreticians. We hope to encourage observers to provide stronger observational constraints where they are needed, and to stimulate theoreticians to isolate the input physics responsible for the different behavior between models and the reasons for the discrepancies with data.

The current status of modeling the evolution and nucleosynthesis of asymptotic giant branch (AGB) stars is reviewed. The principles of AGB evolution have been investigated in recent years leading to improved and refined models, for example with regard to hot-bottom burning or the third dredge-up. The postprocessing s-process model yields quantitative results that reproduce many observations. However, these and most other processes in AGB stars are intimately related to the physics of stellar mixing. Mixing in AGB stars is currently not well-enough understood for accurate yield predictions. Several constraints and methods are available to improve the models. Some regimes of AGB evolution have not yet been studied in sufficient detail. These include the super-AGB stars and AGB stars at extremely low or ultra low metallicity.

The information on the chemical compositions of stars encoded in their spectra plays a central role in contemporary astrophysics. Stellar element abundances are, however, not observed: to decipher the spectral fingerprints in terms of abundances requires realistic models for the stellar atmospheres and the line-formation processes. Still today, the vast majority of abundance analyses of late-type stars rely on one-dimensional (1D), hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE). In this review possible systematic errors in studies of F-, G- and K-type stars introduced by these questionable approximations are discussed. Departures from LTE are commonplace and often quite severe, in particular for low surface gravities or metallicities, with minority species and low-excitation transitions being the most vulnerable. Recently, time-dependent, 3D, hydrodynamical model atmospheres have started to be employed for stellar abundance purposes, with large differences compared with 1D modeling found in particular for metal-poor stars. An assessment of non-LTE and 3D effects for individual elements as well as on the estimated stellar parameters is presented.

We discuss the importance of very metal-poor stars to develop an understanding of the nature of the first stars that formed in the Universe and the nucleosynthesis events associated with them, as well as to refine models of galaxy formation, in particular for large spiral galaxies such as the Milky Way. After briefly reviewing the history of the search for very metal-deficient stars in the Galaxy, we summarize ongoing efforts, concentrating on the two large objective-prism surveys that have led to the discovery of the majority of stars with [Fe/H] < −2.0 known at present: the HK survey of Beers and collaborators and the Hamburg/ESO survey of Christlieb and collaborators. We then consider the wealth of information that can be gleaned from high-resolution spectroscopic study of very metal-poor stars. We close with a list of open questions and a discussion of new survey techniques that will expand the sample of recognized very metal-deficient stars in the Galaxy by several orders of magnitude.

“You ask what is the use of classification, arrangement, systematization. I answer you; order and simplification are the first steps toward the mastery of a subject—the actual enemy is the unknown.”

Settembrini to Hans Castorp in

The Magic Mountain: Thomas Mann

Discussion is made on the philosophy of classification, urging that the classification be made on the basis of morphology alone, not on the basis of supposed physics that some wish to be introduced to “explain” the classification. A history of the development of the present morphological galaxy classification system is given, starting with the purely descriptive systems of Herschel, Wolf, Shapley, and Vorontsov-Velyaminov that have no connective continuity between the classification descriptions. The development of the modern system that has an underlying continuity between the classification bins is reviewed, mainly featuring the 1926 system of Hubble that is the basis of the present extended Hubble system.

In the Galaxy, microwave radiation can be amplified in the interstellar medium in the immediate neighborhood of young stellar objects, or circumstellar envelopes around evolved stars, resulting in cosmic maser emission. Cosmic masers exist because, in contrast to terrestrial conditions, the interstellar gas density is very low so that level population in molecules is typically not in thermal equilibrium, and sometimes inverted. In the nuclear regions of external galaxies, there exist very powerful OH (λ18 cm) and H2O (λ1.35 cm) cosmic masers with line luminosities of ∼ 10²−10⁴L⊙, ≥ 10⁶ times more luminous than typical Galactic maser sources. These are the “mega-masers,” found in high-density molecular gas located within parsecs of active galactic nuclei in the case of H2O mega-masers, or within the central 100 pc of nuclear star-burst regions in the case of OH mega-masers. H2O mega-masers are most frequently found in galactic nuclei with Seyfert2 or LINER spectral characteristics, in spiral and some elliptical galaxies. OH mega-masers are found in ultra-luminous IR galaxies (ULIRG) with the warmest IR colors, and importantly, the OH luminosity is observed to increase with the IR luminosity: LOH ∝ L^1.2IR. Because of the extremely high-surface brightness, H2O mega-maser emission can be mapped at sub-milli-arc-second resolution by Very Long Baseline Interferometry (VLBI), providing a powerful tool to probe spatial and kinematic distributions of molecular gas in distant galactic nuclei at scales below one parsec. An excellent example is the active galaxy, NGC 4258, in which mapping of the H2O mega-maser emission has provided the first direct evidence in an active galactic nucleus for the existence of a thin Keplerian accretion disk with turbulence, as well as highly compelling evidence for the existence of a massive black hole. The NGC 4258 mega-maser has also provided a geometric distance determination of extremely high precision. H2O mega-maser emission is also found to arise from postshocked gas from the impact of nuclear jets or outflows on the surrounding molecular clouds. High-resolution observations have shown that OH mega-masers originate from the molecular gas medium in 100-pc scale nuclear star-burst regions. It is proposed that such extreme star-burst regions, with extensive high-density gas bathed in a very high far-IR radiation field, are conducive to the formation of a very large number of OH maser sources that collectively produce the OH mega-maser emission. In the early Universe, galaxies or mergers could go through a very luminous phase, powered by intensive star-bursts and AGN formation, and could have extremely large OH and H2O maser luminosities, possibly producing giga-masers. With the increasing sensitivity of new telescopes and receivers, surveys and high-resolution studies of mega-masers and giga-masers will be very important tracers and high-resolution probes of active galactic nuclei, dust embedded star-bursts in the earliest galaxies and galaxy mergers in the epoch of very active star formation at z ∼ 2 and beyond. Distance determination of giga-masers at z ∼ 1–2 can provide on independent measure of how fast the universe is expanding.

The Early Universe Molecular Emission Line Galaxies (EMGs) are a population of galaxies with only 36 examples that hold great promise for the study of galaxy formation and evolution at high redshift. The classification, luminosity of molecular line emission, molecular mass, far-infrared (FIR) luminosity, star formation efficiency, morphology, and dynamical mass of the currently known sample are presented and discussed. The star formation rates derived from the FIR luminosity range from about 300 to 5000 M⊙ year ⁻¹ and the molecular mass from 4 × 10⁹ to 1 × 10¹¹M⊙. At the lower end, these star formation rates, gas masses, and diameters are similar to those of local ultraluminous infrared galaxies and represent starbursts in centrally concentrated disks, sometimes, but not always, associated with active galactic nuclei. The evidence for large (>5 kpc) molecular disks is limited. Morphology and several high angular resolution images suggest that some EMGs are mergers with a massive molecular interstellar medium in both components. A critical question is whether the EMGs, in particular those at the higher end of the gas mass and luminosity distribution, represent the formation of massive, giant elliptical galaxies in the early Universe. The sample size is expected to grow explosively in the era of the Atacama Large Millimeter Array (ALMA).

The discovery of the Cosmic Infrared Background (CIB) in 1996, together with recent cosmological surveys from the mid-infrared to the millimeter, have revolutionized our view of star formation at high redshifts. It has become clear, in the last decade, that a population of galaxies that radiate most of their power in the far-infrared (the so-called infrared galaxies) contributes an important part of the whole galaxy build-up in the Universe. Since 1996, detailed (and often painful) investigations of the high-redshift infrared galaxies have resulted in the spectacular progress covered in this review. We outline the nature of the sources of the CIB, including their star-formation rate, stellar and total mass, morphology, metallicity, and clustering properties. We discuss their contribution to the stellar content of the Universe and their origin in the framework of the hierarchical growth of structures. We finally discuss open questions for a scenario of their evolution up to the present-day galaxies.

Galactic winds are the primary mechanism by which energy and metals are recycled in galaxies and are deposited into the intergalactic medium. New observations are revealing the ubiquity of this process, particularly at high redshift. We describe the physics behind these winds, discuss the observational evidence for them in nearby star-forming and active galaxies and in the high-redshift universe, and consider the implications of energetic winds for the formation and evolution of galaxies and the intergalactic medium. To inspire future research, we conclude with a set of observational and theoretical challenges.

Deep surveys of the cosmic X-ray background are reviewed in the context of observational progress enabled by the Chandra X-Ray Observatory and the X-Ray Multi-Mirror Mission-Newton. The sources found by deep surveys are described along with their redshift and luminosity distributions, and the effectiveness of such surveys at selecting active galactic nuclei (AGN) is assessed. Some key results from deep surveys are highlighted, including (a) measurements of AGN evolution and the growth of supermassive black holes, (b) constraints on the demography and physics of high-redshift AGN, (c) the X-ray AGN content of infrared and submillimeter galaxies, and (d) X-ray emission from distant starburst and normal galaxies. We also describe some outstanding problems and future prospects for deep extragalactic X-ray surveys.