Annual Review of Astronomy and Astrophysics - Volume 58, 2020
Volume 58, 2020
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Jack of All
Vol. 58 (2020), pp. 1–25More LessThis article is basically a scientific autobiography from a long and very rewarding career, covering childhood, education, theoretical work, observations, instrumentation, and some social activities. It is not meant to be a review of anything except an incomplete picture of my life, and the relatively few references are to some of my work, work related to mine, and work that had a very large influence on my life and research, so apologies in advance to those I left out in subjects I discuss. I have not in any way attempted to discuss scientific results; those you can go read.
I have used more words on old things than new, with the idea that most readers of this article are much more familiar with the field in the last couple of decades than before. My career spans almost six, and there may be things to learn from antiquity.
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The Assembly of the First Massive Black Holes
Vol. 58 (2020), pp. 27–97More LessThe existence of ∼109M⊙ supermassive black holes (SMBHs) within the first billion years of the Universe has stimulated numerous ideas for the prompt formation and rapid growth of black holes (BHs) in the early Universe. Here, we review ways in which the seeds of massive BHs may have first assembled, how they may have subsequently grown as massive as ∼109M⊙, and how multimessenger observations could distinguish between different SMBH assembly scenarios. We conclude the following:
- ▪ The ultrarare ∼109 M⊙ SMBHs represent only the tip of the iceberg. Early BHs likely fill a continuum from the stellar-mass (∼10M⊙) to the supermassive (∼109) regimes, reflecting a range of initial masses and growth histories.
- ▪ Stellar-mass BHs were likely left behind by the first generation of stars at redshifts as high as ∼30, but their initial growth typically was stunted due to the shallow potential wells of their host galaxies.
- ▪ Conditions in some larger, metal-poor galaxies soon became conducive to the rapid formation and growth of massive seed holes, via gas accretion and by mergers in dense stellar clusters.
- ▪ BH masses depend on the environment (such as the number and properties of nearby radiation sources and the local baryonic streaming velocity) and on the metal enrichment and assembly history of the host galaxy.
- ▪ Distinguishing between assembly mechanisms will be difficult, but a combination of observations by the Laser Interferometer Space Antenna (probing massive BH growth via mergers) and by deep multiwavelength electromagnetic observations (probing growth via gas accretion) is particularly promising.
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Spatially Resolved Spectroscopic Properties of Low-Redshift Star-Forming Galaxies
Vol. 58 (2020), pp. 99–155More LessI review the spatially resolved spectroscopic properties of low-redshift star-forming galaxies (and their retired counterparts) using results from the most recent optical integral field spectroscopy galaxy surveys. First, I briefly summarize the global spectroscopic properties of these galaxies, discussing the main ionization processes and the global relations described by the star-formation rates, gas-phase oxygen abundances, and average properties of their stellar populations (age and metallicity) in comparison with the stellar mass. Second, I present the local distribution of the ionizing processes down to kiloparsec scales, and I show how the global scaling relations found using integrated parameters (like the star-formation main sequence, mass–metallicity relation, and Schmidt–Kennicutt law) have local/resolved counterparts, with the global ones being, for the most part, just integrated/average versions of the local ones. I discuss the local/resolved star-formation histories (SFHs) and chemical-enrichment histories and their implications on the inside-out growth of galaxies. Third, I present the radial distributions of the surface densities of the properties explored globally and how they depend on the integrated galaxy properties.
In conclusion, I find that the evolution of galaxies is mostly governed by local processes but clearly affected by global ones:
- ▪ Many global scaling relations present resolved counterparts (verified down to kiloparsec scales) that can explain them as well as the observed radial gradients in galaxies.
- ▪ These relations are consequences of the local SFHs, the narrow range of the depletion times, and a local metal enrichment.
- ▪ Deviations from these relations are due to dynamical and mixing processes, local exchange of gas (inflows, outflows, and fountains), depletion time differences, and/or differences in the resolved SFHs.
- ▪ Ionization happens at local scales that may be driven by different physical processes, and it cannot be clearly understood using purely integrated quantities. The dominant ionization in galaxies has a stellar origin.
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The Evolution of the Star-Forming Interstellar Medium Across Cosmic Time
Vol. 58 (2020), pp. 157–203More LessOver the past decade, increasingly robust estimates of the dense molecular gas content in galaxy populations between redshift z = 0 and the peak of cosmic galaxy/star formation (z ∼ 1–3) have become available. This rapid progress has been possible due to the advent of powerful ground- and space-based telescopes for the combined study of several millimeter to far-IR, line or continuum tracers of the molecular gas and dust components. The main conclusions of this review are as follows:
- ▪ Star-forming galaxies contained much more molecular gas at earlier cosmic epochs than at the present time.
- ▪ The galaxy-integrated depletion timescale for converting the gas into stars depends primarily on z or Hubble time and, at a given z, on the vertical location of a galaxy along the star-formation rate versus stellar mass main sequence (MS) correlation.
- ▪ Global rates of galaxy gas accretion primarily control the evolution of the cold molecular gas content and star-formation rates of the dominant MS galaxy population, which in turn vary with cosmological expansion. Another key driver may be global disk fragmentation in high-z, gas-rich galaxies, which ties local free-fall timescales to galactic orbital times and leads to rapid radial matter transport and bulge growth. The low star-formation efficiency inside molecular clouds is plausibly set by supersonic streaming motions and internal turbulence, which in turn may be driven by conversion of gravitational energy at high z and/or by local feedback from massive stars at low z.
- ▪ A simple gas regulator model is remarkably successful in predicting the combined evolution of molecular gas fractions, star-formation rates, galactic winds, and gas-phase metallicities.
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Streams, Substructures, and the Early History of the Milky Way
Vol. 58 (2020), pp. 205–256More LessThe advent of the second data release of the Gaia mission, in combination with data from large spectroscopic surveys, is revolutionizing our understanding of the Galaxy. Thanks to these transformational data sets and the knowledge accumulated thus far, a new, more mature picture of the evolution of the early Milky Way is currently emerging.
- ▪ Two of the traditional Galactic components, namely, the stellar halo and the thick disk, appear to be intimately linked: Stars with halo-like kinematics originate in similar proportions from a heated (thick) disk and from debris from a system named Gaia-Enceladus. Gaia-Enceladus was the last big merger event experienced by the Milky Way and was completed around 10 Gyr ago. The puffed-up stars now present in the halo as a consequence of the merger have thus exposed the existence of a disk component at z ∼ 1.8. This is likely related to the previously known metal-weak thick disk and may be traceable to metallicities [Fe/H] −4. As importantly, there is evidence that the merger with Gaia-Enceladus triggered star formation in the early Milky Way, plausibly leading to the appearance of the thick disk as we know it.
- ▪ Other merger events have been characterized better, and new ones have been uncovered. These include, for example, the Helmi streams, Sequoia, and Thamnos, which add to the list of those discovered in wide-field photometric surveys, such as the Sagittarius streams. Current knowledge of their progenitors’ properties, star formation, and chemical evolutionary histories is still incomplete.
- ▪ Debris from different objects shows different degrees of overlap in phase-space. This sometimes confusing situation can be improved by determining membership probabilities via quantitative statistical methods. A task for the next few years will be to use ongoing and planned spectroscopic surveys for chemical labeling and to disentangle events from one another using dimensions other than phase-space, metallicity, or [α/Fe].
- ▪ These large surveys will also provide line-of-sight velocities missing for faint stars in Gaia releases and more accurate distance determinations for distant objects, which in combination with other surveys could also lead to more accurate age dating. The resulting samples of stars will cover a much wider volume of the Galaxy, allowing, for example, the linking of kinematic substructures found in the inner halo to spatial overdensities in the outer halo.
- ▪ All the results obtained so far are in line with the expectations of current cosmological models. Nonetheless, tailored hydrodynamical simulations to reproduce in detail the properties of the merger debris, as well as constrained cosmological simulations of the Milky Way, are needed. Such simulations will undoubtedly unravel more connections between the different Galactic components and their substructures, and will aid in pushing our knowledge of the assembly of the Milky Way to the earliest times.
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Intermediate-Mass Black Holes
Vol. 58 (2020), pp. 257–312More LessWe describe ongoing searches for intermediate-mass black holes with MBH ≈ 10–105 M⊙. We review a range of search mechanisms, both dynamical and those that rely on accretion signatures. We find the following conclusions:
- ▪ Dynamical and accretion signatures alike point to a high fraction of 109–1010 M⊙ galaxies hosting black holes with MBH∼ 105 M⊙. In contrast, there are no solid detections of black holes in globular clusters.
- ▪ There are few observational constraints on black holes in any environment with MBH ≈ 100–104 M⊙.
- ▪ Considering low-mass galaxies with dynamical black hole masses and constraining limits, we find that the MBH–σ* relation continues unbroken to MBH ∼105 M⊙, albeit with large scatter. We believe the scatter is at least partially driven by a broad range in black hole masses, because the occupation fraction appears to be relatively high in these galaxies.
- ▪ We fold the observed scaling relations with our empirical limits on occupation fraction and the galaxy mass function to put observational bounds on the black hole mass function in galaxy nuclei.
- ▪ We are pessimistic that local demographic observations of galaxy nuclei alone could constrain seeding mechanisms, although either high-redshift luminosity functions or robust measurements of off-nuclear black holes could begin to discriminate models.
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Astronomers Engaging with the Education Ecosystem: A Best-Evidence Synthesis
Vol. 58 (2020), pp. 313–361More LessAstronomers have played many roles in their engagement with the larger astronomy education ecosystem. Their activities have served both the formal and informal education communities worldwide, with levels of involvement from the occasional participant to the full-time professional. We discuss these many diverse roles, giving background, context, and perspective on their value in encouraging and improving astronomy education. This review covers the large amounts of new research on best practices for diverse learning environments. This evidence-based perspective can support astronomers in contributing to the broad astronomy education ecosystem in more productive and efficient ways and in identifying new niches and approaches for developing the science capital necessary for a science literate society and for greater involvement of underrepresented groups in the science enterprise. Current research emphasizes the importance of
- ▪ The formation of science ideas in children and the development of their science identity
- ▪ The design of professional development programs for educators
- ▪ Museums and other informal learning institutions and settings
- ▪ The use of astronomical data and immersion experiences
- ▪ Shifting the astronomy education paradigm using new approaches with diverse audiences
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The Cosmic Baryon and Metal Cycles
Vol. 58 (2020), pp. 363–406More LessCharacterizing the relationship between stars, gas, and metals in galaxies is a critical component of understanding the cosmic baryon cycle. We compile contemporary censuses of the baryons in collapsed structures and their chemical makeup and dust content. We show the following:
- ▪ The mass density of the Universe is well determined to redshifts and shows minor evolution with time. New observations of molecular hydrogen reveal its evolution mirrors that of the global star-formation rate density, implying a universal cosmic molecular gas depletion timescale. The low-redshift decline of the star-formation history is thus driven by the lack of molecular gas supply due to a drop in net accretion rate related to the decreased growth of dark matter halos.
- ▪ The metal mass density in cold gas ( K) contains virtually all the metals produced by stars for . At lower redshifts, the contributors to the total amount of metals are more diverse; at , most of the observed metals are bound in stars. Overall, there is little evidence for a “missing metals problem” in modern censuses.
- ▪ We characterize the dust content of neutral gas over cosmic time, finding the dust-to-gas and dust-to-metals ratios fall with decreasing metallicity. We calculate the cosmological dust mass density in the neutral gas up to . There is good agreement between multiple tracers of the dust content of the Universe.
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Magnetohydrodynamics Simulations of Active Galactic Nucleus Disks and Jets
Vol. 58 (2020), pp. 407–439More LessThere is a broad consensus that accretion onto supermassive black holes and consequent jet formation power the observed emission from active galactic nuclei (AGNs). However, there has been less agreement about how jets form in accretion flows, their possible relationship to black hole spin, and how they interact with the surrounding medium. There have also been theoretical concerns about instabilities in standard accretion disk models and lingering discrepancies with observational constraints. Despite seemingly successful applications to X-ray binaries, the standard accretion disk model faces a growing list of observational constraints that challenge its application to AGNs. Theoretical exploration of these questions has become increasingly reliant on numerical simulations owing to the dynamic nature of these flows and the complex interplay between hydrodynamics, magnetic fields, radiation transfer, and curved spacetime. We conclude the following:
- ▪ The advent of general relativistic magnetohydrodynamics (MHD) simulations has greatly improved our understanding of jet production and its dependence on black hole spin.
- ▪ Simulation results show both disks and jets are sensitive to the magnetic flux threading the accretion flow as well as possible misalignment between the angular momentum of the accretion flow and the black hole spin.
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Radiation MHD simulations are providing new insights into the stability of luminous accretion flows and highlighting the potential importance of radiation viscosity, UV opacity from atoms, and spiral density waves in AGNs.
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Magnetohydrodynamic Waves in the Solar Corona
Vol. 58 (2020), pp. 441–481More LessThe corona of the Sun is a unique environment in which magnetohydrodynamic (MHD) waves, one of the fundamental processes of plasma astrophysics, are open to a direct study. There is striking progress in both observational and theoretical research of MHD wave processes in the corona, with the main recent achievements summarized as follows:
- ▪ Both periods and wavelengths of the principal MHD modes of coronal plasma structures, such as kink, slow and sausage modes, are confidently resolved.
- ▪ Scalings of various parameters of detected waves and waveguiding plasma structures allow for the validation of theoretical models. In particular, kink oscillation period scales linearly with the length of the oscillating coronal loop, clearly indicating that they are eigenmodes of the loop. Damping of decaying kink and standing slow oscillations depends on the oscillation amplitudes, demonstrating the importance of nonlinear damping.
- ▪ The dominant excitation mechanism for decaying kink oscillations is associated with magnetized plasma eruptions. Propagating slow waves are caused by the leakage of chromospheric oscillations. Fast wave trains could be formed by waveguide dispersion.
- ▪ The knowledge gained in the study of coronal MHD waves provides ground for seismological probing of coronal plasma parameters, such as the Alfvén speed, the magnetic field and its topology, stratification, temperature, fine structuring, polytropic index, and transport coefficients.
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Observations of Protoplanetary Disk Structures
Vol. 58 (2020), pp. 483–528More LessThe disks that orbit young stars are the essential conduits and reservoirs of material for star and planet formation. Their structures, meaning the spatial variations of the disk physical conditions, reflect the underlying mechanisms that drive those formation processes. Observations of the solids and gas in these disks, particularly at high resolution, provide fundamental insights on their mass distributions, dynamical states, and evolutionary behaviors. Over the past decade, rapid developments in these areas have largely been driven by observations with the Atacama Large Millimeter/submillimeter Array (ALMA). This review highlights the state of observational research on disk structures, emphasizing the following three key conclusions that reflect the main branches of the field:
- ▪ Relationships among disk structure properties are also linked to the masses, environments, and evolutionary states of their stellar hosts.
- ▪ There is clear, qualitative evidence for the growth and migration of disk solids, although the implied evolutionary timescales suggest the classical assumption of a smooth gas disk is inappropriate.
- ▪ Small-scale substructures with a variety of morphologies, locations, scales, and amplitudes—presumably tracing local gas pressure maxima—broadly influence the physical and observational properties of disks.
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The Dust Attenuation Law in Galaxies
Vol. 58 (2020), pp. 529–575More LessUnderstanding the properties of dust attenuation curves in galaxies and the physical mechanisms that shape them are among the fundamental questions of extragalactic astrophysics, with great practical significance for deriving the physical properties of galaxies. Attenuation curves result from a combination of dust grain properties, dust content, and the spatial arrangement of dust and different populations of stars. In this review, we assess the state of the field, paying particular attention to extinction curves as the building blocks of attenuation laws. We introduce a quantitative framework to characterize extinction and attenuation curves, present a theoretical foundation for interpreting empirical results, overview an array of observational methods, and review observational results at low and high redshifts. Our main conclusions include the following:
- ▪ Attenuation curves exhibit a wide range of UV-through-optical slopes, from curves with shallow (Milky Way–like) slopes to those exceeding the slope of the Small Magellanic Cloud extinction curve.
- ▪ The slopes of the curves correlate strongly with the effective optical opacities, in the sense that galaxies with lower dust column density (lower visual attenuation) tend to have steeper slopes, whereas the galaxies with higher dust column density have shallower (grayer) slopes.
- ▪ Galaxies exhibit a range of 2175-Å UV bump strengths, including no bump, but, on average, are suppressed compared with the average Milky Way extinction curve.
- ▪ Theoretical studies indicate that both the correlation between the slope and the dust column as well as variations in bump strength may result from geometric and radiative transfer effects.
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Evidence for Initial Mass Function Variation in Massive Early-Type Galaxies
Vol. 58 (2020), pp. 577–615More LessThe initial mass function (IMF), describing the distribution of birth masses of stars, plays a pivotal role in establishing the observable properties of galaxies. This article reviews the evidence for variation in the IMF of massive early-type galaxies (ETGs), especially from spectroscopic studies and from dynamical and gravitational lensing measurements over the past decade. The principal conclusions are as follows:
- ▪ The spectra of massive ETGs depart from the predictions of models with Milky Way–like IMFs in a way that is best reproduced by assuming a steeper (bottom-heavy) IMF below ∼1 M⊙.
- ▪ Lensing and dynamical models, assuming a constant mass-to-light ratio for the stellar component, infer heavy IMFs, superficially supporting the result from spectra.
- ▪ The spectroscopic signal exhibits a steep gradient, however, and may be confined to the innermost region with scales ≲2 kpc; such internal variation in the stellar mass-to-light ratio would invalidate a key assumption of most dynamics and lensing studies.
- ▪ For masses above the main sequence turnoff in ancient populations (≳1 M⊙), there is little evidence for a steeper IMF in massive ETGs or their high-redshift progenitors; rather, a slightly shallower slope is preferred in this regime from several different arguments.
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Steep internal gradients may be responsible for some of the apparent discrepancies between different methods and also point to the cause of the IMF variation being restricted to conditions specific to the in situ formation phase of ETG cores.
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Observations of the Lyman-α Universe
Vol. 58 (2020), pp. 617–659More LessHydrogen Lyman-α (Lyα) emission has been one of the major observational probes for the high-redshift Universe since the first discoveries of high-z Lyα-emitting galaxies in the late 1990s. Due to the strong Lyα emission originated by resonant scattering and recombination of the most abundant element, Lyα observations witness not only Hii regions of star formation and active galactic nuclei (AGNs) but also diffuse Hi gas in the circumgalactic medium (CGM) and the intergalactic medium (IGM). Here, we review Lyα sources and present theoretical interpretations reached to date. We conclude the following:
- ▪ A typical Lyα emitter (LAE) at z ≳ 2 with a L* Lyα luminosity is a high-z counterpart of a local dwarf galaxy, a compact metal-poor star-forming galaxy (SFG) with an approximate stellar (dark matter halo) mass and star-formation rate of 108−9M⊙ (1010−11M⊙) and 1–10 M⊙ year−1, respectively.
- ▪ High-z SFGs ubiquitously have a diffuse Lyα-emitting halo in the CGM extending to the halo virial radius and beyond.
- ▪ Remaining neutral hydrogen at the epoch of cosmic reionization makes a strong dimming of Lyα emission for galaxies at z > 6 that suggests the late reionization history.
The next-generation large-telescope projects will combine Lyα emission data with Hi Lyα absorptions and 21-cm radio data that map out the majority of hydrogen (Hi+Hii) gas, uncovering the exchanges of (a) matter by outflow and inflow and (b) radiation, relevant to cosmic reionization, between galaxies and the CGM/IGM.
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Star-Forming Galaxies at Cosmic Noon
Vol. 58 (2020), pp. 661–725More LessEver deeper and wider look-back surveys have led to a fairly robust outline of the cosmic star-formation history, which culminated around ; this period is often nicknamed “cosmic noon.” Our knowledge about star-forming galaxies at these epochs has dramatically advanced from increasingly complete population censuses and detailed views of individual galaxies. We highlight some of the key observational insights that influenced our current understanding of galaxy evolution in the equilibrium growth picture:
- ▪ Scaling relations between galaxy properties are fairly well established among massive galaxies at least out to , pointing to regulating mechanisms already acting on galaxy growth.
- ▪ Resolved views reveal that gravitational instabilities and efficient secular processes within the gas- and baryon-rich galaxies at play an important role in the early buildup of galactic structure.
- ▪ Ever more sensitive observations of kinematics at are probing the baryon and dark matter budget on galactic scales and the links between star-forming galaxies and their likely descendants.
- ▪ Toward higher masses, massive bulges, dense cores, and powerful AGNs and AGN-driven outflows are more prevalent and likely play a role in quenching star formation.
We outline emerging questions and exciting prospects for the next decade with upcoming instrumentation, including the James Webb Space Telescope and the next generation of extremely large telescopes.
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Astrochemistry During the Formation of Stars
Vol. 58 (2020), pp. 727–778More LessStar-forming regions show a rich and varied chemistry, including the presence of complex organic molecules—in both the cold gas distributed on large scales and the hot regions close to young stars where protoplanetary disks arise. Recent advances in observational techniques have opened new possibilities for studying this chemistry. In particular, the Atacama Large Millimeter/submillimeter Array has made it possible to study astrochemistry down to Solar System–size scales while also revealing molecules of increasing variety and complexity. In this review, we discuss recent observations of the chemistry of star-forming environments, with a particular focus on complex organic molecules, taking context from the laboratory experiments and chemical models that they have stimulated. The key takeaway points include the following:
- ▪ The physical evolution of individual sources plays a crucial role in their inferred chemical signatures and remains an important area for observations and models to elucidate.
- ▪ Comparisons of the abundances measured toward different star-forming environments (high-mass versus low-mass, Galactic Center versus Galactic disk) reveal a remarkable similarity, which is an indication that the underlying chemistry is relatively independent of variations in their physical conditions.
- ▪ Studies of molecular isotopologues in star-forming regions provide a link with measurements in our own Solar System, and thus may shed light on the chemical similarities and differences expected in other planetary systems.
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Previous Volumes
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Volume 61 (2023)
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Volume 60 (2022)
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Volume 59 (2021)
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Volume 58 (2020)
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Volume 57 (2019)
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Volume 56 (2018)
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Volume 55 (2017)
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Volume 54 (2016)
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Volume 53 (2015)
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Volume 52 (2014)
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Volume 51 (2013)
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Volume 50 (2012)
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Volume 49 (2011)
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Volume 48 (2010)
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Volume 47 (2009)
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Volume 46 (2008)
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Volume 45 (2007)
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Volume 44 (2006)
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Volume 43 (2005)
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Volume 42 (2004)
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Volume 41 (2003)
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Volume 40 (2002)
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Volume 39 (2001)
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Volume 38 (2000)
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Volume 37 (1999)
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Volume 36 (1998)
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Volume 35 (1997)
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Volume 34 (1996)
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Volume 33 (1995)
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Volume 32 (1994)
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Volume 31 (1993)
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Volume 30 (1992)
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Volume 29 (1991)
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Volume 28 (1990)
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Volume 27 (1989)
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Volume 26 (1988)
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Volume 25 (1987)
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Volume 24 (1986)
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Volume 23 (1985)
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Volume 22 (1984)
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Volume 21 (1983)
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Volume 20 (1982)
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Volume 19 (1981)
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Volume 18 (1980)
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Volume 17 (1979)
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Volume 16 (1978)
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Volume 15 (1977)
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Volume 14 (1976)
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Volume 13 (1975)
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Volume 12 (1974)
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Volume 11 (1973)
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Volume 10 (1972)
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Volume 9 (1971)
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Volume 8 (1970)
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Volume 7 (1969)
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Volume 6 (1968)
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Volume 5 (1967)
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Volume 4 (1966)
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Volume 3 (1965)
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Volume 2 (1964)
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Volume 1 (1963)
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Volume 0 (1932)