Pub Date : 2020-10-26DOI: 10.1146/annurev-astro-081817-051905
S. Davis, A. Tchekhovskoy
There 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 agr...
{"title":"Magnetohydrodynamics Simulations of Active Galactic Nucleus Disks and Jets","authors":"S. Davis, A. Tchekhovskoy","doi":"10.1146/annurev-astro-081817-051905","DOIUrl":"https://doi.org/10.1146/annurev-astro-081817-051905","url":null,"abstract":"There 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 agr...","PeriodicalId":8138,"journal":{"name":"Annual Review of Astronomy and Astrophysics","volume":"58 1","pages":"407-439"},"PeriodicalIF":33.3,"publicationDate":"2020-10-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1146/annurev-astro-081817-051905","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44769793","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-10-26DOI: 10.1146/annurev-astro-032320-042940
V. Nakariakov, D. Kolotkov
The 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 progre...
{"title":"Magnetohydrodynamic Waves in the Solar Corona","authors":"V. Nakariakov, D. Kolotkov","doi":"10.1146/annurev-astro-032320-042940","DOIUrl":"https://doi.org/10.1146/annurev-astro-032320-042940","url":null,"abstract":"The 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 progre...","PeriodicalId":8138,"journal":{"name":"Annual Review of Astronomy and Astrophysics","volume":"58 1","pages":"441-481"},"PeriodicalIF":33.3,"publicationDate":"2020-10-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1146/annurev-astro-032320-042940","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49236597","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-08-18DOI: 10.1146/annurev-astro-112119-041947
J. Gunn
This 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.
{"title":"Jack of All","authors":"J. Gunn","doi":"10.1146/annurev-astro-112119-041947","DOIUrl":"https://doi.org/10.1146/annurev-astro-112119-041947","url":null,"abstract":"This 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.","PeriodicalId":8138,"journal":{"name":"Annual Review of Astronomy and Astrophysics","volume":" ","pages":""},"PeriodicalIF":33.3,"publicationDate":"2020-08-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1146/annurev-astro-112119-041947","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45274290","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-08-18DOI: 10.1146/annurev-astro-021820-120014
C. Péroux, C. Péroux, J. Howk
Characterizing 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 [Formula: see text] mass density of the Universe is well determined to redshifts [Formula: see text] 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 ([Formula: see text] K) contains virtually all the metals produced by stars for [Formula: see text]. At lower redshifts, the contributors to the total amount of metals are more diverse; at [Formula: see text], 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 [Formula: see text]. There is good agreement between multiple tracers of the dust content of the Universe.
{"title":"The Cosmic Baryon and Metal Cycles","authors":"C. Péroux, C. Péroux, J. Howk","doi":"10.1146/annurev-astro-021820-120014","DOIUrl":"https://doi.org/10.1146/annurev-astro-021820-120014","url":null,"abstract":"Characterizing 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 [Formula: see text] mass density of the Universe is well determined to redshifts [Formula: see text] 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 ([Formula: see text] K) contains virtually all the metals produced by stars for [Formula: see text]. At lower redshifts, the contributors to the total amount of metals are more diverse; at [Formula: see text], 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 [Formula: see text]. There is good agreement between multiple tracers of the dust content of the Universe.","PeriodicalId":8138,"journal":{"name":"Annual Review of Astronomy and Astrophysics","volume":" ","pages":""},"PeriodicalIF":33.3,"publicationDate":"2020-08-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1146/annurev-astro-021820-120014","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46019461","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-08-18DOI: 10.1146/annurev-astro-032620-021859
M. Ouchi, Y. Ono, T. Shibuya
Hydrogen 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.
{"title":"Observations of the Lyman-α Universe","authors":"M. Ouchi, Y. Ono, T. Shibuya","doi":"10.1146/annurev-astro-032620-021859","DOIUrl":"https://doi.org/10.1146/annurev-astro-032620-021859","url":null,"abstract":"Hydrogen 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.","PeriodicalId":8138,"journal":{"name":"Annual Review of Astronomy and Astrophysics","volume":"1 1","pages":""},"PeriodicalIF":33.3,"publicationDate":"2020-08-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1146/annurev-astro-032620-021859","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41578452","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-06-12DOI: 10.1146/annurev-astro-032620-021927
J. Jørgensen, A. Belloche, R. Garrod
Star-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.
{"title":"Astrochemistry During the Formation of Stars","authors":"J. Jørgensen, A. Belloche, R. Garrod","doi":"10.1146/annurev-astro-032620-021927","DOIUrl":"https://doi.org/10.1146/annurev-astro-032620-021927","url":null,"abstract":"Star-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.","PeriodicalId":8138,"journal":{"name":"Annual Review of Astronomy and Astrophysics","volume":" ","pages":""},"PeriodicalIF":33.3,"publicationDate":"2020-06-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1146/annurev-astro-032620-021927","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48553908","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-02-11DOI: 10.1146/ANNUREV-ASTRO-032620-021917
A. Helmi
The 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] [Formula: see text] −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.
{"title":"Streams, Substructures, and the Early History of the Milky Way","authors":"A. Helmi","doi":"10.1146/ANNUREV-ASTRO-032620-021917","DOIUrl":"https://doi.org/10.1146/ANNUREV-ASTRO-032620-021917","url":null,"abstract":"The 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] [Formula: see text] −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.","PeriodicalId":8138,"journal":{"name":"Annual Review of Astronomy and Astrophysics","volume":" ","pages":""},"PeriodicalIF":33.3,"publicationDate":"2020-02-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1146/ANNUREV-ASTRO-032620-021917","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47262915","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-09DOI: 10.1146/annurev-astro-032620-021933
S. Salim, D. Narayanan
Understanding 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.
{"title":"The Dust Attenuation Law in Galaxies","authors":"S. Salim, D. Narayanan","doi":"10.1146/annurev-astro-032620-021933","DOIUrl":"https://doi.org/10.1146/annurev-astro-032620-021933","url":null,"abstract":"Understanding 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.","PeriodicalId":8138,"journal":{"name":"Annual Review of Astronomy and Astrophysics","volume":" ","pages":""},"PeriodicalIF":33.3,"publicationDate":"2020-01-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1146/annurev-astro-032620-021933","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42027897","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2019-08-27DOI: 10.1146/ANNUREV-ASTRO-081817-052044
M. Carlsson, B. Pontieu, V. Hansteen
The solar chromosphere forms a crucial, yet complex and until recently poorly understood, interface between the solar photosphere and the heliosphere. ▪ Advances in high-resolution instrumentation, adaptive optics, image reconstruction techniques, and space-based observatories allow unprecedented high-resolution views of the finely structured and highly dynamic chromosphere. ▪ Dramatic progress in numerical computations allows 3D radiative magnetohydrodynamic forward models to take the place of the previous generation of 1D semiempirical atmosphere models. These new models provide deep insight into complex nonlocal thermodynamic equilibrium chromospheric diagnostics and enable physics-based interpretations of observations. ▪ This combination of modeling and observations has led to new insights into the role of shock waves, transverse magnetic waves, magnetic reconnection and flux emergence in the chromospheric energy balance, the formation of spicules, the impact of ion-neutral interactions, and the connectivity between chromosphere and transition region. ▪ During the next few years, the advent of new instrumentation (integral-field-unit spectropolarimetry) and observatories (ALMA, DKIST), coupled with novel inversion codes and expansion of existing numerical models to deal with ever more complex physical processes (including multifluid approaches), is expected to lead to major new insights into the dominant heating processes in the chromosphere and beyond.
{"title":"New View of the Solar Chromosphere","authors":"M. Carlsson, B. Pontieu, V. Hansteen","doi":"10.1146/ANNUREV-ASTRO-081817-052044","DOIUrl":"https://doi.org/10.1146/ANNUREV-ASTRO-081817-052044","url":null,"abstract":"The solar chromosphere forms a crucial, yet complex and until recently poorly understood, interface between the solar photosphere and the heliosphere. ▪ Advances in high-resolution instrumentation, adaptive optics, image reconstruction techniques, and space-based observatories allow unprecedented high-resolution views of the finely structured and highly dynamic chromosphere. ▪ Dramatic progress in numerical computations allows 3D radiative magnetohydrodynamic forward models to take the place of the previous generation of 1D semiempirical atmosphere models. These new models provide deep insight into complex nonlocal thermodynamic equilibrium chromospheric diagnostics and enable physics-based interpretations of observations. ▪ This combination of modeling and observations has led to new insights into the role of shock waves, transverse magnetic waves, magnetic reconnection and flux emergence in the chromospheric energy balance, the formation of spicules, the impact of ion-neutral interactions, and the connectivity between chromosphere and transition region. ▪ During the next few years, the advent of new instrumentation (integral-field-unit spectropolarimetry) and observatories (ALMA, DKIST), coupled with novel inversion codes and expansion of existing numerical models to deal with ever more complex physical processes (including multifluid approaches), is expected to lead to major new insights into the dominant heating processes in the chromosphere and beyond.","PeriodicalId":8138,"journal":{"name":"Annual Review of Astronomy and Astrophysics","volume":" ","pages":""},"PeriodicalIF":33.3,"publicationDate":"2019-08-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1146/ANNUREV-ASTRO-081817-052044","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47005801","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2019-08-27DOI: 10.1146/ANNUREV-ASTRO-091918-104438
S. W. Weaver
The recent advancements in far-infrared (far-IR) astronomy brought about by the Herschel, SOFIA, and ALMA observatories have led to technological advancements in millimeterwave and submillimeterwave laboratory spectroscopy that is used to support molecular observations. This review gives an overview of rotational spectroscopy and its relationship with observational astronomy, as well as an overview of laboratory spectroscopic techniques focusing on both historical approaches and new advancements. Additional topics discussed include production and detection techniques for unstable molecular species of astrochemical interest, data analysis approaches that address spectral complexity and line confusion, and the current state of and limitations to spectral line databases. Potential areas for new developments in this field are also reviewed. To advance the field, the following challenges must be addressed: ▪ Data acquisition speed, spectral sensitivity, and analysis approaches for complex mixtures and broadband spectra are the greatest limitations—and hold the greatest promise for advancement—in this field of research. ▪ Full science return from far-IR observatories cannot be realized until laboratory spectroscopy catches up with the data rate for observations. ▪ New techniques building on those used in the microwave and IR regimes are required to fill the terahertz gap.
{"title":"Millimeterwave and Submillimeterwave Laboratory Spectroscopy in Support of Observational Astronomy","authors":"S. W. Weaver","doi":"10.1146/ANNUREV-ASTRO-091918-104438","DOIUrl":"https://doi.org/10.1146/ANNUREV-ASTRO-091918-104438","url":null,"abstract":"The recent advancements in far-infrared (far-IR) astronomy brought about by the Herschel, SOFIA, and ALMA observatories have led to technological advancements in millimeterwave and submillimeterwave laboratory spectroscopy that is used to support molecular observations. This review gives an overview of rotational spectroscopy and its relationship with observational astronomy, as well as an overview of laboratory spectroscopic techniques focusing on both historical approaches and new advancements. Additional topics discussed include production and detection techniques for unstable molecular species of astrochemical interest, data analysis approaches that address spectral complexity and line confusion, and the current state of and limitations to spectral line databases. Potential areas for new developments in this field are also reviewed. To advance the field, the following challenges must be addressed: ▪ Data acquisition speed, spectral sensitivity, and analysis approaches for complex mixtures and broadband spectra are the greatest limitations—and hold the greatest promise for advancement—in this field of research. ▪ Full science return from far-IR observatories cannot be realized until laboratory spectroscopy catches up with the data rate for observations. ▪ New techniques building on those used in the microwave and IR regimes are required to fill the terahertz gap.","PeriodicalId":8138,"journal":{"name":"Annual Review of Astronomy and Astrophysics","volume":" ","pages":""},"PeriodicalIF":33.3,"publicationDate":"2019-08-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1146/ANNUREV-ASTRO-091918-104438","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44398452","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：