The Magellanic Clouds are surrounded by an extended network of gaseous structures. Chief among these is the Magellanic Stream, an interwoven tail of filaments trailing the Clouds in their orbit ...around the Milky Way. When considered in tandem with its Leading Arm, the Stream stretches over 200° on the sky. The Stream is thought to represent the result of tidal interactions between the Clouds and ram-pressure forces exerted by the Galactic corona, and its kinematic properties reflect the dynamical history of the pair of dwarf galaxies closest to the Milky Way. The Stream is a benchmark for hydrodynamical simulations of accreting gas and cloud corona interactions. If the Stream survives these interactions and arrives safely in the Galactic disk, its cargo of over a billion solar masses of gas has the potential to maintain or elevate the Galactic star-formation rate. In this article, we review the current state of knowledge of the Stream, including its chemical composition, physical conditions, origin, and fate. We also review the dynamics of the Magellanic System, including the proper motions and orbital history of the Large and Small Magellanic Clouds, the first-passage and second-passage scenarios, and the evidence for a Magellanic Group of galaxies.
A Promethean technology is one that allows someone of average
resources, skills, and intelligence to carry out actions that were
once only doable by governments, militaries, or institutions with
...considerable resources. Essentially, Promethean technologies allow
users to create their own weapons of mass destruction. These
emerging technologies are increasingly affordable and
accessible-and are no more complicated to operate than a satellite
TV control box or a smart phone. Although these technologies are a
terrifying prospect, the more we know about these dangers, the
better we can prepare to head them off. In The Devil's Toy
Box , Andrew Fox lays out seven decades of preemptive analysis
and shows that while homeland security has explored, in depth, the
possible Promethean threats the world faces, it has failed to
forecast the most likely attacks. Using fictional scenarios Fox
teaches how to predict future threats and how to forecast which
ones are likely to be used by bad actors within the next five to
ten years. Combining the skills of homeland security experts and
the imaginations of speculative fiction writers, he then offers an
analytical method to deter, counter, or abate these threats, rather
than adopting an attitude of resigned fatalism.
•Central extended amygdala (EAc) encompasses Ce and BST.•EAc is engaged by a range of threat-relevant cues.•Optogenetic/chemogenetic studies reveal key molecules and microcircuits.•Next-generation ...nonhuman primate studies….•…bridge the gap between mechanistic work in rodents and imaging studies in humans.
Anxiety disorders impose a staggering burden on public health, underscoring the need to develop a deeper understanding of the distributed neural circuits underlying extreme fear and anxiety. Recent work highlights the importance of the central extended amygdala, including the central nucleus of the amygdala (Ce) and neighboring bed nucleus of the stria terminalis (BST). Anatomical data indicate that the Ce and BST form a tightly interconnected unit, where different kinds of threat-relevant information can be integrated to assemble states of fear and anxiety. Neuroimaging studies show that the Ce and BST are engaged by a broad spectrum of potentially threat-relevant cues. Mechanistic work demonstrates that the Ce and BST are critically involved in organizing defensive responses to a wide range of threats. Studies in rodents have begun to reveal the specific molecules, cells, and microcircuits within the central extended amygdala that underlie signs of fear and anxiety, but the relevance of these tantalizing discoveries to human experience and disease remains unclear. Using a combination of focal perturbations and whole-brain imaging, a new generation of nonhuman primate studies is beginning to close this gap. This work opens the door to discovering the mechanisms underlying neuroimaging measures linked to pathological fear and anxiety, to understanding how the Ce and BST interact with one another and with distal brain regions to govern defensive responses to threat, and to developing improved intervention strategies.
Abstract
We present new simulations of the formation of the Magellanic Stream based on an updated first-passage interaction history for the Magellanic Clouds, including both the Galactic and ...Magellanic Coronae and a live dark matter halo for the Milky Way. This new interaction history is needed because previously successful orbits need updating to account for the Magellanic Corona and the loosely bound nature of the Magellanic Group. These orbits involve two tidal interactions over the last 3.5 Gyr and reproduce the Stream’s position and appearance on the sky, mass distribution, and velocity profile. Most importantly, our simulated Stream is only ∼20 kpc away from the Sun at its closest point, whereas previous first-infall models predicted a distance of 100–200 kpc. This dramatic paradigm shift in the Stream’s 3D position would have several important implications. First, estimates of the observed neutral and ionized masses would be reduced by a factor of ∼5. Second, the stellar component of the Stream is also predicted to be <20 kpc away. Third, the enhanced interactions with the MW’s hot corona at this small distance would substantially shorten the Stream’s lifetime. Finally, the MW’s UV radiation field would be much stronger, potentially explaining the H
α
emission observed along most of the Stream. Our prediction of a 20 kpc Stream could be tested by searching for UV absorption lines toward distant MW halo stars projected onto the Stream.
To thrive in challenging environments, individuals must pursue rewards while avoiding threats. Extensive studies in animals and humans have identified the central extended amygdala (EAc)—which ...includes the central nucleus of the amygdala (Ce) and bed nucleus of the stria terminalis (BST)—as a conserved substrate for defensive behavior. These studies suggest the EAc influences defensive responding and assembles fearful and anxious states. This has led to the proliferation of a view that the EAc is fundamentally a defensive substrate. Yet mechanistic work in animals has implicated the EAc in numerous appetitive and consummatory processes, yielding fresh insights into the microcircuitry of survival- and emotion-relevant response selection. Coupled with the EAc’s centrality in a conserved network of brain regions that encode multisensory environmental and interoceptive information, these findings suggest a broader role for the EAc as an arbiter of survival- and emotion-relevant tradeoffs for action selection. Determining how the EAc optimizes these tradeoffs promises to improve our understanding of common psychiatric illnesses such as anxiety, depression, alcohol- and substance-use disorders, and anhedonia.
It is widely thought that phasic and sustained responses to threat reflect dissociable circuits centered on the central nucleus of the amygdala (Ce) and the bed nucleus of the stria terminalis (BST), ...the two major subdivisions of the central extended amygdala. Early versions of this hypothesis remain highly influential and have been incorporated into the National Institute of Mental Health Research Research Domain Criteria framework. However, new observations encourage a different perspective. Anatomical studies show that the Ce and BST form a tightly interconnected unit, where different kinds of threat-relevant information can be integrated and used to assemble states of fear and anxiety. Imaging studies in humans and monkeys show that the Ce and BST exhibit similar functional profiles. Both regions are sensitive to a range of aversive challenges, including uncertain or temporally remote threat; both covary with concurrent signs and symptoms of fear and anxiety; both show phasic responses to short-lived threat; and both show heightened activity during sustained exposure to diffusely threatening contexts. Mechanistic studies demonstrate that both regions can control the expression of fear and anxiety during sustained exposure to diffuse threat. These observations compel a reconsideration of the central extended amygdala's contributions to fear and anxiety and its role in neuropsychiatric disease.
This review brings together recent research from molecular, neural circuit, animal model, and human studies to help understand the neurodevelopmental mechanisms underlying social anxiety disorder. ...Social anxiety disorder is common and debilitating, and it often leads to further psychopathology. Numerous studies have demonstrated that extremely behaviorally inhibited and temperamentally anxious young children are at marked risk of developing social anxiety disorder. Recent work in human and nonhuman primates has identified a distributed brain network that underlies early-life anxiety including the central nucleus of the amygdala, the anterior hippocampus, and the orbitofrontal cortex. Studies in nonhuman primates have demonstrated that alterations in this circuit are trait-like in that they are stable over time and across contexts. Notably, the components of this circuit are differentially influenced by heritable and environmental factors, and specific lesion studies have demonstrated a causal role for multiple components of the circuit. Molecular studies in rodents and primates point to disrupted neurodevelopmental and neuroplastic processes within critical components of the early-life dispositional anxiety neural circuit. The possibility of identifying an early-life at-risk phenotype, along with an understanding of its neurobiology, provides an unusual opportunity to conceptualize novel preventive intervention strategies aimed at reducing the suffering of anxious children and preventing them from developing further psychopathology.
Spatially resolved gas flows around the Milky Way Clark, Sean; Bordoloi, Rongmon; Fox, Andrew J
Monthly notices of the Royal Astronomical Society,
03/2022, Letnik:
512, Številka:
1
Journal Article
Recenzirano
ABSTRACT
We present spatially resolved measurements of cool gas flowing into and out of the Milky Way (MW), using archival ultraviolet spectra of background quasars from the Hubble Space ...Telescope/Cosmic Origins Spectrograph. We co-add spectra of different background sources at close projected angular separation on the sky. This novel stacking technique dramatically increases the signal-to-noise ratio of the spectra, allowing the detection of low-column-density gas (down to EW > 2 mÅ). We identify absorption as inflowing or outflowing, by using blue/redshifted high-velocity cloud absorption components in the Galactocentric rest frame, respectively. The mass surface densities of both inflowing and outflowing gases vary by more than an order of magnitude across the sky, with mean values of 〈Σin〉 ≳ 104.6 ± 0.1$\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}$ for inflowing gas and 〈Σout〉 ≳ 103.5 ± 0.1$\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}$ for outflowing gas. The mass flow rate surface densities (mass flow rates per unit area) also show large variation across the sky, with $\langle \dot{\Sigma }(d)_{\rm in}\rangle \gtrsim (10^{-3.6\pm 0.1})(d/12\, \mathrm{kpc})^{-1}$ $\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}\, \mathrm{yr}^{-1}$ for inflowing gas and $\langle \dot{\Sigma }(d)_{\rm out}\rangle \gtrsim (10^{-4.8\pm 0.1})(d/12\, \mathrm{kpc})^{-1}$ $\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}\, \mathrm{yr}^{-1}$ for outflowing gas. The regions with highest surface mass density of inflowing gas are clustered at smaller angular scales (θ < 40°). This indicates that most of the mass in inflowing gas is confined to small, well-defined structures, whereas the distribution of outflowing gas is spread more uniformly throughout the sky. Our study confirms that the MW is predominantly accreting gas, but it is also losing a non-negligible mass of gas via outflow.
ABSTRACT The total contribution of diffuse halo gas to the galaxy baryon budget strongly depends on its dominant ionization state. In this paper, we address the physical conditions in the highly ...ionized circumgalactic medium (CGM) traced by absorption lines observed in COS-Halos spectra. We analyze the observed ionic column densities, absorption-line widths and relative velocities, along with the ratios of / for 39 fitted Voigt profile components of O vi. We compare these quantities with the predictions given by a wide range of ionization models. Photoionization models that include only extragalactic UV background radiation are ruled out; conservatively, the upper limits to / and measurements of imply unphysically large path lengths 100 kpc. Furthermore, very broad absorption (b > 40 km s−1) is a defining characteristic of the CGM of star-forming L* galaxies. We highlight two possible origins for the bulk of the observed : (1) highly structured gas clouds photoionized primarily by local high-energy sources or (2) gas radiatively cooling on large scales behind a supersonic wind. Approximately 20% of circumgalactic O vi does not align with any low-ionization state gas within 50 km s−1 and is found only in halos with < 1012 . We suggest that this type of unmatched O vi absorption traces the hot corona itself at a characteristic temperature of K. We discuss the implications of these very distinct physical origins for the dynamical state, gas cooling rates, and total baryonic content of L* gaseous halos.
Abstract
We analyze new far-ultraviolet spectra of 13 quasars from the
COS-Halos survey that cover the H
i
Lyman limit of 14 circumgalactic medium (CGM) systems. These data yield precise estimates or ...more constraining limits than previous COS-Halos measurements on the H
i
column densities
. We then apply a Monte-Carlo Markov chain approach on 32 systems from COS-Halos to estimate the metallicity of the cool (
K) CGM gas that gives rise to low-ionization state metal lines, under the assumption of photoionization equilibrium with the extragalactic UV background. The principle results are: (1) the CGM of field
L
* galaxies exhibits a declining H
i
surface density with impact parameter
(at
confidence), (2) the transmission of ionizing radiation through CGM gas alone is 70 ± 7%; (3) the metallicity distribution function of the cool CGM is unimodal with a median of
and a 95% interval
to
; the incidence of metal-poor (
) gas is low, implying any such gas discovered along quasar sightlines is typically unrelated to
L
* galaxies; (4) we find an unexpected increase in gas metallicity with declining
(at
confidence) and, therefore, also with increasing
; the high metallicity at large radii implies early enrichment; and (5) a non-parametric estimate of the cool CGM gas mass is
, which together with new mass estimates for the hot CGM may resolve the galactic missing baryons problem. Future analyses of halo gas should focus on the underlying astrophysics governing the CGM, rather than processes that simply expel the medium from the halo.
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