We present evidence for cosmological gas accretion on to spiral galaxies in the local universe. The accretion is seen through its effects on the dynamics of the extraplanar neutral gas. The accretion ...rates that we estimate for two nearby spiral galaxies are of the order of their star formation rates. Our model shows that most of the extraplanar gas is produced by supernova feedback (galactic fountain) and only 10–20 per cent comes from accretion. The accreting material must have low specific angular momentum about the disc's spin axis, although the magnitude of the specific angular momentum vector can be higher. We also explore the effects of a hot corona on the dynamics of the extraplanar gas and find that it is unlikely to be responsible for the observed kinematical pattern and the source of accreted gas. However, the interaction with the fountain flow should profoundly affect the hydrodynamics of the corona.
•A neurobiologically constrained, and generalizable account of social cognition.•A characterisation of sociocognitive processing as a case of ‘semantic’ cognition.•A review of multi-method empirical ...studies that form the foundation of the account.•Offers testable predictions regarding social cognition in both health and disease.
Research in social neuroscience has primarily focused on carving up cognition into distinct pieces, as a function of mental process, neural network or social behaviour, while the need for unifying models that span multiple social phenomena has been relatively neglected. Here we present a novel framework that treats social cognition as a case of semantic cognition, and which is neurobiologically constrained and generalizable, with clear, testable predictions regarding sociocognitive processing in the context of both health and disease. According to this framework, social cognition relies on two principal systems of representation and control. These systems are neuroanatomically and functionally distinct, but interact to (1) enable development of foundational, conceptual-level knowledge and (2) regulate access to this information in order to generate flexible and context-appropriate social behaviour. The Social Semantics framework shines new light on the mechanisms of social information processing by maintaining as much explanatory power as prior models of social cognition, whilst remaining simpler, by virtue of relying on fewer components that are “tuned” towards social interactions.
Starting from the hypothesis that the Galaxy's dark halo responded adiabatically to the infall of baryons, we have constructed a self-consistent dynamical model of the Galaxy that satisfies a large ...number of observations, including measurements of gas terminal velocities and masers, the kinematics of 180 000 giant stars from the RAVE (RAdial Velocity Experiment) survey and star-count data from the Sloan Digital Sky Survey. The stellar disc and the dark halo are both specified by distribution functions of the action integrals. The model is obtained by extending the work of Piffl et al. from the construction of a single model to a systematic search of model space. Whereas the model of Piffl et al. violated constraints on the terminal-velocity curve, our model respects these constraints by adopting a long scalelength R
d = 3.66 kpc for the thin and thick discs. The model is, however, inconsistent with the measured optical depth for microlensing of bulge stars because it attributes too large a fraction of the density at R ≲ 3 kpc to dark matter rather than stars. Moreover, it now seems likely that the thick disc's scalelength is significantly shorter than the model implies. Shortening this scalelength would cause the constraints from the rotation curve to be violated anew. We conclude that we can now rule out adiabatic compression of our Galaxy's dark halo.
Supernova-driven gas accretion in the Milky Way Marasco, A; Fraternali, F; Binney, J. J
Monthly notices of the Royal Astronomical Society,
January 2012, Letnik:
419, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We use a model of the Galactic fountain to simulate the neutral-hydrogen emission of the Milky Way Galaxy. The model was developed to account for data on external galaxies with sensitive H i data. ...For appropriate parameter values, the model reproduces well the H i emission observed at intermediate velocities. The optimal parameters imply that cool gas is ionized as it is blasted out of the disc, but becomes neutral when its vertical velocity has been reduced by ∼30 per cent. The parameters also imply that cooling of coronal gas in the wakes of fountain clouds transfers gas from the virial-temperature corona to the disc at ∼2 M⊙ yr−1. This rate agrees, to within the uncertainties, with the accretion rate required to sustain the Galaxy's star formation without depleting the supply of interstellar gas. We predict the radial profile of accretion, which is an important input for models of Galactic chemical evolution. The parameter values required for the model to fit the Galaxy's H i data are in excellent agreement with values estimated from external galaxies and hydrodynamical studies of cloud-corona interaction. Our model does not reproduce the observed H i emission at high velocities, consistent with high-velocity clouds being extragalactic in origin. If our model is correct, the structure of the Galaxy's outer H i disc differs materially from that used previously to infer the distribution of dark matter on the Galaxy's outskirts.
Galactic Astronomy James Binney, Michael Merrifield
2021, 1998, 2021-07-13, Letnik:
9
eBook
This is the definitive treatment of the phenomenology of galaxies--a clear and comprehensive volume that takes full account of the extraordinary recent advances in the field. The book supersedes the ...classic text Galactic Astronomy that James Binney wrote with Dimitri Mihalas, and complements Galactic Dynamics by Binney and Scott Tremaine. It will be invaluable to researchers and is accessible to any student who has a background in undergraduate physics. The book draws on observations both of our own galaxy, the Milky Way, and of external galaxies. The two sources are complementary, since the former tends to be highly detailed but difficult to interpret, while the latter is typically poorer in quality but conceptually simpler to understand. Binney and Merrifield introduce all astronomical concepts necessary to understand the properties of galaxies, including coordinate systems, magnitudes and colors, the phenomenology of stars, the theory of stellar and chemical evolution, and the measurement of astronomical distances. The book's core covers the phenomenology of external galaxies, star clusters in the Milky Way, the interstellar media of external galaxies, gas in the Milky Way, the structure and kinematics of the stellar components of the Milky Way, and the kinematics of external galaxies. Throughout, the book emphasizes the observational basis for current understanding of galactic astronomy, with references to the original literature. Offering both new information and a comprehensive view of its subject, it will be an indispensable source for professionals, as well as for graduate students and advanced undergraduates.
Radial mixing in galactic discs Sellwood, J. A.; Binney, J. J.
Monthly notices of the Royal Astronomical Society,
11/2002, Letnik:
336, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We show that spiral waves in galaxy discs churn the stars and gas in a manner that largely preserves the overall angular momentum distribution and leads to little increase in random motion. Changes ...in the angular momenta of individual stars are typically as large as ∼50 per cent over the lifetime of the disc. The changes are concentrated around the corotation radius for an individual spiral wave, but since transient waves with a wide range of pattern speeds develop in rapid succession, the entire disc is affected. This behaviour has profound consequences for the metallicity gradients with radius in both stars and gas, since the interstellar medium is also stirred by the same mechanism. We find observational support for stirring, propose a simple model for the distribution of stars over metallicity and age, and discuss other possible consequences.
We consider the problem of self-regulated heating and cooling in galaxy clusters and the implications for cluster magnetic fields and turbulence. Viscous heating of a weakly collisional magnetized ...plasma is regulated by the pressure anisotropy with respect to the local direction of the magnetic field. The intracluster medium is a high-beta plasma, where pressure anisotropies caused by the turbulent stresses and the consequent local changes in the magnetic field will trigger very fast microscale instabilities. We argue that the net effect of these instabilities will be to pin the pressure anisotropies at a marginal level, controlled by the plasma beta parameter. This gives rise to local heating rates that turn out to be comparable to the radiative cooling rates. Furthermore, we show that a balance between this heating and bremsstrahlung cooling is thermally stable, unlike the often conjectured balance between cooling and thermal conduction. Given a sufficient (and probably self-regulating) supply of turbulent power, this provides a physical mechanism for mitigating cooling flows and preventing cluster core collapse. For observed density and temperature profiles, the assumed balance of viscous heating and radiative cooling allows us to predict magnetic field strengths, turbulent velocities and turbulence scales as functions of distance from the centre. Specific predictions and comparisons with observations are given for several different clusters. Our predictions can be further tested by future observations of cluster magnetic fields and turbulent velocities.
The uncertainty in Galactic parameters McMillan, Paul J.; Binney, James J.
Monthly notices of the Royal Astronomical Society,
02/2010, Letnik:
402, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We reanalyse the measurements of parallax, proper motion and line-of-sight velocity for 18 masers in high-mass star-forming regions presented by Reid et al. We use a likelihood analysis to ...investigate the distance of the Sun from the Galactic Centre, R0, the rotational speed of the local standard of rest, v0, and the peculiar velocity of the Sun, v⊙, for various models of the rotation curve and models which allow for a typical peculiar motion of the high-mass star-forming regions. We find that these data are best fit by models with non-standard values for v⊙ or a net peculiar motion of the high-mass star-forming regions. We argue that a correction to v⊙ is much more likely and these data support the conclusion of Binney that V⊙ should be revised upwards from 5.2 to 11 km s−1. We find that the values of R0 and v0 that we determine are heavily dependent on the model we use for rotation curve, with model-dependent estimates of R0 ranging from 6.7 ± 0.5 to 8.9 ± 0.9 kpc, and those of v0 ranging from 200 ± 20 to 279 ± 33 km s−1. We argue that these data cannot be thought of as implying any particular values of R0 or v0. However, we find that v0/R0 is better constrained, lying in the range 29.9 − 31.6 km s−1 kpc−1 for all models but one.
Although there is an emerging consensus that the anterior temporal lobes (ATLs) are involved in semantic memory, it is currently unclear which specific parts of this region are implicated in semantic ...representation. Answers to this question are difficult to glean from the existing literature for 3 reasons: 1) lesions of relevant patient groups tend to encompass the whole ATL region; 2) while local effects of repetitive transcranial magnetic stimulation (rTMS) are spatially more specific, only the lateral aspects of the ATL are available to stimulation; and 3) until recently, functional magnetic resonance imaging (fMRI) studies were hindered by technical limitations such as signal distortion and dropout due to magnetic inhomogeneities and also, in some cases, by methodological factors, including a restricted field of view and the choice of baseline contrast for subtraction analysis. By utilizing the same semantic task across semantic dementia, rTMS, and distortion-corrected fMRI in normal participants, we directly compared the results across the 3 methods for the first time. The findings were highly convergent and indicated that crucial regions within the ATL for semantic representation include the anterior inferior temporal gyrus, anterior fusiform gyrus, and the anterior superior temporal sulcus.
We made new estimates of the Galactic escape speed at various Galactocentric radii using the latest data release of the RAdial Velocity Experiment (RAVE DR4). Compared to previous studies we have a ...database that is larger by a factor of 10, as well as reliable distance estimates for almost all stars. Our analysis is based on statistical analysis of a rigorously selected sample of 90 high-velocity halo stars from RAVE and a previously published data set. We calibrated and extensively tested our method using a suite of cosmological simulations of the formation of Milky Way-sized galaxies. Our best estimate of the local Galactic escape speed, which we define as the minimum speed required to reach three virial radii R340, is 533+54-41 km s-1 (90% confidence), with an additional 4% systematic uncertainty, where R340 is the Galactocentric radius encompassing a mean overdensity of 340 times the critical density for closure in the Universe. From the escape speed we further derived estimates of the mass of the Galaxy using a simple mass model with two options for the mass profile of the dark matter halo: an unaltered and an adiabatically contracted Navarro, Frenk & White (NFW) sphere. If we fix the local circular velocity, the latter profile yields a significantly higher mass than the uncontracted halo, but if we instead use the statistics for halo concentration parameters in large cosmological simulations as a constraint, we find very similar masses for both models. Our best estimate for M340, the mass interiorto R340 (dark matter and baryons), is 1.3+0.4-0.3 × 1012 M⊙ (corresponds to M200 = 1.6+0.5-0.4 × 1012 M⊙). This estimate is in good agreement with recently published, independent mass estimates based on the kinematics of more distant halo stars and the satellite galaxy Leo I.
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