1,721,371 research outputs found
Real-world studies provide reliable comparisons of disease modifying therapies in MS – No
No full textDo nuclear rings in barred galaxies form at the shear minimum of the rotation curve?
No full textIt has been recently suggested that (i) nuclear rings in barred galaxies (including our own Milky Way) form at the radius where the shear parameter of the rotation curve reaches a minimum; and (ii) the acoustic instability of Montenegro et al. is responsible for driving the turbulence and angular momentum transport in the central regions of barred galaxies. Here, we test these suggestions by running simple hydrodynamical simulations in a logarithmic barred potential. Since the rotation curve of this potential is scale free, the shear minimum theory predicts that no ring should form. We find that in contrast to this prediction, a ring does form in the simulation, with morphology consistent with that of nuclear rings in real barred galaxies. This proves that the presence of a shear-minimum is not a necessary condition for the formation of a ring. We also find that perturbations that are predicted to be acoustically unstable wind up and eventually propagate off to infinity, so that the system is actually stable. We conclude that (i) the shear-minimum theory is an unlikely mechanism for the formation of nuclear rings in barred galaxies; and (ii) the acoustic instability is a spurious result and may not be able to drive turbulence in the interstellar medium, at least for the case without self-gravity. The question of the role of turbulent viscosity remains open
Modeling the distribution of new MRI cortical lesions in multiple sclerosis longitudinal studies by Sormani MP, Calabrese M, Signori A, Giorgio A, Gallo P, de Stefano N [PLoS One 2011;6(10):e26712. Epub 2011 October 20]
No full textMass inflow rate into the Central Molecular Zone: observational determination and evidence of episodic accretion
No full textIt is well known that the Galactic bar drives a gas inflow into the Central Molecular Zone, which fuels star formation, accretion on to the central supermassive black hole, and large-scale outflows. This inflow happens mostly through two symmetrical dust lanes, similar to those often seen in external barred galaxies. Here, we use the fact that the Milky Way dust lanes have been previously identified in (CO)-C-12 datacubes and a simple geometrical model to derive the first observational determination of the mass inflow rate into the Central Molecular Zone. We find that the time-averaged inflow rate along the near-side dust lane is 1.2(-0.) (+0.7)(8) M-circle dot yr(-1) and along the far-side dust lane is 1.5(-1.0)(+0.9) M-circle dot yr(-1), which gives a total inflow of 2.7(-1.7)(+1.5) M-circle dot yr(-1). We also provide the time series of the inflow rate M for the future few Myr. The latter shows that the inflow rate is variable with time, supporting a scenario of episodic accretion on to the Central Molecular Zone
The effect of rotation on the thermal instability of stratified galactic atmospheres - II. The formation of high-velocity clouds
No full textWhether high-velocity clouds (HVCs) can form by condensation of the hot (T similar to 10(6) K) Galactic corona as a consequence of thermal instabilities has been controversial. Here, we re-examine this problem and we suggest that rotation of the corona might be a missing key ingredient. We do this by studying the evolution of the models of rotating galactic coronae presented in Sormani et al. (2018) under the presence of cooling and thermal conduction. We combine a linear stability analysis with the results of local and global hydrodynamical simulations. We find that condensations are likely to occur in regions where the corona has substantial rotational support. Under reasonably general assumptions on the rotation profile of the corona, the locations where condensations are expected are in remarkable agreement with the observed location of the major non-magellanic HVC complexes in our Galaxy (namely, at distances less than or similar to 15 kpc from the Sun and within 30 degrees from the disc plane). We conclude that HVCs can form by thermal instabilities provided that (i) the corona is rotating substantially in the inner (R less than or similar to 50 kpc) parts, as suggested by current observational data and predicted by cosmological simulations of galaxy formation; and (ii) close to the disc the corona is well represented by a nearly equilibrium stratified rotating structure (as opposed to a fast-cooling flow). Our results also suggest that a better understanding of the disc-halo interface, including supernova feedback, is critical to understand the origin of HVCs
The effect of rotation on the thermal instability of stratified galactic atmospheres - I. Local analysis
No full textObservations show that (i) multiple gas phases can coexist in the atmospheres of galaxies and clusters; (ii) these atmospheres may be significantly rotating in the inner parts, with typical velocities that approach or even exceed the local sound speed. The thermal instability is a natural candidate to explain the formation of cold structures via condensation of a hotter gas phase. Here we systematically study the effect of rotation on the thermal stability of stratified plane-parallel atmospheres, using both analytical arguments and numerical simulations. We find that the formation of cold structures starting from small isobaric perturbations is enhanced in the regions where the rotation of the system is dynamically important (i.e. when the rotational velocity becomes comparable to the sound speed). In particular, the threshold value of the ratio between the cooling and dynamical time t(cool)/t(dyn) below which condensations can form is increased by a factor of up to similar to 10 in the presence of significant rotation. We briefly discuss the implications of our results for galaxies and clusters
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