19,751 research outputs found
Disk-Halo Interaction - I. Three-Dimensional Evolution of the Galactic Disk
The results of a three-dimensional model for disk-halo interaction are
presented here. The model considers explicitly the input of energy and mass by
isolated a nd correlated supernovae in the disk. Once disrupted by the
explosions, the disk never returns to its initial state. Instead it approaches
a state where a thin HI disk is formed in the Galactic plane overlayed by thick
HI and HII gas disk w ith scale heights of 500 pc and of 1 to 1.5 kpc,
respectively. The upper parts o f the thick HII disk (the diffuse ionized
medium) act as a disk-halo interface a nd its formation and stability are
directly correlated to the supernova rate per unit area in the simulated disk.Comment: 19 pages, 18 figures; to appear in Monthly Notices of Royal
Astronomical Societ
Non-relativistic free-free emission due to distribution of electrons - Radiative cooling and thermally averaged and total Gaunt factors
Tracking the thermal evolution of plasmas, characterized by an
n-distribution, using numerical simulations, requires the determination of the
emission spectra and of the radiative losses due to free-free emission from the
correspond- ing temperature averaged and total Gaunt factors. Detailed
calculations of the latter are presented, associated to n-distributed electrons
with the parameter n ranging from 1 (corresponding to the Maxwell-Boltzmann
distribu- tion) to 100. The temperature averaged and total Gaunt factors, with
decreasing n tend to those obtained with the Maxwell-Boltzmann distribution.
Radiative losses due to free-free emission in a plasma evolving under
collisional ionization equilibrium conditions and composed by H, He, C, N, O,
Ne, Mg, Si, S, and Fe ions, are presented. These losses decrease with the
decrease in the parameter n reaching a minimum when n = 1, and, thus converging
to the losses of a thermal plasma.
Tables of the thermal averaged and total Gaunt factors calculated for n
distributions and a wide range electron and photon energies are presented.Comment: Accepted for publication in ApJS. 70 pages, 7 figures and 11 table
The Evolution of the Large-scale ISM: Bubbles, Superbubbles and Non-Equilibrium Ionization
The ISM, powered by SNe, is turbulent and permeated by a magnetic field (with
a mean and a turbulent component). It constitutes a frothy medium that is
mostly out of equilibrium and is ram pressure dominated on most of the
temperature ranges, except for T 1E6 K, where magnetic and
thermal pressures dominate, respectively. Such lack of equilibrium is also
imposed by the feedback of the radiative processes into the ISM flow. Many
models of the ISM or isolated phenomena, such as bubbles, superbubbles, clouds
evolution, etc., take for granted that the flow is in the so-called collisional
ionization equilibrium (CIE). However, recombination time scales of most of the
ions below 1E6 K are longer than the cooling time scale. This implies that the
recombination lags behind and the plasma is overionized while it cools. As a
consequence cooling deviates from CIE. This has severe implications on the
evolution of the ISM flow and its ionization structure. Here, besides reviewing
several models of the ISM, including bubbles and superbubbles, the validity of
the CIE approximation is discussed, and a presentation of recent developments
in modeling the ISM by taking into account the time-dependent ionization
structure of the flow in a full-blown numerical 3D high resolution simulation
is presented.Comment: 15 pages, 6 figures with 15 panels. Invited review for "The Dynamic
ISM: A celebration of the Canadian Galactic Plane Survey" conference;
Naramata BC, Canada June 6-10, 2010. To be published in the ASP Conference
Serie
Testing Global ISM Models: A Detailed Comparison of Ovi Column Densities with FUSE and Copernicus Data
We study the ovi distribution in space and time in a representative section
of the Galactic disk by 3D adaptive mesh refinement HD and MHD simulations of
the ISM, including the disk-halo-disk circulation. The simulations describe a
supernova driven ISM on large (10 kpc) and small (1.25 pc) scales over a
sufficiently large timescale (400 Myrs) in order to establish a global
dynamical equilibrium. The Ovi column density, N(Ovi), is monitored through
lines of sight measurements at different locations in the simulated disk. One
has been deliberately chosen to be inside of a hot bubble, like our own Local
Bubble, while the other locations are random. We obtain a correlation between
N(Ovi) and distance, which is independent of the observer's vantage point in
the disk. In particular, the location of the observer inside a hot bubble does
not have any influence on the correlation, because the contribution of an
individual bubble (with a typical extension of 100 pc) is negligibly small. We
find a remarkable agreement between the Ovi column densities (as a function of
distance) and the averaged Ovi density (~1.8x10^{-8}^{-3}$) in the disk
from our simulations and the values observed with Copernicus, and FUSE. Our
results strongly support the important r\^ole of turbulent mixing in the
distribution of Ovi clumps in the ISM. Supernova induced turbulence is quite
strong and unavoidable due to shearing motions in the ISM and operates on a
large range of scales.Comment: 4 pages using emulateapj5 style, 5 figures including a simulation
image. Accepted for publication in ApJ Letter
High Resolution Simulations of the Global and Local ISM
We present the first to date high resolution calculations of the ISM down to
scales of 0.625 pc of the global and local ISM. The simulations show the
morphology and structure of the different ISM phases and reproduce many of the
features that have been observed in the Milky Way and other galaxies. In
particular, they show that the hot gas has a moderately low volume filling
factor (~20%) even in the absence of magnetic fields. Also, cold gas is mainly
concentrated in filamentary structures running perpendicular to the midplane
forming and dissipating within 10-12 Myr. Compression is the dominant process
for their formation, but thermal instability also plays a role. Also the
evolution of the Local Bubble is simulated by multi-supernova explosions;
calculated extensions after ~13 Myr match observations.Comment: 7 pages, 3 Figures that include 4 simulation images (in jpeg format)
and 2 plots (in ps format), to appear in the proceedings of "Star Formation
Through Time" - A conference to honour Roberto J. Terlevich, ASP Conference
Serie
Temperature-averaged and total free-free Gaunt factors for and Maxwellian distributions of electrons
Aims. Optically thin plasmas may deviate from thermal equilibrium and thus,
electrons (and ions) are no longer described by the Maxwellian distribution.
Instead they can be described by -distributions. The free-free spectrum
and radiative losses depend on the temperature-averaged (over the electrons
distribution) and total Gaunt factors, respectively. Thus, there is a need to
calculate and make available these factors to be used by any software that
deals with plasma emission.
Methods. We recalculated the free-free Gaunt factor for a wide range of
energies and frequencies using hypergeometric functions of complex arguments
and the Clenshaw recurrence formula technique combined with approximations
whenever the difference between the initial and final electron energies is
smaller than in units of . We used double and quadruple
precisions. The temperature- averaged and total Gaunt factors calculations make
use of the Gauss-Laguerre integration with 128 nodes.
Results. The temperature-averaged and total Gaunt factors depend on the
parameter, which shows increasing deviations (with respect to the
results obtained with the use of the Maxwellian distribution) with decreasing
. Tables of these Gaunt factors are provided.Comment: Paper accepted for publication in Astronomy & Astrophysics. Contains
5 pages, 4 figures, and 9 table
3D HD and MHD Adaptive Mesh Refinement Simulations of the Global and Local ISM
We have performed high resolution 3D simulations with adaptive mesh
refinement, following the ISM evolution in a star forming galaxy both on small
(10 kpc) scales, enabling us to track structures in cooling
shock compressed regions as well as the entire Galactic fountain flow. It is
shown in an MHD run that the latter one is not inhibited by a large scale disk
parallel magnetic field. The fountain plays a vital role in limiting the volume
filling factor of the hot gas. Contrary to classical models most of the gas
between 100K and 8000 K is found to be thermally unstable. On scales of
superbubbles we find that the internal temperature structure is rather
inhomogeneous for an old object like our Local Bubble, leading to low OVI
column densities, consistent with observations.Comment: 8 pages, 4 figures that include 1 simulation image in jpeg format and
3 plots, to be published in How Does the Galaxy Works?, E.J. Alfaro, E.Perez,
J. Franco (eds.), Kluwer, Dordrech
The extremely asymmetric radio structure of the z=3.1 radio galaxy B3 J2330+3927
We report on 1.7 and 5.0 GHz observations of the z=3.087 radio galaxy B3
J2330+3927, using the Very Long Baseline Array (VLBA), and archival 1.4 and 8.4
GHz Very Large Array (VLA) data. Our VLBA data identify a compact, flat
spectrum (\alpha_{1.7 GHz}^{5 GHz} = -0.2 +/- 0.1; S_\nu ~ \nu^\alpha) radio
component as the core. The VLA images show that the fraction of core emission
is very large (f_c \approx 0.5 at 8.4 GHz), and reveal a previously undetected,
very faint counterjet, implying a radio lobe flux density ratio R >= 11 and a
radio lobe distance ratio Q \approx 1.9. Those values are much more common in
quasars than in radio galaxies, but the optical/near-IR spectra show a clear
type II AGN for B3 J2330+3927, confirming that it is indeed a radio galaxy.
Unlike all other radio galaxies, the bright Ly-\alpha emitting gas is located
towards the furthest radio arm. We argue against environmental and relativistic
beaming effects being the cause of the observed asymmetry, and suggest this
source has intrinsically asymmetric radio jets. If this is the case, B3
J2330+3927 is the first example of such a source at high redshift, and seems to
be difficult to reconcile with the unified model, which explains the
differences between quasars and radio galaxies as being due to orientation
effects.Comment: 6 pages, 3 figures, to appear as a Letter to MNRA
Critical behavior of a bounded Kardar-Parisi-Zhang equation
A host of spatially extended systems, both in physics and in other
disciplines, are well described at a coarse-grained scale by a Langevin
equation with multiplicative-noise. Such systems may exhibit non-equilibrium
phase transitions, which can be classified into universality classes. Here we
study in detail one of such classes that can be mapped into a
Kardar-Parisi-Zhang (KPZ) interface equation with a positive (negative)
non-linearity in the presence of a bounding lower (upper) wall. The wall limits
the possible values taken by the height variable, introducing a lower (upper)
cut-off, and induce a phase transition between a pinned (active) and a depinned
(absorbing) phase. This transition is studied here using mean field and field
theoretical arguments, as well as from a numerical point of view. Its main
properties and critical features, as well as some challenging theoretical
difficulties, are reported. The differences with other multiplicative noise and
bounded-KPZ universality classes are stressed, and the effects caused by the
introduction of ``attractive'' walls, relevant in some physical contexts, are
also analyzed.Comment: Invited paper to a special issue of the Brazilian J. of Physics. 5
eps Figures. 9 pagres. Revtex
- …