93 research outputs found
Magnetic fields in massive spirals: The role of feedback and initial conditions
Magnetic fields play a very important role in the evolution of galaxies
through their direct impact on star formation and stellar feedback-induced
turbulence. However, their co-evolution with these processes has still not been
thoroughly investigated, and the possible effect of the initial conditions is
largely unknown. This letter presents the first results from a series of
high-resolution numerical models, aimed at deciphering the effect of the
initial conditions and of stellar feedback on the evolution of the galactic
magnetic field in isolated, Milky-Way-like galaxies. The models start with an
ordered, either poloidal or toroidal, magnetic field of varying strength, and
are evolved with and without supernova feedback. They include a dark matter
halo, a stellar and a gaseous disk, as well as the appropriate cooling and
heating processes for the interstellar medium. Independently of the initial
conditions, the galaxies develop a turbulent velocity field and a random
magnetic field component in under 15 Myrs. Supernova feedback is extremely
efficient in building a random magnetic field component up to large galactic
heights. However, a random magnetic field emerges even in runs without
feedback, which points to an inherent instability of the ordered component.
Supernova feedback greatly affects the velocity field of the galaxy up to large
galactic heights, and helps restructure the magnetic field up to 10 kpc above
the disk, independently of the initial magnetic field morphology. On the other
hand, the initial morphology of the magnetic field can accelerate the
development of a random component at large heights. These effects have
important implications for the study of the magnetic field evolution in galaxy
simulations.Comment: A&A Letters, accepte
The effect of ambipolar diffusion on low-density molecular ISM filaments
The filamentary structure of the molecular interstellar medium and the
potential link of this morphology to star formation have been brought into
focus recently by high resolution observational surveys. An especially puzzling
matter is that local interstellar filaments appear to have the same thickness,
independent of their column density. This requires a theoretical understanding
of their formation process and the physics that governs their evolution. In
this work we explore a scenario in which filaments are dissipative structures
of the large-scale interstellar turbulence cascade and ion-neutral friction
(also called ambipolar diffusion) is affecting their sizes by preventing
small-scale compressions. We employ high-resolution, 3D MHD simulations,
performed with the grid code RAMSES, to investigate non-ideal MHD turbulence as
a filament formation mechanism. We focus the analysis on the mass and thickness
distributions of the resulting filamentary structures. Simulations of both
driven and decaying MHD turbulence show that the morphologies of the density
and the magnetic field are different when ambipolar diffusion is included in
the models. In particular, the densest structures are broader and more massive
as an effect of ion-neutral friction and the power spectra of both the velocity
and the density steepen at a smaller wavenumber. The comparison between ideal
and non-ideal MHD simulations shows that ambipolar diffusion causes a shift of
the filament thickness distribution towards higher values. However, none of the
distributions exhibit the pronounced peak found in the observed local
filaments. Limitations in dynamical range and the absence of self-gravity in
these numerical experiments do not allow us to conclude at this time whether
this is due to the different filament selection or due to the physics inherent
of the filament formation.Comment: A&A accepte
Galactic Dynamos
Spiral galaxies, including the Milky Way, have large-scale magnetic fields
with significant energy densities. The dominant theory attributes these
magnetic fields to a large-scale dynamo. We review the current status of dynamo
theory and discuss various numerical simulations designed to explain either
particular aspects of the problem or to reproduce galactic magnetic fields
globally. Our main conclusions can be summarized as follows. (i) Idealized
direct numerical simulations produce mean magnetic fields, whose saturation
energy density tends to decline with increasing magnetic Reynolds number. This
could imply that the observed large-scale galactic magnetic fields might not
entirely originate from a mean-field dynamo. Much of the current numerical
effort is focused on this unsolved problem. (ii) Small-scale dynamos are
important throughout a galaxy's life, and probably provide strong seed fields
at early stages. (iii) Large-scale galactic magnetic fields of microGauss
strengths can probably only be explained if helical magnetic fields of small or
moderate length scales can rapidly be ejected or destroyed. (iv) The
circumgalactic medium (CGM) may play an important role in driving dynamo action
at small and large length scales. These interactions between the galactic disk
and the CGM may be a key aspect in understanding galactic dynamos. We expect
future research in galactic dynamos to focus on the cosmological history of
galaxies and the interaction with the CGM as means of replacing the idealized
boundary conditions used in earlier work.Comment: 46 pages, 15 figures, 4 tables, comments welcome
Shell instability of a collapsing dense core
Understanding the formation of binary and multiple stellar systems largely
comes down to studying the circumstances for the fragmentation of a condensing
core during the first stages of the collapse. However, the probability of
fragmentation and the number of fragments seem to be determined to a large
degree by the initial conditions. In this work we study the fate of the linear
perturbations of a homogeneous gas sphere both analytically and numerically. In
particular, we investigate the stability of the well-known homologous solution
that describes the collapse of a uniform spherical cloud. The difficulty of the
mathematical singularity in the perturbation equations is surpassed here by
explicitly introducing a weak shock next to the sonic point. In parallel, we
perform adaptive mesh refinement (AMR) numerical simulations of the linear
stages of the collapse and compared the growth rates obtained by each method.
With this combination of analytical and numerical tools, we explore the
behavior of both spherically symmetric and non-axisymmetric perturbations. The
numerical experiments provide the linear growth rates as a function of the
core's initial virial parameter and as a function of the azimuthal wave number
of the perturbation. The overlapping regime of the numerical experiments and
the analytical predictions is the situation of a cold and large cloud, and in
this regime the analytically calculated growth rates agree very well with the
ones obtained from the simulations. The use of a weak shock as part of the
perturbation allows us to find a physically acceptable solution to the
equations for a continuous range of growth rates. The numerical simulations
agree very well with the analytical prediction for the most unstable cores,
while they impose a limit of a virial parameter of 0.1 for core fragmentation
in the absence of rotation.Comment: Accepted by A&
A dynamo amplifies the magnetic field of a Milky-Way-like galaxy
The magnetic fields of spiral galaxies are so strong that they cannot be
primordial. Their typical values are over one billion times higher than any
value predicted for the early Universe. Explaining this immense growth and
incorporating it in galaxy evolution theories is one of the long-standing
challenges in astrophysics. So far, the most successful theory for the
sustained growth of the galactic magnetic field is the alpha-omega dynamo. This
theory predicts a characteristic dipolar or quadrupolar morphology for the
galactic magnetic field, which has been observed in external galaxies. However,
so far, there has been no direct demonstration of a mean-field dynamo operating
in direct, multi-physics simulations of spiral galaxies. We do so in this work.
We employ numerical models of isolated, star-forming spiral galaxies that
include a magnetized gaseous disk, a dark matter halo, stars, and stellar
feedback. Naturally, the resulting magnetic field has a complex morphology that
includes a strong random component. Using a smoothing of the magnetic field on
small scales, we are able to separate the mean from the turbulent component and
analyze them individually. We find that a mean-field dynamo naturally occurs as
a result of the dynamical evolution of the galaxy and amplifies the magnetic
field by an order of magnitude over half a Gyr. Despite the highly dynamical
nature of these models, the morphology of the mean component of the field is
identical to analytical predictions. This result underlines the importance of
the mean-field dynamo in galactic evolution. Moreover, by demonstrating the
natural growth of the magnetic field in a complex galactic environment, it
brings us a step closer to understanding the cosmic origin of magnetic fields.Comment: Accepted for publication in Astronomy & Astrophysic
A Young GMC Formed at the Interface of Two Colliding Supershells: Observations Meet Simulations
Dense, star-forming gas is believed to form at the stagnation points of
large-scale ISM flows, but observational examples of this process in action are
rare. We here present a giant molecular cloud (GMC) sandwiched between two
colliding Milky Way supershells, which we argue shows strong evidence of having
formed from material accumulated at the collision zone. Combining 12CO, 13CO
and C18O(J=1-0) data with new high-resolution, 3D hydrodynamical simulations of
colliding supershells, we discuss the origin and nature of the GMC
(G288.5+1.5), favoring a scenario in which the cloud was partially seeded by
pre-existing denser material, but assembled into its current form by the action
of the shells. This assembly includes the production of some new molecular gas.
The GMC is well interpreted as non-self-gravitating, despite its high mass (MH2
~ 1.7 x 10^5 Msol), and is likely pressure confined by the colliding flows,
implying that self-gravity was not a necessary ingredient for its formation.
Much of the molecular gas is relatively diffuse, and the cloud as a whole shows
little evidence of star formation activity, supporting a scenario in which it
is young and recently formed. Drip-like formations along its lower edge may be
explained by fluid dynamical instabilities in the cooled gas.Comment: 13 pages, 9 figures, accepted for publication in Ap
Witnessing the fragmentation of a filament into prestellar cores in Orion B/NGC 2024
Recent Herschel observations of nearby clouds have shown that filamentary
structures are ubiquitous and that most prestellar cores form in filaments.
Probing the density () and velocity () structure of filaments is crucial
for the understanding of the star formation process. To characterize both the
and the field of a fragmenting filament, we mapped NGC2024. 13CO, C18O,
and H13CO+ trace the filament seen in the data. The radial profile
from the data shows ~0.081 pc, which is similar to the
Herschel findings. The from 13CO and C18O are broader, while the
from H13CO+ is narrower, than from Herschel. These results
suggest that 13CO and C18O trace only the outer part of the filament and H13CO+
only the inner part. The H13CO+ map reveals gradients along
both filament axis, as well as oscillations with a period ~0.2 pc
along the major axis. Comparison between the and the distribution shows
a tentative /4 shift in H13CO+ or C18O. This /4 shift is not
simultaneously observed for all cores in any single tracer but is tentatively
seen in either H13CO+ or C18O. We produced a toy model taking into account a
transverse gradient, a longitudinal gradient, and a longitudinal
oscillation mode caused by fragmentation. Examination of synthetic data shows
that the oscillation component produces an oscillation pattern in the velocity
structure function (VSF) of the model. The H13CO+ VSF shows an oscillation
pattern, suggesting that our observations are partly tracing core-forming
motions and fragmentation. We also found that the mean corresponds
to the effective in the filament. This is consistent with a scenario
in which higher-mass cores form in higher line-mass filaments.Comment: accepted in A&
Formation of Cold Filamentary Structure from Wind Blown Superbubbles
The expansion and collision of two wind-blown superbubbles is investigated
numerically. Our models go beyond previous simulations of molecular cloud
formation from converging gas flows by exploring this process with realistic
flow parameters, sizes and timescales. The superbubbles are blown by
time-dependent winds and supernova explosions, calculated from population
synthesis models. They expand into a uniform or turbulent diffuse medium. We
find that dense, cold gas clumps and filaments form naturally in the compressed
collision zone of the two superbubbles. Their shapes resemble the elongated,
irregular structure of observed cold, molecular gas filaments and clumps. At
the end of the simulations, between 65 and 80 percent of the total gas mass in
our simulation box is contained in these structures. The clumps are found in a
variety of physical states, ranging from pressure equilibrium with the
surrounding medium to highly under-pressured clumps with large irregular
internal motions and structures which are rotationally supported.Comment: Submitted to Ap
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