2,821 research outputs found
On the Structure of the Orion A Cloud and the Formation of the Orion Nebula Cluster
We suggest that the Orion A cloud is gravitationally collapsing on large
scales, and is producing the Orion Nebula Cluster due to the focusing effects
of gravity acting within a finite cloud geometry. In support of this
suggestion, we show how an elliptical rotating sheet of gas with a modest
density gradient along the major axis can collapse to produce a structure
qualitatively resembling Orion A, with a fan-shaped structure at one end,
ridges or filaments along the fan, and a narrow curved filament at the other
end reminiscent of the famous integral-shaped filament. The model produces a
local concentration of mass within the narrow filament which in principle could
form a dense cluster of stars like that of the Orion Nebula. We suggest that
global gravitational contraction might be a more common feature of molecular
clouds than previously recognized, and that the formation of star clusters is a
dynamic process resulting from the focusing effects of gravity acting upon the
geometry of finite clouds.Comment: 23 pages, 6 figures, to appear in the Astrophysical Journa
Thermal Instability and the Formation of Clumpy Gas Clouds
The radiative cooling of optically thin gaseous regions and the formation of
a two-phase medium and of cold gas clouds with a clumpy substructure is
investigated. In optically thin clouds, the growth rate of small isobaric
density perturbations is independent of their length scale. However, the growth
of a perturbation is limited by its transition from isobaric to isochoric
cooling. The temperature at which this transition occurs decreases with the
length scale of the perturbation. Consequently small scale perturbations have
the potential to reach higher amplitudes than large scale perturbations. When
the amplitude becomes nonlinear, advection overtakes the pressure gradient in
promoting the compression resulting in an accelerated growth of the
disturbance. The critical temperature for transition depends on the initial
amplitude. The fluctuations which can first reach nonlinearity before their
isobaric to isochoric transition will determine the characteristic size and
mass of the cold dense clumps which would emerge from the cooling of an
initially nearly homogeneous region of gas. Thermal conduction is in general
very efficient in erasing isobaric, small-scale fluctuations, suppressing a
cooling instability. A weak, tangled magnetic field can however reduce the
conductive heat flux enough for low-amplitude fluctuations to grow isobarically
and become non-linear if their length scales are of order 0.01 pc. Finally, we
demonstrate how a 2-phase medium, with cold clumps being pressure confined in a
diffuse hot residual background component, would be sustained if there is
adequate heating to compensate the energy loss.Comment: 26 pages, Latex, 10 postscript figures, ApJ, in pres
Implications of Intermediate Mass Black Hole in globular cluster G1 on Dark Matter detection
Recently there has been a growing evidence in favor of the presence of an
Intermediate Mass Black Hole in the globular cluster G1, in Andromeda Galaxy.
In this paper, we explore whether the adiabatic growth in the dark matter
density due to the presence of a black hole could result in an observable gamma
ray signal due to dark matter annihilation in this globular cluster. Starting
from an initial NFW matter profile, with density parameters consistent with G1
observations, we find that indeed, if the spike in the density has been formed
and has survived till present, the signal could be observed by GLAST and
current ACT detectors.Comment: 5 pages, 1 figur
The Myth of the Molecular Ring
We investigate the structure of the Milky Way by determining how features in
a spatial map correspond to CO features in a velocity map. We examine
structures including logarithmic spiral arms, a ring and a bar. We explore the
available parameter space, including the pitch angle of the spiral arms, radius
of a ring, and rotation curve. We show that surprisingly, a spiral arm provides
a better fit to the observed molecular ring than a true ring feature. This is
because both a spiral arm, and the observed feature known as the molecular
ring, are curved in velocity longitude space. We find that much of the CO
emission in the velocity longitude map can be fitted by a nearly symmetric 2
armed spiral pattern. One of the arms corresponds to the molecular ring, whilst
the opposite arm naturally reproduces the Perseus arm. Multiple arms also
contribute to further emission in the vicinity of the molecular ring and match
other observed spiral arms. Whether the Galactic structure consists primarily
of two, or several spiral arms, the presence of 2 symmetric logarithmic
spirals, which begin in the vicinity of the ends of the bar, suggest a spiral
density wave associated with the bar.Comment: 7 pages, 2 figures, accepted by MNRA
Ionizing Radiation in Smoothed Particle Hydrodynamics
A new method for the inclusion of ionizing radiation from uniform radiation
fields into 3D Smoothed Particle Hydrodynamics (SPHI) simulations is presented.
We calculate the optical depth for the Lyman continuum radiation from the
source towards the SPHI particles by ray-tracing integration. The
time-dependent ionization rate equation is then solved locally for the
particles within the ionizing radiation field. Using test calculations, we
explore the numerical behaviour of the code with respect to the implementation
of the time-dependent ionization rate equation. We also test the coupling of
the heating caused by the ionization to the hydrodynamical part of the SPHI
code.Comment: 9 pages, 9 figures. accepted by MNRA
SAURON's Challenge for the Major Merger Scenario of Elliptical Galaxy Formation
The intrinsic anisotropy delta and flattening epsilon of simulated merger
remnants is compared with elliptical galaxies that have been observed by the
SAURON collaboration, and that were analysed using axisymmetric Schwarzschild
models. Collisionless binary mergers of stellar disks and disk mergers with an
additional isothermal gas component, neglecting star formation cannot reproduce
the observed trend delta = 0.55 epsilon (SAURON relationship). An excellent fit
of the SAURON relationship for flattened ellipticals with epsilon >= 0.25 is
however found for merger simulations of disks with gas fractions >= 20%,
including star formation and stellar energy feedback. Massive black hole
feedback does not strongly affect this result. Subsequent dry merging of merger
remnants however does not generate the slowly-rotating SAURON ellipticals which
are characterized by low ellipticities epsilon < 0.25 and low anisotropies.
This indicates that at least some ellipticals on the red galaxy sequence did
not form by binary mergers of disks or early-type galaxies. We show that
stellar spheroids resulting from multiple, hierarchical mergers of
star-bursting subunits in a cosmological context are in excellent agreement
with the low ellipticities and anisotropies of the slowly rotating SAURON
ellipticals and their observed trend of delta with epsilon. The numerical
simulations indicate that the SAURON relation might be a result of strong
violent relaxation and phase mixing of multiple, kinematically cold stellar
subunits with the angular momentum of the system determining its location on
the relation.Comment: 13 pages, 3 figures, submitted to Ap
On the impact of the magnitude of Interstellar pressure on physical properties of Molecular Cloud
Recently reported variations in the typical physical properties of Galactic
and extra-Galactic molecular clouds (MCs), and in their ability to form stars
have been attributed to local variations in the magnitude of interstellar
pressure. Inferences from these surveys have called into question two
long-standing beliefs that the MCs : 1 are Virialised entities and (2) have
approximately constant surface density i.e., the validity of the Larson's third
law. In this work we invoke the framework of cloud-formation via collisions
between warm gas flows. Post-collision clouds forming in these realisations
cool rapidly and evolve primarily via the interplay between the Non-linear Thin
Shell Instability (NTSI), and the self-gravity. Over the course of these
simulations we traced the temporal evolution of the surface density of the
assembled clouds, the fraction of dense gas, the distribution of gas column
density (NPDF), and the Virial nature of the assembled clouds. We conclude,
these physical properties of MCs not only exhibit temporal variation, but their
respective peak-magnitude also increases in proportion with the magnitude of
external pressure, . The velocity dispersion in assembled clouds
appears to follow the power-law, . Also,
the power-law tail at higher densities becomes shallower with increasing
magnitude of external pressure, for magnitudes,
K cm, at higher magnitudes such as those typically found in the Galactic
CMZ ( K cm), the power-law shows significant
steepening. Thus while our results are broadly consistent with inferences from
various recent observational surveys, it appears, MCs hardly exhibit a unique
set of properties, but rather a wide variety, that can be reconciled with a
range of magnitudes of pressure between 10 K cm - 10 K
cm.Comment: 20 pages, 11 Figures, 1 Table, To appear in Monthly Notice of the RA
On the star-forming ability of Molecular Clouds
The star-forming ability of a molecular cloud depends on the fraction of gas
it can cycle into the dense-phase. Consequently, one of the crucial questions
in reconciling star-formation in clouds is to understand the factors that
control this process. While it is widely accepted that the variation in ambient
conditions can alter significantly the ability of a cloud to spawn stars, the
observed variation in the star-formation rate in nearby clouds that experience
similar ambient conditions, presents an interesting question. In this work we
attempted to reconcile this variation within the paradigm of colliding flows.
To this end we develop self-gravitating, hydrodynamic realisations of identical
flows, but allowed to collide off-centre. Typical observational diagnostics
such as the gas-velocity dispersion, the fraction of dense-gas, the column
density distribution ({\small N-PDF}), the distribution of gas mass as a
function of -band extinction and the strength of compressional/solenoidal
modes in the post-collision cloud were deduced for different choices of the
impact parameter of collision. We find that a strongly sheared cloud is
terribly inefficient in cycling gas into the dense phase and that such a cloud
can possibly reconcile the sluggish nature of star-formation reported for some
clouds. Within the paradigm of cloud-formation via colliding flows this is
possible in case of flows colliding with a relatively large impact parameter.
We conclude that compressional modes - though probably essential - are
insufficient to ensure a relatively higher star-formation efficiency in a
cloud.Comment: 12 pages, 8 figures; To appear in MNRA
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