1,968 research outputs found
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
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
Numerical simulations of the possible origin of the two sub-parsec scale and counter-rotating stellar disks around SgrA*
We present a high resolution simulation of an idealized model to explain the
origin of the two young, counter-rotating, sub-parsec scale stellar disks
around the supermassive black hole SgrA* at the Center of the Milky Way. In our
model, the collision of a single molecular cloud with a circum-nuclear gas disk
(similar to the one observed presently) leads to multiple streams of gas
flowing towards the black hole and creating accretion disks with angular
momentum depending on the ratio of cloud and circum-nuclear disk material. The
infalling gas creates two inclined, counter-rotating sub-parsec scale accretion
disks around the supermassive black hole with the first disk forming roughly 1
Myr earlier, allowing it to fragment into stars and get dispersed before the
second, counter-rotating disk forms. Fragmentation of the second disk would
lead to the two inclined, counter-rotating stellar disks which are observed at
the Galactic Center. A similar event might be happening again right now at the
Milky Way Galactic Center. Our model predicts that the collision event
generates spiral-like filaments of gas, feeding the Galactic Center prior to
disk formation with a geometry and inflow pattern that is in agreement with the
structure of the so called mini-spiral that has been detected in the Galactic
Center.Comment: 14 pages, 12 figures, submitted to Ap
Simulating the impact of the Smith Cloud
We investigate the future evolution of the Smith Cloud by performing
hydrodynamical simulations of the cloud impact onto the gaseous Milky Way
Galactic disk. We assume a local origin for the cloud and thus do not include a
dark matter component to stabilize it. Our main focus is the cloud's influence
on the local and global star formation rate (SFR) of the Galaxy and whether or
not it leads to an observable event in the far future. Our model assumes two
extremes for the mass of the Smith Cloud, an upper mass limit of 10
M and a lower mass limit of 10 M, compared to the
observational value of a few 10 M. In addition, we also make the
conservative assumption that the entirety of the cloud mass of the extended
Smith Cloud is concentrated within the tip of the cloud. We find that the
impact of the low-mass cloud produces no noticeable change in neither the
global SFR nor the local SFR at the cloud impact site within the galactic disk.
For the high-mass cloud we find a short-term (roughly 5 Myr) increase of the
global SFR of up to 1 M yr, which nearly doubles the normal
Milky Way SFR. This highly localized starburst should be observable.Comment: 14 pages, 5 figure
Galacto-forensic of LMC's orbital history as a probe for the dark matter potential in the outskirt of the Galaxy
The 3D observed velocities of the Large and Small Magellanic Clouds(LMC and
SMC) provide an opportunity to probe the Galactic potential in the outskirt of
the Galactic halo. Based on a canonical NFW model of the Galactic potential,
Besla et al.(2007) reconstructed LMC and SMC's orbits and suggested that they
are currently on their first perigalacticon passage about the Galaxy. Motivated
by several recent revisions of the Sun's motion around the Galactic center, we
re-examine the LMC's orbital history and show that it depends sensitively on
the dark-matter's mass distribution beyond its present Galactic distance. We
utilize results of numerical simulations to consider a range of possible
structural and evolutionary models for the Galactic potentials. We find that
within the theoretical and observational uncertainties, it is possible for the
LMC to have had multiple perigalacticon passages on the Hubble time scale,
especially if the Galactic circular velocity at the location of the Sun is
greater than km s. Based on these models, a more accurate
determination of the LMC's motion may be used to determine the dark matter
distribution in the outskirt of the Galactic halo.Comment: 9 pages, 10 figures. Accepted for publication in Ap
Fast Molecular Cloud Destruction Requires Fast Cloud Formation
A large fraction of the gas in the Galaxy is cold, dense, and molecular. If
all this gas collapsed under the influence of gravity and formed stars in a
local free-fall time, the star formation rate in the Galaxy would exceed that
observed by more than an order of magnitude. Other star-forming galaxies behave
similarly. Yet observations and simulations both suggest that the molecular gas
is indeed gravitationally collapsing, albeit hierarchically. Prompt stellar
feedback offers a potential solution to the low observed star formation rate if
it quickly disrupts star-forming clouds during gravitational collapse. However,
this requires that molecular clouds must be short-lived objects, raising the
question of how so much gas can be observed in the molecular phase. This can
occur only if molecular clouds form as quickly as they are destroyed,
maintaining a global equilibrium fraction of dense gas. We therefore examine
cloud formation timescales. We first demonstrate that supernova and superbubble
sweeping cannot produce dense gas at the rate required to match the cloud
destruction rate. On the other hand, Toomre gravitational instability can reach
the required production rate. We thus argue that, although dense, star-forming
gas may last only around a single global free-fall time, the dense gas in
star-forming galaxies can globally exist in a state of dynamic equilibrium
between formation by gravitational instability, and disruption by stellar
feedback. At redshift z >~ 2, the Toomre instability timescale decreases,
resulting in a prediction of higher molecular gas fractions at early times, in
agreement with observations.Comment: 7 pages, no figures, ApJL accepted; v3: corrected several errors,
added discussion, no change in conclusion
About the morphology of dwarf spheroidal galaxies and their dark matter content
The morphological properties of the Carina, Sculptor and Fornax dwarfs are
investigated using new wide field data with a total area of 29 square degrees.
The stellar density maps are derived, hinting that Sculptor possesses tidal
tails indicating interaction with the Milky Way. Contrary to previous studies
we cannot find any sign of breaks in the density profiles for the Carina and
Fornax dwarfs. The possible existence of tidal tails in Sculptor and of King
limiting radii in Fornax and Carina are used to derive global M/L ratios,
without using kinematic data. By matching those M/L ratios to kinematically
derived values we are able to constrain the orbital parameters of the three
dwarfs. Fornax cannot have M/L smaller than 3 and must be close to its
perigalacticon now. The other extreme is Sculptor that needs to be on an orbit
with an eccentricity bigger than 0.5 to be able to form tidal tails despite its
kinematic M/L.Comment: 9 pages, 7 figures, accepted by A&
Globular Cluster Formation from Colliding Substructure
We investigate a scenario where the formation of Globular Clusters (GCs) is
triggered by high-speed collisions between infalling atomic-cooling subhalos
during the assembly of the main galaxy host, a special dynamical mode of star
formation that operates at high gas pressures and is intimately tied to LCDM
hierarchical galaxy assembly. The proposed mechanism would give origin to
"naked" globulars, as colliding dark matter subhalos and their stars will
simply pass through one another while the warm gas within them clashes at
highly supersonic speed and decouples from the collisionless component, in a
process reminiscent of the Bullet galaxy cluster. We find that the resulting
shock-compressed layer cools on a timescale that is typically shorter than the
crossing time, first by atomic line emission and then via fine-structure
metal-line emission, and is subject to gravitational instability and
fragmentation. Through a combination of kinetic theory approximation and
high-resolution -body simulations, we show that this model may produce: (a)
a GC number-halo mass relation that is linear down to dwarf galaxy scales and
agrees with the trend observed over five orders of magnitude in galaxy mass;
(b) a population of old globulars with a median age of 12 Gyr and an age spread
similar to that observed; (c) a spatial distribution that is biased relative to
the overall mass profile of the host; and (d) a bimodal metallicity
distribution with a spread similar to that observed in massive galaxies.Comment: 15 pages, 5 figures, accepted for publication by the Astrophysical
Journa
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