4,165 research outputs found
Gas circulation and galaxy evolution
Galaxies must form and evolve via the acquisition of gas from the
intergalactic environment, however the way this gas accretion takes place is
still poorly understood. Star-forming galaxies are surrounded by multiphase
halos that appear to be mostly produced by internal processes, e.g., galactic
fountains. However, a small fraction of the halo gas shows features that point
to an external origin. Estimates of the halo-gas accretion rate in the local
Universe consistently give values much lower than what would be required to
sustain star formation at the observed rate. Thus, most of the gas accretion
must be "hidden" and not seen directly. I discuss possible mechanisms that can
cause the intergalactic gas to cool and join the star-forming galactic disks. A
possibility is that gas accretion is driven by the galactic-fountain process
via turbulent mixing of the fountain gas with the coronal low-metallicity gas.Comment: 12 pages, 5 figures. Invited review at the conference "Hunting for
the Dark: The Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009. Eds.
V.P. Debattista and C.C. Popescu, AIP Conf. Se
Towards linear modal analysis for an L-shaped beam: equations of motion
We consider an L-shaped beam structure and derive all the equations of motion considering also the rotary inertia terms. We show that the equations are decoupled in two motions, namely the in-plane bending and out-of-plane bending with torsion. In neglecting the rotary inertia terms the torsional equation for the secondary beam is fully decoupled from the other equations for out-of-plane motion. A numerical modal analysis was undertaken for two models of the L-shaped beam, considering two different orientations of the secondary beam, and it was shown that the mode shapes can be grouped into these two motions: in-plane bending and out-of-plane motion. We compared the theoretical natural frequencies of the secondary beam in torsion with finite element results which showed some disagreement, and also it was shown that the torsional mode shapes of the secondary beam are coupled with the other out-of-plane motions. These findings confirm that it is necessary to take rotary inertia terms into account for out-of-plane bending. This work is essential in order to perform accurate linear modal analysis on the L-shaped beam structure
The Galactic fountain as an origin for the Smith Cloud
The recent discovery of an enriched metallicity for the Smith high-velocity
HI cloud (SC) lends support to a Galactic origin for this system. We use a
dynamical model of the galactic fountain to reproduce the observed properties
of the SC. In our model, fountain clouds are ejected from the region of the
disc spiral arms and move through the halo interacting with a pre-existing hot
corona. We find that a simple model where cold gas outflows vertically from the
Perseus spiral arm reproduces the kinematics and the distance of the SC, but is
in disagreement with the cloud's cometary morphology, if this is produced by
ram-pressure stripping by the ambient gas. To explain the cloud morphology we
explore two scenarios: a) the outflow is inclined with respect to the vertical
direction; b) the cloud is entrained by a fast wind that escapes an underlying
superbubble. Solutions in agreement with all observational constraints can be
found for both cases, the former requires outflow angles >40 deg while the
latter requires >1000 km/s winds. All scenarios predict that the SC is in the
ascending phase of its trajectory and have large - but not implausible - energy
requirements.Comment: Submitted to MNRAS letters, revised after referee's comments.
Comments are welcom
Angular momentum, accretion and radial flows in chemodynamical models of spiral galaxies
Gas accretion and radial flows are key ingredients of the chemical evolution
of spiral galaxies. They are also tightly linked to each other (accretion
drives radial flows, due to angular momentum conservation) and should therefore
be modelled simultaneously. We summarise an algorithm that can be used to
consistently compute accretion profiles, radial flows and abundance gradients
under quite general conditions and we describe illustrative applications to the
Milky Way. We find that gas-phase abundance gradients strongly depend on the
angular momentum of the accreting material and, in the outer regions, they are
significantly affected by the choice of boundary conditions.Comment: 4 pages, 2 figures. Proceedings of the 592 WE-Heraeus Seminar. To
appear in Astronomische Nachricthen, special issue "Reconstructing the Milky
Way's history: spectroscopic surveys, asteroseismology and chemodynamical
models", Guest Editors C. Chiappini, J. Montalban and M. Steffe
Modelling the HI halo of the Milky Way
Aims: we studied the global distribution and kinematics of the extra-planar
neutral gas in the Milky Way. Methods: we built 3D models for a series of
Galactic HI layers, projected them for an inside view, and compared them with
the Leiden-Argentina-Bonn 21-cm observations. Results: we show that the Milky
Way disk is surrounded by an extended halo of neutral gas with a vertical
scale-height of 1.6[+0.6/-0.4] kpc and an HI mass of 3.2[+1.0/-0.9]x10^8 solar
masses, which is 5-10% of the total Galactic HI. This HI halo rotates more
slowly than the disk with a vertical velocity gradient of -15[+/-4] km/s/kpc.
We found evidence for a global infall motion, both vertical (20[+5/-7] km/s)
and radial (30[+7/-5]km/s). Conclusions: the Milky Way HI halo shows properties
similar to the halos of external galaxies and is compatible with being
predominantly produced by supernova explosions in the disk. It is most likely
composed of distinct gas complexes with masses of 10^4-10^5 solar masses of
which the Intermediate Velocity Clouds are the local manifestations. The
classical High Velocity Clouds appear to be a separate population.Comment: 13 pages, 9 figures, accepted for publication in A&
On the compact wave dynamics of tensegrity beams in multiple dimensions
This work presents a numerical investigation on the nonlinear wave dynamics
of tensegrity beams in 1D, 2D and 3D arrangements. The simulation of impact
loading on a chain of tensegrity prisms and lumped masses allows us to apply on
a smaller scale recent results on the propagation of compression solitary waves
in 1D tensegrity metamaterials. Novel results on the wave dynamics of 2D and 3D
beams reveal - for the first time - the presence of compact compression waves
in two- and three-dimensional tensegrity lattices with slender aspect ratio.
The dynamics of such systems is characterized by the thermalization of the
lattice nearby the impacted regions of the boundary. The portion of the
absorbed energy moving along the longitudinal direction is transported by
compression waves with compact support. Such waves emerge with nearly constant
speed, and slight modifications of their spatial shape and amplitude, after
collisions with compression waves traveling in opposite direction. The analyzed
behaviors suggest the use of multidimensional tensegrity lattices for the
design and additive manufacturing of novel sound focusing devices
Stationary models for the extra-planar gas in disc galaxies
The kinematics of the extra-planar neutral and ionised gas in disc galaxies
shows a systematic decline of the rotational velocity with height from the
plane (vertical gradient). This feature is not expected for a barotropic gas,
whilst it is well reproduced by baroclinic fluid homogeneous models. The
problem with the latter is that they require gas temperatures (above K)
much higher than the temperatures of the cold and warm components of the
extra-planar gas layer. In this paper, we attempt to overcome this problem by
describing the extra-planar gas as a system of gas clouds obeying the Jeans
equations. In particular, we consider models having the observed extra-planar
gas distribution and gravitational potential of the disc galaxy NGC 891: for
each model we construct pseudo-data cubes and we compare them with the HI data
cube of NGC 891. In all cases the rotational velocity gradients are in
qualitative agreement with the observations, but the synthetic and the observed
data cubes of NGC 891 show systematic differences that cannot be accommodated
by any of the explored models. We conclude that the extra-planar gas in disc
galaxies cannot be satisfactorily described by a stationary Jeans-like system
of gas clouds.Comment: 14 pages, 7 figures, accepted for pubblication in MNRA
Scaling Configuration of Energy Harvesting Sensors with Reinforcement Learning
With the advent of the Internet of Things (IoT), an increasing number of
energy harvesting methods are being used to supplement or supplant battery
based sensors. Energy harvesting sensors need to be configured according to the
application, hardware, and environmental conditions to maximize their
usefulness. As of today, the configuration of sensors is either manual or
heuristics based, requiring valuable domain expertise. Reinforcement learning
(RL) is a promising approach to automate configuration and efficiently scale
IoT deployments, but it is not yet adopted in practice. We propose solutions to
bridge this gap: reduce the training phase of RL so that nodes are operational
within a short time after deployment and reduce the computational requirements
to scale to large deployments. We focus on configuration of the sampling rate
of indoor solar panel based energy harvesting sensors. We created a simulator
based on 3 months of data collected from 5 sensor nodes subject to different
lighting conditions. Our simulation results show that RL can effectively learn
energy availability patterns and configure the sampling rate of the sensor
nodes to maximize the sensing data while ensuring that energy storage is not
depleted. The nodes can be operational within the first day by using our
methods. We show that it is possible to reduce the number of RL policies by
using a single policy for nodes that share similar lighting conditions.Comment: 7 pages, 5 figure
S0 galaxies are faded spirals: clues from their angular momentum content
The distribution of galaxies in the stellar specific angular momentum versus
stellar mass plane (-) provides key insights into their
formation mechanisms. In this paper, we determine the location in this plane of
a sample of ten field/group unbarred lenticular (S0) galaxies from the CALIFA
survey. We performed a bulge-disc decomposition both photometrically and
kinematically to study the stellar specific angular momentum of the disc
components alone and understand the evolutionary links between S0s and other
Hubble types. We found that eight of our S0 discs have a distribution in the
- plane that is fully compatible with that of spiral
discs, while only two have values of lower than the spirals. These
two outliers show signs of recent merging. Our results suggest that merger and
interaction processes are not the dominant mechanisms in S0 formation in
low-density environments. Instead, S0s appear to be the result of secular
processes and the fading of spiral galaxies after the shutdown of star
formation.Comment: 35 pages, 22 figures. Accepted for publication in MNRA
- …