56 research outputs found
Spiral arm triggering of star formation
We present numerical simulations of the passage of clumpy gas through a
galactic spiral shock, the subsequent formation of giant molecular clouds
(GMCs) and the triggering of star formation. The spiral shock forms dense
clouds while dissipating kinetic energy, producing regions that are locally
gravitationally bound and collapse to form stars. In addition to triggering the
star formation process, the clumpy gas passing through the shock naturally
generates the observed velocity dispersion size relation of molecular clouds.
In this scenario, the internal motions of GMCs need not be turbulent in nature.
The coupling of the clouds' internal kinematics to their externally triggered
formation removes the need for the clouds to be self-gravitating. Globally
unbound molecular clouds provides a simple explanation of the low efficiency of
star formation. While dense regions in the shock become bound and collapse to
form stars, the majority of the gas disperses as it leaves the spiral arm.Comment: 6 pages, 4 figures: IAU 237, Triggering of star formation in
turbulent molecular clouds, eds B. Elmegreen and J. Palou
Shocks, cooling and the origin of star formation rates in spiral galaxies
Understanding star formation is problematic as it originates in the large
scale dynamics of a galaxy but occurs on the small scale of an individual star
forming event. This paper presents the first numerical simulations to resolve
the star formation process on sub-parsec scales, whilst also following the
dynamics of the interstellar medium (ISM) on galactic scales. In these models,
the warm low density ISM gas flows into the spiral arms where orbit crowding
produces the shock formation of dense clouds, held together temporarily by
their external pressure. Cooling allows the gas to be compressed to
sufficiently high densities that local regions collapse under their own gravity
and form stars. The star formation rates follow a Schmidt-Kennicutt
\Sigma_{SFR} ~ \Sigma_{gas}^{1.4} type relation with the local surface density
of gas while following a linear relation with the cold and dense gas. Cooling
is the primary driver of star formation and the star formation rates as it
determines the amount of cold gas available for gravitational collapse. The
star formation rates found in the simulations are offset to higher values
relative to the extragalactic values, implying a constant reduction, such as
from feedback or magnetic fields, is likely to be required. Intriguingly, it
appears that a spiral or other convergent shock and the accompanying thermal
instability can explain how star formation is triggered, generate the physical
conditions of molecular clouds and explain why star formation rates are tightly
correlated to the gas properties of galaxies.Comment: 13 pages, 12 figures. MNRAS in pres
The morphology of the Milky Way - II. Reconstructing CO maps from disc galaxies with live stellar distributions
The arm structure of the Milky Way remains somewhat of an unknown, with
observational studies hindered by our location within the Galactic disc. In the
work presented here we use smoothed particle hydrodynamics (SPH) and radiative
transfer to create synthetic longitude-velocity observations. Our aim is to
reverse-engineer a top down map of the Galaxy by comparing synthetic
longitude-velocity maps to those observed. We set up a system of N-body
particles to represent the disc and bulge, allowing for dynamic creation of
spiral features. Interstellar gas, and the molecular content, is evolved
alongside the stellar system. A 3D-radiative transfer code is then used to
compare the models to observational data. The resulting models display arm
features that are a good reproduction of many of the observed emission
structures of the Milky Way. These arms however are dynamic and transient,
allowing for a wide range of morphologies not possible with standard density
wave theory. The best fitting models are a much better match than previous work
using fixed potentials. They favour a 4-armed model with a pitch angle of
approximately 20 degrees, though with a pattern speed that decreases with
increasing Galactic radius. Inner bars are lacking however, which appear
required to fully reproduce the central molecular zone.Comment: 16 pages, 15 figures, accepted by MNRA
The role of previous generations of stars in triggering star formation and driving gas dynamics
We present hydrodynamic and magnetohydrodynamic (MHD) simulations of sub
galactic regions including photoionising and supernova feedack. We aim to
improve the initial conditions of our region extraction models by including an
initial population of stars. We also investigate the reliability of extracting
regions in simulations, and show that with a good choice of region, results are
comparable with using a larger region for the duration of our simulations.
Simulations of star formation on molecular cloud scales typically start with a
turbulent cloud of gas, from which stars form and then undergo feedback. In
reality, a typical cloud or region within a galaxy may already include, or
reside near some population of stars containing massive stars undergoing
feedback. We find the main role of a prior population is triggering star
formation, and contributing to gas dynamics. Early time supernova from the
initial population are important in triggering new star formation and driving
gas motions on larger scales above 100 pc, whilst the ionising feedback
contribution from the initial population has less impact, since many members of
the initial population have cleared out gas around them in the prior model. In
terms of overall star formation rates though, the initial population has a
relatively small effect, and the feedback does not for example suppress
subsequent star formation. We find that MHD has a relatively larger impact than
initial conditions, reducing the star formation rate by a factor of 3 at later
times.Comment: 12 pages, 10 figure
A Synthetic 21-cm Galactic Plane Survey of an SPH Galaxy Simulation
We have created synthetic neutral hydrogen (HI) Galactic Plane Survey data
cubes covering 90 degrees < l < 180 degrees, using a model spiral galaxy from
SPH simulations and the radiative transfer code TORUS. The density, temperature
and other physical parameters are fed from the SPH simulation into TORUS, where
the HI emissivity and opacity are calculated before the 21-cm line emission
profile is determined. Our main focus is the observation of Outer Galaxy
`Perseus Arm' HI, with a view to tracing atomic gas as it encounters shock
motions as it enters a spiral arm interface, an early step in the formation of
molecular clouds. The observation of HI self-absorption features at these shock
sites (in both real observations and our synthetic data) allows us to
investigate further the connection between cold atomic gas and the onset of
molecular cloud formation.Comment: MNRAS accepted; 11 pages, 12 figure
The structure of HI in galactic disks: Simulations vs observations
We generate synthetic HI Galactic plane surveys from spiral galaxy
simulations which include stellar feedback processes. Compared to a model
without feedback we find an increased scale height of HI emission (in better
agreement with observations) and more realistic spatial structure (including
supernova blown bubbles). The synthetic data show HI self-absorption with a
morphology similar to that seen in observations. The density and temperature of
the material responsible for HI self-absorption is consistent with
observationally determined values, and is found to be only weakly dependent on
absorption strength and star formation efficiency.Comment: 12 pages, 7 figures. Accepted for publication in MNRA
Observational Bias and Young Massive Cluster Characterisation II. Can Gaia accurately observe young clusters and associations?
Observations of clusters suffer from issues such as completeness, projection
effects, resolving individual stars and extinction. As such, how accurate are
measurements and conclusions are likely to be? Here, we take cluster
simulations (Westerlund2- and Orion- type), synthetically observe them to
obtain luminosities, accounting for extinction and the inherent limits of Gaia,
then place them within the real Gaia DR3 catalogue. We then attempt to
rediscover the clusters at distances of between 500pc and 4300pc. We show the
spatial and kinematic criteria which are best able to pick out the simulated
clusters, maximising completeness and minimising contamination. We then compare
the properties of the 'observed' clusters with the original simulations. We
looked at the degree of clustering, the identification of clusters and
subclusters within the datasets, and whether the clusters are expanding or
contracting. Even with a high level of incompleteness (e.g. stellar
members identified), similar qualitative conclusions tend to be reached
compared to the original dataset, but most quantitative conclusions are likely
to be inaccurate. Accurate determination of the number, stellar membership and
kinematic properties of subclusters, are the most problematic to correctly
determine, particularly at larger distances due to the disappearance of cluster
substructure as the data become more incomplete, but also at smaller distances
where the misidentification of asterisms as true structure can be problematic.
Unsurprisingly, we tend to obtain better quantitative agreement of properties
for our more massive Westerlund2-type cluster. We also make optical style
images of the clusters over our range of distances.Comment: 19 pages, 10 figures, 6 tables. Accepted for publication in MNRA
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