472 research outputs found
Clouds of Theseus: long-lived molecular clouds are composed of short-lived H2 molecules
We use passive gas tracer particles in an Arepo simulation of a dwarf spiral
galaxy to relate the Lagrangian evolution of star-forming gas parcels and their
H2 molecules to the evolution of their host giant molecular clouds. We find
that the median chemical lifetime of H2 is just 4 Myr, independent of the
lifetime of its host molecular cloud, which may vary from 1 to 90 Myr, with a
substantial portion of all star formation in the galaxy occurring in relatively
long-lived clouds. The rapid ejection of gas from around young massive stars by
early stellar feedback is responsible for this short H2 survival time, driving
down the density of the surrounding gas, so that its H2 molecules are
dissociated by the interstellar radiation field. This ejection of gas from the
H2-dominated state is balanced by the constant accretion of new gas from the
galactic environment, constituting a "competition model" for molecular cloud
evolution. Gas ejection occurs at a rate that is proportional to the molecular
cloud mass, so that the cloud lifetime is determined by the accretion rate,
which may be as high as 4 x 10^4 Msol/Myr in the longest-lived clouds. Our
findings therefore resolve the conflict between observations of rapid gas
ejection around young massive stars and observations of long-lived molecular
clouds in galaxies, that often survive up to several tens of Myr. We show that
the fastest-accreting, longest-lived, highest-mass clouds drive supernova
clustering on sub-cloud scales, which in turn is a key driver of galactic
outflows.Comment: 16 pages, 16 figures. Submitted to MNRAS, comments welcome
Formation of Galactic Disks II: the Physical Drivers of Disk Spin-up
Using a representative sample of Milky Way (MW)-like galaxies from the TNG50
cosmological-volume simulation, we investigate physical processes driving the
formation of galactic disks. A disk forms as a result of the interplay between
inflow and outflow carrying angular momentum in and out of the galaxy.
Interestingly, the inflow and outflow have remarkably similar distributions of
angular momentum, suggesting an exchange of angular momentum and/or outflow
recycling, leading to continuous feeding of pre-aligned material from the
co-rotating circumgalactic medium. We show that disk formation in TNG50 is
correlated with stellar bulge formation, in qualitative agreement with a recent
theoretical model of disk formation facilitated by steep gravitational
potentials. Disk formation is also correlated with the formation of a hot
circumgalactic halo with a significant fraction of the inflow occurring at sub-
and transonic velocities. In the context of recent theoretical works connecting
disk settling and hot halo formation, our results imply that the subsonic part
of the inflow may settle into a disk while the remaining supersonic inflow will
perturb this disk via the chaotic cold accretion. We find that disks tend to
form when the host halos become more massive than , consistent with previous theoretical findings and observational
estimates of the pre-disk protogalaxy remnant in the MW. Our results do not
prove that either co-rotating outflow recycling, gravitational potential
steepening, or hot halo formation cause disk formation but they show that all
these processes occur concurrently and may play an important role in disk
growth.Comment: 22 pages, 15 figures; submitted to ApJ; comments are welcom
Formation of Galactic Disks I: Why did the Milky Way's Disk Form Unusually Early?
Recent results from spectroscopic and astrometric surveys of nearby stars
suggest that the stellar disk of our Milky Way (MW) was formed quite early,
within the first few billion years of its evolution. Chemo-kinematic signatures
of disk formation in cosmological zoom-in simulations appear to be in tension
with these data, suggesting that MW-like disk formation is delayed in
simulations. We investigate the formation of galactic disks using a
representative sample of MW-like galaxies from the cosmological-volume
simulation TNG50. We find that on average MW-mass disks indeed form later than
the local data suggest. However, their formation time and metallicity exhibit a
substantial scatter, such that 10% of MW-mass galaxies form disks early,
similar to the MW. Thus, although the MW is unusual, it is consistent with the
overall population of MW-mass disk galaxies. The direct MW analogs assemble
most of their mass early, Gyr ago, and are not affected by
destructive mergers after that. In addition, these galaxies form their disks
during the early enrichment stage when the ISM metallicity increases rapidly,
with only 25% of early-forming disks being as metal-poor as the MW was at
the onset of disk formation, [Fe/H] . In contrast, most MW-mass
galaxies either form disks from already enriched material or experience late
destructive mergers that reset the signatures of galactic disk formation to
later times and higher metallicities. Finally, we also show that the earlier
disk formation leads to more dominant rotationally-supported stellar disks at
redshift zero.Comment: 18 pages, 13 figures + appendix; submitted to ApJ; comments are
welcom
What Sets the Star Formation Rate of Molecular Clouds? The Density Distribution as a Fingerprint of Compression and Expansion Rates
We use a suite of 3D simulations of star-forming molecular clouds, with and without stellar feedback, magnetic fields, and driven turbulence, to study the compression and expansion rates of the gas as functions of density. We show that, around the mean density, supersonic turbulence promotes rough equilibrium between the amounts of compressing and expanding gas, consistent with continuous gas cycling between high- and low-density states. We find that the inclusion of protostellar jets produces rapidly expanding and compressing low-density gas. We find that the gas mass flux peaks at the transition between the lognormal and power-law forms of the density probability distribution function (PDF). This is consistent with the transition density tracking the post-shock density, which promotes an enhancement of mass at this density (i.e., shock compression and filament formation). At high densities, the gas dynamics are dominated by self-gravity: the compression rate in all of our runs matches the rate of the run with only gravity, suggesting that processes other than self-gravity have little effect at these densities. The net gas mass flux becomes constant at a density below the sink formation threshold, where it equals the star formation rate. The density at which the net gas mass flux equals the star formation rate is one order of magnitude lower than our sink threshold density, corresponds to the formation of the second power-law tail in the density PDF, and sets the overall star formation rates of these simulations
Impact damage in woven carbon fibre/epoxy laminates: Analysis of damage and dynamic strain fields
© 2017 The Authors. Published by Elsevier Ltd. In this study the resultant ballistic dynamic response observed in a 2x2 twill weave T300 carbon fibre/epoxy composite flat-plate specimen is examined, using a combination of non-invasive analysis techniques. The study investigates deformation and damage caused by impacts of two types of projectiles: solid (steel) travelling with velocity of 70-90 m/s, and fragmenting (ice) with the velocity in the range of 300-500 m/s. Digital image correlation was employed to obtain displacement and to estimate dynamic strain fields from the rear surfaces of the specimens during each experiment. 3D X-ray computer tomography (CT) was used to image and visualize the resultant damage inside the samples. It was shown that solid projectiles lead to greater localized deformation and in some cases penetration, whereas fragmenting projects destroyed on impact cause a more distributed impact load but can lead to major front-surface damage depending on the depth of indentation before fragmentation
Understanding Dwarf Galaxies in order to Understand Dark Matter
Much progress has been made in recent years by the galaxy simulation
community in making realistic galaxies, mostly by more accurately capturing the
effects of baryons on the structural evolution of dark matter halos at high
resolutions. This progress has altered theoretical expectations for galaxy
evolution within a Cold Dark Matter (CDM) model, reconciling many earlier
discrepancies between theory and observations. Despite this reconciliation, CDM
may not be an accurate model for our Universe. Much more work must be done to
understand the predictions for galaxy formation within alternative dark matter
models.Comment: Refereed contribution to the Proceedings of the Simons Symposium on
Illuminating Dark Matter, to be published by Springe
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