1,646 research outputs found
Phylogenetic Studies In Oenothera: Further Analysis Of Plants From The Southeastern United States
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141025/1/ajb210440.pd
Hierarchical formation of bulgeless galaxies II: Redistribution of angular momentum via galactic fountains
Within a fully cosmological hydrodynamical simulation, we form a galaxy which
rotates at 140 km/s, and is characterised by two loose spiral arms and a bar,
indicative of a Hubble Type SBc/d galaxy. We show that our simulated galaxy has
no classical bulge, with a pure disc profile at z=1, well after the major
merging activity has ended. A long-lived bar subsequently forms, resulting in
the formation of a secularly-formed "pseudo" bulge, with the final
bulge-to-total light ratio B/T=0.21. We show that the majority of gas which
loses angular momentum and falls to the central region of the galaxy during the
merging epoch is blown back into the hot halo, with much of it returning later
to form stars in the disc. We propose that this mechanism of redistribution of
angular momentum via a galactic fountain, when coupled with the results from
our previous study which showed why gas outflows are biased to have low angular
momentum, can solve the angular momentum/bulgeless disc problem of the cold
dark matter paradigm.Comment: 9 Pages, 10 Figures, accepted MNRAS version. Comments welcom
Within-Season Nest Reuse by Mountain Plovers (\u3ci\u3eCharadrius Montanus\u3c/i\u3e) in Eastern Colorado
The Mountain Plover (Charadrius montanus) is a declining migratory shorebird that nests on shortgrass prairies across the western Great Plains. This ground-nesting species exhibits an uncommon split-clutch mating system, in which each member of a pair simultaneously incubates a nest and uniparental care continues throughout brood-rearing. We report on an observation of within-season nest reuse in this species. To our knowledge, this report includes the first documented occurrence of 2 different Mountain Plovers using the same nest cup in the same breeding season. Nest reuse could represent a time- and energy-saving strategy for renesting individuals
Forming Disk Galaxies in Lambda CDM Simulations
We used fully cosmological, high resolution N-body + SPH simulations to
follow the formation of disk galaxies with rotational velocities between 135
and 270 km/sec in a Lambda CDM universe. The simulations include gas cooling,
star formation, the effects of a uniform UV background and a physically
motivated description of feedback from supernovae. The host dark matter halos
have a spin and last major merger redshift typical of galaxy sized halos as
measured in recent large scale N--Body simulations. The simulated galaxies form
rotationally supported disks with realistic exponential scale lengths and fall
on both the I-band and baryonic Tully Fisher relations. An extended stellar
disk forms inside the Milky Way sized halo immediately after the last major
merger. The combination of UV background and SN feedback drastically reduces
the number of visible satellites orbiting inside a Milky Way sized halo,
bringing it in fair agreement with observations. Our simulations predict that
the average age of a primary galaxy's stellar population decreases with mass,
because feedback delays star formation in less massive galaxies. Galaxies have
stellar masses and current star formation rates as a function of total mass
that are in good agreement with observational data. We discuss how both high
mass and force resolution and a realistic description of star formation and
feedback are important ingredients to match the observed properties of
galaxies.Comment: Revised version after the referee's comments. Conclusions unchanged.
2 new plots. MNRAS in press. 20 plots. 21 page
Combinatorial Bounds and Characterizations of Splitting Authentication Codes
We present several generalizations of results for splitting authentication
codes by studying the aspect of multi-fold security. As the two primary
results, we prove a combinatorial lower bound on the number of encoding rules
and a combinatorial characterization of optimal splitting authentication codes
that are multi-fold secure against spoofing attacks. The characterization is
based on a new type of combinatorial designs, which we introduce and for which
basic necessary conditions are given regarding their existence.Comment: 13 pages; to appear in "Cryptography and Communications
MaGICC baryon cycle: the enrichment history of simulated disc galaxies
Using cosmological galaxy formation simulations from the MaGICC (Making Galaxies in a Cosmological Context) project, spanning stellar mass from ∼107 to 3 × 1010 M⊙, we trace the baryonic cycle of infalling gas from the virial radius through to its eventual participation in the star formation process. An emphasis is placed upon the temporal history of chemical enrichment during its passage through the corona and circumgalactic medium. We derive the distributions of time between gas crossing the virial radius and being accreted to the star-forming region (which allows for mixing within the corona), as well as the time between gas being accreted to the star-forming region and then ultimately forming stars (which allows for mixing within the disc). Significant numbers of stars are formed from gas that cycles back through the hot halo after first accreting to the star-forming region. Gas entering high-mass galaxies is pre-enriched in low-mass proto-galaxies prior to entering the virial radius of the central progenitor, with only small amounts of primordial gas accreted, even at high redshift (z ∼ 5). After entering the virial radius, significant further enrichment occurs prior to the accretion of the gas to the star-forming region, with gas that is feeding the star-forming region surpassing 0.1 Z⊙ by z = 0. Mixing with halo gas, itself enriched via galactic fountains, is thus crucial in determining the metallicity at which gas is accreted to the disc. The lowest mass simulated galaxy (Mvir ∼ 2 × 1010 M⊙, with M⋆ ∼ 107 M⊙), by contrast, accretes primordial gas through the virial radius and on to the disc, throughout its history. Much like the case for classical analytical solutions to the so-called ‘G-dwarf problem’, overproduction of low-metallicity stars is ameliorated by the interplay between the time of accretion on to the disc and the subsequent involvement in star formation – i.e. due to the inefficiency of star formation. Finally, gas outflow/metal removal rates from star-forming regions as a function of galactic mass are presented
Galaxy Formation with local photoionisation feedback I. Methods
We present a first study of the effect of local photoionising radiation on
gas cooling in smoothed particle hydrodynamics simulations of galaxy formation.
We explore the combined effect of ionising radiation from young and old stellar
populations. The method computes the effect of multiple radiative sources using
the same tree algorithm used for gravity, so it is computationally efficient
and well resolved. The method foregoes calculating absorption and scattering in
favour of a constant escape fraction for young stars to keep the calculation
efficient enough to simulate the entire evolution of a galaxy in a cosmological
context to the present day. This allows us to quantify the effect of the local
photoionisation feedback through the whole history of a galaxy`s formation. The
simulation of a Milky Way like galaxy using the local photoionisation model
forms ~ 40 % less stars than a simulation that only includes a standard uniform
background UV field. The local photoionisation model decreases star formation
by increasing the cooling time of the gas in the halo and increasing the
equilibrium temperature of dense gas in the disc. Coupling the local radiation
field to gas cooling from the halo provides a preventive feedback mechanism
which keeps the central disc light and produces slowly rising rotation curves
without resorting to extreme feedback mechanisms. These preliminary results
indicate that the effect of local photoionising sources is significant and
should not be ignored in models of galaxy formation.Comment: Accepted for Publication in MNRAS, 13 pages, 13 figure
Breathing in Low Mass Galaxies: A Study of Episodic Star Formation
We simulate the collapse of isolated dwarf galaxies using SPH + N-Body
simulations including a physically motivated description of the effects of
supernova feedback. As the gas collapses and stars form, the supernova feedback
disrupts enough gas to temporarily quench star formation. The gas flows outward
into a hot halo, where it cools until star formation can continue once more and
the cycle repeats. The star formation histories of isolated Local Group dwarf
galaxies exhibit similar episodic bursts of star formation. We examine the mass
dependence of the stellar velocity dispersions and find that they are no less
than half the velocity of the halos measured at the virial radius.Comment: 5 pages, 3 figures, accepted ApJ. Full resolution figures and movies
available at http://hpcc.astro.washington.edu/feedbac
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