3,864 research outputs found
Red Fraction Among Satellite Galaxies with Disk-like Light Profiles: Evidence for Inflow in the H I Disk
The relationships between color, characterized with respect to the g â r red sequence; stellar structure, as determined using the i-band SĂ©rsic index; and group membership are explored using the Sloan Digital Sky Survey (SDSS). The new results place novel constraints on theories of galaxy evolution, despite the strong correlation between color and stellar structure. Observed correlations are of three independent typesâthose based on stellar structure, on the color of disk-like galaxies, and on the color of elliptical galaxies. Of particular note, the fraction of galaxies residing on the red sequence measured among galaxies with disk-like light profiles is enhanced for satellite galaxies compared to central galaxies. This fraction increases with group mass. When these new results are considered, theoretical treatments of galaxy evolution that adopt a gas accretion model centered on the hot galactic halo cannot consistently account for all observations of disk galaxies. The hypothesis is advanced that inflow within the extended H I disk prolongs star formation in satellite galaxies. When combined with partial ram pressure stripping (RPS) of this disk, this new scenario is consistent with the observations. This is demonstrated by applying an analytical model of RPS of the extended H I disk to the SDSS groups. These results motivate incorporating more complex modes of gas accretion into models of galaxy evolution, including cold mode accretion, an improved treatment of gas dynamics within disks, and disk stripping
Time-dependence in Relativistic Collisionless Shocks: Theory of the Variable "Wisps" in the Crab Nebula
We describe results from time-dependent numerical modeling of the
collisionless reverse shock terminating the pulsar wind in the Crab Nebula. We
treat the upstream relativistic wind as composed of ions and electron-positron
plasma embedded in a toroidal magnetic field, flowing radially outward from the
pulsar in a sector around the rotational equator. The relativistic cyclotron
instability of the ion gyrational orbit downstream of the leading shock in the
electron-positron pairs launches outward propagating magnetosonic waves.
Because of the fresh supply of ions crossing the shock, this time-dependent
process achieves a limit-cycle, in which the waves are launched with
periodicity on the order of the ion Larmor time. Compressions in the magnetic
field and pair density associated with these waves, as well as their
propagation speed, semi-quantitatively reproduce the behavior of the wisp and
ring features described in recent observations obtained using the Hubble Space
Telescope and the Chandra X-Ray Observatory. By selecting the parameters of the
ion orbits to fit the spatial separation of the wisps, we predict the period of
time variability of the wisps that is consistent with the data. When coupled
with a mechanism for non-thermal acceleration of the pairs, the compressions in
the magnetic field and plasma density associated with the optical wisp
structure naturally account for the location of X-ray features in the Crab. We
also discuss the origin of the high energy ions and their acceleration in the
equatorial current sheet of the pulsar wind.Comment: 13 pages, 4 figures, accepted to ApJ. High-resolution figures and
mpeg movies available at http://astron.berkeley.edu/~anatoly/wisp
The relation of maturity to total yield and seasonal distribution of yield in orchardgrass (Dactylis glomerata L.)
Orchardgrass (Dactylis glomerata L.) is grown in the United States from the Canadian border to the northern parts of the Gulf states (8). Although Tennessee is near the southern boundary of orchardgrass adaptation, it ranks seventh among the states with 265,000 acres. Yields of orchardgrass per acre tend to be lower near the southern boundary of its adaptation.
Seasonal distribution of orchardgrass production is not uniform. In Tennessee the most rapid growth occurs in the spring and early summer. Rate of growth is slow in late summer and then increases in the fall. It would be desirable to have an orchardgrass which would produce more during late summer.
Orchardgrass is often grown in association with perennial legumes such as alfalfa. Since the orchardgrass varieties grown in Tennessee mature earlier than these legumes, it is difficult to get maximum yields of high quality forage from orchardgrass-legume mixtures. A later maturing variety of orchardgrass is needed.
The objective of this study was to determine the relationship in orchardgrass of maturity (time of anthesis) to total season production and seasonal distribution of production
Dark Matter Halo Mergers I: Dependence on Environment & Redshift Evolution
This paper presents a study of the specific merger rate as a function of
group membership, local environment, and redshift in a very large, , cosmological N-body simulation, the \textit{Millennium Simulation}. The
goal is to provide environmental diagnostics of major merger populations in
order to test simulations against observations and provide further constraints
on major merger driven galaxy evolution scenarios. A halo sample is defined
using the maximum circular velocity, which is both well defined for subhalos
and closely correlated with galaxy luminosity. Subhalos, including the
precursors of major mergers, are severely tidally stripped. Major mergers
between subhalos are therefore extremely rare. Tidal stripping also suppresses
dynamical friction, resulting in long major merger time scales when the more
massive halo does not host other subhalos. In contrast, when other subhalos are
present major merger time scales are several times shorter. This enhancement is
likely due to inelastic unbound collisions between subhalos. Following these
results, we predict that major mergers in group environments are dominated by
mergers involving the central galaxy, that the specific merger rate is
suppressed in groups, and that the frequency of fainter companions is enhanced
for mergers and their remnants. We also observe an `assembly bias' in the major
merger rate in that mergers of galaxy-like halos are slightly suppressed in
overdense environments while mergers of group-like halos are slightly enhanced.
A dynamical explanation for this trend is advanced which calls on both tidal
effects and interactions between bound halos beyond the virial radii of locally
dynamically dominant halos.Comment: 44 pages, 8 figures, Preprint Submitted to Ap
Birth Kick Distributions and the Spin-Kick Correlation of Young Pulsars
Evidence from pulsar wind nebula symmetry axes and radio polarization
observations suggests that pulsar motions correlate with the spin directions.
We assemble this evidence for young isolated pulsars and show how it can be
used to quantitatively constrain birth kick scenarios. We illustrate by
computing several plausible, but idealized, models where the momentum thrust is
proportional to the neutrino cooling luminosity of the proto-neutron star. Our
kick simulations include the effects of pulsar acceleration and spin-up and our
maximum likelihood comparison with the data constrains the model parameters.
The fit to the pulsar spin and velocity measurements suggests that: i) the
anisotropic momentum required amounts to ~10% of the neutrino flux, ii) while a
pre-kick spin of the star is required, the preferred magnitude is small
10-20rad/s, so that for the best-fit models iii) the bulk of the spin is
kick-induced with ~120rad/s and iv) the models suggest that the
anisotropy emerges on a timescale ~1-3s.Comment: 37 pages, 13 figures, ApJ accepte
Valuing the biodiversity gains from protecting native plant communities from bitou bush (Chrysanthemoides monilifera subsp rotundata (DC.) T.Norl.) in New South Wales: application of the defensive expenditure method
Valuation of the gains from protection of biodiversity is difficult because the services that provide the benefits do not normally pass through markets where prices can form. But the services sometimes pass through markets where consumers or producers behave in a market-oriented manner, and so the values implicit in this behaviour can be identified and derived. Estimates of the benefits of biodiversity protection are derived from the costs of protecting native plant communities from a major weed in Australia, by following this approach. In 1999, invasion of coastal areas of New South Wales by bitou bush (Chrysanthemoides monilifera subsp. rotundata (DC.) T. Norl.) was listed as a key process threatening native plants under the NSW Threatened Species Conservation Act 1995. In accordance with the Act, the Department of Environment and Climate Change prepared a Threat Abatement Plan (TAP) to reduce the impacts of bitou bush on biodiversity at each threatened site. The costs of protecting sites vary closely with the number of priority native species and communities at each site. Following standard economic assumptions about market transactions, these costs are interpreted to provide values the benefits of protecting extra species, communities, and sites. Key words: Bitou bush, Chrysanthemoides monilifera, threat abatement plan, valuation of biodiversity, benefit-cost analysis, weed control, defensive-expenditure method.Bitou bush, Chrysanthemoides monilifera, threat abatement plan, valuation of biodiversity, benefit-cost analysis, weed control, defensive-expenditure method, Demand and Price Analysis, Environmental Economics and Policy,
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