11,012 research outputs found
A kiloparsec-scale nuclear stellar disk in the milky way as a possible explanation of the high velocity peaks in the galactic bulge
The Apache Point Observatory Galactic Evolution Experiment has measured the stellar velocities of red giant stars in the inner Milky Way. We confirm that the line of sight velocity distributions (LOSVDs) in the mid-plane exhibit a second peak at high velocities, whereas those at | b| =2^\circ do not. We use a high resolution simulation of a barred galaxy, which crucially includes gas and star formation, to guide our interpretation of the LOSVDs. We show that the data are fully consistent with the presence of a thin, rapidly rotating, nuclear disk extending to ∼1 kpc. This nuclear disk is orientated perpendicular to the bar and is likely to be composed of stars on x2 orbits. The gas in the simulation is able to fall onto such orbits, leading to stars populating an orthogonal disk
Bloch-Redfield equations for modeling light-harvesting complexes
We challenge the misconception that Bloch-Redfield equations are a less
powerful tool than phenomenological Lindblad equations for modeling exciton
transport in photosynthetic complexes. This view predominantly originates from
an indiscriminate use of the secular approximation. We provide a detailed
description of how to model both coherent oscillations and several types of
noise, giving explicit examples. All issues with non-positivity are overcome by
a consistent straightforward physical noise model. Herein also lies the
strength of the Bloch-Redfield approach because it facilitates the analysis of
noise-effects by linking them back to physical parameters of the noise
environment. This includes temporal and spatial correlations and the strength
and type of interaction between the noise and the system of interest. Finally
we analyze a prototypical dimer system as well as a 7-site Fenna-Matthews-Olson
(FMO) complex in regards to spatial correlation length of the noise, noise
strength, temperature and their connection to the transfer time and transfer
Composition and degradation of salp fecal pellets: Implications for vertical flux in oceanic environments
Changes in the sinking rates, ash-free dry weights, particulate carbon and nitrogen content, and carbon:nitrogen ratios from the fecal pellets of several species of oceanic salps were examined in ten-day decomposition studies. Although bacteria and protozoa became abundant in the incubation vessels, most of the fecal pellets remained physically intact throughout the study. Bacterial activity in the pellets (measured by the rate of uptake of 3H-thymidine) increased, but microbial degradation had little effect on the sinking speeds of most of the fecal pellets. The average losses of ash-free dry weight and carbon and nitrogen content, along with changes in carbon:nitrogen ratio, were small compared to their initial values. We conclude that microbial degradation of large salp fecal pellets would not prevent the vertical flux to the deep ocean of a significant fraction of the particulate organic material contained in the pellets. The fecal pellets of oceanic salps provide a rapid, and potentially important, mechanism for the consolidation and vertical transport of organic and lithogenic material associated with minute particles in the open ocean
Competition in the 1990s
https://deepblue.lib.umich.edu/bitstream/2027.42/154101/1/cole-competition1994.pd
The tilting rate of the Milky Way's disc
We present tilting rates for galaxies comparable to the Milky Way (MW) in a Λ cold dark matter cosmological hydrodynamical simulation, and compare these with the predicted tilting rate detection limit of the Gaia satellite 0.28° Gyr−1. We first identify galaxies with mass comparable to the MW (9 × 1011 ≤ M200 ≤ 1.2 × 1012 M⊙) and consider the tilting rates between z = 0.3 and 0. This sample yields a tilting rate of 7.6° ± 4.5° Gyr−1. We constrain our sample further to exclude any galaxies that have high stellar accretion during the same time. We still find significant tilting, with an average rate of 6.3° Gyr−1. Both subsamples tilt with rates significantly above Gaia's predicted detection limit. We show that our sample of galaxies covers a wide range of environments, including some similar to the MW's. We find galaxies in denser regions tilt with higher rates then galaxies in less dense regions. We also find correlations between the angular misalignment of the hot gas corona and the tilting rate. Gaia is likely to be able to directly measure tilting in the MW. Such a detection will provide an important constraint on the environment of the MW, including the rate of gas cooling on to the disc, the shape and orientation of its dark matter halo, and the mass of the Large Magellanic Cloud. Conversely, failure to detect tilting may suggest the MW is in a very quiet configuration
High-Redshift Galaxies in Cold Dark Matter Models
We use hydrodynamic cosmological simulations to predict the star formation
properties of high-redshift galaxies (z=2-6) in five variants of the
inflationary cold dark matter scenario, paying particular attention to z=3, the
redshift of the largest "Lyman-break galaxy" (LBG) samples. Because we link the
star formation timescale to the local gas density, the rate at which a galaxy
forms stars is governed mainly by the rate at which it accretes cooled gas from
the surrounding medium. At z=3, star formation in most of the simulated
galaxies is steady on 200 Myr timescales, and the instantaneous star formation
rate (SFR) is correlated with total stellar mass. However, there is enough
scatter in this correlation that a sample selected above a given SFR threshold
may contain galaxies with a fairly wide range of masses. The redshift history
and global density of star formation in the simulations depend mainly on the
amplitude of mass fluctuations in the underlying cosmological model. The three
models whose mass fluctuation amplitudes agree with recent analyses of the
Lyman-alpha forest also reproduce the observed luminosity function of LBGs
reasonably well, though the dynamic range of the comparison is small and the
theoretical and observational uncertainties are large. The models with higher
and lower amplitudes appear to predict too much and too little star formation,
respectively, though they are not clearly ruled out. The intermediate amplitude
models predict SFR ~ 30-40 Msun/yr for galaxies with a surface density 1 per
arcmin^2 per unit redshift at z=3. They predict much higher surface densities
at lower SFR, and significant numbers of galaxies with SFR > 10 Msun/yr at z >=
5.Comment: Submitted to ApJ. 31 pages including 10 ps figures. Full resolution
version of Fig 2 available at
http://www.astronomy.ohio-state.edu/~dhw/Sph/zgal.fig2.ps.g
Observed and Expected Mortality in Cohort Studies
Epidemiologists often compare the observed number of deaths in a cohort with the expected number of deaths, obtained by multiplying person-time accrued in the cohort by mortality rates for a reference population (ideally, a reference that represents the mortality rate in the cohort in the absence of exposure). However, if exposure is hazardous (or salutary), this calculation will not consistently estimate the number of deaths expected in the absence of exposure because exposure will have affected the distribution of person-time observed in the study cohort. While problems with interpretation of this standard calculation of expected counts were discussed more than 2 decades ago, these discussions had little impact on epidemiologic practice. The logic of counterfactuals may help clarify this topic as we revisit these issues. In this paper, we describe a simple way to consistently estimate the expected number of deaths in such settings, and we illustrate the approach using data from a cohort study of mortality among underground miners
- …