13,094 research outputs found
Voids in the Local Volume: a limit on appearance of a galaxy in a DM halo
Current explanation of the overabundance of dark matter subhalos in the Local
Group (LG) indicates that there maybe a limit on mass of a halo, which can host
a galaxy. This idea can be tested using voids in the distribution of galaxies:
at some level small voids should not contain any (even dwarf) galaxies. We use
observational samples complete to M_B = -12 with distances less than 8 Mpc to
construct the void function (VF): the distribution of sizes of voids empty of
any galaxies. There are ~30 voids with sizes ranging from 1 to 5 Mpc. We then
study the distribution of dark matter halos in very high resolution simulations
of the LCDM model. The theoretical VF matches the observations remarkably well
only if we use halos with circular velocities larger than 45 +/- 10 km/s. This
agrees with the Local Group predictions. There are smaller halos in the voids,
but they should not produce any luminous matter. Small voids look quite similar
to their giant cousins: the density has a minimum at the center of a void and
it increases as we get closer to the border. Small nonluminous halos inside the
void form a web of tiny filaments. Thus, both the Local Group data and the
nearby voids indicate that isolated halos below 45 +/- 10 km/s must not host
galaxies and that small (few Mpc) voids are truly dark.Comment: 5 pages, 1 figur
Nonadiabatic Dynamics in Open Quantum-Classical Systems: Forward-Backward Trajectory Solution
A new approximate solution to the quantum-classical Liouville equation is
derived starting from the formal solution of this equation in forward-backward
form. The time evolution of a mixed quantum-classical system described by this
equation is obtained in a coherent state basis using the mapping
representation, which expresses quantum degrees of freedom in a
2N-dimensional phase space. The solution yields a simple non-Hamiltonian
dynamics in which a set of coherent state coordinates evolve in forward and
backward trajectories while the bath coordinates evolve under the influence of
the mean potential that depends on these forward and backward trajectories. It
is shown that the solution satisfies the differential form of the
quantum-classical Liouville equation exactly. Relations to other mixed
quantum-classical and semi-classical schemes are discussed.Comment: 28 pages, 1 figur
The Extragalactic Distance Database: Color-Magnitude Diagrams
The CMDs/TRGB (Color-Magnitude Diagrams/Tip of the Red Giant Branch) section
of the Extragalactic Distance Database contains a compilation of observations
of nearby galaxies from the Hubble Space Telescope. Approximately 250 (and
increasing) galaxies in the Local Volume have CMDs and the stellar photometry
tables used to produce them available through the web. Various stellar
populations that make up a galaxy are visible in the CMDs, but our primary
purpose for collecting and analyzing these galaxy images is to measure the TRGB
in each. We can estimate the distance to a galaxy by using stars at the TRGB as
standard candles. In this paper we describe the process of constructing the
CMDs and make the results available to the public.Comment: 8 pages, 5 figures, 1 long table, submitted to Astronomical Journa
The Velocity Field from Type Ia Supernovae Matches the Gravity Field from Galaxy Surveys
We compare the peculiar velocities of nearby SNe Ia with those predicted by
the gravity fields of full sky galaxy catalogs. The method provides a powerful
test of the gravitational instability paradigm and strong constraints on the
density parameter beta = Omega^0.6/b. For 24 SNe Ia within 10,000 km/s we find
the observed SNe Ia peculiar velocities are well modeled by the predictions
derived from the 1.2 Jy IRAS survey and the Optical Redshift Survey (ORS). Our
best is 0.4 from IRAS, and 0.3 from the ORS, with beta>0.7 and
beta<0.15 ruled out at 95% confidence levels from the IRAS comparison.
Bootstrap resampling tests show these results to be robust in the mean and in
its error. The precision of this technique will improve as additional nearby
SNe Ia are discovered and monitored.Comment: 16 pages (LaTex), 3 postscript figure
The pathological role of acrolein in experimental autoimmune encephalomyelitis and multiple sclerosis
Multiple sclerosis (MS) is an autoimmune demyelinating neuropathy that affects nearly 2.5 million people worldwide. Despite substantial efforts, few treatments are currently available largely due to limited knowledge of pathogenic mechanisms underlying the disease. The immune-inflammatory nature of the pathology has prompted investigation of the role of oxidative stress in disease development and progression; however targeting reactive oxygen species for neutralization has had marginal success therapeutically, suggesting that an alternate oxidative stress-related target would prove beneficial. Recently, our lab has implicated acrolein, a highly reactive aldehyde that is both a byproduct and catalyst of lipid peroxidation, as a potential therapeutic target and biomarker for MS diagnosis and symptom monitoring. We have shown that acrolein is elevated in clinical MS cases and experimental autoimmune encephalomyelitis (EAE), a murine model of MS. Furthermore, pharmacological sequestering of acrolein afforded a neuroprotective effect by suppressing tissue acrolein level, slowing disease progression, and decreasing symptom severity. Acrolein can also be produced exogenously as a pollutant from combustion engine exhaust, industrial processing, burning of tobacco and overheated cooking oil vapors. The pathogenic role of endogenous acrolein in MS raises the possibility that environmental exposure to acrolein could potentially increase MS risk or exacerbate MS symptoms. Using a respiratory exposure model in combination with urinary detection of an acrolein metabolite and immunoblotting assessment of tissue acrolein-lysine adducts, we have ascertained that inhalation of acrolein can cause accumulation of acrolein in mice systemically and locally within the CNS. Additionally clinical acrolein assessment using urine and serum samples revealed that MS patients who self-reported as smokers demonstrated higher systemic acrolein levels and demonstrated greater motor deficit compared to MS patients that did not smoke. These observations indicate that acrolein is likely contributing to the mechanisms underlying symptom development in EAE and MS and may serve as a therapeutic target and biomarker for diagnosis, guiding treatment regimens and monitoring relapses
The LCO/Palomar 10,000 km/sec Cluster Survey. I. Properties of the Tully-Fisher Relation
The first results from a Tully-Fisher (TF) survey of cluster galaxies are
presented. The galaxies are drawn from fifteen Abell clusters that lie in the
redshift range 9000-12,000 km/sec and are distributed uniformly around the
celestial sky. The data set consists of R-band CCD photometry and long- slit
H-alpha spectroscopy. The rotation curves (RCs) are characterized by a turnover
radius (r_t) and an asymptotic velocity v_a, while the surface brightness
profiles are characterized in terms of an effective exponential surface
brightness I_e and a scale length r_e. The TF scatter is minimized when the
rotation velocity is measured at 2.0 +/- 0.2 r_e; a significantly larger
scatter results when the rotation velocity is measured at > 3 or < 1.5 scale
lengths. This effect demonstrates that RCs do not have a universal form, as has
been suggested by Persic, Salucci, and Stel. In contrast to previous studies, a
modest but statistically significant surface-brightness dependence of the TF
relation is found, log v = const + 0.28*log L + 0.14*log I_e. This indicates a
stronger parallel between the TF relation and the FP relations of elliptical
galaxies than has previously been recognized. Future papers in this series will
consider the implications of this cluster sample for deviations from Hubble
flow on 100-200 Mpc scales.Comment: 35 pages, 8 figures, uses aaspp4.sty. Submitted to ApJ. Also
available at http://astro.stanford.edu/jeff
Distances from the Correlation between Galaxy Luminosities and Rotation Rates
A large luminosity--linewidth template sample is now available, improved
absorption corrections have been derived, and there are a statistically
significant number of galaxies with well determined distances to supply the
zero point. A revised estimate of the Hubble Constant is H_0=77 +-4 km/s/Mpc
where the error is the 95% probability statistical error. Systematic
uncertainties are potentially twice as large.Comment: 21 pages, 9 figures. Invited chapter for the book `Post-Hipparcos
Cosmic Candles', Eds. F. Caputo and A. Heck (Kluwer Academic Publishers,
Dordrecht
The Structure of Dark Matter Haloes in Dwarf Galaxies
Recent observations indicate that dark matter haloes have flat central
density profiles. Cosmological simulations with non-baryonic dark matter
predict however self similar haloes with central density cusps. This
contradiction has lead to the conclusion that dark matter must be baryonic.
Here it is shown that the dark matter haloes of dwarf spiral galaxies represent
a one parameter family with self similar density profiles. The observed global
halo parameters are coupled with each other through simple scaling relations
which can be explained by the standard cold dark matter model if one assumes
that all the haloes formed from density fluctuations with the same primordial
amplitude. We find that the finite central halo densities correlate with the
other global parameters. This result rules out scenarios where the flat halo
cores formed subsequently through violent dynamical processes in the baryonic
component. These cores instead provide important information on the origin and
nature of dark matter in dwarf galaxies.Comment: uuencoded Z-compressed postscript file, 10 pages, 3 figures included,
to appear in ApJ Letter
Coherent dynamics of photoinduced nucleation processes
We study the dynamics of initial nucleation processes of photoinduced
structural change of molecular crystals. In order to describe the nonadiabatic
transition in each molecule, we employ a model of localized electrons coupled
with a fully quantized phonon mode, and the time-dependent Schr\"odinger
equation for the model is numerically solved. We found a minimal model to
describe the nucleation induced by injection of an excited state of a single
molecule in which multiple types of intermolecular interactions are required.
In this model coherently driven molecular distortion plays an important role in
the successive conversion of electronic states which leads to photoinduced
cooperative phenomena.Comment: 14 pages, 5 figure
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