3,782 research outputs found
Prevalence and Properties of Dark Matter in Elliptical Galaxies
Given the recently deduced relationship between X-ray temperatures and
stellar velocity dispersions (the "T-sigma relation") in an optically complete
sample of elliptical galaxies (Davis & White 1996), we demonstrate that L>L_*
ellipticals contain substantial amounts of dark matter in general. We present
constraints on the dark matter scale length and on the dark-to-luminous mass
ratio within the optical half-light radius and within the entire galaxy. For
example, we find that minimum values of dark matter core radii scale as r_dm >
4(L_V/3L_*)^{3/4}h^{-1}_80 kpc and that the minimum dark matter mass fraction
is >~20% within one optical effective radius r_e and is >~39-85% within 6r_e,
depending on the stellar density profile and observed value of beta_spec. We
also confirm the prediction of Davis & White (1996) that the dark matter is
characterized by velocity dispersions that are greater than those of the
luminous stars: sigma_dm^2 ~ 1.4-2 sigma_*^2. The T-sigma relation implies a
nearly constant mass-to-light ratio within six half-light radii: M/L_V ~ 25h_80
M_sun/L_V_sun. This conflicts with the simplest extension of CDM theories of
large scale structure formation to galactic scales; we consider a couple of
modifications which can better account for the observed T-sigma relation.Comment: 27 pages AASTeX; 15 PostScript figures; to appear in Ap
Constraints on Type Ia Supernova Models from X-ray Spectra of Galaxy Clusters
We present constraints on theoretical models of Type Ia supernovae using
spatially resolved ASCA X-ray spectroscopy of three galaxy clusters: Abell 496,
Abell 2199 and Abell 3571. All three clusters have central iron abundance
enhancements; an ensemble of abundance ratios are used to show that most of the
iron in the central regions of the clusters comes from SN Ia. These
observations are consistent with the suppressed galactic wind scenario proposed
by Dupke and White (1999). At the center of each cluster, simultaneous analysis
of spectra from all ASCA instruments shows that the nickel to iron abundance
ratio (normalized by the solar ratio) is Ni/Fe ~ 4. We use the nickel to iron
ratio as a discriminator between SN Ia explosion models: the Ni/Fe ratio of
ejecta from the "Convective Deflagration" model W7 is consistent with the
observations, while those of "delayed detonation" models are not consistent at
the 90% confidence level.Comment: 20 pages, 2 figures, accepted by The Astrophysical Journa
Chemical enrichment of the complex hot ISM of the Antennae Galaxies: II. Physical properties of the hot gas and supernova feedback
We investigate the physical properties of the interstellar medium (ISM) in
the merging pair of galaxies known as The Antennae (NGC 4038/39), using the
deep coadded ~411 ks Chandra ACIS-S data set. The method of analysis and some
of the main results from the spectral analysis, such as metal abundances and
their variations from ~0.2 to ~20-30 times solar, are described in Paper I
(Baldi et al. submitted). In the present paper we investigate in detail the
physics of the hot emitting gas, deriving measures for the hot-gas mass (~10^
M_sun), cooling times (10^7-10^8 yr), and pressure (3.5x10^-11-2.8x10^-10 dyne
cm^-2). At least in one of the two nuclei (NGC 4038) the hot-gas pressure is
significantly higher than the CO pressure, implying that shock waves may be
driven into the CO clouds. Comparison of the metal abundances with the average
stellar yields predicted by theoretical models of SN explosions points to SNe
of Type II as the main contributors of metals to the hot ISM. There is no
evidence of any correlation between radio-optical star-formation indicators and
the measured metal abundances. Although due to uncertainties in the average gas
density we cannot exclude that mixing may have played an important role, the
short time required to produce the observed metal masses (<=2 Myr) suggests
that the correlations are unlikely to have been destroyed by efficient mixing.
More likely, a significant fraction of SN II ejecta may be in a cool phase, in
grains, or escaping in hot winds. In each case, any such fraction of the ejecta
would remain undetectable with soft X-ray observations.Comment: 29 pages, 6 figures, accepted by the Astrophysical Journa
Perspectives on Astrophysics Based on Atomic, Molecular, and Optical (AMO) Techniques
About two generations ago, a large part of AMO science was dominated by
experimental high energy collision studies and perturbative theoretical
methods. Since then, AMO science has undergone a transition and is now
dominated by quantum, ultracold, and ultrafast studies. But in the process, the
field has passed over the complexity that lies between these two extremes. Most
of the Universe resides in this intermediate region. We put forward that the
next frontier for AMO science is to explore the AMO complexity that describes
most of the Cosmos.Comment: White paper submission to the Decadal Assessment and Outlook Report
on Atomic, Molecular, and Optical (AMO) Science (AMO 2020
The Effect of Proton Temperature Anisotropy on the Solar Minimum Corona and Wind
A semi-empirical, axisymmetric model of the solar minimum corona is developed
by solving the equations for conservation of mass and momentum with prescribed
anisotropic temperature distributions. In the high-latitude regions, the proton
temperature anisotropy is strong and the associated mirror force plays an
important role in driving the fast solar wind; the critical point where the
outflow velocity equals the parallel sound speed is reached already at 1.5 Rsun
from Sun center. The slow wind arises from a region with open field lines and
weak anisotropy surrounding the equatorial streamer belt. The model parameters
were chosen to reproduce the observed latitudinal extent of the equatorial
streamer in the corona and at large distance from the Sun. We find that the
magnetic cusp of the closed-field streamer core lies at about 1.95 Rsun. The
transition from fast to slow wind is due to a decrease in temperature
anisotropy combined with the non-monotonic behavior of the non-radial expansion
factor in flow tubes that pass near the streamer cusp. In the slow wind, the
plasma beta is of order unity and the critical point lies at about 5 Rsun, well
beyond the magnetic cusp. The predicted outflow velocities are consistent with
OVI Doppler dimming measurements from UVCS/SOHO. We also find good agreement
with polarized brightness (pB) measurements from LASCO/SOHO and HI Ly-alpha
images from UVCS/SOHO.Comment: 36 pages, 13 figures. AAS LaTeX Macros v5.0. To appear in The
Astrophysical Journal, Vol. 598, No. 2, Issue December 1, 200
Controls on the composition and lability of dissolved organic matter in Siberia's Kolyma River basin
High-latitude northern rivers export globally significant quantities of dissolved organic carbon (DOC) to the Arctic Ocean. Climate change, and its associated impacts on hydrology and potential mobilization of ancient organic matter from permafrost, is likely to modify the flux, composition, and thus biogeochemical cycling and fate of exported DOC in the Arctic. This study examined DOC concentration and the composition of dissolved organic matter (DOM) across the hydrograph in Siberia's Kolyma River, with a particular focus on the spring freshet period when the majority of the annual DOC load is exported. The composition of DOM within the Kolyma basin was characterized using absorbance-derived measurements (absorbance coefficienta330, specific UV absorbance (SUVA254), and spectral slope ratio SR) and fluorescence spectroscopy (fluorescence index and excitation-emission matrices (EEMs)), including parallel factor analyses of EEMs. Increased surface runoff during the spring freshet led to DOM optical properties indicative of terrestrial soil inputs with high humic-like fluorescence, SUVA254, and low SRand fluorescence index (FI). Under-ice waters, in contrast, displayed opposing trends in optical properties representing less aromatic, lower molecular weight DOM. We demonstrate that substantial losses of DOC can occur via biological (∼30% over 28 days) and photochemical pathways (>29% over 14 days), particularly in samples collected during the spring freshet. The emerging view is therefore that of a more dynamic and labile carbon pool than previously thought, where DOM composition plays a fundamental role in controlling the fate and removal of DOC at a pan-Arctic scale
Coronal Diagnostics from Narrowband Images around 30.4 nm
Images taken in the band centered at 30.4 nm are routinely used to map the
radiance of the He II Ly alpha line on the solar disk. That line is one of the
strongest, if not the strongest, line in the EUV observed in the solar
spectrum, and one of the few lines in that wavelength range providing
information on the upper chromosphere or lower transition region. However, when
observing the off-limb corona the contribution from the nearby Si XI 30.3 nm
line can become significant. In this work we aim at estimating the relative
contribution of those two lines in the solar corona around the minimum of solar
activity. We combine measurements from CDS taken in August 2008 with
temperature and density profiles from semiempirical models of the corona to
compute the radiances of the two lines, and of other representative coronal
lines (e.g., Mg X 62.5 nm, Si XII 52.1 nm). Considering both diagnosed
quantities from line ratios (temperatures and densities) and line radiances in
absolute units, we obtain a good overall match between observations and models.
We find that the Si XI line dominates the He II line from just above the limb
up to ~2 R_Sun in streamers, while its contribution to narrowband imaging in
the 30.4 nm band is expected to become smaller, even negligible in the corona
beyond ~2 - 3 R_Sun, the precise value being strongly dependent on the coronal
temperature profile.Comment: 26 pages, 11 figures; to be published in: Solar Physic
Visual onset expands subjective time
We report a distortion of subjective time perception in which the duration of a first interval is perceived to be longer than the succeeding interval of the same duration. The amount of time expansion depends on the onset type defining the first interval. When a stimulus appears abruptly, its duration is perceived to be longer than when it appears following a stationary array. The difference in the processing time for the stimulus onset and motion onset, measured as reaction times, agrees with the difference in time expansion. Our results suggest that initial transient responses for a visual onset serve as a temporal marker for time estimation, and a systematic change in the processing time for onsets affects perceived time
Significance of left ventricular apical-basal muscle bundle identified by cardiovascular magnetic resonance imaging in patients with hypertrophic cardiomyopathy
Aims Cardiovascular magnetic resonance (CMR) has improved diagnostic and management strategies in hypertrophic cardiomyopathy (HCM) by expanding our appreciation for the diverse phenotypic expression. We sought to characterize the prevalence and clinical significance of a recently identified accessory left ventricular (LV) muscle bundle extending from the apex to the basal septum or anterior wall (i.e. apical-basal). Methods and results CMR was performed in 230 genotyped HCM patients (48 ± 15 years, 69% male), 30 genotype-positive/phenotype-negative (G+/P−) family members (32 ± 15 years, 30% male), and 126 controls. Left ventricular apical-basal muscle bundle was identified in 145 of 230 (63%) HCM patients, 18 of 30 (60%) G+/P− family members, and 12 of 126 (10%) controls (G+/P− vs. controls; P < 0.01). In HCM patients, the prevalence of an apical-basal muscle bundle was similar among those with disease-causing sarcomere mutations compared with patients without mutation (64 vs. 62%; P = 0.88). The presence of an LV apical-basal muscle bundle was not associated with LV outflow tract obstruction (P = 0.61). In follow-up, 33 patients underwent surgical myectomy of whom 22 (67%) were identified to have an accessory LV apical-basal muscle bundle, which was resected in all patients. Conclusion Apical-basal muscle bundles are a unique myocardial structure commonly present in HCM patients as well as in G+/P− family members and may represent an additional morphologic marker for HCM diagnosis in genotype-positive statu
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