2,755 research outputs found
Description of Langley low-frequency noise facility and study of human response to noise frequencies below 50 cps
Test facility for determining human performance in very low frequency noise environment - physiological and psychological response
Response-to-noise studies of some aircraft and spacecraft structures
Fatigue and response to noise of aircraft and spacecraft viscoelastic panels, shell structure, and payload
Description and research capabilities of the Langley Low Frequency Noise Facility
Description and research capabilities of low frequency noise facilit
Mapping the cold molecular gas in a cooling flow cluster: Abell 1795
Cold molecular gas is found in several clusters of galaxies (Edge, 2001,
Salome' & Combes, 2003): single dish telescope observations in CO(1-0) and
CO(2-1) emission lines have revealed the existence of large amounts of cold gas
(up to ~10^11 Msol) in the central region of cooling flow clusters. We present
here interferometric observations performed with the IRAM Plateau de Bure
interferometer in Abell 1795. Comparison with IRAM 30m data shows the cold gas
detected is extended suggesting a cooling flow origin. The CO features
identified are very similar to the structures observed in Halpha and with the
star forming regions observed through UV continuum excess. A large fraction of
the cold gas is not centered on the central cD, but located near brightest
X-ray emitting regions along the North-West orientated radio lobe. The cold gas
kinematics is consistent with the optical nebulosity behaviour in the very
central region. It is not in rotation around the central cD : a velocity
gradient shows the cold gas might be cooled gas from the intra-cluster medium
being accreted by the central galaxy. The optical filaments, aligned with the
cD orbit, are intimately related to the radio jets and lobes. The material
fueling the star formation certainly comes from the deposited gas, cooling more
efficiently along the edge of the radio lobes. Even if some heating mechanisms
are present, these millimetric observations show that an effective cooling to
very low temperatures indeed occurs and is probably accelerated by the presence
of the radio source.Comment: 4 pages, 4 figures, accepted for publication in A&A (Letter
Normalizing the Temperature Function of Clusters of Galaxies
We re-examine the constraints which can be robustly obtained from the
observed temperature function of X-ray cluster of galaxies. The cluster mass
function has been thoroughly studied in simulations and analytically, but a
direct simulation of the temperature function is presented here for the first
time. Adaptive hydrodynamic simulations using the cosmological Moving Mesh
Hydro code of Pen (1997a) are used to calibrate the temperature function for
different popular cosmologies. Applying the new normalizations to the
present-day cluster abundances, we find for a hyperbolic universe, and for a spatially flat universe with a cosmological constant.
The simulations followed the gravitational shock heating of the gas and dark
matter, and used a crude model for potential energy injection by supernova
heating. The error bars are dominated by uncertainties in the heating/cooling
models. We present fitting formulae for the mass-temperature conversions and
cluster abundances based on these simulations.Comment: 20 pages incl 5 figures, final version for ApJ, corrected open
universe \gamma relation, results unchange
Non-hydrostatic gas in the core of the relaxed galaxy cluster A1795
Chandra data on A1795 reveal a mild edge-shaped discontinuity in the gas
density and temperature in the southern sector of the cluster at r=60/h kpc.
The gas inside the edge is 1.3-1.5 times denser and cooler than outside, while
the pressure is continuous, indicating that this is a "cold front", the surface
of contact between two moving gases. The continuity of the pressure indicates
that the current relative velocity of the gases is near zero, making the edge
appear to be in hydrostatic equilibrium. However, a total mass profile derived
from the data in this sector under the equilibrium assumption, exhibits an
unphysical jump by a factor of 2, with the mass inside the edge being lower. We
propose that the cooler gas is "sloshing" in the cluster gravitational
potential well and is now near the point of maximum displacement, where it has
zero velocity but nonzero centripetal acceleration. The distribution of this
non-hydrostatic gas should reflect the reduced gravity force in the
accelerating reference frame, resulting in the apparent mass discontinuity.
Assuming that the gas outside the edge is hydrostatic, the acceleration of the
moving gas can be estimated from the mass jump, a ~ 800 h km/s/(10^8 yr). The
gravitational potential energy of this gas that is available for dissipation is
about half of its current thermal energy. The length of the cool filament
extending from the cD galaxy (Fabian et al.) may give the amplitude of the gas
sloshing, 30-40/h kpc. Such gas bulk motion might be caused by a disturbance of
the central gravitational potential by past subcluster infall.Comment: Minor text clarifications to correspond to published version. 5
pages, 1 figure in color, uses emulateapj.sty. ApJ Letters in pres
Discovery of the Central Excess Brightness in Hard X-rays in the Cluster of Galaxies Abell 1795
Using the X-ray data from \ASCA, spectral and spatial properties of the
intra-cluster medium (ICM) of the cD cluster Abell 1795 are studied, up to a
radial distance of ( kpc). The ICM
temperature and abundance are spatially rather constant, although the cool
emission component is reconfirmed in the central region. The azimuthally-
averaged radial X-ray surface brightness profiles are very similar between soft
(0.7--3 keV) and hard (3--10 keV) energy bands, and neither can be fitted with
a single- model due to a strong data excess within of the
cluster center. In contrast, double- models can successfully reproduce
the overall brightness profiles both in the soft and hard energy bands, as well
as that derived with the \ROSAT PSPC. Properties of the central excess
brightness are very similar over the 0.2--10 keV energy range spanned by \ROSAT
and \ASCA. Thus, the excess X-ray emission from the core region of this cluster
is confirmed for the first time in hard X-rays above 3 keV. This indicates that
the shape of the gravitational potential becomes deeper than the King-type one
towards the cluster center. Radial profiles of the total gravitating matter,
calculated using the double- model, reveal an excess mass of within kpc of the cluster
center. This suggests a hierarchy in the gravitational potential corresponding
to the cD galaxy and the entire cluster.Comment: 27 pages, 8 figures; to appear ApJ 500 (June 20, 1998
A Chandra Study of the Complex Structure in the Core of 2A 0335+096
We present a Chandra observation of the central (r 40 kpc), the X-ray surface brightness is symmetric and slightly elliptical. The cluster has a cool, dense core; the radial temperature gradient varies with position angle. The radial metallicity profile shows a pronounced central drop and an off-center peak. Similarly to many clusters with dense cores, 2A 0335+096 hosts a cold front at r ≈ 40 kpc south of the center. The gas pressure across the front is discontinuous by a factor AP = 1.6 ± 0.3, indicating that the cool core is moving with respect to the ambient gas with a Mach number M ≈ 0.75 ± 0.2. The central dense region inside the cold front shows an unusual X-ray morphology, which consists of a number of X-ray blobs and/or filaments on scales 3 kpc, along with two prominent X-ray cavities. The X-ray blobs are not correlated with either the optical line emission (Hα+[N II]), member galaxies, or radio emission. The deprojected temperature of the dense blobs is consistent with that of the less dense ambient gas, so these gas phases do not appear to be in thermal pressure equilibrium. An interesting possibility is a significant, unseen nonthermal pressure component in the interblob gas, possibly arising from the activity of the central active galactic nucleus (AGN). We discuss two models for the origin of the gas blobs—hydrodynamic instabilities caused by the observed motion of the gas core and "bubbling" of the core caused by multiple outbursts of the central AGN
A Very Hot, High Redshift Cluster of Galaxies: More Trouble for Omega_0 = 1
We have observed the most distant (z=0.829) cluster of galaxies in the
Einstein Extended Medium Sensitivity Survey, with the ASCA and ROSAT
satellites. We find an X-ray temperature of 12.3 +3.1/-2.2 keV for this
cluster, and the ROSAT map reveals significant substructure. The high
temperature of MS1054-0321 is consistent with both its approximate velocity
dispersion, based on the redshifts of 12 cluster members we have obtained at
the Keck and the Canada-France-Hawaii telescopes, and with its weak lensing
signature. The X-ray temperature of this cluster implies a virial mass ~ 7.4 x
10^14 h^-1 solar masses, if the mean matter density in the universe equals the
critical value, or larger if Omega_0 < 1. Finding such a hot, massive cluster
in the EMSS is extremely improbable if clusters grew from Gaussian
perturbations in an Omega_0 = 1 universe. Combining the assumptions that
Omega_0 = 1 and that the intial perturbations were Gaussian with the observed
X-ray temperature function at low redshift, we show that the probability of
this cluster occurring in the volume sampled by the EMSS is less than a few
times 10^{-5}. Nor is MS1054-0321 the only hot cluster at high redshift; the
only two other EMSS clusters already observed with ASCA also have
temperatures exceeding 8 keV. Assuming again that the initial perturbations
were Gaussian and Omega_0 = 1, we find that each one is improbable at the <
10^{-2} level. These observations, along with the fact that these luminosities
and temperatures of the high- clusters all agree with the low-z L_X-T_X
relation, argue strongly that Omega_0 < 1. Otherwise, the initial perturbations
must be non-Gaussian, if these clusters' temperatures do indeed reflect their
gravitational potentials.Comment: 20 pages, 4 figures, To appear in 1 Aug 1998 ApJ (heavily revised
version of original preprint
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