3,784 research outputs found
Copernicus observations of C I and CO in diffuse interstellar clouds
Copernicus was used to observe absorption lines of C I in its ground state and excited fine structure levels and CO toward 29 stars. We use the C I data to infer densities and pressures within the observed clouds, and because our results are of higher precision than previous work, much more precise estimates of the physical conditions in clouds are obtained. In agreement with previous work, the interstellar thermal pressure appears to be variable, with most clouds having values of p/k between 1000/cu cm K and 10,000/cu cm K, but there are some clouds with p/k as high as 100,000/cu cm K. Our results are consistent with the view that the interstellar thermal pressure is so variable that the gas undergoes continuous dynamic evolution. Our observations provide useful constraints on the physical processes on the surfaces of grains. In particular, we find that grains are efficient catalysts of interstellar H2 in the sense that at least half of the hydrogen atoms that strike grains come off as part of H2. Results place strong constraints on models for the formation and destruction of interstellar CO. In many clouds, an order of magnitude less CO than predicted in some models was found
The Nature of Dark Matter in Elliptical Galaxies: Chandra Observations of NGC 4636
We determine the total enclosed mass profile from 0.7 to 35 kpc in the
elliptical galaxy NGC 4636 based on the hot interstellar medium temperature
profile measured using the Chandra X-ray Observatory, and other X-ray and
optical data. The total mass increases as radius to the power 1.2 to a good
approximation over this range in radii, attaining a total mass-to-light ratio
of 40 solar masses per solar visual luminosity at 35 kpc. We find that at least
half, and as much as 80%, of the mass within the optical half-light radius is
non-luminous, implying an exceptionally low baryon fraction in NGC 4636. The
large inferred dark matter concentration and central dark matter density,
consistent with the upper end of the range expected for standard cold dark
matter halos, imply that mechanisms proposed to explain low dark matter
densities in less massive galaxies are not effective in elliptical galaxies.Comment: 4 pages, including 3 embedded figures, to appear in the Proceedings
of the 5th International Symposium on Sources and Detection of Dark Matter
and Dark Energy in the Universe (Dark Matter 2002); Nuclear Physics B, in
pres
Groups and the Entropy Floor- XMM-Newton Observations of Two Groups
Using XMM-Newton spatially resolved X-ray imaging spectroscopy we obtain the
temperature, density, entropy, gas mass, and total mass profiles for two groups
of galaxies out to ~0.3 Rvir (Rvir, the virial radius). Our density profiles
agree well with those derived previously, and the temperature data are broadly
consistent with previous results but are considerably more precise. Both of
these groups are at the mass scale of 2x10^13 Msolar but have rather different
properties. They have considerably lower gas mass fractions at r<0.3 Rvir than
the rich clusters. NGC2563, one of the least luminous groups for its X-ray
temperature, has a very low gas mass fraction of ~0.004 inside 0.1 Rvir, which
rises with radius. NGC4325, one of the most luminous groups at the same average
temperature, has a higher gas mass fraction of 0.02. The entropy profiles and
the absolute values of the entropy as a function of virial radius also differ,
with NGC4325 having a value of ~100 keV cm-2 and NGC2563 a value of ~300 keV
cm-2 at r~0.1 Rvir. For both groups the profiles rise monotonically with radius
and there is no sign of an entropy "floor". These results are inconsistent with
pre-heating scenarios which have been developed to explain the entropy floor in
groups but are broadly consistent with models of structure formation which
include the effects of heating and/or the cooling of the gas. The total entropy
in these systems provides a strong constraint on all models of galaxy and group
formation, and on the poorly defined feedback process which controls the
transformation of gas into stars and thus the formation of structure in the
universe.Comment: 22 pages, 2 figure
RXTE Hard X-ray Observation of A754: Constraining the Hottest Temperature Component and the Intracluster Magnetic Field
Abell 754, a cluster undergoing merging, was observed in hard X-rays with the
Rossi X-ray Timing Explorer (RXTE) in order to constrain its hottest
temperature component and search for evidence of nonthermal emission.
Simultaneous modeling of RXTE data and those taken with previous missions
yields an average intracluster temperature of keV in the 1-50 keV
energy band. A multi-temperature component model derived from numerical
simulations of the evolution of a cluster undergoing a merger produces similar
quality of fit, indicating that the emission measure from the very hot gas
component is sufficiently small that it renders the two models
indistinguishable. No significant nonthermal emission was detected. However,
our observations set an upper limit of
(90% confidence limit) to the nonthermal emission flux at 20 keV. Combining
this result with the radio synchrotron emission flux we find a lower limit of
0.2 G for the intracluster magnetic field. We discuss the implications of
our results for the theories of magnetic field amplifications in cluster
mergers.Comment: Accepted for Publication in the Astrophysical Journal, 22 pages, 5
figure
Strong Turbulence in the Cool Cores of Galaxy Clusters: Can Tsunamis Solve the Cooling Flow Problem?
Based on high-resolution two-dimensional hydrodynamic simulations, we show
that the bulk gas motions in a cluster of galaxies, which are naturally
expected during the process of hierarchical structure formation of the
universe, have a serous impact on the core. We found that the bulk gas motions
represented by acoustic-gravity waves create local but strong turbulence, which
reproduces the complicated X-ray structures recently observed in cluster cores.
Moreover, if the wave amplitude is large enough, they can suppress the
radiative cooling of the cores. Contrary to the previous studies, the heating
is operated by the turbulence, not weak shocks. The turbulence could be
detected in near-future space X-ray missions such as ASTRO-E2.Comment: Movies are available at http://th.nao.ac.jp/tsunami/index.ht
Southern Hemispheric nitrous oxide measurements obtained during 1987 airborne Antarctic ozone experiment
The chemical lifetime of N2O is about 150 years, which makes it an excellent dynamical tracer of air motion on the time scale of the ozone depletion event. For these reasons it was chosen to help test whether dynamical theories of ozone loss over Antarctica were plausible, particularly the theory that upwelling ozone-poor air from the troposphere was replacing ozone-rich stratospheric air. The N2O measurements were made with the Airborne Tunable Laser Absorption Spectrometer (ATLAS) aboard the NASA ER-2 aircraft. The detection technique involves measuring the diffential absorption of the IR laser radiation as it is rapidly scanned over an N2O absorption feature. For the AAOE mission, the instrument was capable of making measurements with a 1 ppb sensitivity, 1 second response time, over an altitude range of 10 to 20 kilometers. The AAOE mission consisted of a series of 12 flights from Punta Arenas (53S) into the polar vortex (approximately 72S) at which time a vertical profile from 65 to 45 km and back was performed. Comparison of the observed profiles inside the vortex with N2O profiles obtained by balloon flights during the austral summer showed that an overall subsidence had occurred during the winter of about 5 to 6 km. Also, over the course of the mission (mid-August to late September), no trend in the N2O vertical profile, either upward or downward, was discernible, eliminating the possibility that upwelling was the cause of the observed ozone decrease
X-ray Evidence for Spectroscopic Diversity of Type Ia Supernovae: XMM observation of the elemental abundance pattern in M87
We present the results of a detailed element abundance study of hot gas in
M87, observed by XMM-Newton. We choose two radial bins, 1'-3' and 8'-16'
(8'-14' for EMOS; hereafter the central and the outer zones), where the
temperature is almost constant, to carry out the detailed abundance
measurements of O, Ne, Mg, Si, S, Ar, Ca, Fe and Ni using EPIC-PN (EPN) and
-MOS (EMOS) data. First, we find that the element abundance pattern in the
central compared to the outer zone in M87 is characterized by SN Ia enrichment
of a high (roughly solar) ratio of Si-group elements (Si, S, Ar, Ca) to Fe,
implying that Si burning in SN Ia is highly incomplete. In nucleosynthesis
modeling this is associated with either a lower density of the
deflagration-detonation transition and/or lower C/O and/or lower central
ignition density and observationally detected as optically subluminous SNe Ia
in early-type galaxies. Second, we find that SN Ia enrichment has a
systematically lower ratio of the Si-group elements to Fe by 0.2 dex in the
outer zone associated with the ICM of the Virgo cluster. We find that such a
ratio and even lower values by another 0.1 dex are a characteristic of the ICM
in many clusters using observed Si:S:Fe ratios as found with ASCA. Third, the
Ni/Fe ratio in the central zone of M87 is 1.5+/-0.3 solar (meteoritic), while
values around 3 times solar are reported for other clusters. In modeling of SN
Ia, this implies a reduced influence of fast deflagration SN Ia models in the
chemical enrichment of M87's ISM. Thus, to describe the SN Ia metal enrichment
in clusters, both deflagration as well as delayed detonation scenarios are
required, supporting a similar conclusion, derived from optical studies on SNe
Ia. Abridged.Comment: 11 pages, A&A, in pres
On Iron Enrichment, Star Formation, and Type Ia Supernovae in Galaxy Clusters
The nature of star formation and Type Ia supernovae (SNIa) in galaxies in the
field and in rich galaxy clusters are contrasted by juxtaposing the build-up of
heavy metals in the universe inferred from observed star formation and
supernovae rate histories with data on the evolution of Fe abundances in the
intracluster medium (ICM). Models for the chemical evolution of Fe in these
environments are constructed, subject to observational constraints, for this
purpose. While models with a mean delay for SNIa of 3 Gyr and standard initial
mass function (IMF) are consistent with observations in the field, cluster Fe
enrichment immediately tracks a rapid, top-heavy phase of star formation --
although transport of Fe into the ICM may be more prolonged and star formation
likely continues to redshifts <1. The source of this prompt enrichment is Type
II supernovae (SNII) yielding at least 0.1 solar masses per explosion (if the
SNIa rate normalization is scaled down from its value in the field according to
the relative number of candidate progenitor stars in the 3-8 solar mass range)
and/or SNIa explosions with short delay times associated with the rapid star
formation mode. Star formation is >3 times more efficient in rich clusters than
in the field, mitigating the overcooling problem in numerical cluster
simulations. Both the fraction of baryons cycled through stars, and the
fraction of the total present-day stellar mass in the form of stellar remnants,
are substantially greater in clusters than in the field.Comment: 51 pages including 26 figures and 2 tables, accepted for publication
in ApJ 5/4/0
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