13,496 research outputs found
Oxygen Absorption in Cooling Flows
The inhomogeneous cooling flow scenario predicts the existence of large
quantities of gas in massive elliptical galaxies, groups, and clusters that
have cooled and dropped out of the flow. Using spatially resolved, deprojected
X-ray spectra from the ROSAT PSPC we have detected strong absorption over
energies ~0.4-0.8 keV intrinsic to the central ~1 arcmin of the galaxy, NGC
1399, the group, NGC 5044, and the cluster, A1795. These systems have amongst
the largest nearby cooling flows in their respective classes and low Galactic
columns. Since no excess absorption is indicated for energies below ~0.4 keV
the most reasonable model for the absorber is warm, collisionally ionized gas
with T=10^{5-6} K where ionized states of oxygen provide most of the
absorption. Attributing the absorption only to ionized gas reconciles the large
columns of cold H and He inferred from Einstein and ASCA with the lack of such
columns inferred from ROSAT, and also is consistent with the negligible atomic
and molecular H inferred from HI, and CO observations of cooling flows. The
prediction of warm ionized gas as the product of mass drop-out in these and
other cooling flows can be verified by Chandra, XMM, and ASTRO-E.Comment: 4 pages (2 figures), Accepted for publication in ApJ Letters, no
significant changes from previous submitted versio
Quantum dynamics of thermalizing systems
We introduce a method "DMT" for approximating density operators of 1D systems
that, when combined with a standard framework for time evolution (TEBD), makes
possible simulation of the dynamics of strongly thermalizing systems to
arbitrary times. We demonstrate that the method performs well for both
near-equilibrium initial states (Gibbs states with spatially varying
temperatures) and far-from-equilibrium initial states, including quenches
across phase transitions and pure states
A Theoretical Model for the Relation for Supermassive Black Holes in Galaxies
We construct a model for the formation of black holes within galactic bulges.
The initial state is a slowly rotating isothermal sphere, characterized by
effective transport speed \aeff and rotation rate . The black hole
mass is determined when the centrifugal radius of the collapse flow exceeds the
capture radius of the central black hole. This model reproduces the observed
correlation between black hole masses and galactic velocity dispersions, \mbh
\approx 10^8 M_\odot (\sigma/200 \kms)^4, where \sigma = \sqrt{2} \aeff.
This model also predicts the ratio \mrat of black hole mass to host mass:
\mrat 0.004 (\sigma/200 \kms).Comment: 9 pages, 2 figures, submitted to Astrophysical Journal Letter
Tectonics of the western Gulf of Oman
Also published as: Journal of
Geophysical Research 84 (1979): 3479-3489The Oman line, running northward from the Strait of Hormuz separates a continentâcontinent plate boundary to the northwest (Persian Gulf region) from an oceanâcontinent plate boundary to the southeast (Gulf of Oman region). A large basement ridge detected on multichannel seismic reflection and gravity profiles to the west of the Oman line is probably a subsurface continuation of the Musandam peninsula beneath the Strait of Hormuz. Collision and underthrusting beneath Iran of the Arabian plate on which this ridge lies has caused many of the large earthquakes that have occurred in this region. Convergence between the oceanic crust of the Arabian plate beneath the Gulf of Oman and the continental Eurasian plate beneath Iran to the north is accommodated by northward dipping subduction. A deformed sediment prism which forms the offshore Makran continental margin and which extends onto land in the Iranian Makran has accumulated above the descending plate. In the western part of the Gulf of Oman, continued convergence has brought the opposing continental margin of Oman into contact with the Makran continental margin. This is an example of the initial stages of a continentâcontinent type collision. A model of imbricate thrusting is proposed to explain the development of the fold ridges and basins on the Makran continental margin. Sediments from the subducting plate are buckled and incorporated into the edge of the Makran continental margin in deformed wedges and subsequently uplifted along major faults that penetrate the accretionary prism further to the north.Prepared for the Office of Naval Research under Contract
N00014-74-C-0262; NR 083-004 and for the National Science
Foundation under Grant 76-10417
X-Ray Emission from M32: X-Ray Binaries or a micro-AGN?
We have analysed archival {\it ROSAT} PSPC data for M32 in order to study the
x-ray emission from this nearest elliptical galaxy. We fit spectra from three
long exposures with Raymond-Smith, thermal bremsstrahlung, and power-law
models. All models give excellent fits. The thermal fits have kT4 keV,
the Raymond-Smith iron abundance is Solar, the power-law
fit has =1.60.1, and all fits have consistent with the
Galactic column. The source is centered on M32 to an accuracy of 9, and
unresolved at 27 FWHM (90 pc). M32 is x-ray variable by a factor of
3--5 on timescales of a decade down to minutes, with evidence for a possible
period of 1.3 days.
There are two plausible interpretations for these results: 1) Emission due to
low-mass x-ray binaries; 2) Emission due to accretion onto a massive central
black hole. Both of these possibilities are supported by arguments based on
previous studies of M32 and other old stellar systems; the {\it ROSAT} PSPC
data do not allow us to unambiguously choose between them. Observations with
the {\it ROSAT} HRI and with {\it ASCA} are required to determine which of
these two very different physical models is correct.Comment: 9 pages, 5 PostScript figures, uses AASTeX style files, Accepted for
publication in Astrophysical Journal Letter
Voigt-Profile Analysis of the Lyman-alpha Forest in a Cold Dark Matter Universe
We use an automated Voigt-profile fitting procedure to extract statistical
properties of the Ly forest in a numerical simulation of an ,
cold dark matter (CDM) universe. Our analysis method is similar to that used in
most observational studies of the forest, and we compare the simulations to
recently published results derived from Keck HIRES spectra. With the
Voigt-profile decomposition analysis, the simulation reproduces the large
number of weak lines (N_{\rm HI}\la 10^{13}\cdunits) found in the HIRES
spectra. The column density distribution evolves significantly between
and , with the number of lines at fixed column density dropping by a
factor in the range where line blending is not severe. At , the
-parameter distribution has a median of 35 \kms and a dispersion of 20
\kms, in reasonable agreement with the observed values. The comparison between
our new analysis and recent data strengthens earlier claims that the \lya
forest arises naturally in hierarchical structure formation as photoionized gas
falls into dark matter potential wells. However, there are two statistically
signficant discrepancies between the simulated forest and the HIRES results:
the model produces too many lines at by a factor , and it
produces more narrow lines (b<20 \kms) than are seen in the data. The first
result is sensitive to our adopted normalization of the mean \lya optical
depth, and the second is sensitive to our assumption that helium reionization
has not significantly raised gas temperatures at . It is therefore too
early to say whether these discrepancies indicate a fundamental problem with
the high-redshift structure of the CDM model or reflect errors of
detail in our modeling of the gas distribution or the observational procedure.Comment: 13 pages, 3 figures, AAS LaTex, accepted to Ap
Cooling Radiation and the Lyman-alpha Luminosity of Forming Galaxies
We examine the cooling radiation from forming galaxies in hydrodynamic
simulations of the LCDM model (cold dark matter with a cosmological constant),
focusing on the Ly-alpha line luminosities of high-redshift systems. Primordial
composition gas condenses within dark matter potential wells, forming objects
with masses and sizes comparable to the luminous regions of observed galaxies.
As expected, the energy radiated in this process is comparable to the
gravitational binding energy of the baryons, and the total cooling luminosity
of the galaxy population peaks at z ~= 2. However, in contrast to the classical
picture of gas cooling from the \sim 10^6 K virial temperature of a typical
dark matter halo, we find that most of the cooling radiation is emitted by gas
with T < 20,000 K. As a consequence, roughly 50% of this cooling radiation
emerges in the Ly-alpha line. While a galaxy's cooling luminosity is usually
smaller than the ionizing continuum luminosity of its young stars, the two are
comparable in the most massive systems, and the cooling radiation is produced
at larger radii, where the Ly-alpha photons are less likely to be extinguished
by dust. We suggest, in particular, that cooling radiation could explain the
two large (\sim 100 kpc), luminous (L_{Ly-alpha} \sim 10^{44} erg s^{-1})
``blobs'' of Ly-alpha emission found in Steidel et al.'s (1999) narrow band
survey of a z = 3 proto-cluster. Our simulations predict objects of the
observed luminosity at about the right space density, and radiative transfer
effects can account for the observed sizes and line widths. We discuss
observable tests of this hypothesis for the nature of the Ly-alpha blobs, and
we present predictions for the contribution of cooling radiation to the
Ly-alpha luminosity function of galaxies as a function of redshift.Comment: Submitted to ApJ. 28 pages including 9 PS figures. Version with color
figures available at
http://donald.astro.umass.edu/~fardal/papers/cooling/cooling.htm
Possession, Transportation, and Use of Firearms by Older Youth in 4-H Shooting Sports Programs
Thirty years ago we would think nothing of driving to school with a jackknife in our pocket or rifle in the gun rack. Since then, the practices of possessing, transporting, and using firearms have been limited by laws, rules, and public perception. Despite restrictions on youth, the Youth Handgun Safety Act does afford 4-H shooting sports members certain rights. This Tools of the Trade highlights the relevant laws and explains the strategy used in Oregon 4-H to help its youth responsibly possess, transport, and use firearms for legitimate purposes
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