12,043 research outputs found
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
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
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Conformational modulation of sequence recognition in synthetic macromolecules
The different triplet sequences in high molecular weight aromatic copolyimides comprising pyromellitimide units ("I") flanked by either ether-ketone ("K") or ether-sulfone residues ("S") show different binding strengths for pyrene-based tweezer-molecules. Such molecules bind primarily to the diimide unit through complementary π-π-stacking and hydrogen bonding. However, as shown by the magnitudes of 1H NMR complexation shifts and tweezer-polymer binding constants, the triplet "SIS" binds tweezer-molecules more strongly than "KIS" which in turn bind such molecules more strongly than "KIK". Computational models for tweezer-polymer binding, together with single-crystal X-ray analyses of tweezer-complexes with macrocyclic ether-imides, reveal that the variations in binding strength between the different triplet sequences arise from the different conformational preferences of aromatic rings at diarylketone and diarylsulfone linkages. These preferences determine whether or not chain-folding and secondary π−π-stacking occurs between the arms of the tweezermolecule and the 4,4'-biphenylene units which flank the central diimide residue
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
Damped Lyman-alpha and Lyman Limit Absorbers in the Cold Dark Matter Model
We study the formation of damped \lya and Lyman limit absorbers in a
hierarchical clustering scenario using a gas dynamical simulation of an , cold dark matter universe. In the simulation, these high column density
systems are associated with forming galaxies. Damped \lya absorption, N_{HI}
\simgt 10^{20.2}\cm^{-2}, arises along lines of sight that pass near the
centers of relatively massive, dense protogalaxies. Lyman limit absorption,
10^{17}\cm^{-2} \simlt N_{HI} \simlt 10^{20.2}\cm^{-2}, develops on lines of
sight that pass through the outer parts of such objects or near the centers of
smaller protogalaxies. The number of Lyman limit systems is less than observed,
while the number of damped \lya systems is quite close to the observed
abundance. Damped absorbers are typically kpc in radius, but the
population has a large total cross section because the systems are much more
numerous than present day galaxies. Our results demonstrate that high
column density systems like those observed arise naturally in a hierarchical
theory of galaxy formation and that it is now possible to study these absorbers
directly from numerical simulations.Comment: compressed postscript, 12 pages including 2 embedded figures. A
version that also includes embedded Figure 1, a 6 Mbyte color postscript
image (which prints reasonable grey scale on a b/w printer) is available from
ftp://bessel.mps.ohio-state.edu/pub/dhw/Preprints Submitted to ApJ 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
Bears Remain Top Summer Predators
In the ten years since wolves (Canis lupus) were restored to Yellowstone National Park (YNP), elk (Cervus elaphus) numbers have substantially decreased. The northern range elk herd is the largest elk herd in Yellowstone, and constitutes the majority of the park’s elk population. During 1994–2005, early winter counts of northern Yellowstone elk decreased from 19,045 to 9,545. Also, during winters 2000–2004, calf:cow ratios declined from 29:100 to 12:100, and were among the lowest recorded during the past several decades. Though many factors (e.g., predation, hunting, and drought) likely contributed to this decreasing abundance and low recruitment, several state and federal legislators continue to speculate that wolves are the primary reason for the recent decrease in elk recruitment rates, and have called for the immediate delisting and liberal control of the abundance and distribution of wolves. Because both wolves and elk are culturally, economically, and ecologically important in the Yellowstone area, it is vital to determine the basis for the decline in the elk population. To help this effort, we initiated a three-year study of northern Yellowstone elk calf mortality in May 2003. Our study was designed to follow up on Dr. Francis Singer et al.’s baseline pre–wolf restoration elk calf mortality study (1987–1990)
The presumption of sociality: Social learning in diverse contexts in brown-headed cowbirds (Molothrus ater).
Data are presented on social and vocal learning in cowbirds (Molothrus ater) housed in large aviaries and given more degrees of freedom than in conventional experimental studies. The studies show that social and vocal outcomes are facultative responses to social contexts. Several findings are reviewed: First, cowbirds quickly self-organize into groups by age and sex; second, opportunities to interact across age and sex do exist and affect courtship competence; third, female cowbirds organize themselves differently in the presence and absence of male competition; and fourth, young, naive cowbirds show rapid and differential sensitivity to group dynamics. Taken as a whole, the data show that social Umwelten are dynamic, developmental ecologies. Solitude is a human presumption. Every quiet step is thunder to beetle life underfoot, a tug of impalpable thread on the web pulling the mate to mate and predator to prey, a beginning or an end. Every choice is a new beginning for the chosen.-Barbara Kingsolver, Prodigal Summer Terra firma provides humans with sure footing but sometimes narrow thinking about the sensory world of other inhabitants. The air-ground complex is one of myriad environments, many neithe
Application of the Biosafety RAM and eProtocol Software Programs to Streamline Institutional Biosafety Committee Processes at the USDA-National Animal Disease Center
The National Animal Disease Center (NADC) conducts basic and applied research on endemic animal diseases of high priority that adversely affect US livestock production or trade. Experiments conducted at this center vary in range and scope with a subset involving synthetic or recombinant nucleic acids (DNA), microorganisms, and/or animals. Historically, the NADC used hard copy paper and filing systems to catalog and monitor these types of experiments, but to improve communication, tracking, searching, reporting, and documentation of Institutional Biosafety Committee (IBC) actions, this institution has transitioned to using a commercially available software tool to digitally manage protocols in our ongoing efforts to maintain excellence in regulatory compliance. In addition, similar to many other research institutions and universities, the scope of the IBC has expanded to include risk assessments on all work conducted at the center. This process has been streamlined using the Biosafety RAM open source software, developed by Sandia National Laboratories, and has stimulated productive discussions on best practices to safely conduct animal and microbiological experiments at the center. Although some initial challenges arose, successful implementation of these two software tools at the NADC has simplified the management of IBC compliance requirements and facilitated review processes at a high-containment government research facility
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