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Neutron powder diffraction at a pulsed neutron source: a study of resolution effects
The General Purpose Powder Diffractometer (GPPD), a high resolution ( d/d = 0.002) time-of-flight instrument, exhibits a resolution function that is almost independent of d-spacing. Some of the special properties of time-of-flight scattering data obtained at a pulsed neutron source will be discussed. A method is described that transforms wavelength dependent data, obtained at a pulsed neutron source, so that standard structural least-squares analyses can be applied. Several criteria are given to show when these techniques are useful in time-of-flight data analysis. 14 refs., 6 figs., 1 tab
Hubble Space Telescope Planetary Camera Images of NGC 1316
We present HST Planetary Camera V and I~band images of the central region of
the peculiar giant elliptical galaxy NGC 1316. The inner profile is well fit by
a nonisothermal core model with a core radius of 0.41" +/- 0.02" (34 pc). At an
assumed distance of 16.9 Mpc, the deprojected luminosity density reaches \sim
2.0 \times 10^3 L_{\sun} pc.
Outside the inner two or three arcseconds, a constant mass-to-light ratio of
is found to fit the observed line width measurements. The
line width measurements of the center indicate the existence of either a
central dark object of mass 2 \times 10^9 M_{\sun}, an increase in the
stellar mass-to-light ratio by at least a factor of two for the inner few
arcseconds, or perhaps increasing radial orbit anisotropy towards the center.
The mass-to-light ratio run in the center of NGC 1316 resembles that of many
other giant ellipticals, some of which are known from other evidence to harbor
central massive dark objects (MDO's).
We also examine twenty globular clusters associated with NGC 1316 and report
their brightnesses, colors, and limits on tidal radii. The brightest cluster
has a luminosity of 9.9 \times 10^6 L_{\sun} (), and the
faintest detectable cluster has a luminosity of 2.4 \times 10^5 L_{\sun}
(). The globular clusters are just barely resolved, but their core
radii are too small to be measured. The tidal radii in this region appear to be
35 pc. Although this galaxy seems to have undergone a substantial merger
in the recent past, young globular clusters are not detected.Comment: 21 pages, latex, postscript figures available at
ftp://delphi.umd.edu/pub/outgoing/eshaya/fornax
Galaxies at z=4 and the Formation of Population II
We report the discovery of four high-redshift objects (3.3 < z < 4) observed
behind the rich cluster CL0939+4713 (Abell 851). One object (DG 433) has a
redshift of z=3.3453; the other three objects have redshifts of z\approx 4: A0
at z=3.9819, DG 353 and P1/P2 at z=3.9822. It is possible that all four objects
are being lensed in some way by the cluster, DG 433 being weakly sheared, A0
being strongly sheared, and DG 353 and P1/P2 being an image pair of a common
source object; detailed modelling of the cluster potential will be necessary to
confirm this hypothesis. The weakness of common stellar wind features like N V
and especially C IV in the spectra of these objects argues for sub-solar
metallicities, at least as low as the SMC. DG 353 and DG 433, which have
ground-based colors, are moderately dusty [E_{int}(B-V) < 0.15], similar to
other z>3 galaxies. Star formation rates range from 2.5 (7.8) h^{-2} to 22.
(78.) h^{-2} M_{\odot}/yr, for q_0=0.5 (0.05), depending on assumptions about
gravitational lensing and extinction, also typical of other z>3 galaxies. These
objects are tenatively identified as the low-metallicity proto-spheroid clumps
that will merge to form the Population II components of today's spheroids.Comment: 16 pages, including 2 PostScript figures. Needs aaspp4.sty
(included). Accepted for publication in the Astrophysical Journa
Empirical modeling of the stellar spectrum of galaxies
An empirical method of modeling the stellar spectrum of galaxies is proposed,
based on two successive applications of Principal Component Analysis (PCA). PCA
is first applied to the newly available stellar library STELIB, supplemented by
the J, H and K magnitudes taken mainly from the 2 Micron All Sky Survey
(2MASS). Next the resultant eigen-spectra are used to fit the observed spectra
of a sample of 1016 galaxies selected from the Sloan Digital Sky Survey Data
Release One (SDSS DR1). PCA is again applied, to the fitted spectra to
construct the eigen-spectra of galaxies with zero velocity dispersion. The
first 9 galactic eigen-spectra so obtained are then used to model the stellar
spectrum of the galaxies in SDSS DR1, and synchronously to estimate the stellar
velocity dispersion, the spectral type, the near-infrared SED, and the average
reddening. Extensive tests show that the spectra of different type galaxies can
be modeled quite accurately using these eigen-spectra. The method can yield
stellar velocity dispersion with accuracies better than 10%, for the spectra of
typical S/N ratios in SDSS DR1.Comment: 34 pages with 18 figures, submitted to A
Hubble Space Telescope Observations of M32: The Color-Magnitude Diagram
We present a V-I color-magnitude diagram for a region 1'-2' from the center
of M32 based on Hubble Space Telescope WFPC2 images. The broad color-luminosity
distribution of red giants shows that the stellar population comprises stars
with a wide range in metallicity. This distribution cannot be explained by a
spread in age. The blue side of the giant branch rises to M_I ~ -4.0 and can be
fitted with isochrones having [Fe/H] ~ -1.5. The red side consists of a heavily
populated and dominant sequence that tops out at M_I ~ -3.2, and extends beyond
V-I=4. This sequence can be fitted with isochrones with -0.2 < [Fe/H] < +0.1,
for ages running from 15 Gyr to 5 Gyr respectively. We do not find the
optically bright asymptotic giant branch stars seen in previous ground-based
work and argue that the majority of them were artifacts of crowding. Our
results are consistent with the presence of the infrared-luminous giants found
in ground-based studies, though their existence cannot be directly confirmed by
our data. There is little evidence for an extended or even a red horizontal
branch, but we find a strong clump on the giant branch itself. If the age
spread is not extreme, the distribution of metallicities in M32 is considerably
narrower than that of the closed-box model of chemical evolution, and also
appears somewhat narrower than that of the solar neighborhood. Overall, the M32
HST color-magnitude diagram is consistent with the average luminosity-weighted
age of 8.5 Gyr and [Fe/H] = -0.25 inferred from integrated spectral indices.Comment: 22 pages, AASTeX, aaspp4 and flushrt style files included, 11
postscript figures, figures 1,2,5,7, and 8 available at
ftp://bb3.jpl.nasa.gov/pub/m32 . Submitted to the Astronomical Journa
Center vortices and confinement vs. screening
We study adjoint and fundamental Wilson loops in the center-vortex picture of
confinement, for gauge group SU(N) with general N. There are N-1 distinct
vortices, whose properties, including collective coordinates and actions, we
study. In d=2 we construct a center-vortex model by hand so that it has a
smooth large-N limit of fundamental-representation Wilson loops and find, as
expected, confinement. Extending an earlier work by the author, we construct
the adjoint Wilson-loop potential in this d=2 model for all N, as an expansion
in powers of , where is the vortex density per unit area and M
is the vortex inverse size, and find, as expected, screening. The leading term
of the adjoint potential shows a roughly linear regime followed by string
breaking when the potential energy is about 2M. This leading potential is a
universal (N-independent at fixed fundamental string tension ) of the form
, where R is the spacelike dimension of a rectangular Wilson
loop. The linear-regime slope is not necessarily related to by Casimir
scaling. We show that in d=2 the dilute vortex model is essentially equivalent
to true d=2 QCD, but that this is not so for adjoint representations; arguments
to the contrary are based on illegal cumulant expansions which fail to
represent the necessary periodicity of the Wilson loop in the vortex flux. Most
of our arguments are expected to hold in d=3,4 also.Comment: 29 pages, LaTex, 1 figure. Minor changes; references added;
discussion of factorization sharpened. Major conclusions unchange
Planetary camera observations of the central parsec of M32
Analysis of V band HST Planetary Camera images of the elliptical galaxy M32 shows that its nucleus is extremely dense and remains unresolved at even the HST diffraction limit. A combined approach of image deconvolution and model fitting is used to investigate the starlight distribution into limiting radii of 0".04 (0.14 pc at 700 kpc). The logarithmic slope of the brightness profile smoothly flattens from y= -1.2 at 3.4 pc to y= -0.5 at 0.34 pc; interior to this radius the profile is equally consistent with a singular ”(r)â r,^(-1/2) cusp or a small nonisothermal core with r_c<0.37 pc. The isophotes maintain
constant ellipticity into tlle center, and there is no evidence for a central point source, disk, dust, or
any other substructures. The cusp model implies central mass densities p_0 > 3 X 10^7 M_â pc^(-3) at the
resolution limit and is consistent with a central M_âą = 3 X 10^6 M_â black hole; the core model implies p_0â4 X 10^6 M_â pc^(-3). From the viewpoint of long-term stability, we argue that a starlight cusp surrounding a central black hole is the more plausible interpretation of the observations. A core at the implied density and size without a black hole has a relaxation time of only ~7 X 10^7 yr and a short stellar oollision timescale implying wholesale stellar merging over the age of the universe. The core would be strongly vulnerable to collapse and concomitant runaway stellar merging. Collapse may lead
to formation of a massive black hole in any case if it cannot be reversed by formation of a binary from high-mass merger products. Regardless of the ultimate fate of the core, however, structural evolution of the core will always be accompanied by strong evolution of the core population-the constant isophote shape and absence of a central color gradient appear to show that such evolution has not occurred. In contrast, the high velocities around a black hole imply long relaxation and stellar collision
times for the cusp population compared to the age of the universe
Hubble Space Telescope Planetary Camera images of R136
The Planetary Camera of the Hubble Space Telescope has been used to obtain broad and narrowband images ofR136, the core of the massive star cluster 30 Doradus in the Large Magellanic Cloud. R136a, the brightest component ofR136, is shown to have at least 12 separate components, including the eight originally identified by speckle interferometry. Three of the 12 components are previously unidentified
close companions of the speckle components. The stars within R136a are found to have luminosities and colors of normal evolved (Wolf-Rayet and blue supergiants) and zero-age main-sequence (ZAMS) massive stars. A narrowband He II filter was used to investigate the Wolf-Rayet stellar population. We find that three stars in R136a are of the Wolf-Rayet type; of the two identified from ground-based data, one is now resolved into two components. We present color-magnitude diagrams and a luminosity function of the stars within the larger region (~2 pc) defined as R136. We find that the stars in R136 are similar in color and luminosity to those of cluster members that lie outside that crowded inner region. The lower end of the color-magnitude diagram corresponds to ZAMS spectral type B3. No red supergiants have been detected within R136. The luminosity per unit area in the inner 1" (0.25 pc) of
R136 is â„ 50 times that of the center of Orion for a comparable area and seven times that of the core
of NGC 3603. The luminosity per unit area of all of R136 is comparable to that of Orion but is sustained over 130 times the area. An F336W surface brightness profile is constructed for R136 based on the stellar photometry. The distribution is found to be consistent with a pure power law with l(r}É r^y with y=-1.72±0.06 or with a small core with r_c 5 X 10^4 M_â pc^(-3). The implied upper limit on the relaxation time for the cluster is much smaller than the age of 3.5 X 10^6 yrs required by the presence of Wolf-Rayet stars. This suggests that relaxation effects have been very important in determining the observed structure of the cluster unless a large population of lower mass stars is also present
Specific Subpopulations of Hypothalamic Leptin Receptor-Expressing Neurons Mediate the Effects of Early Developmental Leptin Receptor Deletion on Energy Balance
ACKNOWLEDGEMENTS We thank MedImmune, Inc. and James Trevaskis, PhD and Christopher Rhodes, PhD for the gift of leptin. We thank members of the Myers and Olson labs for helpful discussions. Research support was provided by the Michigan Diabetes Research Center (NIH P3 0 DK020572, including the Molecular Genetics, Animal Phenotyping, and Clinical Cores), the American Diabetes Association (MGM), the Marilyn H. Vincent Foundation (MGM), the NIH (MGM: D K05673 1; ACR:DK071212; MBA: DK097861), the BBSRC (LKH: BB/NO17838/1) and WellcomeTrust (LKH: 098012).Peer reviewedPublisher PD
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