1,137 research outputs found
WHAM Observations of H-alpha Emission from High Velocity Clouds in the M, A, and C Complexes
The first observations of the recently completed Wisconsin H-Alpha Mapper
(WHAM) facility include a study of emission lines from high velocity clouds in
the M, A, and C complexes, with most of the observations on the M I cloud. We
present results including clear detections of H-alpha emission from all three
complexes with intensities ranging from 0.06 R to 0.20 R. In every observed
direction where there is significant high velocity H I gas seen in the 21 cm
line we have found associated ionized hydrogen emitting the H-alpha line. The
velocities of the H-alpha and 21 cm emission are well correlated in every case
except one, but the intensities are not correlated. There is some evidence that
the ionized gas producing the H-alpha emission envelopes the 21 cm emitting
neutral gas but the H-alpha "halo", if present, is not large. If the H-alpha
emission arises from the photoionization of the H I clouds, then the implied
Lyman continuum flux F_{LC} at the location of the clouds ranges from 1.3 to
4.2 x 10^5 photons cm^{-2} s^{-1}. If, on the other hand, the ionization is due
to a shock arising from the collision of the high-velocity gas with an ambient
medium in the halo, then the density of the pre-shocked gas can be constrained.
We have also detected the [S II] 6716 angstrom line from the M I cloud and have
evidence that the [S II] to H-alpha ratio varies with location on the cloud.Comment: 32 pages, 18 figures, to appear in ApJ (Sept. 10, 1998
Picosecond fluorescence of intact and dissolved PSI-LHCI crystals
Over the last years many crystal structures of photosynthetic pigment-protein complexes have been determined, and used extensively to model spectroscopic results obtained on the same proteins in solution. However, the crystal structure is not necessarily identical to the structure of the protein in solution. Here we studied picosecond fluorescence of Photosystem I-Light Harvesting Complex I (PSI-LHCI), a multisubunit pigment protein complex that catalyzes the first steps of photosynthesis. The ultrafast fluorescence of PSI-LHCI crystals is identical to that of dissolved crystals, but differs considerably from most kinetics presented in literature. In contrast to most studies, the present data can be modeled quantitatively with only 2 compartments: PSI core and LHCI. This yields the rate of charge separation from an equilibrated core (22.5+/-2.5 ps) and rates of excitation energy transfer from LHCI to core (kLC) and vice versa (kCL). The ratio R=kCL/kLC between these rates appears to be wavelength-dependent and scales with the ratio of the absorption spectra of LHCI and core, indicating the validity of a detailed balance relation between both compartments. kLC depends slightly but non systematically on detection wavelength, averaging (9.4+/-4.9 ps)(-1). R ranges from 0.5 (below 690 nm) to around 1.3 above 720 nm
A Model for the Moving `Wisps' in the Crab Nebula
I propose that the moving `wisps' near the center of the Crab Nebula result
from nonlinear Kelvin-Helmholtz instabilities in the equatorial plane of the
shocked pulsar wind. Recent observations suggest that the wisps trace out
circular wavefronts in this plane, expanding radially at speeds approximately
less than c/3. Instabilities could develop if there is sufficient velocity
shear between a faster-moving equatorial zone and a slower moving shocked
pulsar wind at higher latitudes. The development of shear could be related to
the existence of a neutral sheet -- with weak magnetic field -- in the
equatorial zone, and could also be related to a recent suggestion by Begelman
that the magnetic field in the Crab pulsar wind is much stronger than had been
thought. I show that plausible conditions could lead to the growth of
instabilities at the radii and speeds observed, and that their nonlinear
development could lead to the appearance of sharp wisplike features.Comment: 7 pages; 3 postscript figures; LaTex, uses emulateapj.sty; to Appear
in the Astrophysical Journal, Feb. 20, 1999, Vol. 51
Dark matter effects in vacuum spacetime
We analyze a toy model describing an empty spacetime in which the motion of a
test mass (and the trajectories of photons) evidence the presence of a
continuous and homogeneous distribution of matter; however, since the
energy-momentum tensor vanishes, no real matter or energy distribution is
present at all. Thus, a hypothetical observer will conclude that he is immersed
in some sort of dark matter, even though he has no chance to directly detect
it. This suggests yet another possibility of explaining the elusive dark matter
as a purely dynamical effect due to the curvature of spacetime.Comment: 5 pages, 2 figures, expanded with comments about the exact motion and
curvature invariant
A direct empirical proof of the existence of dark matter
We present new weak lensing observations of 1E0657-558 (z=0.296), a unique
cluster merger, that enable a direct detection of dark matter, independent of
assumptions regarding the nature of the gravitational force law. Due to the
collision of two clusters, the dissipationless stellar component and the
fluid-like X-ray emitting plasma are spatially segregated. By using both
wide-field ground based images and HST/ACS images of the cluster cores, we
create gravitational lensing maps which show that the gravitational potential
does not trace the plasma distribution, the dominant baryonic mass component,
but rather approximately traces the distribution of galaxies. An 8-sigma
significance spatial offset of the center of the total mass from the center of
the baryonic mass peaks cannot be explained with an alteration of the
gravitational force law, and thus proves that the majority of the matter in the
system is unseen.Comment: Accepted for publication in ApJ
The Nature of Radio Emission from Distant Galaxies: The 1.4 GHz Observations
We have conducted a deep radio survey with the Very Large Array at 1.4 GHz of
a region containing the Hubble Deep Field. This survey overlaps previous
observations at 8.5 GHz allowing us to investigate the radio spectral
properties of microjansky sources to flux densities greater than 40 Jy at
1.4 GHz and greater than 8 Jy at 8.5 GHz. A total of 371 sources have been
catalogued at 1.4 GHz as part of a complete sample within 20\arcmin ~of the
HDF. The differential source count for this region is only marginally
sub-Euclidean and is given by
srJy. Above about 100 Jy the radio source count is
systematically lower in the HDF as compared to other fields. We conclude that
there is clustering in our radio sample on size scales of 1\arcmin - 40\arcmin
\.
The 1.4 GHz selected sample shows that the radio spectral indices are
preferentially steep () and the sources are
moderately extended with average angular size = 1.8\arcsec . Optical
identification with disk-type systems at 0.5-0.8 suggests that
synchrotron emission, produced by supernovae remnants, is powering the radio
emission in the majority of sources. The 8.5 GHz sample contains primarily
moderately flat spectrum sources (), with less than
15% inverted. We argue that we may be observing an increased fraction of
optically thin bremsstrahlung over synchrotron radiation in these distant
star-forming galaxies.Comment: 31 pages, 5 tables, 10 figures, to appear in Jan. 2000 Ap
3D Distribution of Molecular Gas in the Barred Milky Way
We present a new model of the three-dimensional distribution of molecular gas
in the Milky Way Galaxy, based on CO line data. Our analysis is based on a
gas-flow simulation of the inner Galaxy using smoothed-particle hydrodynamics
(SPH) using a realistic barred gravitional potential derived from the observed
COBE/DIRBE near-IR light distribution. The gas model prescribes the gas orbits
much better than a simple circular rotation model and is highly constrained by
observations, but it cannot predict local details. In this study, we provide a
3D map of the observed molecular gas distribution using the velocity field from
the SPH model. A comparison with studies of the Galactic Center region suggests
that the main structures are reproduced but somewhat stretched along the
line-of-sight, probably on account of limited resolution of the underlying SPH
simulation. The gas model will be publicly available and may prove useful in a
number of applications, among them the analysis of diffuse gamma-ray emission
as measured with GLAST.Comment: ApJ in pres
Lyman-Alpha Absorption Systems and the Nearby Galaxy Distribution
We study the galaxy number density (smoothed on a 5h^{-1} Mpc scale) around
18 low-redshift Lyman-alpha absorbers previously observed with HST. The
absorbers lie in the foregrounds of Mrk 335, Mrk 421, Mrk 501, I Zw 1, and 3C
273, all within regions where there are now complete redshift surveys to
m_{Zw}=15.5. We construct a smoothed galaxy number density field from the
redshift survey data and determine the distribution of densities at the
Lyman-alpha absorber locations. We also find the distribution of galaxy number
density for a variety of test samples: all galaxy locations within the Center
for Astrophysics Redshift Survey (CfA2), CfA2 galaxy locations along randomly
selected lines of sight (LOS), and randomly chosen redshifts along random LOS.
The Lyman-alpha absorbers are present in dense regions of the survey, but
occur far more frequently in underdense regions than do typical luminous
galaxies. The distribution of smoothed galaxy density around the Lyman-alpha
absorbers is inconsistent at the 4-sigma level with the density distribution
around survey galaxies. It is highly consistent with a density distribution at
randomly chosen redshifts along random LOS. This supports earlier evidence that
the nearby, low column density (log N_{HI} < 14) Lyman-alpha forest systems are
spatially distributed at random; they are not well correlated with the local
large-scale structure.Comment: Accepted for publication in ApJ, 38 pages including 10 figure
Relaxation to thermal equilibrium in the self-gravitating sheet model
We revisit the issue of relaxation to thermal equilibrium in the so-called
"sheet model", i.e., particles in one dimension interacting by attractive
forces independent of their separation. We show that this relaxation may be
very clearly detected and characterized by following the evolution of order
parameters defined by appropriately normalized moments of the phase space
distribution which probe its entanglement in space and velocity coordinates.
For a class of quasi-stationary states which result from the violent relaxation
of rectangular waterbag initial conditions, characterized by their virial ratio
R_0, we show that relaxation occurs on a time scale which (i) scales
approximately linearly in the particle number N, and (ii) shows also a strong
dependence on R_0, with quasi-stationary states from colder initial conditions
relaxing much more rapidly. The temporal evolution of the order parameter may
be well described by a stretched exponential function. We study finally the
correlation of the relaxation times with the amplitude of fluctuations in the
relaxing quasi-stationary states, as well as the relation between temporal and
ensemble averages.Comment: 37 pages, 24 figures; some additional discussion of previous
literature and other minor modifications, final published versio
The Radial Extent and Warp of the Ionized Galactic Disk. II. A Likelihood Analysis of Radio-Wave Scattering Toward the Anticenter
We use radio-wave scattering data to constrain the distribution of ionized
gas in the outer Galaxy. Like previous models, our model for the H II disk
includes parameters for the radial scale length and scale height of the H II,
but we allow the H II disk to warp and flare. Our model also includes the
Perseus arm. We use a likelihood analysis on 11 extragalactic sources and 7
pulsars. Scattering in the Perseus arm is no more than 60% of the level
contributed by spiral arms in the inner Galaxy, equivalent to a 1 GHz
scattering diameter of 1.5 mas. Our analysis favors an unwarped, nonflaring
disk with a 1 kpc scale height, though this may reflect the non-uniform and
coarse coverage provided by the available data. The lack of a warp indicates
that VLBI observations near 1 GHz with an orbiting station having baseline
lengths of a few Earth diameters will not be affected by interstellar
scattering at Galactic latitudes |b| ~ 15 degrees. The radial scale length is
15--20 kpc, but the data cannot distinguish between a gradual decrease in the
electron density and a truncated distribution. We favor a truncated one,
because we associate the scattering with massive star formation, which is also
truncated near 20 kpc. The distribution of electron density turbulence
decreases more rapidly with Galactocentric distance than does the hydrogen
distribution. Alternate ionizing and turbulent agents---the intergalactic
ionizing flux and satellite galaxies passing through the disk---do not
contribute significantly to scattering. We cannot exclude the possibility that
a largely ionized, but quiescent disk extends to >~ 100 kpc, similar to that
for some Ly-alpha absorbers.Comment: 34 pages, LaTeX2e with AASTeX aaspp4 macro, 9 figures in 9 PostScript
files, accepted for publication in Ap
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