519 research outputs found
Evidence of a mis-aligned secondary bar in the Large Magellanic Cloud
Evidence of a mis-aligned secondary bar, within the primary bar of the Large
Magellanic Cloud (LMC) is presented. The density distribution and the
de-reddened mean magnitudes () of the red clump stars in the bar obtained
from the OGLE II data are used for this study. The bar region which
predominantly showed wavy pattern in the line of sight in \citet{a03} was
located. These points in the X-Z plane delineate an S-shaped pattern, clearly
indicating a mis-aligned bar. This feature is statistically significant and
does not depend on the considered value of for the LMC center. The rest
of the bar region were not found to show the warp or the wavy pattern. The
secondary bar is found to be considerably elongated in the Z-direction, with an
inclination of 66.5 0.9, whereas the undisturbed part of the
primary bar is found to have an inclination of 15.1 2.7, such
that the eastern sides are closer to us with respect to the western sides of
both the bars. The PA of the secondary bar is found to be 108.4
7.3. The streaming motions found in the H I velocity map close to the
LMC center could be caused by the secondary bar. The recent star formation and
the gas distribution in LMC could be driven by the mis-aligned secondary bar.Comment: 10 pages, to appear in ApJ Letter
Reionization and Cosmology with 21 cm Fluctuations
Measurement of the spatial distribution of neutral hydrogen via the
redshifted 21 cm line promises to revolutionize our knowledge of the epoch of
reionization and the first galaxies, and may provide a powerful new tool for
observational cosmology from redshifts 1<z<4 . In this review we discuss recent
advances in our theoretical understanding of the epoch of reionization (EoR),
the application of 21 cm tomography to cosmology and measurements of the dark
energy equation of state after reionization, and the instrumentation and
observational techniques shared by 21 cm EoR and post reionization cosmology
machines. We place particular emphasis on the expected signal and observational
capabilities of first generation 21 cm fluctuation instruments.Comment: Invited review for Annual Review of Astronomy and Astrophysics (2010
volume
A new Tolman test of a cosmic distance duality relation at 21 cm
Under certain general conditions in an expanding universe, the luminosity
distance (d_L) and angular diameter distance (d_A) are connected by the
Etherington relation as d_L = d_A (1 + z)^2. The Tolman test suggests the use
of objects of known surface brightness, to test this relation. In this letter,
we propose the use of redshifted 21 cm signal from disk galaxies, where neutral
hydrogen (HI) masses are seen to be almost linearly correlated with surface
area, to conduct a new Tolman test. We construct simulated catalogs of
galaxies, with the observed size-luminosity relation and realistic redshift
evolution of HI mass functions, likely to be detected with the planned Square
Kilometer Array (SKA). We demonstrate that these observations may soon provide
the best implementation of the Tolman test to detect any violation of the
Etherington relation.Comment: 4 pages, 2 figures, 1 table, v2: published versio
The enormous outer Galaxy HII region CTB 102
We present new radio recombination line observations of the previously
unstudied HII region CTB 102. Line parameters are extracted and physical
parameters describing the gas are calculated. We estimate the distance to CTB
102 to be 4.3 kpc. Through comparisons with HI and 1.42 GHz radio continuum
data, we estimate the size of CTB 102 to be 100-130 pc, making it one of the
largest HII regions known, comparable to the W4 complex. A stellar wind blown
bubble model is presented as the best explanation for the observed morphology,
size and velocities.Comment: 26 pages, 8 figures. Accepted for publication by The Astrophysical
Journa
A large local rotational speed for the Galaxy found from proper-motions: Implications for the mass of the Milky-Way
Predictions from a Galactic Structure and Kinematic model are compared to the
absolute proper-motions of about 30,000 randomly selected stars with derived from the Southern Proper-Motion Program (SPM) toward
the South Galactic Pole. The absolute nature of the SPM proper-motions allow us
to measure not only the relative motion of the Sun with respect to the local
disk, but also, and most importantly, the overall state of rotation of the
local disk with respect to galaxies. The SPM data are best fit by models having
a solar peculiar motion of +5 km~s in the V-component (pointing in the
direction of Galactic rotation), a large LSR speed of 270 km~s, and a
disk velocity ellipsoid that points towards the Galactic center. We stress,
however, that these results rest crucially on the assumptions of both
axisymmetry and equilibrium dynamics.
The absolute proper-motions in the U-component indicate a solar peculiar
motion of km~s, with no need for a local expansion or
contraction term.
The implications of the large LSR speed are discussed in terms of
gravitational mass of the Galaxy inferred from the most recent and accurate
determination for the proper-motion of the LMC. We find that our derived value
for the LSR is consistent both with the mass of the Galaxy inferred from the
motion of the Clouds ( to kpc), as well
as the timing argument, based on the binary motion of M31 and the Milky Way,
and Leo I and the Milky Way ( to
kpc).Comment: 7 pages (AAS Latex macro v4.0), 2 B&W postscript figures, accepted
for publication on ApJ, Letters sectio
A 490 GHz planar circuit balanced Nb-AlO-Nb quasiparticle mixer for radio astronomy: Application to quantitative local oscillator noise determination
This article presents a heterodyne experiment which uses a 380-520 GHz planar
circuit balanced Nb--Nb
superconductor-insulator-superconductor (SIS) quasiparticle mixer with 4-8 GHz
instantaneous intermediate frequency (IF) bandwidth to quantitatively determine
local oscillator (LO) noise. A balanced mixer is a unique tool to separate
noise at the mixer's LO port from other noise sources. This is not possible in
single-ended mixers. The antisymmetric IV characteristic of a SIS mixer further
helps to simplify the measurements. The double-sideband receiver sensitivity of
the balanced mixer is 2-4 times the quantum noise limit over the
measured frequencies with a maximum LO noise rejection of 15 dB. This work
presents independent measurements with three different LO sources that produce
the reference frequency but also an amount of near-carrier noise power which is
quantified in the experiment as a function of the LO and IF frequency in terms
of an equivalent noise temperature . In a second experiment we use only
one of two SIS mixers of the balanced mixer chip, in order to verify the
influence of near-carrier LO noise power on a single-ended heterodyne mixer
measurement. We find an IF frequency dependence of near-carrier LO noise power.
The frequency-resolved IF noise temperature slope is flat or slightly negative
for the single-ended mixer. This is in contrast to the IF slope of the balanced
mixer itself which is positive due to the expected IF roll-off of the mixer.
This indicates a higher noise level closer to the LO's carrier frequency. Our
findings imply that near-carrier LO noise has the largest impact on the
sensitivity of a receiver system which uses mixers with a low IF band, for
example superconducting hot-electron bolometer (HEB) mixers.Comment: 13 pages, 8 figures, 2 tables, see manuscript for complete abstrac
Supernova Remnants in the Magellanic Clouds. V. The Complex Interior Structure of the N206 SNR
The N206 supernova remnant (SNR) in the Large Magellanic Cloud (LMC) has long
been considered a prototypical "mixed morphology" SNR. Recent observations,
however, have added a new twist to this familiar plot: an elongated,
radially-oriented radio feature seen in projection against the SNR face.
Utilizing the high resolution and sensitivity available with the Hubble Space
Telescope, Chandra, and XMM-Newton, we have obtained optical emission-line
images and spatially resolved X-ray spectral maps for this intriguing SNR. Our
findings present the SNR itself as a remnant in the mid to late stages of its
evolution. X-ray emission associated with the radio "linear feature" strongly
suggests it to be a pulsar-wind nebula (PWN). A small X-ray knot is discovered
at the outer tip of this feature. The feature's elongated morphology and the
surrounding wedge-shaped X-ray enhancement strongly suggest a bow-shock PWN
structure.Comment: 41 pages including 7 figures, accepted for publication by the
Astrophysical Journa
Does the Milky Way have a Maximal Disk?
The Milky Way is often considered to be the best example of a spiral for
which the dark matter not only dominates the outer kinematics, but also plays a
major dynamical role in the inner galaxy: the Galactic disk is therefore said
to be ``sub-maximal.'' This conclusion is important to the understanding of the
evolution of galaxies and the viability of particular dark matter models. The
Galactic evidence rests on a number of structural and kinematic measurements,
many of which have recently been revised. The new constraints indicate not only
that the Galaxy is a more typical member of its class (Sb-Sc spirals) than
previously thought, but also require a re-examination of the question of
whether or not the Milky Way disk is maximal. By applying to the Milky Way the
same definition of ``maximal disk'' that is applied to external galaxies, it is
shown that the new observational constraints are consistent with a Galactic
maximal disk of reasonable . In particular, the local disk column can be
substantially less than the oft-quoted required \Sigma_{\odot} \approx 100
\msolar pc^{-2} - as low as 40 \msolar pc^{-2} in the extreme case - and
still be maximal, in the sense that the dark halo provides negligible rotation
support in the inner Galaxy. This result has possible implications for any
conclusion that rests on assumptions about the potentials of the Galactic disk
or dark halo, and in particular for the interpretation of microlensing results
along both LMC and bulge lines of sight.Comment: Accepted for publication in The Astrophysical Journal. 23
Latex-generated pages, one (new) table, three figures (two new). A few
additions to the bibliography, an expanded discussion, and slight
quantitative changes, none of which affect the conclusion
Experimental Demonstration of Squeezed State Quantum Averaging
We propose and experimentally demonstrate a universal quantum averaging
process implementing the harmonic mean of quadrature variances. The harmonic
mean protocol can be used to efficiently stabilize a set of fragile squeezed
light sources with statistically fluctuating noise levels. The averaged
variances are prepared probabilistically by means of linear optical
interference and measurement induced conditioning. We verify that the
implemented harmonic mean outperforms the standard arithmetic mean strategy.
The effect of quantum averaging is experimentally tested both for uncorrelated
and partially correlated noise sources with sub-Poissonian shot noise or
super-Poissonian shot noise characteristics.Comment: 4 pages, 5 figure
Supernova Remnants in the Magellanic Clouds. IV. X-Ray Emission from the Largest SNR in the LMC
We present the first X-ray detection of SNR 0450-70.9 the largest known
supernova remnant (SNR) in the Large Magellanic Cloud. To study the physical
conditions of this SNR, we have obtained XMM-Newton X-ray observations, optical
images and high-dispersion spectra, and radio continuum maps. Optical images of
SNR 0450-70.9 show a large, irregular elliptical shell with bright filaments
along the eastern and western rims and within the shell interior. The interior
filaments have higher [S II]/Halpha ratios and form an apparent inner shell
morphology. The X-ray emission region is smaller than the full extent of the
optical shell, with the brightest X-ray emission found within the small
interior shell and on the western rim of the large shell. The expansion
velocity of the small shell is ~220 km/s, while the large shell is ~120 km/s.
The radio image shows central brightening and a fairly flat radio spectral
index over the SNR. However, no point X-ray or radio source corresponding to a
pulsar is detected and the X-ray emission is predominantly thermal. Therefore,
these phenomena can be most reasonably explained in terms of the advanced age
of the large SNR. Using hydrodynamic models combined with a nonequilibrium
ionization model for thermal X-ray emission, we derived a lower limit on the
SNR age of about 45,000 yr, well into the later stages of SNR evolution.
Despite this, the temperature and density derived from spectral fits to the
X-ray emission indicate that the remnant is still overpressured, and thus that
the development is largely driven by hot gas in the SNR interior.Comment: Accepted for publication in The Astrophysical Journa
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