637 research outputs found
Combining Supernovae and LSS Information with the CMB
Observations of the Cosmic Microwave Background (CMB), large scale structure
(LSS) and standard candles such as Type 1a Supernovae (SN) each place different
constraints on the values of cosmological parameters. We assume an inflationary
Cold Dark Matter model with a cosmological constant, in which the initial
density perturbations in the universe are adiabatic. We discuss the parameter
degeneracies inherent in interpreting CMB or SN data, and derive their
orthogonal nature. We then present our preliminary results of combining CMB and
SN likelihood functions. The results of combining the CMB and IRAS 1.2 Jy
survey information are given, with marginalised confidence regions in the H_0,
Omega_m, b_IRAS and Q_rms-ps directions assuming n=1, Omega_Lambda+Omega_m=1
and Omega_b h^2=0.024. Finally we combine all three likelihood functions and
find that the three data sets are consistent and suitably orthogonal, leading
to tight constraints on H_0, Omega_m, b_IRAS and Q_rms-ps, given our
assumptions.Comment: 7 pages, 4 figures, submitted to ``The CMB and the Planck Mission'',
proceedings of the workshop held in Santander, Spain, June 199
Radio jet refraction in galactic atmospheres with static pressure gradients
A theory of double radio sources which have a 'Z' or 'S' morphology is proposed, based on the refraction of radio jets in the extended atmosphere of an elliptical galaxy. The model describes a collimated jet of supersonic material bending self-consistently under the influence of external static pressure gradients. Gravity and magnetic fields are neglected in the simplest case except insofar as they determine the static pressure distribution. The calculation is a straightforward extension of a method used to calculate a ram-pressure model for twin radio trails ('C' morphology). It may also be described as a continuous-jet version of a buoyancy model proposed in 1973. The model has the added virtue of invoking a galactic atmosphere similar to those already indicated by X-ray measurements of some other radio galaxies and by models for the collimation of other radio jets
Synchrotron brightness distribution of turbulent radio jets
Radio jets are considered as turbulent mixing regions and it is proposed that the essential small scale viscous dissipation in these jets is by emission of MHD waves and by their subsequent strong damping due, at least partly, to gyro-resonant acceleration of supra-thermal particles. A formula relating the synchrotron surface brightness of a radio jet to the turbulent power input is deduced from physical postulates, and is tested against the data for NGC315 and 3C31 (NGC383). The predicted brightness depends essentially on the collimation behavior of the jet, and, to a lesser extent, on the CH picture of a 'high' nozzle with accelerating flow. The conditions for forming a large scale jet at a high nozzle from a much smaller scale jet are discussed. The effect of entrainment on the prediction is discussed with the use of similarity solutions. Although entrainment is inevitably associated with the turbulent jet, it may or may not be a dominant factor depending on the ambient density profile
A Comprehensive Radio and Optical Study of Abell 2256: Activity from an Infalling Group
Abell 2256 is a nearby (z~0.06), rich cluster of galaxies with fascinating
observed properties across a range of wavelengths. Long believed to represent a
cluster merger, recent X-ray and optical results have suggested that in
addition to the primary cluster and subcluster there is evidence for a third,
poorer system. We present wide-field, high sensitivity 1.4 GHz VLA radio
observations of Abell 2256 in conjunction with optical imaging and additional
spectroscopy. Over 40 cluster radio galaxies are identified, with optical
spectroscopy indicating the emission source (star formation or AGN) for most of
them. While the overall fraction of galaxies exhibiting radio emission is
consistent with a large sample of other nearby clusters, we find an increase in
the activity level of galaxies belonging to the third system (hereafter, the
``Group''). Specifically, the Group has relatively more star formation than
both the primary cluster and main subcluster. The position of the Group is also
coincident with the observed cluster radio relic. We suggest that the Group
recently (~0.3 Gyr) merged with the primary cluster and that this merger, not
the ongoing merger of the primary and the main subcluster, might be responsible
for many of the unusual radio properties of Abell 2256. Furthermore, the
greater star formation activity of the Group suggests that the infall of groups
is an important driver of galaxy evolution in clusters.Comment: 21 pages plus 13 JPEG figures; to appear in the Astronomical Journa
Bayesian `Hyper-Parameters' Approach to Joint Estimation: The Hubble Constant from CMB Measurements
Recently several studies have jointly analysed data from different
cosmological probes with the motivation of estimating cosmological parameters.
Here we generalise this procedure to take into account the relative weights of
various probes. This is done by including in the joint \chi^2 function a set of
`Hyper-Parameters', which are dealt with using Bayesian considerations. The
resulting algorithm (in the case of uniform priors on the log of the
Hyper-Parameters) is very simple: instead of minimising \sum \chi_j^2 (where
\chi_j^2 is per data set j) we propose to minimise \sum N_j \ln (\chi_j^2)
(where N_j is the number of data points per data set j). We illustrate the
method by estimating the Hubble constant H_0 from different sets of recent CMB
experiments (including Saskatoon, Python V, MSAM1, TOCO and Boomerang).Comment: submitted to MNRAS, 6 pages, Latex, with 3 figures embedde
Cosmological Parameters from Velocities, CMB and Supernovae
We compare and combine likelihood functions of the cosmological parameters
Omega_m, h and sigma_8, from peculiar velocities, CMB and type Ia supernovae.
These three data sets directly probe the mass in the Universe, without the need
to relate the galaxy distribution to the underlying mass via a "biasing"
relation. We include the recent results from the CMB experiments BOOMERANG and
MAXIMA-1. Our analysis assumes a flat Lambda CDM cosmology with a
scale-invariant adiabatic initial power spectrum and baryonic fraction as
inferred from big-bang nucleosynthesis. We find that all three data sets agree
well, overlapping significantly at the 2 sigma level. This therefore justifies
a joint analysis, in which we find a joint best fit point and 95 per cent
confidence limits of Omega_m=0.28 (0.17,0.39), h=0.74 (0.64,0.86), and
sigma_8=1.17 (0.98,1.37). In terms of the natural parameter combinations for
these data sigma_8 Omega_m^0.6 = 0.54 (0.40,0.73), Omega_m h = 0.21
(0.16,0.27). Also for the best fit point, Q_rms-ps = 19.7 muK and the age of
the universe is 13.2 Gyr.Comment: 8 pages, 5 figures. Submitted to MNRA
The unequal-time matter power spectrum: impact on weak lensing observables
We investigate the impact of a common approximation of weak lensing power
spectra: the use of single-epoch matter power spectra in integrals over redshift. We disentangle this from the closely connected Limber’s approximation. We derive the unequal-time
matter power spectrum at one-loop in standard perturbation theory and effective field theory
to deal with non-linear physics. We compare these formalisms and conclude that the unequaltime power spectrum using effective field theory breaks for larger scales. As an alternative
we introduce the midpoint approximation. We also provide, for the first time, a fitting function for the time evolution of the effective field theory counterterms based on the Quijote
simulations. Then we compute the angular power spectrum using a range of approaches: the
Limber approximation, and the geometric and midpoint approximations. We compare our
results with the exact calculation at all angular scales using the unequal-time power spectrum. We use DES Y1 and LSST-like redshift distributions for our analysis. We find that the
use of the Limber’s approximation in weak lensing diverges from the exact calculation of the
angular power spectrum on large-angle separations, ` < 10. Even though this deviation is of
order 2% maximum for cosmic lensing, we find the biggest effect for galaxy clustering and
galaxy-galaxy lensing. We show that not only is this true for upcoming galaxy surveys, but
also for current data such as DES Y1. Finally, we make our pipeline and analysis publicly
available as a Python package called unequalpy
Measuring the Reduced Shear
Neglecting the second order corrections in weak lensing measurements can lead
to a few percent uncertainties on cosmic shears, and becomes more important for
cluster lensing mass reconstructions. Existing methods which claim to measure
the reduced shears are not necessarily accurate to the second order when a
point spread function (PSF) is present. We show that the method of Zhang (2008)
exactly measures the reduced shears at the second order level in the presence
of PSF. A simple theorem is provided for further confirming our calculation,
and for judging the accuracy of any shear measurement method at the second
order based on its properties at the first order. The method of Zhang (2008) is
well defined mathematically. It does not require assumptions on the
morphologies of galaxies and the PSF. To reach a sub-percent level accuracy,
the CCD pixel size is required to be not larger than 1/3 of the Full Width at
Half Maximum (FWHM) of the PSF. Using a large ensemble (> 10^7) of mock
galaxies of unrestricted morphologies, we find that contaminations to the shear
signals from the noise of background photons can be removed in a well defined
way because they are not correlated with the source shapes. The residual shear
measurement errors due to background noise are consistent with zero at the
sub-percent level even when the amplitude of such noise reaches about 1/10 of
the source flux within the half-light radius of the source. This limit can in
principle be extended further with a larger galaxy ensemble in our simulations.
On the other hand, the source Poisson noise remains to be a cause of systematic
errors. For a sub-percent level accuracy, our method requires the amplitude of
the source Poisson noise to be less than 1/80 ~ 1/100 of the source flux within
the half-light radius of the source, corresponding to collecting roughly 10^4
source photons.Comment: 18 pages, 3 figures, 4 tables, minor changes from the previous
versio
Chandra observtaion of A2256 - a cluster at the early stage of merging
We present here \chandra observations of the rich cluster of galaxies A2256.
In addition to the known cool subcluster, a new structure was resolved 2
east of the peak of the main cluster. Its position is roughtly at the center of
a low-brightness radio halo. Spectral analysis shows that the "shoulder" has
high iron abundance ( 1). We suggest that this structure is either
another merging component or an internal structure of the main cluster. The
X-ray redshifts of several regions were measured. The results agree with the
optical ones and suggest that the main cluster, the subcluster and the
"shoulder" are physically associated and interacting. The subcluster has low
temperature ( 4.5 keV) and high iron abundance ( 0.6) in the
central 150 kpc. The \chandra image shows a relatively sharp brightness
gradient at the south of the subcluster peak running south-south-east (SSE). A
temperature jump was found across the edge, with higher temperature ahead of
the edge in the low density region. This phenomenon is qualitatively similar to
the "cold fronts" found in A2142 and A3667. If the "shoulder" is ignored, the
temperature map resembles those simulations at the early stage of merging while
the subcluster approached the main cluster from somewhere west. This fact and
the observed edge, in combination with the clear iron abundance contrast
between the center of the subcluster ( 0.6) and the main cluster (
0.2), all imply that the ongoing merger is still at the early stage. At least
three member galaxies, including a radio head-tail galaxy, were found to have
corresponding X-ray emission.Comment: The revised version. The shown abstract is shrunk. Accepted by ApJ.
If it is possible, please try to look at the high-resolution version is
http://cfa160.harvard.edu/~sunm/a2256.tar.g
Limitation of spiral microchannels for particle separation in heterogeneous mixtures: impact of particles’ size and deformability
Spiral microchannels have shown promising results for separation applications. Hydrodynamic particle-particle interactions are a known factor strongly influencing focussing behaviours in inertial devices, with recent work highlighting how the performance of bidisperse mixtures is altered when compared with pure components, in square channels. This phenomenon has not been previously investigated in detail for spiral channels. Here, we demonstrate that, in spiral channels, both the proportion and deformability of larger particles (13 μm diameter) impact upon the recovery (up to 47% decrease) of small rigid particles (4 μm). The effect, observed at low concentrations (volume fraction <0.0012), is attributed to the hydrodynamic capture of beads by larger cells. These changes in particles focussing behaviour directly impede the efficiency of the separation – diverting beads from locations expected from measurements with pure populations to co-collection with larger cells – and could hamper deployment of the technology for certain applications. Similar focussing behaviour alterations were noted when working with purification of stem cell end products
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