954 research outputs found
Weak Gravitational Lensing by a Sample of X-Ray Luminous Clusters of Galaxies -- II. Comparison with Virial Masses
Dynamic velocity dispersion and mass estimates are given for a sample of five
X-ray luminous rich clusters of galaxies at intermediate redshifts (z~0.3)
drawn from a sample of 39 clusters for which we have obtained gravitational
lens mass estimates. The velocity dispersions are determined from between 9 and
20 redshifts measured with the LDSS spectrograph of the William Herschel
Telescope, and virial radii are determined from imaging using the UH8K mosaic
CCD camera on the University of Hawaii 2.24m telescope.
Including clusters with velocity dispersions taken from the literature, we
have velocity dispersion estimates for 12 clusters in our gravitational lensing
sample. For this sample we compare the dynamical velocity dispersion estimates
with our estimates of the velocity dispersions made from gravitational lensing
by fitting a singular isothermal sphere profile to the observed tangential weak
lensing distortion as a function of radius. In all but two clusters, we find a
good agreement between the velocity dispersion estimates based on spectroscopy
and on weak lensing.Comment: 9 pages, 4 figures, accepted for publication in ApJ. Version in
emulateapj format with only minor change
Radial Redshift Space Distortions
The radial component of the peculiar velocities of galaxies cause
displacements in their positions in redshift space. We study the effect of the
peculiar velocities on the linear redshift space two point correlation
function. Our analysis takes into account the radial nature of the redshift
space distortions and it highlights the limitations of the plane parallel
approximation. We consider the problem of determining the value of \beta and
the real space two point correlation function from the linear redshift space
two point correlation function. The inversion method proposed here takes into
account the radial nature of the redshift space distortions and can be applied
to magnitude limited redshift surveys that have only partial sky coverage.Comment: 26 pages including 11 figures, to appear in Ap
The Correlation Function in Redshift Space: General Formula with Wide-angle Effects and Cosmological Distortions
A general formula for the correlation function in redshift space is derived
in linear theory. The formula simultaneously includes wide-angle effects and
cosmological distortions. The formula is applicable to any pair with arbitrary
angle between lines of sight, and arbitrary redshifts, , ,
which are not necessarily small. The effects of the spatial curvature both on
geometry and on fluctuation spectrum are properly taken into account, and thus
our formula holds in a Friedman-Lema\^{\i}tre universe with arbitrary
cosmological parameters and . We illustrate the pattern
of the resulting correlation function with several models, and also show that
validity region of the conventional distant observer approximation is .Comment: 45 pages including 9 figures, To Appear in Astrophys. J. 535 (2000
The Radio-Optical Correlation in Steep-Spectrum Quasars
Using complete samples of steep-spectrum quasars, we present evidence for a
correlation between radio and optical luminosity which is not caused by
selection effects, nor caused by an orientation dependence (such as
relativistic beaming), nor a byproduct of cosmic evolution. We argue that this
rules out models of jet formation in which there are no parameters in common
with the production of the optical continuum. This is arguably the most direct
evidence to date for a close link between accretion onto a black hole and the
fuelling of relativistic jets. The correlation also provides a natural
explanation for the presence of aligned optical/radio structures in only the
most radio luminous high-redshift galaxies.Comment: MNRAS in press. Uses BoxedEPS (included
CMB component separation by parameter estimation
We propose a solution to the CMB component separation problem based on
standard parameter estimation techniques. We assume a parametric spectral model
for each signal component, and fit the corresponding parameters pixel by pixel
in a two-stage process. First we fit for the full parameter set (e.g.,
component amplitudes and spectral indices) in low-resolution and high
signal-to-noise ratio maps using MCMC, obtaining both best-fit values for each
parameter, and the associated uncertainty. The goodness-of-fit is evaluated by
a chi^2 statistic. Then we fix all non-linear parameters at their
low-resolution best-fit values, and solve analytically for high-resolution
component amplitude maps. This likelihood approach has many advantages: The
fitted model may be chosen freely, and the method is therefore completely
general; all assumptions are transparent; no restrictions on spatial variations
of foreground properties are imposed; the results may be rigorously monitored
by goodness-of-fit tests; and, most importantly, we obtain reliable error
estimates on all estimated quantities. We apply the method to simulated Planck
and six-year WMAP data based on realistic models, and show that separation at
the muK level is indeed possible in these cases. We also outline how the
foreground uncertainties may be rigorously propagated through to the CMB power
spectrum and cosmological parameters using a Gibbs sampling technique.Comment: 20 pages, 10 figures, submitted to ApJ. For a high-resolution
version, see http://www.astro.uio.no/~hke/docs/eriksen_et_al_fgfit.p
Engaging students in scenario-based assessment for final exams
We present our approaches to enhancing the authenticity of final exams across large first-year first semester biology units of cohort sizes between 300-1200 students. Historically exams were primarily used as an instrument that mainly assessed knowledge retention with limited provision of feedback to students. The necessity to shift to online learning during the height of the COVID-19 pandemic provided us with a challenging, yet opportune moment to transform our final examinations into an authentic learning experience for undergraduate biology students. We placed a large focus on integrating scenario-based questions in the final exam thereby assessing students’ ability to apply knowledge to real-world contexts. To enhance engagement with the assessment, we also provided personalised feedback for each student. With additional challenges around access to artificial intelligence and academic integrity, we share our experiences returning to in-person final examinations and evaluate the relevancy and benefits of scenario-based questions for student assessment and learning. We also share our approaches to feedforwarding initiatives to prepare students for examinations that is different to what most students would have experienced in their secondary schooling
Correlations of Richness and Global Properties in Galaxy Clusters
We measure the optical richness of galaxy clusters from the CNOC1 cluster
redshift survey using the galaxy-cluster center correlation amplitude B_gc. We
show that the B_gc values measured using photometric catalogs are consistent
with those derived from redshift catalogs, indicating that richness can be
measured reliably from photometric data alone, even at moderate redshifts of
\~0.6. We establish the correlations between optical richness and other
important attributes of a galaxy cluster, such as velocity dispersion, mass,
radius, and X-ray temperature and luminosity. We find that the scaling
relations of these quantities with richness are entirely consistent with those
derived by assuming a simple mass density profile at 0.5 Mpc of rho~r^{-1.8}.
The excellent correlations between B_gc and velocity dispersion and X-ray
temperature allow one to use richness, an easily measurable quantity using
relatively shallow optical imaging data alone, as a predictor of these
quantities at moderate redshifts. The B_gc parameter can be used to estimate
the velocity dispersion of a cluster to a precision of approximately 15%
(~+/-100 km/s), and X-ray temperature to about 20%. Similar correlations, but
with larger scatter, are also obtained between richness and the characteristic
radius and mass of the clusters. We compare the relative merits of B_gc, T_x,
and L_x as predictors of the dynamical mass, and find that they are comparable,
providing estimates at an accuracy of ~30%. We also perform similar analyses of
correlations between richness and velocity dispersion, T_x and L_x with a
sample of low-redshift Abell clusters and find consistent results, but with
larger scatter, which may be the result of a less homogeneous database, or
sample-dependent effects.Comment: 18 pages. Accepted; to appear in ApJ, March 200
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