4,509 research outputs found
Radio Galaxy Clustering at z~0.3
Radio galaxies are uniquely useful as probes of large-scale structure as
their uniform identification with giant elliptical galaxies out to high
redshift means that the evolution of their bias factor can be predicted. As the
initial stage in a project to study large-scale structure with radio galaxies
we have performed a small redshift survey, selecting 29 radio galaxies in the
range 0.19<z<0.45 from a contiguous 40 square degree area of sky. We detect
significant clustering within this sample. The amplitude of the two-point
correlation function we measure is consistent with no evolution from the local
(z<0.1) value. This is as expected in a model in which radio galaxy hosts form
at high redshift and thereafter obey a continuity equation, although the
signal:noise of the detection is too low to rule out other models. Larger
surveys out to z~1 should reveal the structures of superclusters at
intermediate redshifts and strongly constrain models for the evolution of
large-scale structure.Comment: 7 pages, 3 figures, accepted by ApJ Letter
Dynamical Masses in Modified Gravity
Differences in masses inferred from dynamics, such as velocity dispersions or
X-rays, and those inferred from lensing are a generic prediction of modified
gravity theories. Viable models however must include some non-linear mechanism
to restore General Relativity (GR) in dense environments, which is necessary to
pass Solar System constraints on precisely these deviations. In this paper, we
study the dynamics within virialized structures in the context of two modified
gravity models, f(R) gravity and DGP. The non-linear mechanisms to restore GR,
which f(R) and DGP implement in very different ways, have a strong impact on
the dynamics in bound objects; they leave distinctive signatures in the
dynamical mass-lensing mass relation as a function of mass and radius. We
present measurements from N-body simulations of f(R) and DGP, as well as
semi-analytical models which match the simulation results to surprising
accuracy in both cases. The semi-analytical models are useful for making the
connection to observations. Our results confirm that the environment- and
scale-dependence of the modified gravity effects have to be taken into account
when confronting gravity theories with observations of dynamics in galaxies and
clusters.Comment: 18 pages, 16 figures; submitted to PRD; v2: typos corrected,
references added, minor additions (Sec. IID
Simulated Extragalactic Observations with a Cryogenic Imaging Spectrophotometer
In this paper we explore the application of cryogenic imaging
spectrophotometers. Prototypes of this new class of detector, such as
superconducting tunnel junctions (STJs) and transition edge sensors (TESs),
currently deliver low resolution imaging spectrophotometry with high quantum
efficiency (70-100%) and no read noise over a wide bandpass in the visible to
near-infrared. In order to demonstrate their utility and the differences in
observing strategy needed to maximize their scientific return, we present
simulated observations of a deep extragalactic field. Using a simple analytic
technique, we can estimate both the galaxy redshift and spectral type more
accurately than is possible with current broadband techniques. From our
simulated observations and a subsequent discussion of the expected migration
path for this new technology, we illustrate the power and promise of these
devices.Comment: 30 pages, 10 figures, accepted for publication in the Astronomical
Journa
The Evolution of Radio Galaxies at Intermediate Redshift
We describe a new estimate of the radio galaxy 1.4 GHz luminosity function
and its evolution at intermediate redshifts (z~0.4). Photometric redshifts and
color selection have been used to select Bj<23.5 early-type galaxies from the
Panoramic Deep Fields, a multicolor survey of two 25 sq deg fields.
Approximately 230 radio galaxies have then been selected by matching early-type
galaxies with NVSS radio sources brighter than 5 mJy. Estimates of the 1.4 GHz
luminosity function of radio galaxies measure significant evolution over the
observed redshift range. For an Omega_M=1 cosmology the evolution of the radio
power is consistent with luminosity evolution where P(z)=P(0)(1+z)^{k_L} and
3<k_L<5. The observed evolution is similar to that observed for UVX and X-ray
selected AGN and is consistent with the same physical process being responsible
for the optical and radio luminosity evolution of AGN.Comment: 26 pages, 9 Figures, Accepted for Publication in A
Measuring the galaxy power spectrum with future redshift surveys
Precision measurements of the galaxy power spectrum P(k) require a data
analysis pipeline that is both fast enough to be computationally feasible and
accurate enough to take full advantage of high-quality data. We present a
rigorous discussion of different methods of power spectrum estimation, with
emphasis on the traditional Fourier method, the linear (Karhunen-Loeve; KL),
and quadratic data compression schemes, showing in what approximations they
give the same result. To improve speed, we show how many of the advantages of
KL data compression and power spectrum estimation may be achieved with a
computationally faster quadratic method. To improve accuracy, we derive
analytic expressions for handling the integral constraint, since it is crucial
that finite volume effects are accurately corrected for on scales comparable to
the depth of the survey. We also show that for the KL and quadratic techniques,
multiple constraints can be included via simple matrix operations, thereby
rendering the results less sensitive to galactic extinction and mis-estimates
of the radial selection function. We present a data analysis pipeline that we
argue does justice to the increases in both quality and quantity of data that
upcoming redshift surveys will provide. It uses three analysis techniques in
conjunction: a traditional Fourier approach on small scales, a pixelized
quadratic matrix method on large scales and a pixelized KL eigenmode analysis
to probe anisotropic effects such as redshift-space distortions.Comment: Major revisions for clarity. Matches accepted ApJ version. 23 pages,
with 2 figs included. Color figure and links at
http://www.sns.ias.edu/~max/galpower.html (faster from the US), from
http://www.mpa-garching.mpg.de/~max/galpower.html (faster from Europe) or
from [email protected]
Designing a mobile augmented memory system for people with traumatic brain injuries
Augmented memory systems help people remember events in their lives. Individuals with Traumatic Brain Injury (TBI) often have memory impairments. We conducted a user study to learn about strategies individuals with TBI use to remember events in their lives. We explored what characteristics individuals with TBI expect of an augmented memory system. We then investigated these aspects in an initial mobile app design, and propose here a concept for a rehearsal application that addresses the issues found in our studies
Clustering Analyses of 300,000 Photometrically Classified Quasars--I. Luminosity and Redshift Evolution in Quasar Bias
Using ~300,000 photometrically classified quasars, by far the largest quasar
sample ever used for such analyses, we study the redshift and luminosity
evolution of quasar clustering on scales of ~50 kpc/h to ~20 Mpc/h from
redshifts of z~0.75 to z~2.28. We parameterize our clustering amplitudes using
realistic dark matter models, and find that a LCDM power spectrum provides a
superb fit to our data with a redshift-averaged quasar bias of b_Q =
2.41+/-0.08 () for . This represents a better
fit than the best-fit power-law model (; ). We find b_Q increases with redshift.
This evolution is significant at >99.6% using our data set alone, increasing to
>99.9999% if stellar contamination is not explicitly parameterized. We measure
the quasar classification efficiency across our full sample as a = 95.6 +/-
^{4.4}_{1.9}%, a star-quasar separation comparable with the star-galaxy
separation in many photometric studies of galaxy clustering. We derive the mean
mass of the dark matter halos hosting quasars as MDMH=(5.2+/-0.6)x10^{12}
M_solar/h. At z~1.9 we find a deviation from luminosity-independent
quasar clustering; this suggests that increasing our sample size by a factor of
1.8 could begin to constrain any luminosity dependence in quasar bias at z~2.
Our results agree with recent studies of quasar environments at z < 0.4, which
detected little luminosity dependence to quasar clustering on proper scales >50
kpc/h. At z < 1.6, our analysis suggests that b_Q is constant with luminosity
to within ~0.6, and that, for g < 21, angular quasar autocorrelation
measurements are unlikely to have sufficient statistical power at z < 1.6 to
detect any luminosity dependence in quasars' clustering.Comment: 13 pages, 9 figures, 2 tables; uses amulateapj; accepted to Ap
Recovering the Primordial Density Fluctuations: A comparison of methods
We present a comparative study of six different methods for reversing the
gravitational evolution of a cosmological density field to recover the
primordial fluctuations: linear theory, the Gaussianization mapping scheme, two
different quasi-linear dynamical schemes based on the Zel'dovich approximation,
a Hybrid dynamical-Gaussianization method and the Path Interchange Zel'dovich
Approximation (PIZA). The final evolved density field from an N-body simulation
constitutes our test case. We use a variety of statistical measures to compare
the initial density field recovered from it to the true initial density field,
using each of the six different schemes. These include point-by-point
comparisons of the density fields in real space, the individual modes in
Fourier space, as well as global statistical properties such as the genus, the
PDF of the density, and the distribution of peak heights and their shapes. We
find linear theory to be the most inaccurate of all the schemes. The
Gaussianization scheme is the least accurate after linear theory. The two
quasi-linear dynamical schemes are more accurate than Gaussianization, although
they break down quite drastically when used outside their range of validity -
the quasi-linear regime. The complementary beneficial aspects of the dynamical
and the Gaussianization schemes are combined in the Hybrid method. We find this
Hybrid scheme to be more accurate and robust than either Gaussianization or the
dynamical method alone. The PIZA scheme performs substantially better than the
others in all point-by-point comparisons. However, it produces an oversmoothed
initial density field, with a smaller number of peaks than expected, but
recovers the PDF of the initial density with impressive accuracy on scales as
small as 3Mpc/h.Comment: 39 pages, including 13 Figures, submitted to Ap
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