2,356 research outputs found
Optimizing future imaging survey of galaxies to confront dark energy and modified gravity models
We consider the extent to which future imaging surveys of galaxies can
distinguish between dark energy and modified gravity models for the origin of
the cosmic acceleration. Dynamical dark energy models may have similar
expansion rates as models of modified gravity, yet predict different growth of
structure histories. We parameterize the cosmic expansion by the two
parameters, and , and the linear growth rate of density fluctuations
by Linder's , independently. Dark energy models generically predict
, while the DGP model . To determine
if future imaging surveys can constrain within 20 percent (or
), we perform the Fisher matrix analysis for a weak lensing
survey such as the on-going Hyper Suprime-Cam (HSC) project. Under the
condition that the total observation time is fixed, we compute the Figure of
Merit (FoM) as a function of the exposure time \texp. We find that the
tomography technique effectively improves the FoM, which has a broad peak
around \texp\simeq {\rm several}\sim 10 minutes; a shallow and wide survey is
preferred to constrain the parameter. While
cannot be achieved by the HSC weak-lensing survey alone, one can improve the
constraints by combining with a follow-up spectroscopic survey like WFMOS
and/or future CMB observations.Comment: 18 pages, typos correcte
Optical/Near-Infrared Observations of GRO J1744-28
We present results from a series of optical (g and r-band) and near-infrared
(K'-band) observations of the region of the sky including the entire XTE and
ROSAT error circles for the ``Bursting Pulsar'' GRO J1744-28. These data were
taken with the Astrophysical Research Consortium's 3.5-m telescope at Apache
Point Observatory and with the 2.2-m telescope at the European Southern
Observatory. We see no new object, nor any significant brightening of any known
object, in these error circles, with the exception of an object detected in our
8 February 1996 image. This object has already been proposed as a near-infrared
counterpart to GRO J1744-28. While it is seen in only two of our ten 8 February
frames, there is no evidence that this is an instrumental artifact, suggesting
the possibility of near-infrared flares from GRO J1744-28, similar to those
that have been reported from the Rapid Burster. The distance to the ``Bursting
Pulsar'' must be more than 2 kpc, and we suggest that it is more than 7 kpc.Comment: 21 pages, 5 JPEG plates, 2 postscript figures. This paper will appear
in the May 1, 1997 edition of the Astrophysical Journa
High-rate, high-fidelity entanglement of qubits across an elementary quantum network
We demonstrate remote entanglement of trapped-ion qubits via a
quantum-optical fiber link with fidelity and rate approaching those of local
operations. Two Sr qubits are entangled via the polarization
degree of freedom of two photons which are coupled by high-numerical-aperture
lenses into single-mode optical fibers and interfere on a beamsplitter. A novel
geometry allows high-efficiency photon collection while maintaining unit
fidelity for ion-photon entanglement. We generate remote Bell pairs with
fidelity at an average rate (success
probability ).Comment: v2 updated to include responses to reviewers, as published in PR
Superclustering at Redshift Z=0.54
We present strong evidence for the existence of a supercluster at a redshift
of z=0.54 in the direction of Selected Area 68. From the distribution of
galaxies with spectroscopic redshifts we find that there is a large
over-density of galaxies (a factor of four over the number expected in an
unclustered universe) within the redshift range 0.530 < z < 0.555. By
considering the spatial distribution of galaxies within this redshift range
(using spectroscopic and photometric redshifts) we show that the galaxies in
SA68 form a linear structure passing from the South-West of the survey field
through to the North-East (with a position angle of approximately 35 deg East
of North). This position angle is coincident with the positions of the X-ray
clusters CL0016+16, RX J0018.3+1618 and a new X-ray cluster, RX J0018.8+1602,
centered near the radio source 54W084. All three of these sources are at a
redshift of approximately z=0.54 and have position angles, derived from their
X-ray photon distributions, consistent with that measured for the supercluster.
Assuming a redshift of 0.54 for the distribution of galaxies and a FWHM
dispersion in redshift of 0.020 this represents a coherent structure with a
radial extent of 31 Mpc, transverse dimension of 12 Mpc, and a thickness of
approximately 4 Mpc. The detection of this possible supercluster demonstrates
the power of using X-ray observations, combined with multicolor observations,
to map the large scale distribution of galaxies at intermediate redshifts.Comment: 12 pages, 3 figures, Latex, aaspp4.sty, accepted for publication in
Ap J Letters. Figure 3 and followup observations can be found at
http://tarkus.pha.jhu.edu/~ajc/papers/supercluster
Galaxy Zoo : Building the low-mass end of the red sequence with local post-starburst galaxies
We present a study of local post-starburst galaxies (PSGs) using the photometric and spectroscopic observations from the Sloan Digital Sky Survey and the results from the Galaxy Zoo project. We find that the majority of our local PSG population have neither early- nor late-type morphologies but occupy a well-defined space within the colour-stellar mass diagram, most notably, the low-mass end of the 'green valley' below the transition mass thought to be the mass division between low-mass star-forming galaxies and high-mass passively evolving bulge-dominated galaxies. Our analysis suggests that it is likely that local PSGs will quickly transform into 'red', low-mass early-type galaxies as the stellar morphologies of the 'green' PSGs largely resemble that of the early-type galaxies within the same mass range. We propose that the current population of PSGs represents a population of galaxies which is rapidly transitioning between the star-forming and the passively evolving phases. Subsequently, these PSGs will contribute towards the build-up of the low-mass end of the 'red sequence' once the current population of young stars fade and stars are no longer being formed. These results are consistent with the idea of 'downsizing' where the build-up of smaller galaxies occurs at later epochs.Peer reviewe
Colors, magnitudes and velocity dispersions in early-type galaxies: Implications for galaxy ages and metallicities
We present an analysis of the color-magnitude-velocity dispersion relation
for a sample of 39320 early-type galaxies within the Sloan Digital Sky Survey.
We demonstrate that the color-magnitude relation is entirely a consequence of
the fact that both the luminosities and colors of these galaxies are correlated
with stellar velocity dispersions. Previous studies of the color-magnitude
relation over a range of redshifts suggest that the luminosity of an early-type
galaxy is an indicator of its metallicity, whereas residuals in color from the
relation are indicators of the luminosity-weighted age of its stars. We show
that this, when combined with our finding that velocity dispersion plays a
crucial role, has a number of interesting implications. First, galaxies with
large velocity dispersions tend to be older (i.e., they scatter redward of the
color-magnitude relation). Similarly, galaxies with large dynamical mass
estimates also tend to be older. In addition, at fixed luminosity, galaxies
which are smaller, or have larger velocity dispersions, or are more massive,
tend to be older. Second, models in which galaxies with the largest velocity
dispersions are also the most metal poor are difficult to reconcile with our
data. However, at fixed velocity dispersion, galaxies have a range of ages and
metallicities: the older galaxies have smaller metallicities, and vice-versa.
Finally, a plot of velocity dispersion versus luminosity can be used as an age
indicator: lines of constant age run parallel to the correlation between
velocity dispersion and luminosity.Comment: 12 pages, 9 figures. Accepted by A
- âŚ