182 research outputs found
Segmented waveguides in thin silicon-on-insulator
We have developed new silicon-on-insulator waveguide designs for simultaneously achieving both low-loss optical confinement and electrical contacts, and we present a design methodology based on calculating the Bloch modes of such segmented waveguides. With this formalism, waveguides are designed in a single thin layer of silicon-on-insulator to achieve both optical confinement and minimal insertion loss. Waveguides were also fabricated and tested, and the measured data were found to closely agree with theoretical predictions, demonstrating input insertion loss and propagation loss better than 0.1 dB and -16 dB/cm, respectively
How neutral is the intergalactic medium surrounding the redshift z=7.085 quasar ULAS J1120+0641?
The quasar ULAS J1120+0641 at redshift z=7.085 has a highly ionised near zone
which is smaller than those around quasars of similar luminosity at z~6. The
spectrum also exhibits evidence for a damping wing extending redward of the
systemic Lya redshift. We use radiative transfer simulations in a cosmological
context to investigate the implications for the ionisation state of the
inhomogeneous IGM surrounding this quasar. Our simulations show that the
transmission profile is consistent with an IGM in the vicinity of the quasar
with a volume averaged HI fraction of f_HI>0.1 and that ULAS J1120+0641 has
been bright for 10^6--10^7 yr. The observed spectrum is also consistent with
smaller IGM neutral fractions, f_HI ~ 10^-3--10-4, if a damped Lya system in an
otherwise highly ionised IGM lies within 5 proper Mpc of the quasar. This is,
however, predicted to occur in only ~5 per cent of our simulated sight-lines
for a bright phase of 10^6--10^7 yr. Unless ULAS J1120+0641 grows during a
previous optically obscured phase, the low age inferred for the quasar adds to
the theoretical challenge of forming a 2x10^9 M_sol black hole at this high
redshift.Comment: 5 pages, 4 figures, accepted to MNRAS letter
The Multi-Object, Fiber-Fed Spectrographs for SDSS and the Baryon Oscillation Spectroscopic Survey
We present the design and performance of the multi-object fiber spectrographs
for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon
Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999
on the 2.5-m aperture Sloan Telescope at Apache Point Observatory, the
spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II
surveys, enabling a wide variety of Galactic and extra-galactic science
including the first observation of baryon acoustic oscillations in 2005. The
spectrographs were upgraded in 2009 and are currently in use for BOSS, the
flagship survey of the third-generation SDSS-III project. BOSS will measure
redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyman-alpha
absorption of 160,000 high redshift quasars over 10,000 square degrees of sky,
making percent level measurements of the absolute cosmic distance scale of the
Universe and placing tight constraints on the equation of state of dark energy.
The twin multi-object fiber spectrographs utilize a simple optical layout
with reflective collimators, gratings, all-refractive cameras, and
state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in
two channels over a bandpass covering the near ultraviolet to the near
infrared, with a resolving power R = \lambda/FWHM ~ 2000. Building on proven
heritage, the spectrographs were upgraded for BOSS with volume-phase
holographic gratings and modern CCD detectors, improving the peak throughput by
nearly a factor of two, extending the bandpass to cover 360 < \lambda < 1000
nm, and increasing the number of fibers from 640 to 1000 per exposure. In this
paper we describe the original SDSS spectrograph design and the upgrades
implemented for BOSS, and document the predicted and measured performances.Comment: 43 pages, 42 figures, revised according to referee report and
accepted by AJ. Provides background for the instrument responsible for SDSS
and BOSS spectra. 4th in a series of survey technical papers released in
Summer 2012, including arXiv:1207.7137 (DR9), arXiv:1207.7326 (Spectral
Classification), and arXiv:1208.0022 (BOSS Overview
The discovery of an M4+T8.5 binary system
The original article can be found at: http://www3.interscience.wiley.com Copyright Blackwell Publishing / Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.14620.xWe report the discovery of a T8.5 dwarf, which is a companion to the M4 dwarf Wolf 940. [Please see original online abstract for complete version with correct notation]Peer reviewe
Post-starburst galaxies: more than just an interesting curiosity
From the VIMOS VLT DEEP Survey (VVDS) we select a sample of 16 galaxies with
spectra which identify them as having recently undergone a strong starburst and
subsequent fast quenching of star formation. These post-starburst galaxies lie
in the redshift range 0.510^9.75Msun. They have a number
density of 1x10^-4 per Mpc^3, almost two orders of magnitude sparser than the
full galaxy population with the same mass limit. We compare with simulations to
show that the galaxies are consistent with being the descendants of gas rich
major mergers. Starburst mass fractions must be larger than ~5-10% and decay
times shorter than ~10^8 years for post-starburst spectral signatures to be
observed in the simulations. We find that the presence of black hole feedback
does not greatly affect the evolution of the simulated merger remnants through
the post-starburst phase. The multiwavelength spectral energy distributions of
the post-starburst galaxies show that 5/16 have completely ceased the formation
of new stars. These 5 galaxies correspond to a mass flux entering the
red-sequence of rhodot(A->Q, PSB) = 0.0038Msun/Mpc^3/yr, assuming the defining
spectroscopic features are detectable for 0.35Gyr. If the galaxies subsequently
remain on the red sequence, this accounts for 38(+4/-11)% of the growth rate of
the red sequence. Finally, we compare our high redshift results with a sample
of galaxies with 0.05<z<0.1 observed in the SDSS and UKIDSS surveys. We find a
very strong redshift evolution: the mass density of strong post-starburst
galaxies is 230 times lower at z~0.07 than at z~0.7.Comment: 18 pages, 12 figures, to match version accepted to MNRAS. Minor
reordering of text in places and Sec 2.2 on SPH simulation comparisons
expande
The End of the Rainbow: What Can We Say About the Extragalactic Sub-Megahertz Radio Sky?
The Galactic disc is opaque to radio waves from extragalactic sources with
frequencies nu less than ~3 MHz. However, radio waves with kHz, Hz, and even
lower frequencies may propagate through the intergalactic medium (IGM). I argue
that the presence of these waves can be inferred by using the Universe as our
detector. I discuss possible sub-MHz sources and set new non-trivial upper
limits on the energy density of sub-MHz radio waves in galaxy clusters and the
average cosmic background. Limits based on five effects are considered: (1)
changes in the expansion of the Universe from the radiation energy density (2)
heating of the IGM by free-free absorption; (3) radiation pressure squeezing of
IGM clouds by external radio waves; (4) synchrotron heating of electrons in
clusters; and (5) Inverse Compton upscattering of sub-MHz radio photons. Any
sub-MHz background must have an energy density much smaller than the CMB at
frequencies below 1 MHz. The free-free absorption bounds from the Lyman-alpha
forest are potentially the strongest, but are highly dependent on the
properties of sub-MHz radio scattering in the IGM. I estimate an upper limit of
6 * 10^4 L_sun Mpc^-3 for the emissivity within Lyman-alpha forest clouds in
the frequency range 5 - 200 Hz. The sub-MHz energy density in the Coma cluster
is constrained to be less than ~10^-15 erg cm^-3. At present, none of the
limits is strong enough to rule out a maximal T_b = 10^12 K sub-MHz synchrotron
background, but other sources may be constrained with a better knowledge of
sub-MHz radio propagation in the IGM.Comment: Accepted by MNRAS, 4 figures, 20 pages
Galaxy Environments in the UKIDSS Ultra Deep Survey (UDS)
We present a study of galaxy environments to z~2, based on a sample of over
33,000 K-band selected galaxies detected in the UKIDSS Ultra Deep Survey (UDS).
The combination of infrared depth and area in the UDS allows us to extend
previous studies of galaxy environment to z>1 without the strong biases
associated with optical galaxy selection. We study the environments of galaxies
divided by rest frame (U-B) colours, in addition to `passive' and
`star-forming' subsets based on template fitting. We find that galaxy colour is
strongly correlated with galaxy overdensity on small scales (<1Mpc diameter),
with red/passive galaxies residing in significantly denser environments than
blue/star-forming galaxies to z~1.5. On smaller scales (<0.5Mpc diameter) we
also find a relationship between galaxy luminosity and environment, with the
most luminous blue galaxies at z~1 inhabiting environments comparable to red,
passive systems at the same redshift. Monte Carlo simulations demonstrate that
these conclusions are robust to the uncertainties introduced by photometric
redshift errors.Comment: Accepted for publication in MNRAS. The paper contains 9 pages, 11
figures and 1 tabl
Signatures of Hierarchical Clustering in Dark Matter Detection Experiments
In the cold dark matter model of structure formation, galaxies are assembled
hierarchically from mergers and the accretion of subclumps. This process is
expected to leave residual substructure in the Galactic dark halo, including
partially disrupted clumps and their associated tidal debris. We develop a
model for such halo substructure and study its implications for dark matter
(WIMP and axion) detection experiments. We combine the Press-Schechter model
for the distribution of halo subclump masses with N-body simulations of the
evolution and disruption of individual clumps as they orbit through the
evolving Galaxy to derive the probability that the Earth is passing through a
subclump or stream of a given density. Our results suggest that it is likely
that the local complement of dark matter particles includes a 1-5% contribution
from a single clump. The implications for dark matter detection experiments are
significant, since the disrupted clump is composed of a `cold' flow of
high-velocity particles. We describe the distinctive features due to halo
clumps that would be seen in the energy and angular spectra of detection
experiments. The annual modulation of these features would have a different
signature and phase from that for a smooth halo and, in principle, would allow
one to discern the direction of motion of the clump relative to the Galactic
center.Comment: 26 pages, 18 figure
The SDSS Quasar Survey: Quasar Luminosity Function from Data Release Three
We determine the number counts and z=0-5 luminosity function for a
well-defined, homogeneous sample of quasars from the Sloan Digital Sky Survey
(SDSS). We conservatively define the most uniform statistical sample possible,
consisting of 15,343 quasars within an effective area of 1622 deg^2 that was
derived from a parent sample of 46,420 spectroscopically confirmed broad-line
quasars in the 5282 deg^2 of imaging data from SDSS Data Release Three. The
sample extends from i=15 to i=19.1 at z3. The number
counts and luminosity function agree well with the results of the 2dF QSO
Survey, but the SDSS data probe to much higher redshifts than does the 2dF
sample. The number density of luminous quasars peaks between redshifts 2 and 3,
although uncertainties in the selection function in this range do not allow us
to determine the peak redshift more precisely. Our best fit model has a flatter
bright end slope at high redshift than at low redshift. For z<2.4 the data are
best fit by a redshift-independent slope of beta = -3.1 (Phi(L) propto L^beta).
Above z=2.4 the slope flattens with redshift to beta=-2.37 at z=5. This slope
change, which is significant at a >5-sigma level, must be accounted for in
models of the evolution of accretion onto supermassive black holes.Comment: 57 pages, 21 figures (9 color); minor changes to reflect the version
accepted by AJ; higher resolution version available at
ftp://ftp.astro.princeton.edu/gtr/dr3qlf/Feb1306
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