108,253 research outputs found
Nonlinear Design Technique for High-Power Switching-Mode Oscillators
A simple nonlinear technique for the design of high-efficiency and high-power switching-mode oscillators is presented. It combines existing quasi-nonlinear methods and the use of an auxiliary generator (AG) in harmonic balance. The AG enables the oscillator optimization to achieve high output power and dc-to-RF conversion efficiency without affecting the oscillation frequency. It also imposes a sufficient drive on the transistor to enable the switching-mode operation with high efficiency. Using this AG, constant-power and constant-efficiency contour plots are traced in order to determine the optimum element values. The oscillation startup condition and the steady-state stability are analyzed with the pole-zero identification technique. The influence of the gate bias on the output power, efficiency, and stability is also investigated. A class-E oscillator is demonstrated using the proposed technique. The oscillator exhibits 75 W with 67% efficiency at 410 MHz
Synthetic Galaxy Images and Spectra from the Illustris Simulation
We present our methods for generating a catalog of 7,000 synthetic images and
40,000 integrated spectra of redshift z = 0 galaxies from the Illustris
Simulation. The mock data products are produced by using stellar population
synthesis models to assign spectral energy distributions (SED) to each star
particle in the galaxies. The resulting synthetic images and integrated SEDs
therefore properly reflect the spatial distribution, stellar metallicity
distribution, and star formation history of the galaxies. From the synthetic
data products it is possible to produce monochromatic or color-composite
images, perform SED fitting, classify morphology, determine galaxy structural
properties, and evaluate the impacts of galaxy viewing angle. The main
contribution of this paper is to describe the production, format, and
composition of the image catalog that makes up the Illustris Simulation
Obsevatory. As a demonstration of this resource, we derive galactic stellar
mass estimates by applying the SED fitting code FAST to the synthetic galaxy
products, and compare the derived stellar masses against the true stellar
masses from the simulation. We find from this idealized experiment that
systematic biases exist in the photometrically derived stellar mass values that
can be reduced by using a fixed metallicity in conjunction with a minimum
galaxy age restriction.Comment: 21 pages, 17 figures, submitted to MNRAS. Comments welcom
The extended epoch of galaxy formation: age dating of ~3600 galaxies with 2<z<6.5 in the VIMOS Ultra-Deep Survey
We aim at improving constraints on the epoch of galaxy formation by measuring
the ages of 3597 galaxies with spectroscopic redshifts 2<z<6.5 in the VIMOS
Ultra Deep Survey (VUDS). We derive ages and other physical parameters from the
simultaneous fitting with the GOSSIP+ software of observed UV rest-frame
spectra and photometric data from the u-band up to 4.5 microns using composite
stellar population models. We conclude from extensive simulations that at z>2
the joint analysis of spectroscopy and photometry combined with restricted age
possibilities when taking into account the age of the Universe substantially
reduces systematic uncertainties and degeneracies in the age derivation. We
find galaxy ages ranging from very young with a few tens of million years to
substantially evolved with ages up to ~1.5-2 Gyr. The formation redshifts z_f
derived from the measured ages indicate that galaxies may have started forming
stars as early as z_f~15. We produce the formation redshift function (FzF), the
number of galaxies per unit volume formed at a redshift z_f, and compare the
FzF in increasing redshift bins finding a remarkably constant 'universal' FzF.
The FzF is parametrized with (1+z)^\zeta, with \zeta~0.58+/-0.06, indicating a
smooth 2 dex increase from z~15 to z~2. Remarkably this observed increase is of
the same order as the observed rise in the star formation rate density (SFRD).
The ratio of the SFRD with the FzF gives an average SFR per galaxy of
~7-17Msun/yr at z~4-6, in agreement with the measured SFR for galaxies at these
redshifts. From the smooth rise in the FzF we infer that the period of galaxy
formation extends from the highest possible redshifts that we can probe at z~15
down to redshifts z~2. This indicates that galaxy formation is a continuous
process over cosmic time, with a higher number of galaxies forming at the peak
in SFRD at z~2 than at earlier epochs. (Abridged)Comment: Submitted to A&A, 24 page
Recovering Stellar Population Properties and Redshifts from Broad-Band Photometry of Simulated Galaxies: Lessons for SED Modeling
We present a detailed analysis of our ability to determine stellar masses,
ages, reddening and extinction values, and star formation rates of
high-redshift galaxies by modeling broad-band SEDs with stellar population
synthesis. In order to do so, we computed synthetic optical-to-NIR SEDs for
model galaxies taken from hydrodynamical merger simulations placed at redshifts
1.5 < z < 3. Viewed under different angles and during different evolutionary
phases, the simulations represent a wide variety of galaxy types (disks,
mergers, spheroids). We show that simulated galaxies span a wide range in SEDs
and color, comparable to these of observed galaxies. In all star-forming
phases, dust attenuation has a large effect on colors, SEDs, and fluxes. The
broad-band SEDs were then fed to a standard SED modeling procedure and
resulting stellar population parameters were compared to their true values.
Disk galaxies generally show a decent median correspondence between the true
and estimated mass and age, but suffer from large uncertainties. During the
merger itself, we find larger offsets (e.g., log M_recovered - log M_true =
-0.13^{+0.10}_{-0.14}). E(B-V) values are generally recovered well, but the
estimated total visual absorption Av is consistently too low, increasingly so
for larger optical depths. Since the largest optical depths occur during the
phases of most intense star formation, it is for the highest SFRs that we find
the largest underestimates. The masses, ages, E(B-V), Av, and SFR of merger
remnants (spheroids) are very well reproduced. We discuss possible biases in
SED modeling results caused by mismatch between the true and template star
formation history, dust distribution, metallicity variations and AGN
contribution.Comment: Accepted for publication in the Astrophysical Journal, 24 pages, 19
figure
Polarised foreground removal at low radio frequencies using rotation measure synthesis: uncovering the signature of hydrogen reionisation
Measurement of redshifted 21-cm emission from neutral hydrogen promises to be
the most effective method for studying the reionisation history of hydrogen
and, indirectly, the first galaxies. These studies will be limited not by raw
sensitivity to the signal, but rather, by bright foreground radiation from
Galactic and extragalactic radio sources and the Galactic continuum. In
addition, leakage due to gain errors and non-ideal feeds conspire to further
contaminate low-frequency radio obsevations. This leakage leads to a portion of
the complex linear polarisation signal finding its way into Stokes I, and
inhibits the detection of the non-polarised cosmological signal from the epoch
of reionisation. In this work, we show that rotation measure synthesis can be
used to recover the signature of cosmic hydrogen reionisation in the presence
of contamination by polarised foregrounds. To achieve this, we apply the
rotation measure synthesis technique to the Stokes I component of a synthetic
data cube containing Galactic foreground emission, the effect of instrumental
polarisation leakage, and redshifted 21-cm emission by neutral hydrogen from
the epoch of reionisation. This produces an effective Stokes I Faraday
dispersion function for each line of sight, from which instrumental
polarisation leakage can be fitted and subtracted. Our results show that it is
possible to recover the signature of reionisation in its late stages (z ~ 7) by
way of the 21-cm power spectrum, as well as through tomographic imaging of
ionised cavities in the intergalactic medium.Comment: 22 pages including 11 figures. Minor revisions following referee's
report. MNRAS, in pres
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