2,870 research outputs found
Cloning Dropouts: Implications for Galaxy Evolution at High Redshift
The evolution of high redshift galaxies in the two Hubble Deep Fields, HDF-N
and HDF-S, is investigated using a cloning technique that replicates z~ 2-3 U
dropouts to higher redshifts, allowing a comparison with the observed B and V
dropouts at higher redshifts (z ~ 4-5). We treat each galaxy selected for
replication as a set of pixels that are k-corrected to higher redshift,
accounting for resampling, shot-noise, surface-brightness dimming, and the
cosmological model. We find evidence for size evolution (a 1.7x increase) from
z ~ 5 to z ~ 2.7 for flat geometries (Omega_M+Omega_LAMBDA=1.0). Simple scaling
laws for this cosmology predict that size evolution goes as (1+z)^{-1},
consistent with our result. The UV luminosity density shows a similar increase
(1.85x) from z ~ 5 to z ~ 2.7, with minimal evolution in the distribution of
intrinsic colors for the dropout population. In general, these results indicate
less evolution than was previously reported, and therefore a higher luminosity
density at z ~ 4-5 (~ 50% higher) than other estimates. We argue the present
technique is the preferred way to understand evolution across samples with
differing selection functions, the most relevant differences here being the
color cuts and surface brightness thresholds (e.g., due to the (1+z)^4 cosmic
surface brightness dimming effect).Comment: 56 pages, 22 figures, accepted for publication in Ap
Comparison of lightning location data and polarisation radar observations of clouds
Simultaneous observations of both the precipitation and the lightning associated with thunderstorms show that the lightning is within 3 km of the maximum precipitation echo. The intensity and type of the precipitation is observed with 500 m spatial accuracy using an S-band polarization radar and the position of the lightning is inferred from a low frequency magnetic direction finding location system. Empirical adjustment to the angles using the redundancy of the lightning data reduce this error. Radar echoes above 45dBZ may be caused by soft hail or hailstones, but similarly intense echoes may result from melting snow. The data show that a new polarization radar parameter, the linear depolarization ratio, can distinguish between soft hail and melting snow, and that the intense radar echoes associated with melting snow pose no threat of lightning. A lightning risk only exists when the radar indicates that the clouds contain soft hail or hailstones
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Estimating drizzle drop size and precipitation rate using two-colour lidar measurements
A method to estimate the size and liquid water content of drizzle drops using lidar measurements at two wavelengths is described. The method exploits the differential absorption of infrared light by liquid water at 905 nm and 1.5 μm, which leads to a different backscatter cross section for water drops larger than ≈50 μm. The ratio of backscatter measured from drizzle samples below cloud base at these two wavelengths (the colour ratio) provides a measure of the median volume drop diameter D0. This is a strong effect: for D0=200 μm, a colour ratio of ≈6 dB is predicted. Once D0 is known, the measured backscatter at 905 nm can be used to calculate the liquid water content (LWC) and other moments of the drizzle drop distribution.
The method is applied to observations of drizzle falling from stratocumulus and stratus clouds. High resolution (32 s, 36 m) profiles of D0, LWC and precipitation rate R are derived. The main sources of error in the technique are the need to assume a value for the dispersion parameter μ in the drop size spectrum (leading to at most a 35% error in R) and the influence of aerosol returns on the retrieval (≈10% error in R for the cases considered here). Radar reflectivities are also computed from the lidar data, and compared to independent measurements from a colocated cloud radar, offering independent validation of the derived drop size distributions
Theory and observations of ice particle evolution in cirrus using Doppler radar: evidence for aggregation
Vertically pointing Doppler radar has been used to study the evolution of ice
particles as they sediment through a cirrus cloud. The measured Doppler fall
speeds, together with radar-derived estimates for the altitude of cloud top,
are used to estimate a characteristic fall time tc for the `average' ice
particle. The change in radar reflectivity Z is studied as a function of tc,
and is found to increase exponentially with fall time. We use the idea of
dynamically scaling particle size distributions to show that this behaviour
implies exponential growth of the average particle size, and argue that this
exponential growth is a signature of ice crystal aggregation.Comment: accepted to Geophysical Research Letter
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Feedback control of oscillations in combustion and cavity flows
This thesis considers the control of combustion oscillations, motivated by the susceptibility of lean premixed combustion to such oscillations, and the long and expensive development and commissioning times that this is giving rise to. The controller used is both closed-loop, employing an actuator to modify some system parameter in response to a measured signal, and adaptive, meaning that it is able to maintain control over a wide range of operating conditions. The controller is applied to combustion systems with annular geometries, where instabilities can occur both longitudinally and azimuthally, and which require multiple sensors and multiple actuators for control.
One of the requirements of Lyapunov-based adaptive control which is particularly troublesome for combustion systems is then addressed: that the sign of the high-frequency gain of the open-loop system is known. We address it by using an adaptive controller which employs a Nussbaum gain, and successfully apply it experimentally to combustion oscillations in a Rijke tube.
Another type of fluid-acoustic resonance is then considered: the compressible flow past a shallow cavity. We start by finding a linear model of the cavity flow's dynamics, or its `transfer function', which we identify from direct numerical simulations. We compare this measured transfer function to that given by a conceptual model which is based on the Rossiter mechanism, and which models each component of the flow physics separately.
We then look at using closed-loop control to eliminate these cavity oscillations. We start by designing a robust controller based on a balanced reduced order model of the system, the model being provided by the Eigensystem Realization Algorithm (ERA). The robust controller provides closed-loop stability over a much wider Mach number range than seen in previous studies. Finally, we look at the suitability of the adaptive controller, earlier developed for combustion oscillations, for the cavity. Based on some general properties of the cavity flow, and by using collocated control, the oscillations are eliminated at all Mach numbers tested in the range .This work was supported by the EPSRC and Rolls-Royce plc
Constraints on z~10 Galaxies from the Deepest HST NICMOS Fields
We use all available fields with deep NICMOS imaging to search for J dropouts
(H<28) at z~10. Our primary data set for this search were the two J+H NICMOS
parallel fields taken with the ACS HUDF. The 5 sigma limiting mags were 28.6 in
J and 28.5 in H. Several shallower fields were also used: J+H NICMOS frames
available over the HDF North, the HDF South NICMOS parallel, and the ACS HUDF.
The primary selection criterion was (J-H)>1.8. 11 such sources were found in
all search fields using this criterion. 8 of these were clearly ruled out as
credible z~10 sources, either as a result of detections (>2 sigma) blueward of
J or their colors redward of the break (H-K~1.5). The nature of the 3 remaining
sources could not be determined from the data. The number appears consistent
with the expected contamination from low-z interlopers. Analysis of the stacked
images for the 3 candidates also suggests contamination. Regardless of their
true redshifts, the actual number of z~10 sources must be <=3. To assess the
significance of these results, two lower redshift samples (a z~3.8 B-dropout
and z~6 i-dropout sample) were projected to z~8-12 using a (1+z)^{-1} size
scaling. They were added to the image frames, and the selection repeated,
giving 15.6 and 4.8 J-dropouts, respectively. This suggests that to the limit
of this probe (0.3 L*) there has been evolution from z~3.8 and possibly from
z~6. This is consistent with the strong evolution already noted at z~6 and
z~7.5 relative to z~3-4. Even assuming that 3 sources from this probe are at
z~10, the rest-frame continuum UV (~1500 A) luminosity density at z~10
(integrated down to 0.3 L*) is just 0.19_{-0.09}^{+0.13}x that at z~3.8 (or
0.19_{-0.10}^{+0.15}x including cosmic variance). However, if none of our
sources is at z~10, this ratio has a 1 sigma upper limit of 0.07. (abridged)Comment: 13 pages, 3 figures, 2 tables, accepted for publication in the
Astrophysical Journal Letter
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