27 research outputs found
Beyond maximum entropy: Fractal Pixon-based image reconstruction
We have developed a new Bayesian image reconstruction method that has been shown to be superior to the best implementations of other competing methods, including Goodness-of-Fit methods such as Least-Squares fitting and Lucy-Richardson reconstruction, as well as Maximum Entropy (ME) methods such as those embodied in the MEMSYS algorithms. Our new method is based on the concept of the pixon, the fundamental, indivisible unit of picture information. Use of the pixon concept provides an improved image model, resulting in an image prior which is superior to that of standard ME. Our past work has shown how uniform information content pixons can be used to develop a 'Super-ME' method in which entropy is maximized exactly. Recently, however, we have developed a superior pixon basis for the image, the Fractal Pixon Basis (FPB). Unlike the Uniform Pixon Basis (UPB) of our 'Super-ME' method, the FPB basis is selected by employing fractal dimensional concepts to assess the inherent structure in the image. The Fractal Pixon Basis results in the best image reconstructions to date, superior to both UPB and the best ME reconstructions. In this paper, we review the theory of the UPB and FPB pixon and apply our methodology to the reconstruction of far-infrared imaging of the galaxy M51. The results of our reconstruction are compared to published reconstructions of the same data using the Lucy-Richardson algorithm, the Maximum Correlation Method developed at IPAC, and the MEMSYS ME algorithms. The results show that our reconstructed image has a spatial resolution a factor of two better than best previous methods (and a factor of 20 finer than the width of the point response function), and detects sources two orders of magnitude fainter than other methods
Photometry and Photometric Redshifts of Faint Galaxies in the Hubble Deep Field South NICMOS Field
We present a catalog of photometry and photometric redshifts of 335 faint
objects in the HDF-S NICMOS field. The analysis is based on (1) infrared images
obtained with the Hubble Space Telescope (HST) using the Near Infrared Camera
and Multi-Object Spectrograph (NICMOS) with the F110W, F160W, and F222M
filters, (2) an optical image obtained with HST using the Space Telescope
Imaging Spectrograph (STIS) with no filter, and (3) optical images obtained
with the European Southern Observatory (ESO) Very Large Telescope (VLT) with U,
B, V, R, and I filters. The primary utility of the catalog of photometric
redshifts is as a survey of faint galaxies detected in the NICMOS F160W and
F222M images. The sensitivity of the survey varies significantly with position,
reaching a limiting depth of AB(16,000) ~ 28.7 and covering 1.01 arcmin^2 to
AB(16,000) = 27 and 1.05 arcmin^2 to AB(16,000) = 26.5. The catalog of
photometric redshifts identifies 21 galaxies (or 6% of the total) of redshift z
> 5, 8 galaxies (or 2% of the total) of redshift z > 10, and 11 galaxies (or 3%
of the total) of best-fit spectral type E/S0, of which 5 galaxies (or 1% of the
total) are of redshift z > 1.Comment: 33 pages, 10 figures, accepted for publication in the Astrophysical
Journal, August 1, 2000 issu
Validation of Tikhonov adaptively regularized gamma variate fitting with 24-h plasma clearance in cirrhotic patients with ascites
PURPOSE: The aim was to compare late-time extrapolation of plasma clearance (CL) from Tikhonov adaptively regularized gamma variate fitting (Tk-GV) and from mono-exponential (E1) fitting. METHODS: Ten (51)Cr-ethylenediaminetetraacetic acid bolus IV studies in adults--8 with ascites--assessed for liver transplantation, with 12-16 plasma samples drawn from 5-min to 24-h, were fit with Tk-GV and E1 models and CL results were compared using Passing-Bablok fitting. RESULTS: The 24-h CL(Tk-GV) values ranged from 11.4 to 79.7 ml/min. Linear regression of 4- versus 24-h CL(Tk-GV) yielded no significant departure from a slope of 1, whereas the 4- versus 24-h CL(E1) slope, 1.56, was significantly increased. For CL(Tk-GV-24-h) versus CL(E1-24-h), there was a biased slope and intercept (0.85, 5.97 ml/min). Moreover, the quality of fitting of 24-h data was significantly better for Tk-GV than for E1, as follows. For 10 logarithm of concentration curves, higher r values were obtained for each Tk-GV fit (median 0.998) than for its corresponding E1 fit (median 0.965), with p < 0.0001 (paired t-test of z-statistics from Fisher r-z transformations). The E1 fit quality degraded with increasing V/W [volume of distribution (l) per kg body weight, p = 0.003]. However, Tk-GV fit quality versus V/W was uncorrelated (p = 0.8). CONCLUSION: CL(E1) values were dependent on sample time and the quality of fit was poor and degraded with increasing ascites, consistent with current opinion that CL(E1) is contraindicated in ascitic patients. CL(Tk-GV) was relatively more accurate and the good quality of fit was unaffected by ascites. CL(Tk-GV) was the preferred method for the accurate calculation of CL and was useful despite liver failure and ascites
Detection of Near-IR CO Absorption Bands in R Coronae Borealis Stars
R Coronae Borealis (RCB) stars are hydrogen-deficient, carbon-rich pulsating
post-AGB stars that experience massive irregular declines in brightness caused
by circumstellar dust formation. The mechanism of dust formation around RCB
stars is not well understood. It has been proposed that CO molecules play an
important role in cooling the circumstellar gas so that dust may form. We
report on a survey for CO in a sample of RCB stars. We obtained H- and K-band
spectra including the first and second overtone CO bands for eight RCB stars,
the RCB-like star, DY Per and the final-helium-flash star, FG Sge. The first
and second overtone CO bands were detected in the cooler (T(eff)<6000 K) RCB
stars, Z Umi, ES Aql, SV Sge and DY Per. The bands are not present in the
warmer (T(eff)>6000 K) RCB stars, R CrB, RY Sgr, SU Tau, XX Cam. In addition,
first overtone bands are seen in FG Sge, a final-helium-flash star that is in
an RCB-like phase at present. Effective temperatures of the eight RCB stars
range from 4000 to 7250 K. The observed photospheric CO absorption bands were
compared to line-blanketed model spectra of RCB stars. As predicted by the
models, the CO bands are strongest in the coolest RCB stars and not present in
the warmest. No correlation was found between the presence or strength of the
CO bands and dust formation activity in the stars.Comment: 13 oages, 3 figures, AJ in pres
Visible and Near-Infrared Spectrophotometry of the Deep Impact Ejecta of Comet 9P/Tempel 1
We have obtained optical spectrophotometry of the evolution of comet
9P/Tempel 1 after the impact of the Deep Impact probe, using the Supernova
Integral Field Spectrograph (SNIFS) at the UH 2.2m telescope, as well as
simultaneous optical and infrared spectra using the Lick
Visible-to-Near-Infrared Imaging Spectrograph (VNIRIS) spectrograph. The
spatial distribution and temporal evolution of the "violet band" CN (0-0)
emission and of the 630 nm [OI] emission was studied. We found that CN emission
centered on the nucleus increased in the two hours after impact, but that this
CN emission was delayed compared to the light curve of dust-scattered sunlight.
The CN emission also expanded faster than the cloud of scattering dust. The
emission of [OI] at 630 nm rose similarly to the scattered light, but then
remained nearly constant for several hours after impact. On the day following
the impact, both CN and [OI] emission concentrated on the comet nucleus had
returned nearly to pre-impact levels. We have also searched for differences in
the scattering properties of the dust ejected by the impact compared to the
dust released under normal conditions. Compared to the pre-impact state of the
comet, we find evidence that the color of the comet was slightly bluer during
the post-impact rise in brightness. Long after the impact, in the following
nights, the comet colors returned to their pre-impact values. This can be
explained by postulating a change to a smaller particle size distribution in
the ejecta cloud, in agreement with the findings from mid-infrared observatons,
or by postulating a large fraction of clean ice particles, or by a combination
of these two.Comment: 28 pages of text and 8 figures. Paper is accepted for publication in
Icaru
Dust Emission Features in NGC 7023 between 0.35 and 2.5 micron: Extended Red Emission (0.7 micron) and Two New Emission Features (1.15 and 1.5 micron)
We present 0.35 to 2.5 micron spectra of the south and northwest filaments in
the reflection nebula NGC 7023. These spectra were used to test the theory of
Seahra & Duley that carbon nanoparticles are responsible for Extended Red
Emission (ERE). Our spectra fail to show their predicted second emission band
at 1.0 micron even though both filaments exhibit strong emission in the
familiar 0.7 micron ERE band. The northwest filament spectrum does show one,
and possibly two, new dust emission features in the near-infrared. We clearly
detect a strong emission band at 1.5 micron which we tentatively attribute to
beta-FeSi_2 grains. We tentatively detect a weaker emission band at 1.15 micron
which coincides with the location expected for transitions from the conduction
band to mid-gap defect states of silicon nanoparticles. This is added evidence
that silicon nanoparticles are responsible for ERE as they already can explain
the observed behavior of the main visible ERE band.Comment: 9 pages, color figures, accepted to the ApJ, color and b/w versions
available at http://dirty.as.arizona.edu/~kgordon/papers/ere_1um.htm
Multi-wavelength analysis of high energy electrons in solar flares: a case study of August 20, 2002 flare
A multi-wavelength spatial and temporal analysis of solar high energy
electrons is conducted using the August 20, 2002 flare of an unusually flat
(gamma=1.8) hard X-ray spectrum. The flare is studied using RHESSI, Halpha,
radio, TRACE, and MDI observations with advanced methods and techniques never
previously applied in the solar flare context. A new method to account for
X-ray Compton backscattering in the photosphere (photospheric albedo) has been
used to deduce the primary X-ray flare spectra. The mean electron flux
distribution has been analysed using both forward fitting and model independent
inversion methods of spectral analysis. We show that the contribution of the
photospheric albedo to the photon spectrum modifies the calculated mean
electron flux distribution, mainly at energies below 100 keV. The positions of
the Halpha emission and hard X-ray sources with respect to the current-free
extrapolation of the MDI photospheric magnetic field and the characteristics of
the radio emission provide evidence of the closed geometry of the magnetic
field structure and the flare process in low altitude magnetic loops. In
agreement with the predictions of some solar flare models, the hard X-ray
sources are located on the external edges of the Halpha emission and show
chromospheric plasma heated by the non-thermal electrons. The fast changes of
Halpha intensities are located not only inside the hard X-ray sources, as
expected if they are the signatures of the chromospheric response to the
electron bombardment, but also away from them.Comment: 26 pages, 9 figures, accepted to Solar Physic
Variability of Disk Emission in Pre-Main Sequence and Related Stars. I. HD 31648 and HD 163296 - Isolated Herbig Ae Stars Driving Herbig-Haro Flows
Infrared photometry and spectroscopy covering a time span of a quarter
century are presented for HD 31648 (MWC 480) and HD 163296 (MWC 275). Both are
isolated Herbig Ae stars that exhibit signs of active accretion, including
driving bipolar flows with embedded Herbig-Haro (HH) objects. HD 163296 was
found to be relatively quiescent photometrically in its inner disk region, with
the exception of a major increase in emitted flux in a broad wavelength region
centered near 3 microns in 2002. In contrast, HD 31648 has exhibited sporadic
changes in the entire 3-13 micron region throughout this span of time. In both
stars the changes in the 1-5 micron flux indicate structural changes in the
region of the disk near the dust sublimation zone, possibly causing its
distance from the star to vary with time. Repeated thermal cycling through this
region will result in the preferential survival of large grains, and an
increase in the degree of crystallinity. The variability observed in these
objects has important consequences for the interpretation of other types of
observations. For example, source variability will compromise models based on
interferometry measurements unless the interferometry observations are
accompanied by nearly-simultaneous photometric data.Comment: 55 pages, 18 figures, 2 tables, Accepted by Ap