110,296 research outputs found
Calibrating Array Detectors
The development of sensitive large format imaging arrays for the infrared
promises to provide revolutionary capabilities for space astronomy. For
example, the Infrared Array Camera (IRAC) on SIRTF will use four 256 x 256
arrays to provide background limited high spatial resolution images of the sky
in the 3 to 8 micron spectral region. In order to reach the performance limits
possible with this generation of sensitive detectors, calibration procedures
must be developed so that uncertainties in detector calibration will always be
dominated by photon statistics from the dark sky as a major system noise
source. In the near infrared, where the faint extragalactic sky is observed
through the scattered and reemitted zodiacal light from our solar system,
calibration is particularly important. Faint sources must be detected on this
brighter local foreground.
We present a procedure for calibrating imaging systems and analyzing such
data. In our approach, by proper choice of observing strategy, information
about detector parameters is encoded in the sky measurements. Proper analysis
allows us to simultaneously solve for sky brightness and detector parameters,
and provides accurate formal error estimates.
This approach allows us to extract the calibration from the observations
themselves; little or no additional information is necessary to allow full
interpretation of the data. Further, this approach allows refinement and
verification of detector parameters during the mission, and thus does not
depend on a priori knowledge of the system or ground calibration for
interpretation of images.Comment: Scheduled for ApJS, June 2000 (16 pages, 3 JPEG figures
A Slow Merger History of Field Galaxies Since z~1
Using deep infrared observations conducted with the CISCO imager on the
Subaru Telescope, we investigate the field-corrected pair fraction and the
implied merger rate of galaxies in redshift survey fields with Hubble Space
Telescope imaging. In the redshift interval, 0.5 < z < 1.5, the fraction of
infrared-selected pairs increases only modestly with redshift to 7% +- 6% at
z~1. This is nearly a factor of three less than the fraction, 22% +- 8%,
determined using the same technique on HST optical images and as measured in a
previous similar study. Tests support the hypothesis that optical pair
fractions at z~1 are inflated by bright star-forming regions that are unlikely
to be representative of the underlying mass distribution. By determining
stellar masses for the companions, we estimate the mass accretion rate
associated with merging galaxies. At z~1, we estimate this to be 2x10^{9 +-
0.2} solar masses per galaxy per Gyr. Although uncertainties remain, our
results suggest that the growth of galaxies via the accretion of pre-existing
fragments remains as significant a phenomenon in the redshift range studied as
that estimated from ongoing star formation in independent surveys.Comment: 5 pages, accepted for publication in ApJ Letter
Near-infrared luminosity function and colours of dwarf galaxies in the Coma Cluster
We present K-band observations of the low-luminosity galaxies in the Coma
cluster, which are responsible for the steep upturn in the optical luminosity
function at M_R ~ -16, discovered recently. The main results of this study are
(i) The opticalnear-infrared colours of these galaxies imply that they are
dwarf spheroidals. The median M-K colour for galaxies with -19.3 < M_K < -16.3
is 3.6 mag. (ii) The K-band luminosity function in the Coma cluster at the
faint-end is not wee constrained, because of the uncertainties due to the
field-to-field variance of the background. However, within the estimate large
errors, it is consistent with the R-band luminosity function, shifted by
magnitudes. (iii) Many of the cluster dwarfs lie in a region of the B-K
vs. B-R colour-colour diagram where background galaxies are rare Local dwarf
spheroidal galaxies lie in this region too. This suggests that a better
measurement of the K-band cluster luminosity function can be made if the
field-to-field variance of the background can be measured as a function of
colour. (iv) If we assume that none of the galaxies in the region of the B-K
vs. B-R plane given in (iii) in our cluster fields are background, and that all
the cluster galaxies with lie in this region of the plane,
then we measure alpha = -1.41 +/- 0.35 for -19.3 < M_K < -16.3, where alpha is
the logarithmic slope of the luminosity function.Comment: 6 pages, 8 figs, 2 tabs, MNRAS in press; email: [email protected],
[email protected]
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PERSIANN-MSA: A precipitation estimation method from satellite-based multispectral analysis
Visible and infrared data obtained from instruments onboard geostationary satellites have been extensively used for monitoring clouds and their evolution. The Advanced Baseline Imager (ABI) that will be launched onboard the Geostationary Operational Environmental Satellite-R (GOES-R) series in the near future will offer a larger range of spectral bands; hence, it will provide observations of cloud and rain systems at even finer spatial, temporal, and spectral resolutions than are possible with the current GOES. In this paper, a new method called Precipitation Estimation from Remotely Sensed information using Artificial Neural Networks-Multispectral Analysis (PERSIANN-MSA) is proposed to evaluate the effect of using multispectral imagery on precipitation estimation. The proposed approach uses a self-organizing feature map (SOFM) to classify multidimensional input information, extracted from each grid box and corresponding textural features of multispectral bands. In addition, principal component analysis (PCA) is used to reduce the dimensionality to a few independent input features while preserving most of the variations of all input information. The above method is applied to estimate rainfall using multiple channels of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard the Meteosat Second Generation (MSG) satellite. In comparison to the use of a single thermal infrared channel, the analysis shows that using multispectral data has the potential to improve rain detection and estimation skills with an average of more than 50% gain in equitable threat score for rain/no-rain detection, and more than 20% gain in correlation coefficient associated with rain-rate estimation. © 2009 American Meteorological Society
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Daytime precipitation estimation using bispectral cloud classification system
Two previously developed Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN) algorithms that incorporate cloud classification system (PERSIANN-CCS) and multispectral analysis (PERSIANN-MSA) are integrated and employed to analyze the role of cloud albedo from Geostationary Operational Environmental Satellite-12 (GOES-12) visible (0.65 ÎŒm) channel in supplementing infrared (10.7 mm) data. The integrated technique derives finescale (0.04° Ă 0.04° latitudelongitude every 30 min) rain rate for each grid box through four major steps: 1) segmenting clouds into a number of cloud patches using infrared or albedo images; 2) classification of cloud patches into a number of cloud types using radiative, geometrical, and textural features for each individual cloud patch; 3) classification of each cloud type into a number of subclasses and assigning rain rates to each subclass using a multidimensional histogram matching method; and 4) associating satellite gridbox information to the appropriate corresponding cloud type and subclass to estimate rain rate in grid scale. The technique was applied over a study region that includes the U.S. landmass east of 115°W. One reference infrared-only and three different bis-pectral (visible and infrared) rain estimation scenarios were compared to investigate the technique's ability to address two major drawbacks of infrared-only methods: 1) underestimating warm rainfall and 2) the inability to screen out no-rain thin cirrus clouds. Radar estimates were used to evaluate the scenarios at a range of temporal (3 and 6 hourly) and spatial (0.04°, 0.08°, 0.12°, and 0.24° latitude-longitude) scales. Overall, the results using daytime data during June-August 2006 indicate that significant gain over infrared-only technique is obtained once albedo is used for cloud segmentation followed by bispectral cloud classification and rainfall estimation. At 3-h, 0.04° resolution, the observed improvement using bispectral information was about 66% for equitable threat score and 26% for the correlation coefficient. At coarser 0.24° resolution, the gains were 34% and 32% for the two performance measures, respectively. © 2010 American Meteorological Society
Observational constraints on dust disk lifetimes : implications for planet formation
Thus far our impressions regarding the evolutionary time scales for young circumstellar disks
have been based on small number statistics. Over the past decade, however, in addition to preci-
sion study of individual star/disk systems, substantial observational effort has been invested in
obtaining less detailed data on large numbers of objects in young star clusters. This has resulted
in a plethora of information now enabling statistical studies of disk evolutionary diagnostics.
Along an ordinate one can measure disk presence or strength through indicators such as ul-
traviolet/blue excess or spectroscopic emission lines tracing accretion, infrared excess tracing
dust, or millimeter flux measuring mass. Along an abscissa one can track stellar age. While
bulk trends in disk indicators versus age are evident, observational errors affecting both axes,
combined with systematic errors in our understanding of stellar ages, both cloud and bias any
such trends. Thus detailed understanding of the physical processes involved in disk dissipation
and of the relevant time scales remains elusive. Nevertheless, a clear effect in current data that
is unlikely to be altered by data analysis improvements is the dispersion in disk lifetimes. Inner
accretion disks are traced by near-infrared emission. Moderating a generally declining trend in
near-infared continuum excess and excess frequency with age over <1 to 8±4 Myr, is the fact
that a substantial fraction of rather young (<1 Myr old) stars apparently have already lost their
inner accretion disks while a significant number of rather old (8-16 Myr) stars apparently still
retain inner accretion disks. The age at which evidence for inner accretion disks ceases to be
apparent for the vast majority (~90%) of stars is in the range 3-8 Myr. More distant, terrestrial
zone dust is traced by mid-infrared emission where sufficient sensitivity and uniform data collec-
tion are only now being realized with data return from the Spitzer Space Telescope. Constraints
on mid-disk dissipation and disk clearing trends with radius are forthcoming
NICMOS images of JVAS/CLASS gravitational lens systems
We present Hubble Space Telescope (HST) infrared images of four gravitational
lens systems from the JVAS/CLASS gravitational lens survey and compare the new
infrared HST pictures with previously published WFPC2 HST optical images and
radio maps. Apart from the wealth of information that we get from the flux
ratios and accurate positions and separations of the components of the lens
systems that we can use as inputs for better constraints on the lens models we
are able to discriminate between reddening and optical/radio microlensing as
the possible cause of differences observed in the flux ratios of the components
across the three wavelength bands. Substantial reddening has been known to be
present in the lens system B1600+434 and has been further confirmed by the
present infrared data. In the two systems B0712+472 and B1030+074 microlensing
has been pinpointed down as the main cause of the flux ratio discrepancy both
in the optical/infrared and in the radio, the radio possibly caused by the
substructure revealed in the lensing galaxies. In B0218+357 however the results
are still not conclusive. If we are actually seeing the two "true" components
of the lens system then the flux ratio differences are attributed to a
combination of microlensing and reddening or alternatively due to some
variability in at least one of the images. Otherwise the second "true"
component of B0218+357 maybe completely absorbed by a molecular cloud and the
anomalous flux density ratios and large difference in separation between the
optical/infrared and radio that we see can be explained by emission from either
a foreground object or from part of the lensing galaxy.Comment: 10 pages, 4 figures (original higher resolution figures can be
obtained at the e-mail above), to appear in MNRAS (accepted
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