3,591 research outputs found
Surface Brightness Fluctuations as Primary and Secondary Distance Indicators
The surface brightness fluctuations (SBF) method measures the variance in a
galaxy's light distribution arising from fluctuations in the numbers and
luminosities of individual stars per resolution element. Once calibrated for
stellar population effects, SBF measurements with HST provide distances to
early-type galaxies with unrivaled precision. Optical SBF data from HST for the
Virgo and Fornax clusters give the relative distances of these nearby fiducial
clusters with 2% precision and constrain their internal structures.
Observations in hand will allow us to tie the Coma cluster, the standard of
comparison for distant cluster studies, into the same precise relative distance
scale. The SBF method can be calibrated in an absolute sense either empirically
from Cepheids or theoretically from stellar population models. The agreement
between the model and empirical zero points has improved dramatically,
providing an independent confirmation of the Cepheid distance scale. SBF is
still brighter in the near-IR, and an ongoing program to calibrate the method
for the F110W and F160W passbands of the WFC3 IR channel will enable accurate
distance derivation whenever a large early-type galaxy or bulge is observed in
these passbands at distances reaching well out into the Hubble flow.Comment: 8 pages, invited review at the conference "The Fundamental Cosmic
Distance Scale: State of the Art and Gaia Perspective", to appear in
Astrophysics and Space Scienc
The Dependence of Globular Cluster Number on Density for Abell Cluster Central Galaxies
A study of the globular cluster systems of 23 brightest, or central, galaxies
in 19 Abell clusters has recently been completed. This Letter presents some of
the newly discovered correlations of the globular cluster specific frequency
in these galaxies with other galaxy and cluster properties and puts forth
an interpretation of these results. is found to correlate strongly with
measures of the cluster density, such as the velocity dispersion of the cluster
galaxies and the cluster X-ray temperature and luminosity (especially ``local''
X-ray luminosity). Within a cluster, galaxies at smaller projected distances
from the X-ray center are found to have higher values of . Taken together,
the scaling of with cluster density and the relative constancy of central
galaxy luminosity suggest a scenario in which globular clusters form in
proportionate numbers to the available mass, but galaxy luminosity
``saturates'' at a maximum threshold, resulting in higher \sn\ values for
central galaxies in denser clusters as well as the suitability of these
galaxies as ``standard candles.'' Thus, these galaxies do not have too many
globular clusters for their luminosity; rather, they are underluminous for
their number of globular clusters.Comment: 10 pages, with 4 included postscript figures; AASTeX (aaspp4.sty); to
appear in ApJ Letter
ER-2 investigations of lightning and thunderstorms
The primary objective of the ER-2 lightning program is to investigate relationships between lightning and storm electrification and a number of underlying and interrelated phenomena including the structure, dynamics, and evolution of thunderstorms and thunderstorm systems, precipitation distribution and amounts, atmospheric chemistry processes, and the global electric circuit. This research is motivated by the desire to develop an understanding needed for the effective utilization and interpretation of data from the Lighting Imaging Sensor (LIS), the Lightning Mapper Sensor (LMS), and other satellite-based lightning detectors planned for the late 1900's and early 2000's. These satellite lightning detection systems will be characterized by high detection efficiencies (i.e., 90 percent) and the capability to detect both intracloud and cloud-to-ground discharges during day and night. The Lightning Imaging Sensor (LIS) is being developed by NASA for the Tropical Rainfall Measuring Mission (TRMM) satellite. In the ER-2 and related investigations, the emphasis is on establishing quantitative relationships and developing practical algorithms that employ lightning data, such as could be derived from satellite observations of optical lightning emissions, as the independent variable. Significant accomplishments made during the past year are presented
Experimental evidence for mixed reality states
Recently researchers at the University of Illinois coupled a real pendulum to
its virtual counterpart. They observed that the two pendulums suddenly start to
move in synchrony if their lengths are sufficiently close. In this synchronized
state, the boundary between the real system and the virtual system is blurred,
that is, the pendulums are in a mixed reality state. An instantaneous,
bidirectional coupling is a prerequisite for mixed reality states. In this
article we explore the implications of mixed reality states in the context of
controlling real-world systems.Comment: 2 pages, 2 figure
Control of superlattice morphology in GaAs sub1-subxPsubx cascade cells
Superlattices of GaAs(1-x)p(x) are being incorporated into cascade solar cell structures in order to reduce the dislocation density in the top cells and thus reduce recombination loss and increase output voltage. For a superlattice to effectively block the propagation of dislocations its average composition must be equal to that of the layer beneath it (from efficiency considerations for a cascade cell, the average composition should be about GaAs(.7)P(.3). When superlattices of this approximate composition were grown on GaAs by MOCVD, severe distortion of the crystal layers was observed. The essential features of this distortion are nonplanar morphology and accelerated etching in regions containing excess phosphorus and clusters of dislocations. Similar observations have been made with superlattices grown with two other MOCVD systems, indicating that the problem is of fundamental technological significance, not just an artifact of one particular growth system. The nature of the distortion effect is described, and several strategies for preventing its occurrence are presented
Method and apparatus for determining return stroke polarity of distant lightning
A method is described for determining the return stroke polarity of distant lightning for distances beyond 600 km by detecting the electric field associated with a return stroke of distant lightning, and processing the electric field signal to determine the polarity of the slow tail of the VLF waveform signal associated with the detected electric field. The polarity of the return stroke of distant lightning is determined based upon the polarity of the slow tail portion of the waveform
Stellar Populations and Surface Brightness Fluctuations: New Observations and Models
We examine the use of surface brightness fluctuations (SBF) for both stellar
population and distance studies. New V-band SBF data are reported for five
Fornax cluster galaxies and combined with literature data to define a new
V-band SBF distance indicator. We use new stellar population models, based on
the latest Padua isochrones transformed empirically to the observational plane,
to predict SBF magnitudes and integrated colours for a wide range of population
ages and metallicities. We examine the sensitivity of the predictions to
changes in the isochrones, transformations, and IMF. The new models reproduce
the SBF data for globular clusters fairly well, especially if higher
metallicity globulars are younger. The models also give a good match to the
"fluctuation colors" of elliptical galaxies. In order to obtain theoretical
calibrations of the SBF distance indicators, we combine our single-burst models
into composite population models. These models reproduce the observed behavior
of the SBF magnitudes as a function of stellar population parameters, including
the steep colour dependence found for HST/WFPC2 F814W SBF data. Because the
theoretical SBF calibrations are fairly sensitive to uncertain details of
stellar evolution, the empirical calibrations are more secure. However, the
sensitivity of SBF to these finer details potentially makes it a powerful
constraint for stellar evolution and population synthesis. [abbridged]Comment: 24 pages with 17 embedded figures. MNRAS, in pres
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