2,410 research outputs found
The Properties of Brightest Cluster Galaxies in X-Ray Selected Clusters
We present the K-band Hubble diagram for 162 brightest cluster galaxies
(BCGs) in X-ray selected clusters, 0.01<z<0.83. The sample incorporates that of
Burke, Collins, & Mann (2000) and includes additional infrared data from the
2MASS extended source catalogue. We show that below z=0.1 the BCGs show no
correlation with their environment, however, above z=0.1 BCGs in more X-ray
luminous clusters are more uniform in their photometric properties. This
suggests that there may be two populations of BCGs which have different
evolutionary histories.Comment: 2 pages, to appear in the proceedings of the Sesto 2001 conference on
tracing cosmic evolution with galaxy cluster
The Butcher-Oemler Effect in High Redshift X-ray Selected Clusters
We are engaged in a wide-field, multi-colour imaging survey of X-ray selected
clusters at intermediate and high redshift. We present blue fractions for the
first 8 out of 29 clusters, covering almost a factor of 100 in X-ray
luminosity. We find no correlation of blue fraction with redshift or X-ray
luminosity. The lack of a correlation with L, places strong constraints
on the importance of ram-pressure stripping as a driver of the Butcher-Oemler
effect.Comment: 4 pages, 4 figures, to be puplished in the proceedings of the ''Sesto
2001-Tracing Cosmic Evolution with Galaxy Clusters'', Sesto 3-6 July 2001,
Italy, eds, Stefano Borgan
Policy opportunities
Recommendations are given regarding National Science Foundation (NSF) astronomy programs and the NASA Space Astrophysics program. The role of ground based astronomy is reviewed. The role of National Optical Astronomy Observatories (NOAO) in ground-based night-time astronomical research is discussed. An enhanced Explored Program, costs and management of small and moderate space programs, the role of astrophysics within NASA's space exploration initiative, suborbital and airborne astronomical research, the problems of the Hubble Space Telescope, and astronomy education are discussed. Also covered are policy issues related to the role of science advisory committees, international cooperation and competition, archiving and distribution of astronomical data, and multi-wavelength observations of variable sources
Ultracold collision properties of metastable alkaline-earth atoms
Ultra-cold collisions of spin-polarized 24Mg,40Ca, and 88Sr in the metastable
3P2 excited state are investigated. We calculate the long-range interaction
potentials and estimate the scattering length and the collisional loss rate as
a function of magnetic field. The estimates are based on molecular potentials
between 3P2 alkaline-earth atoms obtained from ab initio atomic and molecular
structure calculations. The scattering lengths show resonance behavior due to
the appearance of a molecular bound state in a purely long-range interaction
potential and are positive for magnetic fields below 50 mT. A loss-rate model
shows that losses should be smallest near zero magnetic field and for fields
slightly larger than the resonance field, where the scattering length is also
positive.Comment: 4 pages, 4 figure
Orbital-free Bond Breaking via Machine Learning
Machine learning is used to approximate the kinetic energy of one dimensional
diatomics as a functional of the electron density. The functional can
accurately dissociate a diatomic, and can be systematically improved with
training. Highly accurate self-consistent densities and molecular forces are
found, indicating the possibility for ab-initio molecular dynamics simulations
Traffic Aware Planner for Cockpit-Based Trajectory Optimization
The Traffic Aware Planner (TAP) software application is a cockpit-based advisory tool designed to be hosted on an Electronic Flight Bag and to enable and test the NASA concept of Traffic Aware Strategic Aircrew Requests (TASAR). The TASAR concept provides pilots with optimized route changes (including altitude) that reduce fuel burn and/or flight time, avoid interactions with known traffic, weather and restricted airspace, and may be used by the pilots to request a route and/or altitude change from Air Traffic Control. Developed using an iterative process, TAP's latest improvements include human-machine interface design upgrades and added functionality based on the results of human-in-the-loop simulation experiments and flight trials. Architectural improvements have been implemented to prepare the system for operational-use trials with partner commercial airlines. Future iterations will enhance coordination with airline dispatch and add functionality to improve the acceptability of TAP-generated route-change requests to pilots, dispatchers, and air traffic controllers
Initial Implementation and Operational Use of TASAR in Alaska Airlines Flight Operations
NASA has been developing and testing the Traffic Aware Strategic Aircrew Requests (TASAR) concept for aircraft operations featuring a NASA-developed cockpit automation tool, the Traffic Aware Planner (TAP), which computes route changes compatible with nearby traffic and airspace constraints to improve flight efficiency. The TAP technology is anticipated to save fuel, flight time, and operating costs and thereby provide immediate and pervasive benefits to the aircraft operator. Alaska Airlines is partnered with NASA to implement and evaluate TASAR in revenue service. This paper will describe activities undertaken to achieve TASAR operational status at Alaska Airlines, and it will present preliminary results from initial flight operations
The environmental dependence of galaxy colors in intermediate-redshift Xârayâselected clusters
We present a wide-field imaging study of the colors of bright galaxies (â + 2) in a sample of 12 X-rayâselected clusters and groups of galaxies at z ~ 0:3. The systems cover one of the largest ranges in X-ray luminosity (Lx ~ 1043 1045 ergs s-1), and hence mass, of any sample studied at this redshift. We find that the ââredââ galaxies form a tight color-magnitude relation (CMR) and that neither the slope nor zero point of this relation changes significantly over the factor of 100 in X-ray luminosity covered by our sample. Using stellar population synthesis models, we find that our data allow a maximum possible change of 2 Gyr in the typical age of the ââredââ galaxies on the CMR over the range of Lx of our sample.We also measure the fraction of blue galaxies (fb) relative to the CMR in our clusters and find a low value of fb ~ 0.1 consistent with other X-rayâselected cluster samples.We find that there is no correlation between fb and Lx over our large Lx range. However, we do find that both the CMR and fb depend significantly on cluster radius, with the zero point of the CMR shifting blueward in B - R by 0.10 ± 0.036 mag out to a radius of 0.75 times the virial radius. This color change is equivalent to a luminosity-weighted age gradient of ~2.5 Gyr per log (radius) and is consistent with previous studies of the radial change in the zero point of the CMR. It thus appears
that the global cluster environment, in the form of cluster mass (Lx), has little influence on the properties of the bright cluster galaxies, whereas the local environment, in the form of galaxy density (radius), has a strong effect. The range of ~100 in Lx corresponds to a factor of ~40 in ram pressure efficiency, thus suggesting that ram pressure stripping or other mechanisms that depend on cluster mass, like tidal stripping or harassment, are unlikely to be solely responsible for changing the galaxy population from the ââblueââ star-forming galaxies, which dominate low-density
environments, to the ââredââ passive galaxies, which dominate cluster cores
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