2,974 research outputs found
Spectral Classification of Galaxies
We investigate the integrated spectra of a sample of 24 normal galaxies. A
principal component analysis suggests that most of the variance present in the
spectra is due to the differences in morphology of the galaxies in the sample.
We show that spectroscopic parameters extracted from the spectra, like the
amplitude of the 4000 \AA~ break or of the CN band, correlate well with Hubble
types and are useful for quantitative classification.Comment: 7 pages uuencoded compressed PostScript file. To appear in Vistas in
Astronomy, special issue on Artificial Neural Networks in Astronom
Velocity-modulation atomization of liquid jets
A novel atomizer based on high-amplitude velocity atomization has been developed. Presently, the most common methods of atomization can use only the Rayleigh instability of a liquid cylinder and the Kelvin-Helmholtz instability of a liquid sheet. Our atomizer is capable of atomizing liquid jets by the excitation and destabilization of many other higher-order modes of surface deformation. The potential benefits of this sprayer are more uniform fuel air mixtures, faster fuel-air mixing, extended flow ranges for commercial nozzles, and the reduction of nozzle plugging by producing small drops from large nozzles
A First Comparison of the SBF Survey Distances with the Galaxy Density Field: Implications for H_0 and Omega
We compare the peculiar velocities measured in the SBF Survey of Galaxy
Distances with the predictions from the density fields of the IRAS 1.2 Jy
flux-limited redshift survey and the Optical Redshift Survey (ORS) to derive
simultaneous constraints on the Hubble constant and the density parameter
, where is the linear bias. We find
and for the IRAS and ORS
comparisons, respectively, and \kmsMpc (with an additional 9%
uncertainty due to the Cepheids themselves). The match between predicted and
observed peculiar velocities is good for these values of and , and
although there is covariance between the two parameters, our results clearly
point toward low-density cosmologies. Thus, the unresolved discrepancy between
the ``velocity-velocity'' and ``density-density'' measurements of
continues.Comment: 4 pages with 3 embedded ps figures; uses emulateapj.sty (included).
Accepted for publication in ApJ Letter
Research on the design of adaptive control systems, volume 1 Final report
Adaptive control systems - combined optimization and adaptive control, analysis-synthesis and passive adaptive systems, learning systems, and measurement adaptive system
Threshold Resummation for W-Boson Production at RHIC
We study the resummation of large logarithmic perturbative corrections to the
partonic cross sections relevant for the process pp -> W^+- X at the BNL
Relativistic Heavy Ion Collider (RHIC). At RHIC, polarized protons are
available, and spin asymmetries for this process will be used for precise
measurements of the up and down quark and anti-quark distributions in the
proton. The corrections arise near the threshold for the partonic reaction and
are associated with soft-gluon emission. We perform the resummation to
next-to-leading logarithmic accuracy, for the rapidity-differential cross
section. We find that resummation leads to relatively moderate effects on the
cross sections and spin asymmetries.Comment: 25 pages, 15 figures as eps files. One reference added and typo
correcte
The Tully-Fisher relation of distant cluster galaxies
We have measured maximum rotation velocities (Vrot) for a sample of 111
emission-line galaxies with 0.1 < z < 1, observed in the fields of 6 clusters.
From these data we construct 'matched' samples of 58 field and 22 cluster
galaxies, covering similar ranges in redshift (0.25 < z < 1.0) and luminosity
(M_B < -19.5 mag), and selected in a homogeneous manner. We find the
distributions of M_B, Vrot, and scalelength, to be very similar for the two
samples. However, using the Tully-Fisher relation (TFR) we find that cluster
galaxies are systematically offset with respect to the field sample by
-0.7+-0.2 mag. This offset is significant at 3 sigma and persists when we
account for an evolution of the field TFR with redshift. Extensive tests are
performed to investigate potential differences between the measured emission
lines and derived rotation curves of the cluster and field samples. However, no
such differences which could affect the derived Vrot values and account for the
offset are found. The most likely explanation for the TFR offset is that giant
spiral galaxies in distant clusters are on average brighter, for a given
rotation velocity, than those in the field. We discuss the potential mechanisms
responsible for this, and consider alternative explanations.Comment: 19 pages, 13 figures, accepted by MNRA
The Origin of [OII] in Post-Starburst and Red-Sequence Galaxies in High-Redshift Clusters
We present the first results from a near-IR spectroscopic campaign of the
Cl1604 supercluster at z~0.9 and the cluster RX J1821.6+6827 at z~0.82 to
investigate the nature of [OII] 3727A emission in cluster galaxies at high
redshift. Of the 401 members in the two systems, 131 galaxies have detectable
[OII] emission with no other signs of current star-formation, as well as strong
absorption features indicative of a well-established older stellar population.
The combination of these features suggests that the primary source of [OII]
emission in these galaxies is not the result of star-formation, but rather due
to the presence of a LINER or Seyfert component. Using the NIRSPEC spectrograph
on the Keck II 10-m telescope, 19 such galaxies were targeted, as well as six
additional [OII]-emitting cluster members that exhibited other signs of ongoing
star-formation. Nearly half (~47%) of the 19 [OII]-emitting, absorption-line
dominated galaxies exhibit [OII] to Ha equivalent width ratios higher than
unity, the typical value for star-forming galaxies. A majority (~68%) of these
19 galaxies are classified as LINER/Seyfert based on the emission-line ratio of
[NII] and Ha, increasing to ~85% for red [OII]-emitting, absorption-line
dominated galaxies. The LINER/Seyfert galaxies exhibit L([OII])/L(Ha) ratios
significantly higher than that observed in populations of star-forming
galaxies, suggesting that [OII] is a poor indicator of star-formation in a
large fraction of high-redshift cluster members. We estimate that at least ~20%
of galaxies in high-redshift clusters contain a LINER/Seyfert component that
can be revealed with line ratios. We also investigate the effect this
population has on the star formation rate of cluster galaxies and the
post-starburst fraction, concluding that LINER/Seyferts must be accounted for
if these quantities are to be meaningful.Comment: 33 pages, 17 figures, to appear in Ap
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