3,144 research outputs found
Star forming regions of the southern galaxy
A catalog of southern dust cloud properties is being compiled to aid in the planning and analysis of radio spectral line surveys in the southern hemisphere. Ultimately, images of dust temperature and column density will be produced. For the interim, a list of the 60 and 100 micron fluxes was prepared for the cores and adjacent backgrounds of 65 prominent dust clouds. Dust temperatures and column densities were derived
Low frequency VLBI in space using GAS-Can satellites: Report on the May 1987 JPL Workshop
Summarized are the results of a workshop held at JPL on May 28 and 29, 1987, to study the feasibility of using small, very inexpensive spacecraft for a low-frequency radio interferometer array. Many technical aspects of a mission to produce high angular resolution images of the entire sky at frequencies from 2 to 20 MHz were discussed. The workshop conclusion was that such a mission was scientifically valuable and technically practical. A useful array could be based on six or more satellites no larger than those launched from Get-Away-Special canisters. The cost of each satellite could be $1-2M, and the mass less than 90 kg. Many details require further study, but as this report shows, there is good reason to proceed. No fundamental problems have been discovered involving the use of untraditional, very inexpensive spacecraft for this type of mission
The host galaxy of the z=2.4 radio-loud AGN MRC 0406-244 as seen by HST
We present multicolour Hubble Space Telescope images of the powerful z=2.4
radio galaxy MRC 0406-244 and model its complex morphology with several
components including a host galaxy, a point source, and extended nebular and
continuum emission. We suggest that the main progenitor of this radio galaxy
was a normal, albeit massive (M ~10^{11} solar masses), star-forming galaxy.
The optical stellar disc of the host galaxy is smooth and well described by a
S\'ersic profile, which argues against a recent major merger, however there is
also a point-source component which may be the remnant of a minor merger. The
half-light radius of the optical disc is constrained to lie in the range 3.5 to
8.2kpc, which is of similar size to coeval star forming galaxies.
Biconical shells of nebular emission and UV-bright continuum extend out from
the host galaxy along the radio jet axis, which is also the minor axis of the
host galaxy. The origin of the continuum emission is uncertain, but it is most
likely to be young stars or dust-scattered light from the AGN, and it is
possible that stars are forming from this material at a rate of
200^{+1420}_{-110} solar masses per year.Comment: Accepted for publication in MNRA
The Spectral Variability of Cygnus X-1 at MeV Energies
In previous work, we have used data from the first three years of the CGRO
mission to assemble a broad-band -ray spectrum of the galactic black
hole candidate Cygnus X-1. Contemporaneous data from the COMPTEL, OSSE and
BATSE experiments on CGRO were selected on the basis of the hard X-ray flux
(45--140 keV) as measured by BATSE. This provided a spectrum of Cygnus X-1 in
its canonical low X-ray state (as measured at energies below 10 keV), covering
the energy range from 50 keV to 5 MeV. Here we report on a comparison of this
spectrum to a COMPTEL-OSSE spectrum collected during a high X-ray state of
Cygnus X-1 (May, 1996). These data provide evidence for significant spectral
variability at energies above 1 MeV. In particular, whereas the hard X-ray flux
{\it decreases} during the high X-ray state, the flux at energies above 1 MeV
{\it increases}, resulting in a significantly harder high energy spectrum. This
behavior is consistent with the general picture of galactic black hole
candidates having two distinct spectral forms at soft -ray energies.
These data extend this picture, for the first time, to energies above 1 MeV.Comment: 5 pages, 4 figures, to be published in AIP Conf. Proc., "The Fifth
Compton Symposium
A z~3 radio galaxy and its protocluster: evidence for a superstructure?
We present spectroscopic follow-up observations of Lyman Break Galaxies
(LBGs) selected in the field surrounding the radio galaxy MRC0316-257 at z~3.13
(0316). Robust spectroscopic redshifts are determined for 20 out of 24 objects.
Three of the spectroscopically confirmed galaxies have 3.12<z<3.13 indicating
that these objects reside in a protocluster structure previously found around
the radio galaxy. An additional 5 objects are found 1600 km/s blue-shifted with
respect to the main protocluster structure. This is in addition to three [OIII]
emitters found at this redshift in a previous study. This is further evidence
that a structure exists directly in front of the 0316 protocluster. We estimate
that the foreground structure is responsible for half of the surface
overdensity of LBGs found in the field as a whole. The foreground structure is
associated with a strong surface density peak 1.4 Mpc to the North-West of the
radio galaxy and a 2D Kolmogorov-Smirnov test indicates that the spatial
distributions of the 0316 and foreground galaxies differ at the 3 sigma level.
In addition, we compare the properties of protocluster, foreground structure
and field galaxies, but we find no significant differences. In terms of the
nature of the two structures, a merger scenario is a possible option. Simple
merger dynamics indicates that the observed relative velocity of 1600 km/s can
be reproduced if the two structures have masses of ~5x10^14 Msun and have
starting separations of around 2.5 to 3 Mpc. It is also possible that the
foreground structure is unrelated to the 0316 protocluster in which case the
two structures will not interact before z=0.Comment: 13 pages, 6 figures, accepted for publication in MNRA
Statistical Studies of Giant Pulse Emission from the Crab Pulsar
We have observed the Crab pulsar with the Deep Space Network (DSN) Goldstone
70 m antenna at 1664 MHz during three observing epochs for a total of 4 hours.
Our data analysis has detected more than 2500 giant pulses, with flux densities
ranging from 0.1 kJy to 150 kJy and pulse widths from 125 ns (limited by our
bandwidth) to as long as 100 microseconds, with median power amplitudes and
widths of 1 kJy and 2 microseconds respectively. The most energetic pulses in
our sample have energy fluxes of approximately 100 kJy-microsecond. We have
used this large sample to investigate a number of giant-pulse emission
properties in the Crab pulsar, including correlations among pulse flux density,
width, energy flux, phase and time of arrival. We present a consistent
accounting of the probability distributions and threshold cuts in order to
reduce pulse-width biases. The excellent sensitivity obtained has allowed us to
probe further into the population of giant pulses. We find that a significant
portion, no less than 50%, of the overall pulsed energy flux at our observing
frequency is emitted in the form of giant pulses.Comment: 19 pages, 17 figures; to be published in Astrophysical Journa
Systematic X-ray absorption study of hole doping in BSCCO - phases
X-ray absorption spectroscopy (XAS) on the O 1s threshold was applied to
Bi-based, single crystalline high temperature superconductors (HTc's), whose
hole densities in the CuO2 planes was varied by different methods. XAS gives
the intensity of the so-called pre-peak of the O 1s line due to the unoccupied
part of the Zhang-Rice (ZR) singlet state. The effects of variation of the
number n of CuO2 - planes per unit cell (n = 1,2,3) and the effect of
La-substitution for Sr for the n = 1 and n = 2 phase were studied
systematically. Furthermore the symmetry of the states could be probed by the
polarization of the impinging radiation.Comment: 4 pages, 2 figures, to appear in the proceedings of SCES2001, Ann
Arbor, August 6-10, 200
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