47 research outputs found
0319 - 454: an FR II giant radio galaxy with twin jets
We present radio images of the edge-brightened giant radio galaxy 0319 – 454 made with the Australia Telescope at frequencies 1.5, 2.4 and 4.8 GHz. The observations show a jet and a counterjet out to exceptional distances of 380 and 590 kpc, respectively, from the radio core. 0319–454 is therefore one of only three powerful radio galaxies observed to date to have twin quasi-continuous jets. The NE lobe of the double radio structure has an extremely rare configuration of five compact hotspots that may be classified into a primary and a cluster of four secondaries. Optical imaging of the parent galaxy reveals a prominent, warped dust lane and a highly disturbed structure that is indicative of past merger activity
Multiscale probability mapping: groups, clusters and an algorithmic search for filaments in SDSS
We have developed a multiscale structure identification algorithm for the
detection of overdensities in galaxy data that identifies structures having
radii within a user-defined range. Our "multiscale probability mapping"
technique combines density estimation with a shape statistic to identify local
peaks in the density field. This technique takes advantage of a user-defined
range of scale sizes, which are used in constructing a coarse-grained map of
the underlying fine-grained galaxy distribution, from which overdense
structures are then identified. In this study we have compiled a catalogue of
groups and clusters at 0.025 < z < 0.24 based on the Sloan Digital Sky Survey,
Data Release 7, quantifying their significance and comparing with other
catalogues. Most measured velocity dispersions for these structures lie between
50 and 400 km/s. A clear trend of increasing velocity dispersion with radius
from 0.2 to 1 Mpc/h is detected, confirming the lack of a sharp division
between groups and clusters. A method for quantifying elongation is also
developed to measure the elongation of group and cluster environments. By using
our group and cluster catalogue as a coarse-grained representation of the
galaxy distribution for structure sizes of <~ 1 Mpc/h, we identify 53 filaments
(from an algorithmically-derived set of 100 candidates) as elongated unions of
groups and clusters at 0.025 < z < 0.13. These filaments have morphologies that
are consistent with previous samples studied.Comment: 22 pages, 14 figures and 6 tables. Accepted for publication in MNRAS.
Data products, three-dimensional visualisations and further information about
MSPM can be found at http://www.physics.usyd.edu.au/sifa/Main/MSPM/ . v2
contains two additional references. v3 has a slightly altered title and
updated reference
Optical Spectroscopy of GX339-4 during the High-Soft and Low-Hard States II: Line Ionisation and Emission Region
We have carried out observations of the X-ray transient GX339-4 during its
high-soft and low-hard X-ray spectral states. Our high-resolution spectroscopic
observation in 1999 April suggests that the H-alpha line has a single-peaked
profile in the low-hard state as speculated in our previous paper. The HeII
4686 line, however, has a double-peaked profile in both the high-soft and
low-hard states. This suggests that the line-emission mechanism is different in
the two states. Our interpretation is that double-peaked lines are emitted from
a temperature-inversion layer on the accretion-disk surface when it is
irradiatively heated by soft X-rays. Single-peaked lines may be emitted from
outflow/wind matter driven by hard X-ray heating. We have constructed a simple
plane-parallel model and we use it to illustrate that a temperature-inversion
layer can be formed at the disk surface under X-ray illumination. We also
discuss the conditions required for the formation of temperature inversion and
line emission. Based on the velocity separations measured for the double-peaked
lines in the high-soft state, we propose that GX339-4 is a low-inclination
binary system. The orbital inclination is about 15 deg if the orbital period is
14.8 hours.Comment: accepted by mnras, 1 aug 200
Tunable-filter imaging of quasar fields at z~1. I. A cluster around MRC B0450-221
Using a combination of multicolour broad- and narrow-band imaging techniques
and follow-up spectroscopy, we have detected an overdensity of galaxies in the
field of quasar MRC B0450-221, whose properties are consistent with a cluster
at the quasar redshift z=0.9. An excess of red galaxies (V-I>2.2, I-K'>3.8) is
evident within 1' of the quasar, with the colours expected for galaxies at
z=0.9 that have evolved passively for 3 Gyr or more. A number of line-emitting
galaxies (nine candidates with equivalent widths EW>70A) are also detected in
the field using the TAURUS Tunable Filter (TTF). Three have been confirmed
spectroscopically to indeed lie at z=0.9. The TTF candidates with the strongest
[O II] line emission cluster in a group which lies 200-700 kpc away from the
quasar and the red galaxy excess, and therefore most likely on the outskirts of
the cluster. These observations are the first in a series probing quasar
environments at z~1 with TTF.Comment: Accepted for publication in AJ. 25 pages, 24 figs (large files in jpg
or gif format), uses emulateapj.st
Dust in High Redshift Galaxies
Measurements of Zn and Cr abundances in damped Lyman alpha systems at
absorption redshifts between 0.692 and 3.390 show that metals and dust are much
less abundant in high redshift galaxies than in the Milky Way today. We
conclude that the overall degree of metal enrichment of DLA galaxies
approximately 13.5 Gyr ago is 1/15 solar. The depletion of Cr is approximately
2, significantly less than in local interstellar clouds. We propose this
reflects an overall lower abundance of dust, and deduce a typical dust-to-gas
ratio of 1/30 of the Milky Way value, still sufficient to explain the weakness
of Lyman alpha emission from star-forming regions. We show that, despite claims
to the contrary, these conclusions are not inconsistent with recent high
resolution observations of DLAs with the Keck Telescope.Comment: 23 pages, LaTeX type, 1 Postscript figure Submitted to Astrophysical
Journa
The Molonglo reference catalog 1 Jy radio source survey. III. Identification of a complete quasar sample
We present a new complete sample of 111 radio quasars (including six BL Lac objects) selected from the Molonglo Reference Catalog (MRC) at 408 MHz. The sample, which we call the Molonglo Quasar Sample (MQS), forms part of a complete survey of 557 MRC radio sources with S408 ≥ 0.95 Jy in the declination range -30 ° < δ < -20 ° , b > 20 ° but excluding the R.A. range 14h03m-20h20m. Quasar classifications are based on high-resolution radio images, deep optical identifications, and follow-up spectroscopy of sources in the strip. The relatively low radio frequency of the finding survey and the complete optical identification of quasars to faint magnitudes ensure that the MQS is relatively free from orientation biases that affect most other samples of radio-loud quasars. The MQS is therefore particularly well suited to investigating the effects of radio axis orientation on quasar properties. This paper describes in detail the formation of the MQS and presents basic radio and optical data, including VLA images of extended radio sources in the sample and a complete set of optical finding charts