212 research outputs found
Infrared imaging of WENSS radio sources
We have performed deep imaging in the IR J- and K-bands for three sub-samples
of radio sources extracted from the Westerbork Northern Sky Survey, a large
low-frequency radio survey containing Ultra Steep Spectrum (USS), Gigahertz
Peaked Spectrum (GPS) and Flat Spectrum (FS) sources. We present the results of
these IR observations, carried out with the ARcetri Near Infrared CAmera
(ARNICA) at the Nordic Optical Telescope (NOT), providing photometric and
morphologic information on high redshift radio galaxies and quasars. We find
that the radio galaxies contained in our sample do not show the pronounced
radio/IR alignment claimed for 3CR sources. IR photometric measurements of the
gravitational lens system 1600+434 are also presented.Comment: 8 pages, 54 Postscript figures, to be published in Astronomy and
Astrophysics, Supplement Serie
Radio jet interactions in the radio galaxy PKS 2152-699
We present radio observations of the radio galaxy PKS 2152-699 obtained with
the Australia Telescope Compact Array (ATCA). The much higher resolution and
s/n of the new radio maps reveals the presence of a bright radio component
about 10 arcsec NE of the nucleus. This lies close to the highly ionized cloud
previously studied in the optical and here shown in a broadband red snapshot
image with the HST PC2. It suggests that PKS 2152-699 may be a jet/cloud
interaction similar to 3C277.3. This could cause the change in the position
angle (of ~20 deg) of the radio emission from the inner to the outer regions.
On the large scale, the source has Fanaroff & Riley type II morphology although
the presence of the two hot-spots in the centres of the lobes is unusual. The
northern lobe shows a particularly relaxed structure while the southern one has
an edge-brightened, arc-like structure.Comment: 7 pages, 5 encapsulated figures, 1 JPEG figure, accepted for MNRA
GRB 000911: Evidence for an Associated Supernova?
We present photometric and spectroscopic observations of the late afterglow of GRB 000911. We detect a moderately significant re-brightening in the R, I and J lightcurves, associated with a sizable reddening of the spectrum. This can be explained through the presence of an underlying supernova, outshining the afterglow ~ 30 days after the burst event
A Uniform Analysis of the Ly-alpha Forest at z=0 - 5: V. The extragalactic ionizing background at low redshift
In Paper III of our series "A Uniform Analysis of the Ly-alpha forest at z=0
- 5", we presented a set of 270 quasar spectra from the archives of the Faint
Object Spectrograph on the Hubble Space Telescope. A total of 151 of these
spectra, yielding 906 lines, are suitable for using the proximity effect
signature to measure J(\nu_0), the mean intensity of the hydrogen-ionizing
background radiation field, at low redshift. Using a maximum likelihood
technique and the best estimates possible for each QSO's Lyman limit flux and
systemic redshift, we find J(\nu_0)= 7.6^+9.4_-3.0 x 10^-23 ergs s^-1 cm^-2
Hz^-1 sr^-1 at at 0.03 < z < 1.67. This is in good agreement with the mean
intensity expected from models of the background which incorporate only the
known quasar population. When the sample is divided into two subsamples,
consisting of lines with z 1, the values of J(\nu_0) found are
6.5^+38._-1.6 x 10^-23 ergs s^-1 cm^-2 Hz^-1 sr^-1, and 1.0^+3.8_-0.2 x 10^-22
ergs s^-1 cm^-2 Hz^-1 sr^-1, respectively, indicating that the mean intensity
of the background is evolving over the redshift range of this data set.
Relaxing the assumption that the spectral shapes of the sample spectra and the
background are identical, the best fit HI photoionization rates are found to be
6.7 x 10^-13 s^-1 for all redshifts, and 1.9 x 10^-13 s^-1 and 1.3 x 10^-12
s^-1 for z 1, respectively. This work confirms that the evolution
of the number density of Ly-alpha lines is driven by a decrease in the ionizing
background from z ~ 2 to z ~ 0 as well as by the formation of structure in the
intergalactic medium. (Abridged)Comment: 71 LaTeX pages, 20 encapsulated Postscript figures, Accepted for
publication in ApJ, Figure 4 available at
http://lithops.as.arizona.edu/~jill/QuasarSpectra/ or
http://hea-www.harvard.edu/QEDT/QuasarSpectra
Ricci flow for homogeneous compact models of the universe
Using quaternions, we give a concise derivation of the Ricci tensor for
homogeneous spaces with topology of the 3-dimensional sphere. We derive
explicit and numerical solutions for the Ricci flow PDE and discuss their
properties. In the collapse (or expansion) of these models, the interplay of
the various components of the Ricci tensor are studied. We dedicate this paper
to honor the work of Josh Goldberg.Comment: 18 pages, 2 figure
Polarized Narrow-Line Emission from the Nucleus of NGC 4258
The detection of polarized continuum and line emission from the nucleus of
NGC 4258 by Wilkes et al. (1995) provides an intriguing application of the
unified model of Seyfert nuclei to a galaxy in which there is known to be an
edge-on, rotating disk of molecular gas surrounding the nucleus. Unlike most
Seyfert nuclei, however, NGC 4258 has strongly polarized narrow emission lines.
To further investigate the origin of the polarized emission, we have obtained
spectropolarimetric observations of the NGC 4258 nucleus at the Keck-II
telescope. The narrow-line polarizations range from 1.0% for [S II] 6716 to
13.9% for the [O II] 7319,7331 blend, and the position angle of polarization is
oriented nearly parallel to the projected plane of the masing disk. A
correlation between critical density and degree of polarization is detected for
the forbidden lines, indicating that the polarized emission arises from
relatively dense (n_e > 10^4 cm^-3) gas. An archival Hubble Space Telescope
narrow-band [O III] image shows that the narrow-line region has a compact,
nearly unresolved core, implying a FWHM size of <2.5 pc. We discuss the
possibility that the polarized emission might arise from the accretion disk
itself and become polarized by scattering within the disk atmosphere. A more
likely scenario is an obscuring torus or strongly warped disk surrounding the
inner portion of a narrow-line region which is strongly stratified in density.
The compact size of the narrow-line region implies that the obscuring structure
must be smaller than ~2.5 pc in diameter.Comment: To appear in the Astronomical Journal. 13 pages, including 1 table
and 4 figures. Uses emulateapj.st
Emission-Line Galaxies from the HST PEARS Grism Survey I: The South Fields
We present results of a search for emission-line galaxies in the Southern
Fields of the Hubble Space Telescope PEARS (Probing Evolution And Reionization
Spectroscopically) grism survey. The PEARS South Fields consist of five ACS
pointings (including the Hubble Ultra Deep Field) with the G800L grism for a
total of 120 orbits, revealing thousands of faint object spectra in the
GOODS-South region of the sky. Emission-line galaxies (ELGs) are one subset of
objects that are prevalent among the grism spectra. Using a 2-dimensional
detection and extraction procedure, we find 320 emission lines orginating from
226 galaxy "knots'' within 192 individual galaxies. Line identification results
in 118 new grism-spectroscopic redshifts for galaxies in the GOODS-South Field.
We measure emission line fluxes using standard Gaussian fitting techniques. At
the resolution of the grism data, the H-beta and [OIII] doublet are blended.
However, by fitting two Gaussian components to the H-beta and [OIII] features,
we find that many of the PEARS ELGs have high [OIII]/H-beta ratios compared to
other galaxy samples of comparable luminosities. The star-formation rates
(SFRs) of the ELGs are presented, as well as a sample of distinct giant
star-forming regions at z~0.1-0.5 across individual galaxies. We find that the
radial distances of these HII regions in general reside near the galaxies'
optical continuum half-light radii, similar to those of giant HII regions in
local galaxies.Comment: 15 pages, 13 figures; Accepted for publication in A
Polarimetry and Unification of Low-Redshift Radio Galaxies
We have made high-quality measurements of the polarization spectra of 13 FR
II radio galaxies and taken polarization images for 11 of these with the Keck
telescopes. Seven of the eight narrow-line radio galaxies (NLRG) are polarized,
and six of the seven show prominent broad Balmer lines in polarized light. The
broad lines are also weakly visible in total flux. Some of the NLRG show
bipolar regions with roughly circumferential polarization vectors, revealing a
large reflection nebula illuminated by a central source. Our observations
powerfully support the hidden quasar hypothesis for some NLRG. Classification
as NLRG, broad-line radio galaxy (BLRG), or quasar therefore depends on
orientation. However, not all objects fit into this unification scheme. Our
sample is biased towards objects known in advance to be polarized, but the
combination of our results with those of Hill, Goodrich and DePoy (1996) show
that at least 6 out of a complete, volume and flux-limited sample of 9 FR II
NLRG have broad lines, seen either in polarization or P_alpha.Comment: To appear in November 1999 Astronomical Journal. 49 pages, 13 figure
Cosmological Birefringence: an Astrophysical test of Fundamental Physics
We review the methods used to test for the existence of cosmological
birefringence, i.e. a rotation of the plane of linear polarization for
electromagnetic radiation traveling over cosmological distances, which might
arise in a number of important contexts involving the violation of fundamental
physical principles. The main methods use: (1) the radio polarization of radio
galaxies and quasars, (2) the ultraviolet polarization of radio galaxies, and
(3) the cosmic microwave background polarization. We discuss the main results
obtained so far, the advantages and disadvantages of each method, and future
prospects.Comment: To appear in the Proceedings of the JENAM 2010 Symposium "From
Varying Couplings to Fundamental Physics", held in Lisbon, 6-10 Sept. 201
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