16 research outputs found
Central Powering of the Largest Lyman-alpha Nebula is Revealed by Polarized Radiation
High-redshift Lyman-alpha blobs are extended, luminous, but rare structures
that appear to be associated with the highest peaks in the matter density of
the Universe. Their energy output and morphology are similar to powerful radio
galaxies, but the source of the luminosity is unclear. Some blobs are
associated with ultraviolet or infrared bright galaxies, suggesting an extreme
starburst event or accretion onto a central black hole. Another possibility is
gas that is shock excited by supernovae. However some blobs are not associated
with galaxies, and may instead be heated by gas falling into a dark matter
halo. The polarization of the Ly-alpha emission can in principle distinguish
between these options, but a previous attempt to detect this signature returned
a null detection. Here we report on the detection of polarized Ly-alpha from
the blob LAB1. Although the central region shows no measurable polarization,
the polarized fraction (P) increases to ~20 per cent at a radius of 45 kpc,
forming an almost complete polarized ring. The detection of polarized radiation
is inconsistent with the in situ production of Ly-alpha photons, and we
conclude that they must have been produced in the galaxies hosted within the
nebula, and re-scattered by neutral hydrogen.Comment: Published in the August 18 issue of Nature. 1750 words, 3 figures,
and full Supplementary Information. Version has not undergone proofing.
Reduced and processed data products are available here:
http://obswww.unige.ch/people/matthew.hayes/LymanAlpha/LabPol
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
Evolutionary Map of the Universe: Tracing Clusters to High Redshift
The Australian SKA Pathfinder (ASKAP) is a new radio-telescope being built in
Western Australia. One of the key surveys for which it is being built is EMU
(Evolutionary Map of the Universe), which will make a deep (~10 {\mu}Jy/bm rms)
radio continuum survey covering the entire sky as far North as +30\circ. EMU
may be compared to the NRAO VLA Sky Survey (NVSS), except that it will have
about 45 times the sensitivity, and five times the resolution. EMU will also
have much better sensitivity to diffuse emission than previous large surveys,
and is expected to produce a large catalogue of relics, tailed galaxies, and
haloes, and will increase the number of known clusters by a significant factor.
Here we describe the EMU project and its impact on the astrophysics of
clusters.Comment: Accepted by J. Astrophys. Ast
Baryons in the relativistic jets of the stellar-mass black-hole candidate 4U 1630-47
Accreting black holes are known to power relativistic jets, both in stellar-mass binary systems and at the centres of galaxies. The power carried away by the jets, and, hence, the feedback they provide to their surroundings, depends strongly on their composition. Jets containing a baryonic component should carry significantly more energy than electron–positron jets. Energetic considerations1, 2 and circular-polarization measurements3 have provided conflicting circumstantial evidence for the presence or absence of baryons in jets, and the only system in which they have been unequivocally detected is the peculiar X-ray binary SS 433 (refs 4, 5). Here we report the detection of Doppler-shifted X-ray emission lines from a more typical black-hole candidate X-ray binary, 4U 1630-47, coincident with the reappearance of radio emission from the jets of the source. We argue that these lines arise from baryonic matter in a jet travelling at approximately two-thirds the speed of light, thereby establishing the presence of baryons in the jet. Such baryonic jets are more likely to be powered by the accretion disk6 than by the spin of the black hole7, and if the baryons can be accelerated to relativistic speeds, the jets should be strong sources of γ-rays and neutrino emission
The nature of jets: evidence from circular polarization observations
We review recent observations of circularly polarized radiation from AGN made with the VLBA and with the ATCA. We also discuss briefly the detections of the Galactic sources Sgr A* and SS 433. The origin of the circular polarization is still an open question in most cases, and we discuss four possible mechanisms. Detectable circular polarization is a common property of quasars, but not of radio galaxies, and is always associated with the compact core. There is growing evidence that the sign of the circular polarization stays the same over at least 20 - 30 years, suggesting that it is a fundamental property of the jet
Detecting relativistic X-ray jets in high-redshift quasars
We analyze Chandra X-ray images of a sample of 11 quasars that are known to contain kiloparsec scale radio jets. The sample consists of five high-redshift (z ≥ 3.6) flat-spectrum radio quasars, and six intermediate redshift (2.1 < z < 2.9) quasars. The dataset includes four sources with integrated steep radio spectra and three with flat radio spectra. A total of 25 radio jet features are present in this sample. We apply a Bayesian multi-scale image reconstruction method to detect and measure the X-ray emission from the jets. We compute deviations from a baseline model that does not include the jet, and compare observed X-ray images with those computed with simulated images where no jet features exist. This allows us to compute p-value upper bounds on the significance that an Xray jet is detected in a pre-determined region of interest. We detected 12 of the features unambiguously, and an additional 6 marginally. We also find residual emission in the cores of 3 quasars and in the background of 1 quasar that suggest the existence of unresolved X-ray jets. The dependence of the X-ray to radio luminosity ratio on redshift is a potential diagnostic of the emission mechanism, since the inverse Compton scattering of cosmic microwave background photons (IC/CMB) is thought to be redshift dependent, whereas in synchrotron models no clear redshift dependence is expected. We find that the high-redshift jets have X-ray to radio flux ratios that are marginally inconsistent with those from lower redshifts, suggesting that either the X-ray emissions is due to the IC/CMB rather than the synchrotron process, or that high redshift jets are qualitatively different