69 research outputs found
Magnetic field tomography, helical magnetic fields and Faraday depolarization
Wide-band radio polarization observations offer the possibility to recover
information about the magnetic fields in synchrotron sources, such as details
of their three-dimensional configuration, that has previously been
inaccessible. The key physical process involved is the Faraday rotation of the
polarized emission in the source (and elsewhere along the wave's propagation
path to the observer). In order to proceed, reliable methods are required for
inverting the signals observed in wavelength space into useful data in Faraday
space, with robust estimates of their uncertainty. In this paper, we examine
how variations of the intrinsic angle of polarized emission with the
Faraday depth within a source affect the observable quantities. Using
simple models for the Faraday dispersion and , along
with the current and planned properties of the main radio interferometers, we
demonstrate how degeneracies among the parameters describing the magneto-ionic
medium can be minimised by combining observations in different wavebands. We
also discuss how depolarization by Faraday dispersion due to a random component
of the magnetic field attenuates the variations in the spectral energy
distribution of the polarization and shifts its peak towards shorter
wavelengths. This additional effect reduces the prospect of recovering the
characteristics of the magnetic field helicity in magneto-ionic media dominated
by the turbulent component of the magnetic field.Comment: Published: 2014, MNRAS 441, 2049. 9 pages, 5 figures. Major changes
since previous version: added section 2.4 (Spectral dependence) and section
3.2 and appendix (Faraday dispersion
A LABOCA survey of submillimeter galaxies behind galaxy clusters
Context: Submillimeter galaxies are a population of dusty star-forming
galaxies at high redshift. Measuring their properties will help relate them to
other types of galaxies, both at high and low redshift. This is needed in order
to understand the formation and evolution of galaxies. Aims: We use
gravitational lensing by galaxy clusters to probe the faint and abundant
submillimeter galaxy population down to a lower flux density level than what
can be achieved in blank-field observations. Methods: We use the LABOCA
bolometer camera on the APEX telescope to observe five cluster of galaxies at a
wavelength of 870 micron. The final maps have an angular resolution of 27.5
arcsec and a point source noise level of 1.2-2.2 mJy. We model the mass
distribution in the clusters as superpositions of spherical NFW halos and
derive magnification maps that we use to calculate intrinsic flux densities as
well as area-weighted number counts. We also use the positions of Spitzer MIPS
24 micron sources in four of the fields for a stacking analysis. Results: We
detected 37 submm sources, out of which 14 have not been previously reported.
One source has a sub-mJy intrinsic flux density. The derived number counts are
consistent with previous results, after correction for gravitational
magnification and completeness levels. The stacking analysis reveals an
intrinsic 870 micron signal of 390 \pm 27 microJy at 14.5 sigma significance.
We study the S_{24 micron} - S_{870 micron} relation by stacking on subsamples
of the 24 micron sources and find a linear relation at S_{24 micron} < 300
microJy, followed by a flattening at higher 24 micron flux densities. The
signal from the significantly detected sources in the maps accounts for 13% of
the Extragalactic Background Light discovered by COBE, and the stacked signal
accounts for 11%.Comment: 15 pages, 10 figures, Accepted by A&
CO deficiency in galaxies of the Fornax cluster?
There is ample observational evidence that cluster galaxies are different from those in the field. Interaction with the hot intracluster medium affects the morphology of the galaxies, their gaseous content and possibly their star-formation activity. Tidal encounters between galaxies also play an important role. The atomic component has been investigated in detail for several clusters, among them our neighbor Virgo. With the Swedish-ESO 15 m telescope, we have observed in the 12CO(1-0) transition the 23 brightest spirals and lenticulars of the Formax cluster
Assessing the galaxy population out to z ~ 2 using the Hubble Deep Field South
In this work we use the Hubble Deep Field South (HDF-S) version 2 images to
assess the galaxy population out to z ~ 2. We have used two methods of
templates fitting of the spectral energy distributions to obtain photometric
redshifts and classify the objects. The Bayesian photometric redshifts gave
better results when compared with 54 spectroscopic redshifts available in the
literature. Analysis of the rest-frame colour distribution shows a bimodality
out to z ~ 1.4. We separated our sample in a blue and a red population at B-V =
0.29. At low redshifts (0.2 0.29
whereas at higher redshifts ~ 60% of the galaxies are bluer than B-V < 0.29.
Although in low numbers, a population of early-type galaxies (or heavily
obscured low redshift galaxies) is seen out to z ~ 2.Comment: 7 pages, 7 figures, online material (the gallery) available at
http://www.oso.chalmers.se/~theresaw/Deep/gallery.htm
Radio astronomy in Africa: the case of Ghana
South Africa has played a leading role in radio astronomy in Africa with the
Hartebeesthoek Radio Astronomy Observatory (HartRAO). It continues to make
strides with the current seven-dish MeerKAT precursor array (KAT-7), leading to
the 64-dish MeerKAT and the giant Square Kilometer Array (SKA), which will be
used for transformational radio astronomy research. Ghana, an African partner
to the SKA, has been mentored by South Africa over the past six years and will
soon emerge in the field of radio astronomy. The country will soon have a
science-quality 32m dish converted from a redundant satellite communication
antenna. Initially, it will be fitted with 5 GHz and 6.7 GHz receivers to be
followed later by a 1.4 - 1.7 GHz receiver. The telescope is being designed for
use as a single dish observatory and for participation in the developing
African Very Long Baseline Interferometry (VLBI) Network (AVN) and the European
VLBI Network. Ghana is earmarked to host a remote station during a possible SKA
Phase 2. The location of the country on 5 degree north of the Equator gives it
the distinct advantage of viewing the entire plane of the Milky Way galaxy and
nearly the whole sky. In this article, we present the case of Ghana in the
radio astronomy scene and the science/technology that will soon be carried out
by engineers and astronomers.Comment: 6 pages, 3 figures, Full Referred Journal Article accepted for
publication in the South African Institute of Physics (SAIP 2014) Conference
Proceeding
The signature of dark energy on the local Hubble flow
Using N-body simulations of flat, dark energy-dominated cosmologies, we show that galaxies around simulated binary systems resembling the Local Group (LG) have low peculiar velocities, in good agreement with observational data. We have compared results for LG-like systems selected from large, high-resolution simulations of three cosmologies: a ΛCDM model, a ΛWDM model with a 2-keV warm dark matter candidate, and a quintessence (QCDM) model with an equation-of-state parameter w=−0.6. The Hubble flow is significantly colder around LGs selected in a flat, Λ-dominated cosmology than around LGs in open or critical models, showing that a dark energy component manifests itself on the scales of nearby galaxies, cooling galaxy peculiar motions. Flows in the ΛWDM and QCDM models are marginally colder than in the ΛCDM one. The results of our simulations have been compared to existing data and to a new data set of 28 nearby galaxies with robust distance measures (Cepheids and surface brightness fluctuations). The measured line-of-sight velocity dispersion is given by σH= (88 ± 20 km s−1) × (R/7 Mpc). The best agreement with observations is found for LGs selected in the ΛCDM cosmology in environments with −0.1 < δρ/ρ < 0.6 on scales of 7 Mpc, in agreement with existing observational estimates on the local matter density. These results provide new, independent evidence for the presence of dark energy on scales of a few megaparsecs, corroborating the evidence gathered from observations of distant objects and the early Univers
The magnetized disk-halo transition region of M 51
The grand-design face-on spiral galaxy M 51 is an excellent laboratory for studying magnetic fields in galaxies. Due to wavelength-dependent Faraday depolarization, linearly polarized synchrotron emission at different radio frequencies yields a picture of the galaxy at different depths: observations in the L-band (1-2 GHz) probe the halo region, while at 4.85 GHz (C-band) and 8.35 GHz (X-band), the linearly polarized emission mostly emerges from the disk region of M 51. We present new observations of M 51 using the Karl G. Jansky Very Large Array at the intermediate frequency range of the S-band (2-4 GHz), where previously no high-resolution broadband polarization observations existed, to shed new light on the transition region between the disk and the halo. We present the S-band radio images of the distributions of the total intensity, polarized intensity, degree of polarization, and rotation measure (RM). The RM distribution in the S-band shows a fluctuating pattern without any apparent large-scale structure. We discuss a model of the depolarization of synchrotron radiation in a multi-layer magneto-ionic medium and compare the model predictions to the multi-frequency polarization data of M 51 between 1-8 GHz. The model makes distinct predictions of a two-layer (disk-halo) and three-layer (far-side halo "disk"near-side halo) system. Since the model predictions strongly differ within the wavelength range of the S-band, the new S-band data are essential for distinguishing between the different systems. A two-layer model of M 51 is preferred. The parameters of the model are adjusted to fit to the data of polarization fractions in a few selected regions. In three spiral arm regions, the turbulent field in the disk dominates with strengths between 18 μG and 24 μG, while the regular field strengths are 8 - 16 μG. In one inter-arm region, the regular field strength of 18 μG exceeds that of the turbulent field of 11 μG. The regular field strengths in the halo are 3 - 5 μG. The observed RMs in the disk-halo transition region are probably dominated by tangled regular fields, as predicted from models of evolving dynamos, and/or vertical fields, as predicted from numerical simulations of Parker instabilities or galactic winds. Both types of magnetic fields have frequent reversals on scales similar to or larger than the beam size (∼550 pc) that contribute to an increase of the RM dispersion and to distortions of any large-scale pattern of the regular field. Our study devises new ways of analyzing and interpreting broadband multi-frequency polarization data that will be applicable to future data from, for example, the Square Kilometre Array
Evidence of AGN feedback and sloshing in the X-ray luminous NGC 1550 galaxy group
We present results from GMRT and Chandra observations of the NGC 1550 galaxy
group. Although previously thought of as relaxed, we show evidence that gas
sloshing and active galactic nucleus (AGN) heating have affected the structure
of the system. The 610 and 235 MHz radio images show an asymmetric jet-lobe
structure with a total size of 33 kpc, with a sharp kink at the base of
the more extended western jet, and bending of the shorter eastern jet as it
enters the lobe. The 235610 MHz spectral index map shows that both radio
lobes have steep spectral indices () indicating the
presence of an old electron population. The X-ray images reveal an asymmetric
structure in the hot gas correlated with the radio structure, as well as
potential cavities coincident with the radio lobes, with rims and arms of gas
that may have been uplifted by the cavity expansion. The X-ray residual map
reveals an arc shaped structure to the east that resembles a sloshing cold
front. Radio spectral analysis suggests a radiative age of about 33 Myr for the
source, comparable to the sloshing timescale and dynamical estimates of the age
of the lobes. An estimate of the mechanical energy required to inflate the
cavities suggests that the AGN of NGC 1550 is capable of balancing radiative
losses from the intragroup medium (IGM) and preventing excessive cooling,
providing that the AGN jets are efficiently coupled to the IGM gas. In
conclusion, we find evidence of sloshing motions from both radio and X-ray
structures, suggesting that NGC 1550 was perturbed by a minor merger or
infalling galaxy about 33 Myr ago.Comment: Accepted for publication in MNRAS, 17 pages with 13 figures and 10
table
The redshift evolution of extragalactic magnetic fields
Faraday rotation studies of distant radio sources can constrain the evolution
and the origin of cosmic magnetism. We use data from the LOFAR Two Metre Sky
Survey: Data Release 2 (LoTSS DR2) to study the dependence of the Faraday
rotation measure (RM) on redshift. By focusing on radio sources that are close
in terms of their projection on the sky, but physically unrelated (random
pairs), we measure the RM difference, RM, between the two sources.
Thus, we isolate the extragalactic contribution to RM from other
contributions. We present a statistical analysis of the resulting sample of
random pairs and find a median absolute RM difference |RM| rad/m , with |RM| uncorrelated both with respect to
the redshift difference of the pair and the redshift of the nearer source, and
a median excess of random pairs over physical pairs of
rad/m. We seek to reproduce this result with Monte Carlo simulations
assuming a non vanishing seed cosmological magnetic field and a redshift
evolution of the comoving magnetic field strength that varies as . We find the best fitting results nG and that we
conservatively quote as upper limits due to an unmodelled but non vanishing
contribution of local environments to the RM difference. A comparison with
cosmological simulations shows our results to be incompatible with primordial
magnetogenesis scenarios with uniform seed fields of order nG
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