72 research outputs found
Steady-state hadronic gamma-ray emission from 100-MYR-old Fermi Bubbles
Fermi Bubbles are enigmatic γ-ray features of the Galactic bulge. Both putative activity (within few × Myr) connected to the Galactic center super-massive black hole and, alternatively, nuclear star formation have been claimed as the energizing source
A Unified Model of the Fermi Bubbles, Microwave Haze, and Polarized Radio Lobes: Reverse Shocks in the Galactic Center's Giant Outflows
The Galactic Center's giant outflows are manifest in three different,
non-thermal phenomena: i) the hard-spectrum, \gamma-ray `Fermi Bubbles'
emanating from the nucleus and extending to |b| ~ 50 degrees; ii) the
hard-spectrum, total-intensity microwave (~ 20-40 GHz) `Haze' extending to |b|
~ 35 degrees in the lower reaches of the Fermi Bubbles; and iii) the steep
spectrum, polarized, `S-PASS' radio (~ 2-20 GHz) Lobes that envelop the Bubbles
and extend to |b| ~ 60 degrees. We find that the nuclear outflows inflate a
genuine bubble in each Galactic hemisphere that has the classical structure,
working outwards, of reverse shock, contact discontinuity, and forward shock.
Expanding into the finite pressure of the halo and given appreciable cooling
and gravitational losses, the contact discontinuity of each bubble is now
expanding only very slowly. We find observational signatures in both
hemispheres of giant, reverse shocks at heights of ~ 1 kpc above the nucleus;
their presence ultimately explains all three of the non-thermal phenomena
mentioned above. Synchrotron emission from shock-reaccelerated cosmic-ray
electrons explains the spectrum, morphology, and vertical extent of the
microwave Haze and the polarized radio Lobes. Collisions between
shock-reaccelerated hadrons and denser gas in cooling condensations that form
inside the contact discontinuity account for most of the Bubbles' \gamma-ray
emissivity.Comment: Accepted for publication in the Astrophysical Journal. 14 figures.
Qualitative results unchanged from vers. 1 but calculations made more robust
and text clarifie
Ghost of a Shell: Magnetic Fields of Galactic Supershell GSH 006157
We identify a counterpart to a Galactic supershell in diffuse radio
polarisation, and use this to determine the magnetic fields associated with
this object. GSH 006157 has perturbed the polarised emission at
2.3GHz, as observed in the S-band Polarisation All Sky Survey (S-PASS),
acting as a Faraday screen. We model the Faraday rotation over the shell, and
produce a map of Faraday depth over the area across it. Such models require
information about the polarised emission behind the screen, which we obtain
from the Wilkinson Microwave Anisotropy Probe (WMAP), scaled from 23GHz to
2.3GHz, to estimate the synchrotron background behind GSH 006157.
Using the modelled Faraday thickness we determine the magnitude and the
plane-of-the-sky structure of the line-of-sight magnetic field in the shell. We
find a peak line-of-sight field strength of G. Our measurement probes weak magnetic fields
in a low-density regime (number densities of cm) of the ISM,
thus providing crucial information about the magnetic fields in the
partially-ionised phase.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 19 pages, 19 figure
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
Estimating extragalactic Faraday rotation
(abridged) Observations of Faraday rotation for extragalactic sources probe
magnetic fields both inside and outside the Milky Way. Building on our earlier
estimate of the Galactic contribution, we set out to estimate the extragalactic
contributions. We discuss the problems involved; in particular, we point out
that taking the difference between the observed values and the Galactic
foreground reconstruction is not a good estimate for the extragalactic
contributions. We point out a degeneracy between the contributions to the
observed values due to extragalactic magnetic fields and observational noise
and comment on the dangers of over-interpreting an estimate without taking into
account its uncertainty information. To overcome these difficulties, we develop
an extended reconstruction algorithm based on the assumption that the
observational uncertainties are accurately described for a subset of the data,
which can overcome the degeneracy with the extragalactic contributions. We
present a probabilistic derivation of the algorithm and demonstrate its
performance using a simulation, yielding a high quality reconstruction of the
Galactic Faraday rotation foreground, a precise estimate of the typical
extragalactic contribution, and a well-defined probabilistic description of the
extragalactic contribution for each data point. We then apply this
reconstruction technique to a catalog of Faraday rotation observations. We vary
our assumptions about the data, showing that the dispersion of extragalactic
contributions to observed Faraday depths is most likely lower than 7 rad/m^2,
in agreement with earlier results, and that the extragalactic contribution to
an individual data point is poorly constrained by the data in most cases.Comment: 20 + 6 pages, 19 figures; minor changes after bug-fix; version
accepted for publication by A&A; results are available at
http://www.mpa-garching.mpg.de/ift/faraday
Magnetic field evolution in cosmic filaments with LOFAR data
Measuring the magnetic field in cosmic filaments reveals how the Universe is
magnetised and the process that magnetised it. Using the Rotation Measures (RM)
at 144-MHz from the LoTSS DR2 data, we analyse the rms of the RM extragalactic
component as a function of redshift to investigate the evolution with redshift
of the magnetic field in filaments. From previous results, we find that the
extragalactic term of the RM rms at 144-MHz is dominated by the contribution
from filaments (more than 90 percent). Including an error term to account for
the minor contribution local to the sources, we fit the data with a model of
the physical filament magnetic field, evolving as
and with a density drawn from cosmological simulations of five magnetogenesis
scenarios. We find that the best-fit slope is in the range with uncertainty of --0.5, which is consistent with
no evolution. The comoving field decreases with redshift with a slope of
. The mean field strength at is in
the range --84~nG. For a typical filament gas overdensity of
the filament field strength at is in the range
--26~nG. A primordial stochastic magnetic field model with
initial comoving field of --0.11~nG is favoured. The
primordial uniform field model is rejected
S-PASS view of polarized Galactic synchrotron at 2.3 GHz as a contaminant to CMB observations
We have analyzed the southern sky emission in linear polarization at 2.3 GHz as observed by the S-band Polarization All Sky Survey (S-PASS). Our purpose is to study the properties of the diffuse Galactic polarized synchrotron as a contaminant to B-mode observations of the cosmic microwave background (CMB) polarization. We studied the angular distribution of the S-PASS signal at intermediate and high Galactic latitudes by means of the polarization angular power spectra. The power spectra, computed in the multipole interval 20 64 ` 641000, show a decay of the spectral amplitude as a function of multipole for ` . 200, typical of the diffuse emission. At smaller angular scales, power spectra are dominated by the radio point source radiation. We find that, at low multipoles, spectra can be approximated by a power law CEE;BB ` /, with ' 3, and characterized by a B-To-E ratio of about 0.5. We measured the polarized synchrotron spectral energy distribution (SED) in harmonic space, by combining S-PASS power spectra with low frequency WMAP and Planck ones, and by fitting their frequency dependence in six multipole bins, in the range 20 64 \u2113 64 140. Results show that the recovered SED, in the frequency range 2.333 GHz, is compatible with a power law with \u3b2 s =-3:22 \ub1 0:08, which appears to be constant over the considered multipole range and in the different Galactic cuts. Combining the S-PASS total polarized intensity maps with those coming from WMAP and Planck we derived a map of the synchrotron spectral index \u3b2 s at angular resolution of 2\ub0 on about 30% of the sky. The recovered \u3b2s distribution peaks at the value around-3.2. It exibits an angular power spectrum which can be approximated with a power law C`\u2113 ` with \u3b3-2:6. We also measured a significant spatial correlation between synchrotron and thermal dust signals, as traced by the Planck 353 GHz channel. This correlation reaches about 40% on the larger angular scales, decaying considerably at the degree scales. Finally, we used the S-PASS maps to assess the polarized synchrotron contamination to CMB observations of the B-modes at higher frequencies. We divided the sky in small patches (with fsky ' 1%) and find that, at 90 GHz, the minimal contamination, in the cleanest regions of the sky, is at the level of an equivalent tensor-To-scalar ratio rsynch 4810--3-. Moreover, by combining S-PASS data with Planck 353 GHz observations, we recover a map of the minimum level of total polarized foreground contamination to B-modes, finding that there is no region of the sky, at any frequency, where this contamination lies below equivalent tenor-To-scalar ratio rFG ' 10-3. This result confirms the importance of observing both high and low frequency foregrounds in CMB B-mode measurements
Measuring magnetism in the Milky Way with the Square Kilometre Array
Magnetic fields in the Milky Way are present on a wide variety of sizes and
strengths, influencing many processes in the Galactic ecosystem such as star
formation, gas dynamics, jets, and evolution of supernova remnants or pulsar
wind nebulae. Observation methods are complex and indirect; the most used of
these are a grid of rotation measures of unresolved polarized extragalactic
sources, and broadband polarimetry of diffuse emission. Current studies of
magnetic fields in the Milky Way reveal a global spiral magnetic field with a
significant turbulent component; the limited sample of magnetic field
measurements in discrete objects such as supernova remnants and HII regions
shows a wide variety in field configurations; a few detections of magnetic
fields in Young Stellar Object jets have been published; and the magnetic field
structure in the Galactic Center is still under debate.
The SKA will unravel the 3D structure and configurations of magnetic fields
in the Milky Way on sub-parsec to galaxy scales, including field structure in
the Galactic Center. The global configuration of the Milky Way disk magnetic
field, probed through pulsar RMs, will resolve controversy about reversals in
the Galactic plane. Characteristics of interstellar turbulence can be
determined from the grid of background RMs. We expect to learn to understand
magnetic field structures in protostellar jets, supernova remnants, and other
discrete sources, due to the vast increase in sample sizes possible with the
SKA. This knowledge of magnetic fields in the Milky Way will not only be
crucial in understanding of the evolution and interaction of Galactic
structures, but will also help to define and remove Galactic foregrounds for a
multitude of extragalactic and cosmological studies.Comment: 19 pages, 2 figures; to appear as part of 'Cosmic Magnetism' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14)09
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