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Planck 2015 results. XXV. Diffuse low-frequency Galactic foregrounds
Abstract and Figures
(abridged) We discuss the Galactic foreground emission between 20 and 100GHz
based on observations by Planck/WMAP. The Commander component-separation tool
has been used to separate the various astrophysical processes in total
intensity. Comparison with RRL templates verifies the recovery of the free-free
emission along the Galactic plane. Comparison of the high-latitude Halpha
emission with our free-free map shows residuals that correlate with dust
optical depth, consistent with a fraction (~30%) of Halpha having been
scattered by high-latitude dust. We highlight a number of diffuse spinning dust
morphological features at high latitude. There is substantial spatial variation
in the spinning dust spectrum, with the emission peak ranging from below 20GHz
to more than 50GHz. There is a strong tendency for the spinning dust component
near many prominent HII regions to have a higher peak frequency, suggesting
that this increase in peak frequency is associated with dust in the
photodissociation regions around the nebulae. The emissivity of spinning dust
in these diffuse regions is of the same order as previous detections in the
literature. Over the entire sky, the commander solution finds more anomalous
microwave emission than the WMAP component maps, at the expense of synchrotron
and free-free emission. This can be explained by the difficulty in separating
multiple broadband components with a limited number of frequency maps. Future
surveys (5-20GHz), will greatly improve the separation by constraining the
synchrotron spectrum. We combine Planck/WMAP data to make the highest S/N ratio
maps yet of the intensity of the all-sky polarized synchrotron emission at
frequencies above a few GHz. Most of the high-latitude polarized emission is
associated with distinct large-scale loops and spurs, and we re-discuss their
structure...
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The characterization of the Galactic foregrounds has been shown to be the main obstacle in the challenging quest to detect primordial B-modes in the polarized microwave sky. We make use of the Planck-HFI 2015 data release at high frequencies to place new constraints on the properties of the polarized thermal dust emission at high Galactic latitudes. Here, we specifically study the spatial variability of the dust polarized spectral energy distribution, and its potential impact on the determination of the tensor-to-scalar ratio. We use the correlation ratio of the angular power spectra between the 217- and 353-GHz channels as a tracer of these potential variations, computed on different high Galactic latitude regions, ranging from 80% to 20% of the sky. The new insight from Planck data is a departure of the correlation ratio from unity that cannot be attributed to a spurious decorrelation due to the cosmic microwave background, instrumental noise, or instrumental systematics. The effect is marginally detected on each region, but the statistical combination of all the regions gives more than 99% confidence for this variation in polarized dust properties. In addition, we show that the decorrelation increases when there is a decrease in the mean column density of the region of the sky being considered, and we propose a simple power-law empirical model for this dependence, which matches what is seen in the Planck data. We explore the effect that this measured decorrelation has on simulations of the BICEP2-Keck Array/Planck analysis and show that the 2015 constraints from those data still allow a decorrelation between the dust at 150 and 353GHz of the order of the one we measure. Finally we show that either spatial variation of the dust SED or of the dust polarization angle could produce decorrelations between 217- and 353-GHz data similar to those we observe in the data.
Recent models for the large-scale Galactic magnetic fields in the literature
were largely constrained by synchrotron emission and Faraday rotation measures.
We select three different but representative models and compare their predicted
polarized synchrotron and dust emission with that measured by the Planck
satellite. We first update these models to match the Planck synchrotron
products using a common model for the cosmic-ray leptons. We discuss the impact
on this analysis of the ongoing problems of component separation in the Planck
microwave bands and of the uncertain cosmic-ray spectrum. In particular, the
inferred degree of ordering in the magnetic fields is sensitive to these
systematic uncertainties. We then compare the resulting simulated emission to
the observed dust emission and find that the dust predictions do not match the
morphology in the Planck data, particularly the vertical profile in latitude.
We show how the dust data can then be used to further improve these magnetic
field models, particularly in the thin disc of the Galaxy where the dust is
concentrated. We demonstrate this for one of the models and present it as a
proof of concept for how we will advance these studies in future using
complementary information from ongoing and planned observational projects.
We present a description of the pipeline used to calibrate the Planck Low Frequency Instrument (LFI) timelines into thermodynamic temperatures for the Planck 2015 data release, covering four years of uninterrupted operations. As in the 2013 data release, our calibrator is provided by the spin-synchronous modulation of the cosmic microwave background dipole, but we now use the orbital component, rather than adopting the Wilkinson Microwave Anisotropy Probe (WMAP) solar dipole. This allows our 2015 LFI analysis to provide an independent Solar dipole estimate, which is in excellent agreement with that of HFI and within 1σ (0.3 % in amplitude) of the WMAP value. This 0.3 % shift in the peak-to-peak dipole temperature from WMAP and a general overhaul of the iterative calibration code increases the overall level of the LFI maps by 0.45 % (30 GHz), 0.64 % (44 GHz), and 0.82 % (70 GHz) in temperature with respect to the 2013 Planck data release, thus reducing the discrepancy with the power spectrum measured by WMAP. We estimate that the LFI calibration uncertainty is now at the level of 0.20 % for the 70 GHz map, 0.26 % for the 44 GHz map, and 0.35 % for the 30 GHz map. We provide a detailed description of the impact of all the changes implemented in the calibration since the previous data release.
We present a Radio Recombination Line (RRL) survey of the Galactic plane from the H i Parkes All-sky Survey and associated Zone of Avoidance survey, which mapped the region l = 196°–0°–52° and |b| ≤ 5° at 1.4 GHz and 14.4 arcmin resolution. We combine three RRLs, H168α, H167α, and H166α to derive fully sampled maps
of the diffuse ionized emission along the inner Galactic plane. The velocity information, at a resolution of 20 km s−1, allows us to study the spatial distribution of the ionized gas and compare it with that of the molecular gas, as traced
by CO. The longitude–velocity diagram shows that the RRL emission is mostly associated with CO gas from the molecular ring
and is concentrated within the inner 30° of longitude. A map of the free–free emission in this region of the Galaxy is derived
from the line-integrated RRL emission, assuming an electron temperature gradient with Galactocentric radius of 496 ± 100 K kpc−1. Based on the thermal continuum map, we extracted a catalogue of 317 compact (≲15 arcmin) sources, with flux densities, sizes,
and velocities. We report the first RRL observations of the southern ionized lobe in the Galactic Centre. The line profiles
and velocities suggest that this degree-scale structure is in rotation. We also present new evidence of diffuse ionized gas
in the 3-kpc arm. Helium and carbon RRLs are detected in this survey. The He line is mostly observed towards H ii regions, whereas the C line is also detected further away from the source of ionization. These data represent the first observations
of diffuse C RRLs in the Galactic plane at a frequency of 1.4 GHz.
We analyze the 5.5 year all-sky data from the Fermi Large Area Telescope
restricted to gamma-ray photons with energies between 0.6--307.2 GeV. Raw count
maps show a superposition of diffuse and point-like contributions and are
subject to shot noise and instrumental artifacts. Using the D3PO inference
algorithm, we model the observed photon counts as the sum of a diffuse and a
point-like photon flux, convolved with the instrumental beam and subject to
Poissonian shot noise. The D3PO algorithm performs a Bayesian inference in this
setting without the use of spatial or spectral templates; i.e., it removes the
shot noise, deconvolves the instrumental response, and yields estimates for the
two flux components separately. The non-parametric reconstruction uncovers the
morphology of the diffuse photon flux up to several hundred GeV. We present an
all-sky spectral index map for the diffuse component. We show that the diffuse
gamma-ray flux can phenomenologically be described by only two distinct
components: a soft component, presumably dominated by hadronic processes,
tracing the dense, cold interstellar medium and a hard component, presumably
dominated by leptonic interactions, following the hot and dilute medium and
outflows such as the Fermi bubbles. A comparison of the soft component to the
Galactic dust emission indicates that dust-to-soft-gamma ratio in the
interstellar medium decreases with latitude. The hard component exists in a
thick Galactic disk and tends to flow out of the Galaxy at some locations.
Furthermore, we find the angular power spectrum of the diffuse flux to roughly
follow a power-law with an index of 2.45 on large scales, independent of
energy. Our preliminary point source catalog includes 2522 candidates of which
we associate 1269 with known sources from the second Fermi source catalog. We
provide upper limit fluxes for galaxy clusters with known radio halos.
We employ the all-sky map of the anomalous microwave emission (AME) produced
by component separation of the microwave sky to study correlations between the
AME and Galactic dust properties. We find that while the AME is highly
correlated with all tracers of dust emission, fluctuations in the AME intensity
per dust optical depth are uncorrelated with fluctuations in the emission from
polycyclic aromatic hydrocarbons (PAHs), casting doubt on the association
between AME and PAHs. Further, we find that the best predictor of the AME
strength is the dust radiance and that the AME intensity increases with
increasing radiation field strength, at variance with predictions from the
spinning dust hypothesis. A reconsideration of other emission mechanisms, such
as magnetic dipole emission, is warranted.
Soft X-ray intensity at 0.89 keV along the North Polar Spur is shown to
follow the extinction law due to the interstellar gas in the Aquila Rift by
analyzing the ROSAT archival data, which proves that the NPS is located behind
the rift. The Aquila-Serpens molecular clouds, where the X-ray optical depth
exceeds unity, are shown to have a mean LSR velocity of v=7.33 +/- 1.94 km/s,
corresponding to a kinematic distance of r=0.642 +/- 0.174 kpc. Assuming a
shell structure, a lower limit of the distance to NPS is derived to be 1.01 +/-
0.25 kpc, with the shell center being located farther than 1.1 kpc. Based on
the distance estimation, we argue that the NPS is a galactic halo object.
The all-sky 408 MHz map of Haslam et al. is one the most important
total-power radio surveys. It has been widely used to study diffuse synchrotron
radiation from our Galaxy and as a template to remove foregrounds in cosmic
microwave background data. However, there are a number of issues associated
with it that must be dealt with, including large-scale striations and
contamination from extragalactic radio sources. We have re-evaluated and
re-processed the rawest data available to produce a new and improved 408 MHz
all-sky map. We first quantify the positional accuracy ( arcmin) and
effective beam ( arcmin) of the four individual surveys from which
it was assembled. Large-scale striations associated with 1/f noise in the
scan direction are reduced to a level K using a Fourier-based filtering
technique. The most important improvement results from the removal of
extragalactic sources. We have used an iterative combination of two techniques
-- two-dimensional Gaussian fitting and minimum curvature spline surface
inpainting -- to remove the brightest sources ( Jy), which provides
a significant improvement over previous versions of the map. We quantify the
impact with power spectra and a template fitting analysis of foregrounds to the
WMAP data. The new map is publicly available and is recommended as the template
of choice for large-scale diffuse Galactic synchrotron emission. We also
provide a destriped-only version as well as a higher resolution map with
small-scale fluctuations added, assuming a power-law angular power spectrum
down to the pixel scale (1.7 arcmin). This should prove useful in simulations
used for studying the feasibility of detecting HI fluctuations from the Epoch
of Reionization.
Polarisation amplitude measurements are affected by a positive noise bias,
particularly important in regions with low signal-to-noise ratio. We present a
new method, the 'known- angle estimator' to correct for the bias in the general
case where the uncertainties in the Q,U Stokes parameters are not symmetric and
there is an independent measurement of the polarisation angle. We show using
Monte Carlo simulations for a general case that this method successfully
corrects the polarisation bias, outperforming methods described in the
literature when the polarised signal is only marginally detected. We also test
our method with realistic data, using the noise properties of the three lower
frequency maps of WMAP. In this case, the known-angle estimator produces better
results than other methods.
We used data from the WMAP satellite at 23, 33 and 41 GHz to study the
diffuse polarised emission over the entire sky. This emission is due to
synchrotron radiation and it originates mostly from filamentary structures with
well-ordered magnetic fields. Some of these structures have been known for
decades in radio continuum maps: the 'radio loops', with the North Polar Spur
being the most studied. The origin of these filaments is not clear and there
are many filaments that are visible for the first time with these polarisation
data. We have identified 11 filaments and studied their observational
properties. We find that the polarisation spectral indices, averaged over 18
regions in the sky is , which is consistent with
synchrotron radiation, although there are significant variations in
over the sky (). The polarisation fraction of some of
the filaments can be as high as 40%, which is a signature of a well ordered
magnetic field.
We explore the link between the large-scale filaments and the local ISM,
using the model of an expanding shell in the vicinity of the Sun. We compared
the observed polarisation angles with the predictions from the model and found
good agreement over most of the sky. We also studied the level of contamination
added by the diffuse filaments to the CMB E- and B-modes power spectra. We find
that the power measured at low of the B-mode spectrum is times
larger than the power measured after masking the filaments from the maps. We
conclude that, even though these filaments present low radio brightness, a
careful removal will be necessary for future all-sky CMB polarisation analysis.