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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 large-scale radio/microwave sky has been mapped over a range of frequencies from tens of MHz to tens of GHz, in intensity and polarization. The emission is primarily synchrotron radiation from cosmic ray electrons spiralling in the Galactic magnetic field, in addition to free–free radiation from warm ionized gas. Away from the Galactic plane, the radio sky is dominated by very large (tens of degrees) loops, arcs, spurs and filaments, including the well-known North Polar Spur (NPS), which forms part of Loop I with a diameter of ∼120∘. In polarization data, such features are often more discernible due to their high polarization fractions suggesting ordered magnetic fields, while the polarization angles suggest fields that are parallel to the filament. The exact nature of these features are poorly understood. We give a brief review of these features, focussing on the NPS/Loop I, whose polarization directions can be explained using a simple expanding shell model, placing the centre of the shell at a distance of ∼100–200 pc. However, there is significant evidence for a larger distance in the range ∼500–1000 pc, while larger distances including the Galactic Centre are unlikely. We also briefly discuss other large-scale curiosities in the radio sky such as the microwave haze and anti-correlation of Hα filaments and synchrotron polarized intensity.
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 the first results from the MALT-45 (Millimetre Astronomer's Legacy Team-45 GHz) Galactic Plane survey. We have
observed 5 square degrees (l = 330°–335°, b = ±0 5) for spectral lines in the 7 mm band (42–44 and 48–49 GHz), including CS (1–0), class I CH3OH masers in the 7(0,7)–6(1,6) A+ transition and SiO (1–0) v = 0, 1, 2, 3. MALT-45 is the first unbiased, large-scale, sensitive spectral line survey in this frequency range. In this
paper, we present data from the survey as well as a few intriguing results; rigorous analyses of these science cases are reserved
for future publications. Across the survey region, we detected 77 class I CH3OH masers, of which 58 are new detections, along with many sites of thermal and maser SiO emission and thermal CS. We found
that 35 class I CH3OH masers were associated with the published locations of class II CH3OH, H2O and OH masers but 42 have no known masers within 60 arcsec. We compared the MALT-45 CS with NH3 (1,1) to reveal regions of CS depletion and high opacity, as well as evolved star-forming regions with a high ratio of CS
to NH3. All SiO masers are new detections, and appear to be associated with evolved stars from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE). Generally, within SiO regions of multiple vibrational
modes, the intensity decreases as v = 1, 2, 3, but there are a few exceptions where v = 2 is stronger than v = 1.
Anomalous microwave emission (AME) has been observed in numerous sky regions,
in the frequency range ~10-60 GHz. One of the most scrutinized regions is
G159.6-18.5, located within the Perseus molecular complex. In this paper we
present further observations of this region (194 hours in total over ~250
deg^2), both in intensity and in polarization. They span four frequency
channels between 10 and 20 GHz, and were gathered with QUIJOTE, a new CMB
experiment with the goal of measuring the polarization of the CMB and Galactic
foregrounds. When combined with other publicly-available intensity data, we
achieve the most precise spectrum of the AME measured to date, with 13
independent data points being dominated by this emission. The four QUIJOTE data
points provide the first independent confirmation of the downturn of the AME
spectrum at low frequencies, initially unveiled by the COSMOSOMAS experiment in
this region. We accomplish an accurate fit of these data using models based on
electric dipole emission from spinning dust grains, and also fit some of the
parameters on which these models depend.
We also present polarization maps with an angular resolution of ~1 deg and a
sensitivity of ~25 muK/beam. From these maps, which are consistent with zero
polarization, we obtain upper limits of Pi<6.3% and <2.8% (95% C.L.)
respectively at 12 and 18 GHz, a frequency range where no AME polarization
observations have been reported to date. These constraints are compatible with
theoretical predictions of the polarization fraction from electric dipole
emission originating from spinning dust grains. At the same time, they rule out
several models based on magnetic dipole emission from dust grains ordered in a
single magnetic domain, which predict higher polarization levels. Future
QUIJOTE data in this region may allow more stringent constraints on the
polarization level of the AME.
We describe a simple but efficient method for deriving a consistent set of
monopole and dipole corrections for multi-frequency sky map data sets, allowing
robust parametric component separation with the same data set. The
computational core of this method is linear regression between pairs of
frequency maps, often called "T-T plots". Individual contributions from
monopole and dipole terms are determined by performing the regression locally
in patches on the sky, while the degeneracy between different frequencies is
lifted when ever the dominant foreground component exhibits a significant
spatial spectral index variation. Based on this method, we present two
different, but each internally consistent, sets of monopole and dipole
coefficients for the 9-year WMAP, Planck 2013, SFD 100 um, Haslam 408 MHz and
Reich & Reich 1420 MHz maps. The two sets have been derived with different
analysis assumptions and data selection, and provides an estimate of residual
systematic uncertainties. In general, our values are in good agreement with
previously published results. Among the most notable results are a relative
dipole between the WMAP and Planck experiments of 10-15 uK (depending on
frequency), an estimate of the 408 MHz map monopole of 8.9 +- 1.3 K, and a
non-zero dipole in the 1420 MHz map of $0.15 +- 0.03 K pointing towards
Galactic coordinates (l,b) = (308,-36) +- 14 degrees. These values represent
the sum of any instrumental and data processing offsets, as well as any
Galactic or extra-Galactic component that is spectrally uniform over the full
sky.
The Orion Molecular Complex is the nearest site of ongoing high-mass star
formation, making it one of the most extensively studied molecular complexes in
the Galaxy. We have developed a new technique for mapping the 3D distribution
of dust in the Galaxy using Pan-STARRS1 photometry. We isolate the dust at the
distance to Orion using this technique, revealing a large (100 pc, 14 degree
diameter), previously unrecognized ring of dust, which we term the "Orion dust
ring." The ring includes Orion A and B, and is not coincident with current
H-alpha features. The circular morphology suggests formation as an ancient
bubble in the interstellar medium, though we have not been able to conclusively
identify the source of the bubble. This hint at the history of Orion may have
important consequences for models of high-mass star formation and triggered
star formation.
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 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.