arXiv:1501.06069v1 [astro-ph.GA] 24 Jan 2015
Mon. Not. R. Astron. Soc.
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C-Band All-Sky Survey: A First Look at the Galaxy
M. O. Irfan,
1
∗
C. Dickinson,
1
†
R. D. Davies,
1
C. Copley,
2
,
3
,
4
R. J. Davis,
1
P. G. Ferreira,
4
C. M. Holler,
4
,
5
J. L. Jonas,
2
,
3
Michael E. Jones,
4
O. G. King,
4
,
6
J. P. Leahy,
1
J. Leech,
4
E. M. Leitch,
7
S. J. C. Muchovej,
6
T. J. Pearson,
6
M. W. Peel,
1
A. C. S. Readhead,
6
M. A. Stevenson,
6
D. Sutton,
4
,
8
,
9
Angela C. Taylor,
4
J. Zuntz
1
,
4
1
Jodrell Bank Centre for Astrophysics, School of Physics and
Astronomy, The University of Manchester, Oxford Road
Manchester, M13 9PL, Manchester, U.K.
2
Department of Physics and Electronics, Rhodes University,
Drostdy Road, Grahamstown, 6139, South Africa
3
SKA SA, 3rd Floor, The Park, Park Road, Pinelands, 7405, Sout
h Africa
4
Sub-department of Astrophysics, University of Oxford, Den
ys Wilkinson Building, Keble Road, Oxford OX1 3RH, U.K.
5
Munich University of Applied Sciences, Lothstr. 34, 80335 M
unich, Germany
6
California Institute of Technology, Pasadena, CA 91125, US
A
7
University of Chicago, Chicago, Illinois 60637, USA
8
Institute of Astronomy, University of Cambridge, Madingle
y Road, Cambridge CB3 0HA, U.K.
9
Kavli Institute for Cosmology, Cambridge, Madingley Road,
Cambridge, CB3 0HA, U.K.
27 January 2015
ABSTRACT
We present an analysis of the diffuse emission at 5 GHz in the first quad
rant of the
Galactic plane using two months of preliminary intensity data taken wit
h the C-Band
All Sky Survey (C-BASS) northern instrument at the Owens Valley R
adio Observa-
tory, California.
Combining C-BASS maps with ancillary data to make temperature-tem
perature
plots we find synchrotron spectral indices of
β
=
−
2
.
65
±
0
.
05 between 0.408 GHz and
5 GHz and
β
=
−
2
.
72
±
0
.
09 between 1.420 GHz and 5 GHz for
−
10
◦
<
|
b
|
<
−
4
◦
,
20
◦
< l <
40
◦
. Through the subtraction of a radio recombination line (RRL) free-
free
template we determine the synchrotron spectral index in the Galac
tic plane (
|
b
|
<
4
◦
)
to be
β
=
−
2
.
56
±
0
.
07 between 0.408 GHz and 5 GHz, with a contribution of 53
±
8
per cent from free-free emission at 5 GHz. These results are cons
istent with previous
low frequency measurements in the Galactic plane.
By including C-BASS data in spectral fits we demonstrate the prese
nce of anoma-
lous microwave emission (AME) associated with the H
ii
complexes W43, W44 and
W47 near 30 GHz, at 4.4
σ
, 3.1
σ
and 2.5
σ
respectively. The CORNISH VLA 5 GHz
source catalogue rules out the possibility that the excess emission d
etected around
30 GHz may be due to ultra-compact H
ii
regions. Diffuse AME was also identified at
a 4
σ
level within 30
◦
< l <
40
◦
,
−
2
◦
< b <
2
◦
between 5 GHz and 22.8 GHz.
Key words:
radiation mechanisms: non-thermal – radiation mechanism: therma
l –
diffuse radiation – radio continuum: ISM.
1 INTRODUCTION
Diffuse Galactic radio emission is a combination of free-fre
e,
synchrotron, anomalous microwave and thermal dust emis-
sion. Below 10 GHz, free-free and synchrotron contribution
s
account for the majority of the emission, while anomalous
microwave emission (AME) reaches its peak between 10 and
30 GHz (
Gold et al. 2009
;
Planck Collaboration et al. 2014e
).
∗
E-mail:mirfan@jb.man.ac.uk
†
E-mail:Clive.Dickinson@manchester.ac.uk
At frequencies
>
100 GHz, the total Galactic emission is
dominated by thermal emission from dust (
Finkbeiner &
Schlegel 1999
). Measurements of the diffuse Galactic emis-
sion are used for both the interpretation of cosmological da
ta
and the understanding of our Galaxy. Synchrotron emission
can reveal information on the Galactic magnetic field and
cosmic ray physics (
Jaffe et al. 2011
) while free-free emis-
sion measurements can be used to explore H
ii
regions and
the warm ionised medium (
Davies et al. 2006
). The cosmo-
logical demand for increasingly sensitive, large-scale ma
ps
of Galactic emission is driven by the need to remove these
c
2014 RAS
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M. O. Irfan et al.
‘foreground’ emissions to obtain an accurate measurement
of the cosmic microwave background (CMB) signal. This
is particularly the case for imaging the CMB polarization
signal and identifying
B
-modes, where the intrinsic cosmo-
logical signal is a small fraction of the foregrounds (
Dunkley
et al. 2009
;
Betoule et al. 2009
;
Bennett et al. 2013
;
Er-
rard & Stompor 2012
). This issue has been exemplified by
the recent claim of a detection of intrinsic CMB
B
-modes
(
Ade et al. 2014
), which now appears to be contaminated
by foregrounds (
Planck Collaboration et al. 2014a
).
As different foregrounds dominate over different spec-
tral and spatial ranges, it is important to have all-sky data
available covering a range of frequencies and with
∼
1
◦
res-
olution if CMB polarization experiments are to live up to
their full potential in determining cosmological paramete
rs
(
Planck Collaboration et al. 2014f
). The low frequency all-
sky intensity surveys currently most used for this purpose
are at 0.408 GHz (
Haslam et al. 1982
), 1.420 GHz (
Reich &
Reich 1986
;
Testori et al. 2001
;
Reich, Testori & Reich 2001
),
22.8 GHz (
Bennett et al. 2013
) and 28.4 GHz (
Planck Collab-
oration et al. 2014b
).
Jonas, Baart & Nicolson
(
1998
) and
Carretti et al.
(
2013
) provide southern Hemisphere maps
at 2.3 GHz in intensity and polarization, respectively.
Sun
et al.
(
2007
) present a polarization survey of the Galactic
plane at 5 GHz with a FWHM of 9.5 arcmin so as to cap-
ture the small-scale structures. However the lack of all-sk
y,
degree scale surveys between 1.420 GHz and 22.8 GHz intro-
duces great uncertainties in the spectral behaviour of free
-
free, synchrotron and anomalous microwave emission across
the full sky (
Peel et al. 2012
).
C-BASS, the C-Band All Sky Survey, is a project cur-
rently mapping Galactic intensity and linear polarization
at a frequency of 5 GHz (Jones et al. in prep,
King et al.
2010
). At this frequency the polarized emission is negligi-
bly affected by Faraday rotation at high latitudes (∆
ψ
.
1
◦
across most of the
|
b
|
>
10 deg latitude sky). C-BASS con-
sists of two independent telescopes: C-BASS North observes
from the Owens Valley Radio Observatory (OVRO) in Cal-
ifornia, USA (latitude 37
◦
.
2 N) whilst C-BASS South is lo-
cated at the Klerefontein support base for the Karoo Radio
Astronomy Observatory (latitude 30
◦
.
7 S). C-BASS North
achieved first light in 2010 and entered its final survey mode
in late 2012 after a period of commissioning and upgrades
(Muchovej et al. in prep). C-BASS South is currently being
commissioned. The final all-sky maps will have a FWHM
resolution of 43
.
′
8 and a nominal sensitivity in polariza-
tion of
≈
0.1 mK per beam. On completion the C-BASS
data will be used to probe the Galactic magnetic field us-
ing synchrotron radiation, constrain the spectral behavio
ur
of AME, and provide a polarized foreground template for
CMB polarization experiments. The large angular scale and
high sensitivity of the final survey will be of particular use
for confirming
B
-mode detections at low multipoles.
In this paper we present some of the Galactic science
that can be achieved using preliminary C-BASS intensity
results. We focus on the composition of the Galactic plane
at 5 GHz, determining the spectral index of synchrotron
emission between 0.408 and 5 GHz and the fractional com-
position of free-free and synchrotron emission present. We
extend this analysis to include higher frequency data to
constrain the AME amplitude and spectrum towards a few
compact regions. For this analysis we use only two months
Table 1.
C-BASS North observational parameters. The full-beam
area is defined as being within 3
.
◦
5 of the main beam peak and
the first main-beam null is at 1
◦
from the main-beam peak.
Parameter
C-BASS North
Latitude
37
.
◦
2 N
Antenna optics
symmetric Gregorian
Antenna geometry
Az/El
Diameter
6.1 m
Primary beamwidth (FWHM)
44 arcmin
First Null
1
.
◦
5
Main-beam efficiency (
<
1
.
◦
0)
72.8 %
Full-beam efficiency (
<
3
.
◦
5)
89.0 %
Intensity centre frequency
4.76 GHz
Intensity noise-equivalent bandwidth
0.489 GHz
Noise equivalent temperature
2 mK
√
s
of preliminary C-BASS North intensity data (January and
February 2012), which has a sufficient signal-to-noise ratio
to constrain the properties of the bright emission near the
Galactic plane. The main observational parameters for C-
BASS North are shown in Table
1
.
This paper is organized as follows: Section 2 describes
the C-BASS and ancillary data, and presents a derivation
of the free-free template used in subsequent analysis. The
quality of the C-BASS data calibration is verified in Sec-
tion 3. In Section 4 we investigate the synchrotron spectral
index of the Galactic plane (
|
b
|
<
4 deg), determining the in-
termediate latitude (4
◦
<
|
b
|
<
10
◦
) synchrotron-dominated
spectral index between 0.408/1.420 GHz and 5 GHz, as well
as the in-plane (0
◦
<
|
b
|
<
4
◦
) synchrotron spectral index
once we account for free-free emission in this region. In Sec
-
tion 5 we use the C-BASS data alongside higher-frequency
data from the
WMAP
and
Planck
satellites to constrain
the spectral behaviour of anomalous microwave emission in
−
2
◦
< b <
2
◦
, 30
◦
< l <
40
◦
. Conclusions are presented in
Section 6.
2 DATA
2.1 C-BASS
C-BASS North observes by repeatedly scanning over 360
◦
in azimuth at a constant elevation. Several different scan
speeds close to 4
◦
per second are used, to ensure that a
given feature in the time-ordered data does not always cor-
respond to the same angular frequency on the sky, in case
of data corruption by effects such as microphonics. The
C-BASS North data are subject to 1.2 Hz (and harmon-
ics) microphonic oscillations but these effects are reduced
to a negligible level within the intensity data, especially
within the high signal-to-noise regions investigated here
, by
our data reduction pipeline (Muchovej et al., in prep.). We
scan at an elevation of 37
.
◦
2 (through the North Celestial
Pole) for about two-thirds of the time, and at higher ele-
vations (mostly 47
.
◦
2) for the remainder of the time, in or-
der to even out the sky coverage. The northern receiver is
a continuous-comparison receiver (
King et al. 2014
), which
measures the difference between sky brightness temperature
and a temperature-stabilized resistive load. This archite
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ture reduces receiver 1/
f
noise, which would otherwise con-
taminate the sky signal.
The northern receiver has a nominal bandpass of
4.5 – 5.5 GHz, but in-band filters to remove terrestrial radio
frequency interference (RFI) reduce the effective bandwidt
h
to 0.489 GHz, with a central frequency of 4.76 GHz (
King
et al. 2014
). The finite and relatively large (20 per cent)
bandwidth, means that adopting a single central frequency
is a simplification. This is because the effective frequency
is a convolution of the instrument bandpass with the sky
signal across this bandpass:
ν
β
eff
=
∫
f
(
ν
)
G
m
(
ν
)
ν
β
dν
∫
f
(
ν
)
G
m
(
ν
)
dν
,
(1)
where
f
(
ν
) is the bandpass response and
G
m
(
ν
) is the for-
ward gain (e.g.,
Jarosik et al. 2011
). Any source that has a
spectral shape different to that of the calibrator source, wi
ll
result in a slightly different effective frequency. One can co
r-
rect for this by making ‘colour corrections’ (e.g.,
Leahy et al.
2010
) assuming the bandpass shape and the spectral shape
of the source being observed. For C-BASS, we estimate that
these colour corrections are
≈
1 per cent for typical spectra
(
Irfan 2014
); only spectral shapes significantly different to
that of our calibrators (primarily Tau A and Cas A) require
corrections. For this analysis, we do not make any colour
corrections and assume an effective frequency of 4.76 GHz.
We include an additional 1 per cent uncertainty, which is
added in quadrature with the other uncertainties.
The raw telescope time-ordered data are processed by a
data reduction pipeline that identifies RFI events, perform
s
amplitude and polarization calibration and applies atmo-
spheric opacity corrections (Muchovej et al. in prep). The
northern receiver includes a noise diode which, when acti-
vated, injects a signal of constant noise temperature. The
noise diode excess noise ratio (ENR) is stable to within
1 per cent over time periods of several months. The data
reduction pipeline calibrates the intensity signal onto th
e
noise diode scale, and the noise diode is subsequently cali-
brated against the astronomical sources Cas A, Tau A and
Cyg A. The calibrator point-source flux densities are calcu-
lated from the spectral forms given in
Weiland et al.
(
2011
),
Baars et al.
(
1977
) and
Vinyaikin
(
2007
) and converted to
an equivalent antenna temperature. Next, the models of the
beam are used to convert the antenna temperature scale
(
T
A
) to a ‘full beam’ temperature scale (
T
F B
):
T
F B
=
T
OF F
+
Ω
A
Ω
F B
T
A
,
(2)
where Ω
F B
is the full beam solid angle, Ω
A
is the total beam
solid angle and
T
OF F
is the survey zero level (e.g.,
Jonas,
Baart & Nicolson 1998
). Following
Reich
(
1982
) and
Jonas,
Baart & Nicolson
(
1998
), we define the extent of the full-
beam to be 3
.
◦
5 from boresight. Using GRASP physical op-
tics simulations (
Holler et al. 2013
) we calculate the C-BASS
North antenna temperature to full-beam brightness temper-
ature conversion factor to be 1.124. The initial temperatur
e
scale of the C-BASS maps is referenced to the antenna tem-
perature of the noise diode, and this is subsequently con-
verted antenna temperature and then the more useful full-
beam brightness temperature to account for power lost in
the far sidelobes. The level of power lost is dependent on
the observed source morphology and so this factor varies
depending on what is being observed. It is the generally ac-
cepted practice to convert from antenna temperature to the
full-beam scale, as described above, in order to fully repre
-
sent the large-scale structures of the map. While using
T
F B
is correct for diffuse emission, as demonstrated in Section
3
, it underestimates point source emission by
≈
5 per cent
and so
T
A
is used for the analysis conducted in Section
5.2
.
The conversion between flux density and full-beam
brightness temperature required for this calibration sche
me,
alongside the opacity (calculated from skydips) and colour
corrections, carry with them a few per cent uncertainty. The
main source of uncertainty in the calibration comes from the
telescope aperture efficiency, which is required to convert b
e-
tween flux density and antenna temperature. The aperture
efficiency of the northern telescope has been determined by
simulations, to an accuracy that we estimate to be 5 per
cent, dominated by uncertainties in the modeling of stand-
ing waves between the feed and the subreflector. Further
work is being done on both simulation and measurement of
the aperture efficiency for the calibration of the full survey
.
In the meantime a conservative 5 per cent calibration error
has been assigned to these preliminary data.
The C-BASS maps are constructed from the pipeline-
processed time-ordered data using the DEStriping CARTog-
rapher,
Descart
(
Sutton et al. 2010
).
Descart
performs a
maximum likelihood fit to the data, by modelling the contri-
bution of 1/
f
noise in the timestreams as a series of offsets
of a given length. The multiple crossings of each pixel over
a range of parallactic angles in long sets of observations al
-
low these offsets to be well determined. The characteristic
timescale (‘knee frequency’) of typical 1
/f
fluctuations in
the C-BASS intensity data is tens of mHz (corresponding
to tens of seconds), depending on the atmospheric condi-
tions. We adopt an offset length of 5 seconds, although we
achieve similar results with 10 seconds. We use power spec-
tra of the time-ordered data to estimate the white-noise var
i-
ance and knee frequency, providing a model of instrumental
noise which is used in the mapping process. The data are
inverse-variance weighted in each pixel to achieve the op-
timal weighting for signal-to-noise ratio. No striations a
re
visible in our final map. The data are gridded into
HEALPix
(
G ́orski et al. 2005
) maps with
N
side
= 256, corresponding
to 13.7 arcmin pixels.
The completed intensity survey made using all avail-
able data will be confusion limited, but the maps used in
this analysis were made with just two months data and are
noise limited. The C-BASS confusion noise limit in inten-
sity is estimated to be
≈
130 mJy beam
−
1
(
Condon 2002
) or
0.8 mK, while the thermal noise limit for the two month pre-
liminary data used in this analysis is
≈
6 mK. The Galactic
signals analysed in this paper are
>
500 mK.
The four regions listed in Table
2
and outlined in Fig.
1
.
The four regions can be considered as off-plane (Area 1,
which is split into two) and in-plane (Areas 2, 3 and 3).
Fig.
1
shows the Galactic plane and higher latitudes as seen
at 4.76 GHz by C-BASS North as well as the northern part
of the Gould Belt, a star forming disc tilted at 18
◦
to the
Galactic plane. The Gould Belt is known for its increased su-
pernova activity (
Grenier 2000
) and includes the Ophiuchus
cloud complex.
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M. O. Irfan et al.
Figure 1.
C-BASS 4.76 GHz intensity map using data taken in
January and February 2012 showing a region in the first Galact
ic
quadrant. The data are at
N
side
= 256 and FWHM of 44 arcmin.
The white dashed rectangles highlight the four regions, use
d in
this analysis and the white solid curved lines delineate the
north-
ern part of the Gould Belt. The empty map region, bottom right
,
is due to an absence of C-BASS North data below declination =
−
30
◦
; this area will be mapped by C-BASS South observations in
the future. The colour bar scale is in kelvin and the black con
tours
are spaced at 0.2 K intervals.
Table 2.
The four Galactic plane regions investigated in this
work.
Area Long. (
◦
)
Lat. (
◦
)
Motivation
1 20 – 40
−
10
→ −
4 & 4
→
10 Synchrotron dominated
2 20 – 40
−
4
→
4
The RRL data region
3 21 – 26
−
4
→
4
Typical diffuse region
4 30 – 39
−
2
→
2
AME region
2.2 Ancillary Data
We have compared the C-BASS maps with the all-sky maps
at other frequencies listed in Table
3
. We will now briefly
discuss each data set in turn. All maps were used in
HEALPix
format. Those maps that were originally in a different format
were converted to
HEALPix
using a nearest-neighbour pixel
interpolation. The maps were first smoothed to a common
1
◦
resolution, assuming that the original survey beams were
Gaussian with the FWHM specified in Table
3
. Depending
on the application, the maps were resampled as required to
HEALPix
N
side
= 256 (13
.
′
7 pixels) or
N
side
= 64 (55
′
pixels).
2.2.1 Low frequency data
We use the
Haslam et al.
(
1982
) 408 MHz,
Reich & Re-
ich
(
1986
) 1.42 GHz, and
Jonas, Baart & Nicolson
(
1998
)
2.326 GHz radio maps to provide additional low frequency
information on synchrotron/free-free emission. The Hasla
m
data are used in their unfiltered form so point sources and
striations are still present. These data, alongside the
Jonas,
Baart & Nicolson
(
1998
) data are also used to determine
the spectral index between 0.408 and 4.76 GHz outside of
the Galactic plane. The
Jonas, Baart & Nicolson
(
1998
)
data only cover the southern sky (
−
83
◦
< δ <
13
◦
) but
fortunately the survey extends to the Galactic plane region
under investigation in this work.
Haslam et al.
(
1982
) estimate an uncertainty of 3 K in
their global zero level. Although the true random noise is
far less than this, striations present in the data are typica
lly
0.5 K and worsen to 3 K in some regions, so we take 3 K as
a conservative
local
uncertainty in the pixel values.
The temperature scale of the 1.420 GHz map is problem-
atic, as extensively discussed by
Reich & Reich
(
1988
). The
map is nominally on the full-beam scale. Because of the near-
sidelobe response within 3
.
◦
5, fluxes of point sources obtained
by fitting a Gaussian to the central lobe of the beam are ex-
pected to be underestimates, and this may be corrected by
converting to a ‘main-beam’ brightness temperature. Based
on the observed beam profile, this correction was expected
to be about 1
.
25
±
0
.
15; however,
Reich & Reich
(
1988
) find
1
.
55
±
0
.
08 from direct observations of calibrators, so either
the measured beam area or the original temperature scale is
in error by around 24 per cent. In our application we multi-
plied the data by the empirically deduced correction factor
of 1.55. We too find this factor to be consistent with our
calibration observations of Barnard’s Loop (Section 3). We
assign a 10 per cent uncertainty, mainly due to the unquanti-
fied variation of the temperature scale with the angular size
of the source. We note that
Reich & Reich
(
1988
) estimate
a full-to-main beam factor of 1
.
04
±
0
.
05 for the 0.408 GHz
survey, so it suffers much less from this ‘pedestal’ effect.
To allow for scanning artefacts in the 1.42 GHz maps
we use a conservative local uncertainty of 0.5 K equal to the
original estimate of the zero-level uncertainty.
2.2.2 High frequency data
We use the
Planck
and
WMAP
9-year data to help con-
strain the spectral form of AME. The
WMAP
K-band data
are specifically used to determine the ratio of AME to to-
tal emission between 5 and 23 GHz. The
WMAP
thermal
noise in each
HEALPix
pixel (
σ
) was calculated using the re-
lation (
Bennett et al. 2013
)
σ
=
σ
0
/
√
N
obs
, where
N
obs
is
the map hit count and
σ
0
is the thermal noise per sample.
Therefore the r.m.s. thermal noise values listed in Table
3
for
WMAP
are average values across the sky. The
Planck
100 GHz and 217 GHz maps were not used as these data in-
clude unwanted and significant contributions from the CO
lines (
Planck Collaboration et al. 2014c
).
An additional 3 per cent uncertainty was assigned to
both the
WMAP
and
Planck
data, as in
Planck Collabora-
tion et al.
(
2011
), to account for residual beam asymmetries
after smoothing. Both
WMAP
and
Planck
have unblocked
and severely under-illuminated apertures to reduce sidelo
bes
to extremely low levels, and so the difference between the
antenna temperature and full-beam brightness temperature
scale is less than 0.5 per cent (in the case of
WMAP
, the
data are corrected for far sidelobe contamination and the
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Table 3.
The data used in this paper alongside their frequency, FWHM,
HEALPix
N
side
, calibration uncertainty, average thermal noise
(in mK Rayleigh-Jeans) unless stated otherwise) and refere
nce. The two bottom rows are derived component separation pr
oducts.
Survey
ν
(GHz) Res.(arcmin)
N
side
Calibration (%)
σ
Reference
Haslam
0.408
51
512
10 700
Haslam et al.
(
1982
)
RRLs
1.4
14.8
512
15
5
Alves et al.
(
2012
)
Reich
1.420
35
512
10 140
Reich & Reich
(
1986
)
Jonas
2.3
20.0
256
5
30
Jonas, Baart & Nicolson
(
1998
)
C-BASS
4.76
43.8
256
5
6
King et al.
(
2010
)
WMAP
22.8
49
512
0.2
0.04
Bennett et al.
(
2013
)
WMAP
33.0
40
512
0.2
0.04
Bennett et al.
(
2013
)
WMAP
40.7
31
512
0.2
0.04
Bennett et al.
(
2013
)
WMAP
60.7
21
512
0.2
0.04
Bennett et al.
(
2013
)
WMAP
93.5
13
512
0.2
0.03
Bennett et al.
(
2013
)
Planck
28.4
32.65
1024
0.4
0.009
Planck Collaboration et al.
(
2014b
)
Planck
44.1
27.92
1024
0.4
0.009
Planck Collaboration et al.
(
2014b
)
Planck
70.4
13.01
1024
0.4
0.008
Planck Collaboration et al.
(
2014b
)
Planck
143
7.04
2048
0.4
0.0006
Planck Collaboration et al.
(
2014b
)
Planck
353
4.43
2048
0.4
0.0003
Planck Collaboration et al.
(
2014b
)
Planck
545
3.80
2048
7
0.0001
Planck Collaboration et al.
(
2014b
)
Planck
857
3.67
2048
7
0.00006
Planck Collaboration et al.
(
2014b
)
COBE
-DIRBE 1249
37.1
1024
13.5
0.5 MJy sr
−
1
Hauser et al.
(
1998
)
COBE
-DIRBE 2141
38.0
1024
10.6 32.8 MJy sr
−
1
Hauser et al.
(
1998
)
COBE
-DIRBE 2997
38.6
1024
11.6 10.7 MJy sr
−
1
Hauser et al.
(
1998
)
Derived
MEM
22.8
60
128
-
-
Bennett et al.
(
2013
)
MCMC
22.8
60
64
-
-
Bennett et al.
(
2013
)
corrections from the
WMAP
‘main-beam’ to the C-BASS
full-beam are less than 0.6 per cent (
Jarosik et al. 2007
),
and we have not made any correction in this analysis.
We use the
COBE
-DIRBE band 8, 9 and 10 data to help
constrain the spectral form of thermal dust emission. The
COBE
-DIRBE data used are the Zodi-Subtracted Mission
Average (ZSMA) data (
Hauser et al. 1998
).
2.2.3 Free-Free templates
At low Galactic latitudes (
|
b
|
<
4
◦
), the intensities of the
diffuse free-free and synchrotron emission are comparable a
t
gigahertz frequencies. We separate the synchrotron and fre
e-
free contributions to total emission in the Galactic plane a
t
5 GHz; to do this a free-free emission template was required
as well as the C-BASS data. Free-free emission is unpolarize
d
and characterised by a spectral index of
β
≈ −
2
.
1 (
Gold
et al. 2009
;
Dickinson, Davies & Davis 2003
), for optically
thin regions. At high frequencies (
≈
90 GHz), the free-free
spectral index will steepen (
Draine 2011
) to
−
2
.
14 due to
the Gaunt factor, which accounts for quantum mechanical
corrections, which become important at higher frequencies
.
The source of the diffuse Galactic free-free emission is
the Warm Ionized Medium (WIM), with its intensity pro-
portional to the emission measure (EM), given by EM =
∫
(n
e
)
2
dl, where
n
e
is the electron density. The WIM
also radiates H
α
recombination lines, including the optical
Balmer-alpha line (656.28 nm) and high-n radio recombina-
tion lines (RRLs) whose strength also depends on the EM.
As a result, H
α
lines can be used to calculate free-free emis-
sion intensities provided the electron temperature is know
n.
However, H
α
lines are not satisfactory free-free tracers in
the Galactic plane: dust absorption along the line-of-sigh
t
of the H
α
lines (which needs to be corrected for through-
out the Galaxy) reaches a maximum in the plane due to the
increased density of dusty star forming regions. RRLs from
ionised hydrogen provide an absorption-free alternative f
or
tracing free-free emission in the Galactic plane (
Alves et al.
2012
). We derive the free-free template used in this analy-
sis from RRL data constructed from the HI Parkes All-Sky
Survey (HIPASS;
Barnes et al. 2001
).
The RRL data have a beam FWHM of 14.8 arcmin, and
have thermal noise and calibration uncertainties of 5 mK
and 15 per cent respectively. The RRL data (
Alves et al.
2012
) cover 20
◦
< l <
40
◦
,
−
4
◦
< b <
4
◦
and were used
by Alves et al. to produced a free-free emission template
from the data by using an average electron temperature of
6000 K across the whole region. This average value carries a
large uncertainty (
±
1000 K), as it does not account for the
presence of warmer and cooler regions. This uncertainty is
the dominant factor in the 15 per cent uncertainty assigned
to the free-free template.
For this analysis the RRL data provide our free-free
template of choice. As this is a novel approach to deriving
free-free templates, we find it illuminating to compare the
RRL-derived result with those from
WMAP
. All-sky free-
free models are available from the
WMAP
9-year product re-
lease constructed using a maximum entropy method (MEM)
and Markov Chain Monte Carlo simulation (MCMC)
1
. The
MCMC free-free model (
Gold et al. 2009
) at 22.8 GHz used
in this work (‘model f’) fits the total emission model for each
pixel as a sum of a power-law with a free spectral index pa-
1
http://lambda.gsfc.nasa.gov/product/map/dr5
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rameter for synchrotron emission, a fixed-index power-law
for the free-free emission, a CMB term, a power-law with
free spectral index for the thermal dust emission, and a the-
oretical spinning dust curve with amplitude. The MEM free-
free model at 22.8 GHz is a similar spectral pixel-by-pixel fi
t,
which uses Bayesian priors for pixels where the data do not
constrain the parameters very well (see
Gold et al. 2009
for
details).
3 DATA VALIDATION
We have considered the effect of pointing errors, absolute
intensity calibration, colour (bandpass) corrections and
at-
mospheric opacity on the C-BASS temperature scale, but
the dominant error is the uncertainty on the aperture effi-
ciency, which we estimate at 5 per cent. To assess the con-
sistency of the C-BASS temperature scale, we compare C-
BASS data with other radio surveys. We focus on Barnard’s
Loop, an H
ii
shell within the Orion complex (
Heiles et al.
2000
). This region is known to be dominated by optically
thin free-free emission at these frequencies, which allows
us
to use the well-defined spectral index to compare data at
a range of frequencies. Fig.
2
shows Barnard’s Loop in H
α
Balmer line emission and in 4.76 GHz radio continuum as
seen by C-BASS North.The boxed region shows the specific
area selected for analysis in Fig.
3
. The close morpholog-
ical similarity indicates that free-free emission from war
m
ionized gas dominates.
The temperature spectrum of a diffuse Galactic emis-
sion is generally approximated in the form of a power-law:
T
(
ν
)
∝
ν
β
,
(3)
where
T
(
ν
) represents the brightness temperature at fre-
quency
ν
and
β
is the spectral index of the temperature
distribution. Given two intensity data sets taken at dif-
ferent frequencies (
ν
1
and
ν
2
) a temperature-temperature
(
T
-
T
) plot of these two sets will reveal a linear relationship,
the gradient of which relates to the emission spectral index
(
Turtle et al. 1962
) via:
T
(
ν
1
) =
(
ν
1
ν
2
)
β
T
(
ν
2
) + baseline offsets
.
(4)
An advantage of the
T
-
T
plot method is its insensitivity to
zero-level uncertainties, though if several emission mech
a-
nisms are present several linear relationships will be seen
so
the region for analysis must be chosen with care. Optically
thin free-free emission has a well-documented spectral ind
ex
of
−
2
.
12
±
0
.
02 (
Draine 2011
), therefore a simple and effec-
tive validation of the C-BASS data quality can be achieved
using
T
-
T
plots of this area.
The compact sources Orion A (M42), Orion B and
NVSS J060746-062303 have been masked out of the C-BASS
image using a diameter 3
.
5
×
the C-BASS FWHM for Orion
A and B and 2
×
the C-BASS FWHM for NVSS J060746-
062303 (see Fig.
2
). A smaller mask size could be used for
NVSS J060746-062303 as the source is fainter than Orion A
or Orion B.
Fig.
3
displays
T
-
T
plots using the C-BASS data for
Barnard’s Loop against
Haslam et al.
(
1982
),
Reich & Reich
(
1986
) and
WMAP
K-band data. The
T
-
T
plots are made
Figure 2.
Top
: Barnard’s Loop as seen at 4.76 GHz by C-BASS
North.
N
side
= 256 and FWHM resolution of 0
.
◦
73. Orion A,
Orion B and NVSS J060746-062303 are masked out. The colour
scale in in kelvin and the contours are spaced at 0.01 K interv
als.
Bottom:
Barnard’s Loop in H
α
emission at
HEALPix
N
side
= 512
and resolution 6.1 arcmin. The colour scale is in rayleighs a
nd
the contours spaced at 100 rayleigh intervals between 100 an
d
500 (
Finkbeiner 2003
). The morphological similarity between the
maps suggests that free-free emission is dominant at 4.76 GH
z.
The white box encapsulates the region selected for temperat
ure
analysis.
at
N
side
= 64 to reduce correlations between the pixels – at
a FWHM of 1
◦
, this gives
<
2 pixels per beam.
The error bars in Fig.
3
represent the pixel noise for
the respective radio maps calculated using 1000 iteration
Monte Carlo simulations. These include the effects of con-
fusion noise (0.8 mK for the C-BASS data), zero-level un-
certainties (3 K for the
Haslam et al.
(
1982
) and 0.5 K for
the
Reich & Reich
(
1986
) data), CMB anisotropies (for the
WMAP
data),
HEALPix
smoothing and degrading as well as
the r.m.s. thermal noise. The CMB anisotropies were simu-
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lated using the
CAMB
web interface
2
and the standard, pre-set
cosmological parameters. It was determined that calibrati
on
uncertainty, and not pixel noise, was the dominant source of
uncertainty. The uncertainties quoted for the spectral in-
dices are a quadrature combination of the pixel errors and
calibration uncertainties associated with each data set.
The 0.408–4.76 GHz, 1.42–4.76 GHz and 4.76–22.8 GHz
T
-
T
plots in Fig.
3
show linear fits; the reduced
χ
2
values are
1.9, 0.5 and 1.3 for the 0.408–4.76 GHz, 1.42–4.76 GHz and
4.76–22.8 GHz data, respectively. The average spectral ind
ex
of
β
=
−
2
.
15
±
0
.
03 confirms a free-free dominated spectrum
for Barnard’s Loop. This spectral index is consistent withi
n
1
σ
of the expected value (
β
≈ −
2
.
12).
The multiplicative factor required to bring C-BASS
data to be in perfect agreement with
WMAP
22.8 GHz data
(assuming all the emission is due to free-free emission) is
0
.
95
±
0
.
05. This validation gives us confidence that there
are no significant systematic calibration errors in excess o
f
the 5 per cent calibration error assigned to the preliminary
C-BASS data.
4 DIFFUSE EMISSION BETWEEN 0.408 AND
4.76 GHz
The total diffuse Galactic plane radio continuum emission
seen at 4.76 GHz is primarily a mixture of free-free and
synchrotron emission. Free-free emission is known to have
a narrow latitude distribution while synchrotron emission
is broader (
Alves et al. 2012
;
Planck Collaboration et al.
2014d
). The narrow width of the free-free distribution shown
in Section
4.2
suggests that the free-free contribution is neg-
ligible compared to the synchrotron contribution at latitu
des
higher than four degrees. This suggestion is supported by
the H
α
free-free template (
Dickinson, Davies & Davis 2003
),
which covers off-plane regions. It is therefore possible to d
e-
termine the spectral index of synchrotron emission between
0.408 and 4.76 GHz at intermediate and high Galactic lati-
tudes without having to perform any free-free/synchrotron
emission separation.
4.1 Intermediate latitude total emission spectral
indices
The spectral index of the Galactic synchrotron emission
varies significantly, both spatially and with frequency.
Strong, Orlando & Jaffe
(
2011
) review results from sky maps
between 22 MHz and 23 GHz at intermediate latitudes, find-
ing a steepening in the synchrotron spectrum from
β
≈ −
2
.
5
at 0.1 GHz to
β
≈ −
3 at 5 GHz. Between 0.408 GHz and
3.8 GHz an index of
≈ −
2
.
7 has been estimated in the Galac-
tic plane whereas between 1.42 and 7.5 GHz the spectrum
appears to steepen to
≈ −
3.0 (
Platania et al. 1998
).
Jaffe
et al.
(
2011
),
Lawson et al.
(
1987
) and
Bennett et al.
(
2003
)
also discuss the spectral steepening of synchrotron emissi
on
in the Galactic plane and it is clear that between 2.3 and
22.8 GHz the spectral index undergoes a steepening from
≈ −
2
.
7 to
≈ −
3
.
1. Due to the relationship between electron
energy distribution
N
(
E
)
∝
E
δ
, and synchrotron spectral
2
http://lambda.gsfc.nasa.gov/toolbox
Figure 3.
T
-
T
plots of the Barnard’s Loop region between
4.76 GHz and (a) 0.408 GHz, (b) 1.42 GHz and (c) 22.8 GHz. The
spectral indices confirm a free-free emission dominated reg
ion.
index
β
=
−
(
δ
+ 3)
/
2, this spectral index steepening re-
sults from a steepening of the cosmic ray electron spectrum.
It resembles that expected from synchrotron losses, i.e. a
‘break’ (actually a rather smooth steepening) of 0.5–1 in
β
assuming a steady state between injection of fresh particle
s,
radiative loss, and transport out of the Galaxy (e.g.
Bulanov
& Dogel 1974
;
Strong, Orlando & Jaffe 2011
). However, the
timescale for electrons to leave the Galaxy implied by this
interpretation is far shorter than the cosmic ray residence
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timescales inferred from the presence of spallation produc
t
nuclei in the cosmic rays, so in current models (e.g.
Orlando
& Strong 2013
) the steepening is explained by a break in the
electron injection energy spectrum; the break due to radia-
tive losses is predicted to occur at a much lower frequency
than is actually observed.
The steepening of the synchrotron spectrum at a few
GHz is consistent with the measured steepening of the local
cosmic ray electron spectrum (
Abdo et al. 2009
), with
δ
varying from
<
2
.
3 to
≈
3 between 1 and 10 GeV (solar
modulation prevents accurate measurement of
δ
much below
1 GeV).
Reich & Reich
(
1988
) observed a steepening along
the Galactic plane of the synchrotron
β
(between 0.408
and 1.420 GHz) with
β
=
−
2
.
86 at (
l,b
) = (20
◦
,
0
◦
) and
β
=
−
2
.
48 at (
l,b
) = (85
◦
,
0
◦
).
Peel et al.
(
2012
) comment
on the lack of flatter-spectrum (
β
≈ −
2
.
7) emission at high
latitudes between 2.3 GHz and 30 GHz, in contrast to the
Galactic plane. Conversely, the most sophisticated models
of cosmic ray propagation in the Galaxy to date (
Orlando
& Strong 2013
) predict variations in
β
of no more than
0.1 between different directions, e.g. between the plane and
higher latitudes. These models are explicitly of the large-
scale diffuse emission only, and so will omit any spectral var
i-
ations caused by features on sub-kpc scales, including the
radio “loops” that dominate the high-latitude synchrotron
emission. These are expected to cause variations in
β
be-
cause they may be sites of particle injection, hence with
flatter spectra (as for most supernova remnants) and be-
cause they have stronger magnetic fields, so emission at a
given frequency is from lower energy electrons (which will
also flatten the spectrum). Spectra
steeper
than the diffuse
emission require that they contain old electrons confined by
a very low effective diffusion coefficient in magnetic fields
much stronger than typical interstellar fields, so that sign
ifi-
cant
in situ
radiative losses can occur without compensating
particle acceleration. It is also worth noting that the low-
latitude regions claimed to show flatter synchrotron spectr
a
are the ones most contaminated by free-free emission, e.g.
the Cygnus X region at
l
= 75
◦
–86
◦
.
Fig.
4
shows
T
-
T
plots of semi-correlated pixels (6 pixels
per beam), which were made in the off-plane latitude range
of 4
◦
<
|
b
|
<
10
◦
with no masking of specific regions. We
expect this region to be dominated by synchrotron emission
between 0.408/1.420 GHz and 4.76 GHz. The positive and
negative latitude ranges are plotted separately to differen
ti-
ate between Galactic plane emission and emission from the
North Polar Spur and the Gould Belt. The region in Figs.
4
b
and
4
d encompass part of the Gould Belt and the North Po-
lar Spur. The variation in synchrotron spectral index acros
s
the positive latitude range results in a bulged point distri
-
bution in Fig.
4
b between 0.12 and 0.16 K on the x-axis.
The mean spectral indices shown in Fig.
4
are sum-
marised in Table
4
and range between
β
=
−
2
.
64 and
β
=
−
2
.
72. The weighted mean spectral indices between
0.408–4.76 GHz and 1.420–4.76 GHz are
−
2
.
67
±
0
.
04 and
−
2
.
68
±
0
.
06, respectively. These results are consistent with,
but more accurate than, those in the literature, which typ-
ically give an average synchrotron spectral index of
≈ −
2
.
7
both within the galactic plane and away from the radio
“loops”. The C-BASS data provide a longer lever-arm for
constraining spectral indices with uncertainties ∆
β <
0
.
1.
Table 4.
The off-plane (4
◦
<
|
b
|
<
10
◦
) spectral indices between
0.408/1.420 GHz and 4.76 GHz.
Lon. (
◦
)
Lat. (
◦
)
ν
(GHz)
β
20 – 40
−
10
→ −
4 0.408 – 4.76
−
2
.
65
±
0
.
05
20 – 40
4
→
10
0.408 – 4.76
−
2
.
69
±
0
.
05
20 – 40
−
10
→ −
4 1.420 – 4.76
−
2
.
72
±
0
.
09
20 – 40
4
→
10
1.420 – 4.76
−
2
.
64
±
0
.
09
The larger uncertainties on the 1.420–4.76 spectral indice
s
are due to the larger uncertainties in the 1.42 GHz survey.
We expect the full C-BASS survey to have a significantly
reduced uncertainty in its calibration, and in conjunction
with other modern surveys such as S-PASS (
Carretti et al.
2013
) we should be able to constrain the spectral index and
possible curvature (steepening/flattening) of the spectru
m
even more accurately.
4.2 Free-free cleaned maps at 0.408/1.420/4.76
GHz
At low Galactic latitudes (
|
b
|
<
4
◦
) the free-free emis-
sion cannot be assumed to be negligible; the synchrotron
and free-free components need to be separated. Assuming
that below 5 GHz only synchrotron and free-free emission
are present at a detectable level, synchrotron maps can be
formed from the
Haslam et al.
(
1982
),
Reich & Reich
(
1986
)
and C-BASS data using a pixel-by-pixel subtraction of a
free-free template of choice. More sophisticated componen
t
separation methods, such as parametric fitting, could be
used to acquire synchrotron maps but this would require
an accurate estimate of the C-BASS zero-level. Therefore,
until the full survey is complete we use simpler methods of
component separation.
In Fig.
5
, we present two latitude profiles for the RRL,
MEM, and MCMC free-free templates over-plotted on the
C-BASS data for the same region. The RRL, MCMC and
MEM templates have been scaled to 4.76 GHz using the free-
free spectral index of
β
=
−
2.1. The free-free profiles all
have their baselines subtracted so that the in-plane emis-
sions can be compared without biasing from different survey
zero-levels. The baseline level was established using a cos
e-
cant (1
/
sin(
|
b
|
)) plus a slope to the
|
b
|
<
10
◦
points for each
of the curves. We then subtract the slope (offset and gradi-
ent) to leave the plane emission. The 1
◦
FWHM maps were
resampled at
HEALPix
N
side
= 64 to match the MCMC tem-
plate. The two latitude profiles average over a) 21
◦
< l <
26
◦
and b) the full RRL longitude range 20
◦
< l <
40
◦
. The 21
◦
to 26
◦
longitude range was selected to be a typical bright
diffuse region as it contained the fewest bright, compact ar-
eas in the C-BASS map. Both profiles span the full RRL
latitude range of
−
4
◦
< b <
4
◦
.
The C-BASS data were used as a ‘testbed’ for these
templates and help to identify regions over which they fail t
o
describe the measured emission. The RRL free-free template
is systematically lower in temperature than the MEM and
MCMC templates because the electron temperature used
is 1000 K lower than the 7000 K assumed by
WMAP
. The
RRLs are a more direct free-free measure as they are not
subject to the same parameter degeneracies faced by the
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Figure 4.
Top row:
T
-
T
plots between 0.408 GHz and 4.76 GHz for (a)
−
10
◦
< b <
−
4
◦
(Below Plane) and (b) 10
◦
> b >
4
◦
(Above
Plane).
Bottom row:
T
-
T
plot for 1.420 GHz and 4.76 GHz for (c)
−
10
◦
< b <
−
4
◦
and (d) 10
◦
> b >
4
◦
. The off-plane spectral indices
between 0.408 and 4.76 GHz can be seen to range between
−
2
.
67 and
−
2
.
72.
MCMC and MEM templates but of course are so far only
available for
|
b
|
<
4
◦
and a limited range of longitudes.
Three synchrotron maps at 0.408 GHz, 1.420 GHz and
4.76 GHz were made by subtracting the RRL free-free tem-
plate from the
Haslam et al.
(
1982
),
Reich & Reich
(
1986
)
and C-BASS maps. The RRL free-free template was scaled
to the different frequencies using a single power-law with
β
=
−
2
.
1. The synchrotron maps, scaled to 4.76 GHz (using
β
values given in Table
5
), are shown as latitude profiles
in Fig.
6
. The best-fit synchrotron spectral indices required
to match the 0.408/1.420 GHz latitude profiles with their
corresponding 4.76 GHz values are shown in Table
5
. The
data are shown at
HEALPix
N
side
= 256 so as to fully sam-
ple the latitude profile, resulting in correlated pixel erro
rs.
The thermal noise is shown as error bars while the average
calibration errors (based on a 0.4 K signal) are shown in the
legend. Fig.
6
a averages over the 21
◦
to 26
◦
longitude range
while
6
b averages over the entire 20
◦
to 40
◦
longitude range.
As in Fig.
5
, the synchrotron curves are shown after the sub-
traction of their zero-levels. Both Fig.
6
a and Fig.
6
b dis-
play the broad peaks typically associated with synchrotron
emission. Although one average synchrotron spectral index
is clearly a good approximation across the
|
b
|
= 4
◦
latitude
range, small scale differences are seen between the curves. A
full analysis of the intensity and polarization data sets wi
ll
reveal which of these differences are physically interestin
g.
These in- and off-plane results confirm a synchrotron
spectral index of
−
2
.
72
< β <
−
2
.
64 between 0.408 and
4.76 GHz for
−
10
◦
< b <
10
◦
. Once again no significant
spectral steepening of the synchrotron spectral index is ob
-
served and the values are in agreement with the expected
index of
−
2
.
7.
At 1.5 GHz, diffuse Galactic emission has been shown
to be roughly 30 per cent free-free and 70 per cent syn-
chrotron emission (
Platania et al. 1998
). The ratio between
free-free and synchrotron emission at 4.76 GHz can now be
determined using the derived latitude distributions of the
4.76 GHz pure synchrotron map. Fig.
7
shows latitude pro-
files across
−
4
◦
< b <
4
◦
for the total 4.76 GHz emission
as measured by C-BASS, the RRL free-free map scaled to
4.76 GHz using
β
=
−
2
.
1 and the 4.76 GHz pure synchrotron
map. Extrapolating our results to 1.5 GHz, the fraction of
synchrotron emission present is 70
±
10 per cent while at
4.76 GHz it was found to be 53
±
8 per cent for 20
◦
< l <
40
◦
,
−
4
◦
< b <
4
◦
.
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M. O. Irfan et al.
Figure 5.
Free-free latitude distribution for
|
b
|
<
4
◦
using aver-
aged longitude data from the RRL, MCMC, MEM free-free tem-
plates all scaled to 4.76 GHz for the longitude range of a) 21
◦
– 26
◦
and b) 20
◦
– 40
◦
. The maps have been smoothed to 1
◦
resolution and downgraded to
HEALPix
N
side
= 64.
5 IDENTIFICATION OF ANOMALOUS
EMISSION AT 22.8 GHz
At the
WMAP
K-band frequency of 22.8 GHz, diffuse emis-
sion in the Galactic plane is a combination of free-free, syn
-
chrotron and anomalous microwave emission (AME). The
favoured emission mechanism for AME is electric dipole ra-
diation from spinning dust grains (e.g.
Draine & Lazarian
1998
;
Planck Collaboration et al. 2011
;
Davies et al. 2006
).
AME has been readily identified in dark clouds and molec-
ular clouds (
Vidal et al. 2011
;
Watson et al. 2005
;
Casassus
et al. 2008
;
AMI Consortium et al. 2011
). However it is also
present throughout the Galactic plane and adds to the com-
plexity of component separation of CMB data (
Planck Col-
laboration et al. 2011
,
2014e
). For example, with only full-
sky data at 1.4 and 22.8 GHz, the presence of AME, which
peaks in flux density at
∼
30 GHz (
Planck Collaboration
et al. 2011
), will appear only as a flattening of the total-
emission power-law. Here we use the preliminary C-BASS
data to help characterise the synchrotron signal and so de-
tect the presence of AME. The 4.76 GHz C-BASS data pro-
vides a higher-frequency measure of the synchrotron emis-
sion, which is uncontaminated by AME, and can thus be
Figure 6.
Latitude profiles of Galactic synchrotron emission for
|
b
|
<
4
◦
using averaged longitude data from the RRL free-free
subtracted
Haslam et al.
(
1982
),
Reich & Reich
(
1986
) and C-
BASS data, all scaled to 4.76 GHz for the longitude range of
a) 21
◦
– 26
◦
and b) 20
◦
– 40
◦
(see Table
5
). The maps have
been downgraded to
HEALPix
N
side
= 256 and are smoothed to
1
◦
resolution. The thermal noise is shown as error bars on the
data points while the calibration uncertainty (based on a 0.
4 K
signal) are shown at the bottom of each panel. The broad profil
e
expected for synchrotron emission is clearly visible.
Table 5.
The Galactic plane synchrotron spectral indices as de-
termined using synchrotron maps formed using the RRL free-f
ree
templates.
Long. (
◦
)
ν
range
β
21 – 26 0.408 – 4.76
−
2
.
59
±
0
.
08
21 – 26 1.420 – 4.76
−
2
.
69
±
0
.
17
20 – 40 0.408 – 4.76
−
2
.
56
±
0
.
07
20 – 40 1.420 – 4.76
−
2
.
62
±
0
.
14
used in combination with 22.8 GHz data to separate the dif-
ferent spectral components.
5.1 Diffuse AME
For this analysis the following three regions were chosen fo
r
their combination of bright diffuse and compact regions:
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