MNRAS
000
, 1–21 (2019)
Preprint 11 June 2020
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The C-Band All-Sky Survey (C-BASS): Total intensity
point-source detection over the northern sky
R.D.P. Grumitt,
1
?
Angela C. Taylor,
1
Luke Jew,
1
Michael E. Jones,
1
C. Dickinson,
2
,
4
A. Barr,
2
R. Cepeda-Arroita,
2
H. C. Chiang,
3
S. E. Harper,
2
H. M. Heilgendorff,
5
J. L. Jonas,
6
,
7
J. P. Leahy,
2
J. Leech,
1
T. J. Pearson,
4
M. W. Peel,
8
,
9
A. C. S. Readhead,
4
J. Sievers
3
1
Sub-department of Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
2
Jodrell Bank Centre for Astrophysics, Alan Turing Building, Department of Physics and Astronomy, School of Natural Sciences,
The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.
3
Department of Physics, McGill University, 3600 Rue University, Montr ́eal, QC H3A 2T8, Canada
4
Cahill Centre for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
5
Astrophysics & Cosmology Research Unit, School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal,
Westville Campus, Private Bag X54001, Durban 4000, South Africa
6
Department of Physics and Electronics, Rhodes University, Grahamstown, 6139, South Africa
7
South African Radio Astronomy Observatory, 2 Fir Road, Observatory, Cape Town, 7925, South Africa
8
Instituto de Astrof ́ısica de Canarias, E-38205 La Laguna, Tenerife, Spain
9
Departamento de Astrof ́ısica, Universidad de La Laguna (ULL), E-38206 La Laguna, Tenerife, Spain
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
We present a point-source detection algorithm that employs the second order Spher-
ical Mexican Hat wavelet filter (SMHW2), and use it on C-BASS northern intensity
data to produce a catalogue of point-sources. This catalogue allows us to cross-check
the C-BASS flux-density scale against existing source surveys, and provides the basis
for a source mask which will be used in subsequent C-BASS and cosmic microwave
background (CMB) analyses. The SMHW2 allows us to filter the entire sky at once,
avoiding complications from edge effects arising when filtering small sky patches. The
algorithm is validated against a set of Monte Carlo simulations, consisting of dif-
fuse emission, instrumental noise, and various point-source populations. The simu-
lated source populations are successfully recovered. The SMHW2 detection algorithm
is used to produce a
4
.
76 GHz
northern sky source catalogue in total intensity, con-
taining 1784 sources and covering declinations
δ
≥ −
10
◦
. The C-BASS catalogue is
matched with the Green Bank 6 cm (GB6) and Parkes-MIT-NRAO (PMN) catalogues
over their areas of common sky coverage. From this we estimate the
90
per cent com-
pleteness level to be approximately
610 mJy
, with a corresponding reliability of
98
per
cent, when masking the brightest
30
per cent of the diffuse emission in the C-BASS
northern sky map. We find the C-BASS and GB6 flux-density scales to be consistent
with one another to within approximately
4
per cent.
Key words:
catalogues – surveys – cosmic microwave background – methods: data
analysis – radio continuum: general – cosmology: observations
1 INTRODUCTION
C-BASS is an experiment to observe the whole sky in total
intensity and polarization at
4
.
76 GHz
, at
45 arcmin
angu-
lar resolution (Jones et al. 2018). The primary purpose of
?
E-mail: richard.grumitt@physics.ox.ac.uk
the experiment is to provide data on synchrotron emission
for CMB polarization experiments at low frequencies com-
pared to the peak of the CMB, whilst avoiding significant
de-polarization due to Faraday rotation. C-BASS will also
provide improved measurements of low-frequency emission
components, enabling a detailed study of the Galactic Mag-
netic Field. The northern sky survey has now been com-
©
2019 The Authors
arXiv:1910.08583v2 [astro-ph.CO] 9 Jun 2020
2
R. D. P. Grumitt et al.
pleted, with detailed analysis of these data currently un-
derway. In this paper we present a
4
.
76 GHz
northern sky
point-source catalogue, produced using a point-source de-
tection algorithm employing the SMHW2.
Extragalactic radio sources are a key contaminant in
CMB studies, with their detection and removal being an im-
portant step in CMB component separation (Brandt et al.
1994; Taylor et al. 2001; Waldram et al. 2003, 2007; de Zotti
et al. 2010). For future CMB experiments, it has been shown
that point-source contamination has the potential to signifi-
cantly complicate measurements of the
B
-mode polarization
power spectrum on scales
`
&
50
(Mesa et al. 2002; Curto
et al. 2013; Puglisi et al. 2018; Remazeilles et al. 2018).
Given the large beam of C-BASS, the instrument is not well
suited to point-source detection, with detections of all but
the brightest sources (
&
1 Jy
) being limited by source confu-
sion and diffuse emission. For the C-BASS analysis, perform-
ing our own dedicated source detection allows us to carry out
important data quality checks on the C-BASS flux-density
scale and pointing accuracy, by comparing the C-BASS cat-
alogue with pre-existing catalogues around
4
.
76 GHz
. The
resulting catalogue will also allow us to construct an accu-
rate mask of bright source emission in the C-BASS maps,
which will be key to avoid biasing CMB component sepa-
ration analyses with C-BASS. Fainter sources can either be
subtracted using deeper, pre-existing source catalogues, or
can be treated statistically at the power spectrum level. As-
suming sources down to some flux density
S
max
have been
removed, and that sources are Poisson-distributed on the
sky, the point-source contribution to the power spectrum is
given by
C
PS
`
=
π
S
max
0
d
N
d
S
S
2
d
S
,
(1)
where
S
is the flux density and
d
N
/
d
S
is the differential
source count, i.e., the number of sources per unit flux density,
per unit steradian (Tegmark & Efstathiou 1996).
Numerous source-detection algorithms have been de-
veloped for CMB experiments, see e.g., Tegmark & de
Oliveira-Costa (1998); Vielva et al. (2003); Arg
̈
ueso et al.
(2009); Bennett et al. (2013); Herranz et al. (2009); Carvalho
et al. (2012); L ́opez-Caniego et al. (2006); Gonz ́alez-Nuevo
et al. (2006); Planck Collaboration et al. (2014, 2016c). The
Planck Catalogue of compact sources (PCCS) used an al-
gorithm based on filtering small patches of sky using the
flat-sky second-order Mexican Hat wavelet as an approxi-
mation to a matched filter, followed by threshold detection
based on the signal-to-noise ratio (SNR). Here we employ a
similar scheme but using the equivalent spherical function on
the whole sky at once, implemented in the Healpix pixeliza-
tion scheme (G ́orski et al. 2005). Previous applications of the
first-order SMHW (SMHW1) to searches for non-gaussianity
can be found in Cay ́on et al. (2001); Mart ́ınez-Gonz ́alez et al.
(2002); Curto et al. (2011), and to point-source detection in
Vielva et al. (2003). The details of this implementation are
discussed in Section 3.
The outline of this paper is as follows: In Section 2 we
summarize pre-existing source catalogues directly relevant
to our C-BASS analysis. In Section 3 we discuss the C-BASS
point-source detection algorithm, giving an overview of the
method and the Monte Carlo simulations used to validate
the algorithm. In Section 4 we discuss the C-BASS total in-
tensity, northern sky point-source catalogue obtained using
our detection algorithm. We compare the C-BASS catalogue
with the GB6 and PMN catalogues, and calculate the differ-
ential source counts for the C-BASS sources as a cross-check
on the statistical properties of the bright source population.
We summarize our results in Section 5. Whilst analysis of
polarized sources is important for future CMB polarization
studies, this falls beyond the scope of this paper where we
focus on the C-BASS total intensity results. In addition, due
to the low level of source polarization, only a small number
of polarized sources (
O(
10
)
) will be detected.
2 PRE-EXISTING SOURCE CATALOGUES
In order to construct an accurate template for masking out
the point sources in the C-BASS maps it is necessary to con-
struct a point-source catalogue based on the C-BASS obser-
vations themselves and any useful information that can be
gleaned from other more sensitive, higher resolution surveys.
Relevant to our work in this paper are the GB6 (Gregory
et al. 1996), PMN (Griffith & Wright 1993; Wright et al.
1994; Griffith et al. 1994, 1995; Wright et al. 1996a), Effels-
berg S5 (Kuehr et al. 1981), RATAN-600 (Mingaliev et al.
2007) and Combined Radio All-Sky Targeted Eight-GHz
Survey (CRATES) (Healey et al. 2007) catalogues. These
catalogues are primarily used in this paper to make com-
parisons with the C-BASS catalogue, for the purposes of
data validation and estimation of the statistical properties
of the C-BASS catalogue. However, the catalogues are also
useful for characterizing the faint point-source population
in any C-BASS analysis (i.e., sources with flux-densities be-
low
∼
1 Jy
), where reliable extraction from the C-BASS map
becomes more challenging. These radio surveys are summa-
rized in Table 1.
The GB6 and PMN
4
.
85 GHz
source catalogues cover
declinations
−
87
.
5
◦
≤
δ
≤
75
◦
. The GB6 catalogue was pro-
duced using the NRAO seven-beam receiver on the
91 m
telescope, and the PMN catalogue was produced using the
Parkes
64 m
radio telescope. The GB6 catalogue has a flux-
density limit of approximately
18 mJy
, whilst the PMN cat-
alogue has an average flux-density limit of approximately
35 mJy
over the sky. The GB6 and PMN catalogues provide
far deeper flux-density coverage than can be achieved with
C-BASS, given the differing resolutions and hence confu-
sion levels. However, it is still necessary for us to obtain our
own source catalogue, such that we can construct an accu-
rate mask for the brightest sources in the C-BASS maps,
and account for source variability between the C-BASS and
GB6/PMN surveys. The GB6 and PMN catalogues remain
useful in accounting for fainter sources (below
∼
1 Jy
) in any
C-BASS analysis.
Source catalogues covering the North Celestial Pole
(NCP) region are more limited, with GB6 only covering de-
clinations up to
δ
=
75
◦
. The Effelsberg S5 catalogue covers
declinations
δ
≥
70
◦
and is complete down to
250 mJy
(Kuehr
et al. 1981). In comparing to the S5 catalogue there are sig-
nificant issues from the variability of flat-spectrum sources,
given the large separation in time between the S5 survey
and the C-BASS survey. The RATAN-600 catalogue includes
measurements at
4
.
8 GHz
, and observed 504 sources in the
NCP region with NRAO VLA Sky Survey (NVSS) flux-
MNRAS
000
, 1–21 (2019)
C-BASS Northern Sources
3
Table 1.
Summary of radio surveys relevant to the work in this paper. In stating the sky coverage,
δ
denotes declination, and
b
denotes
Galactic latitude.
Survey Name
Reference
Frequency
Sky
FWHM
Flux Limit
Number of
[GHz]
Coverage
[arcmin]
[mJy]
Sources
C-BASS
Jones et al. (2018)
4.76
−
90
◦
≤
δ
≤
90
◦
a
45
∼
500
b
1784
GB6
Gregory et al. (1996)
4.85
0
◦
≤
δ
≤
75
◦
3
∼
18
75,162
PMN
Wright et al. (1996b)
4.85
−
87
.
5
◦
≤
δ
≤
10
◦
5
∼
35
50,814
Effelsberg S5
Kuehr et al. (1981)
4.9
70
◦
≤
δ
≤
90
◦
2.7
∼
250
476
RATAN-600
Mingaliev et al. (2007)
4
.
8
(
1
.
1
−
21
.
7
)
c
75
◦
≤
δ
≤
88
◦
0
.
67
×
6
.
6
d
(
200
)
e
504
CRATES
Healey et al. (2007)
8
.
4
(
4
.
85
)
f
|
b
|
>
10
◦
2.4
∼
65
11,131
a
Here we state the sky coverage of the whole C-BASS experiment. The results concerning point sources in this paper were
obtained for the C-BASS northern intensity data, covering declinations
−
10
◦
≤
δ
≤
90
◦
.
b
For C-BASS we state the value
3
.
5
〈
σ
〉
, where
〈
σ
〉
is the mean background fluctuation level found across the map, applying
the CG30 mask described in Section 3.5. Details on the estimation of background fluctuation levels are given in Section 3.1.
c
The RATAN-600 catalogue covers 6 frequencies from
1
.
1
−
21
.
7 GHz
, including
4
.
8 GHz
.
d
We state
FHWM
RA
×
FWHM
δ
for RATAN-600, determined from the values given in Kovalev et al. (1999).
e
The RATAN-600 catalogue was produced by pre-selecting NVSS sources with
S
1
.
4 GHz
>
200 mJy
.
f
The CRATES catalogue is primarily at
8
.
4 GHz
, with
4
.
85 GHz
sources used as the basis for observations of
8
.
4 GHz
counterparts. The properties of these
4
.
85 GHz
sources are provided with the CRATES catalogue. Further observations were
also made at
4
.
85 GHz
to fill in gaps at
δ >
88
◦
.
densities,
S
1
.
4 GHz
≥
200mJy
(Mingaliev et al. 2001, 2007).
The RATAN-600 catalogue was used in a previous anal-
ysis of diffuse emission in the NCP region with C-BASS
in Dickinson et al. (2019). Whilst this catalogue provides
deeper flux-density coverage than C-BASS, it was produced
by pre-selecting sources for study from the NVSS catalogue
at
1
.
4 GHz
(Condon et al. 1998). This can potentially miss
rising-spectrum sources that would otherwise be observable
in the C-BASS catalogue. Given the C-BASS flux-density
limit of approximately
500 mJy
, we can use the NVSS source
counts and the
1
.
4
−
4
.
85 GHz
spectral index distributions
in Tucci et al. (2011) to estimate that there may be
O(
1
)
sources that could be observed by C-BASS, whilst being
missed by RATAN-600.
It is also worth noting the CRATES catalogue (Healey
et al. 2007). CRATES is an
8
.
4 GHz
catalogue of flat-
spectrum sources with flux-densities
S
ν
=
4
.
85 GHz
>
65 mJy
,
covering Galactic latitudes
|
b
|
>
10
◦
. The catalogue there-
fore serves as a useful proxy for flat-spectrum sources in
the C-BASS catalogue, which are the primary contributor
to source variability. Healey et al. (2009) made additional
observations of the NCP region at declinations,
δ >
88
◦
to
supplement the original CRATES catalogue. The purpose of
this was to bring the flux-density limit in this region down
to the CRATES flux-density limit of
∼
65 mJy
. Three sources
were observed in this region at
4
.
85 GHz
, with flux-densities
of
67 mJy
,
58 mJy
and
142 mJy
.
3 C-BASS SOURCE DETECTION
ALGORITHM
To detect sources in a sky map we need to remove obscur-
ing diffuse emission and noise. For a source with a known
point-spread function (PSF) embedded in additive noise, the
matched filter (MF) is the optimal filter that can be applied
to maximize the source SNR. The matched filter is given by
Ψ
MF
(
k
)
=
[
2
π
π
d
k k
τ
2
(
k
)
P
(
k
)
]
−
1
τ
(
k
)
P
(
k
)
,
(2)
where
k
is the Fourier wavenumber,
τ
(
k
)
is the source pro-
file and
P
(
k
)
is the power spectrum of the unfiltered map
(Tegmark & de Oliveira-Costa 1998; L ́opez-Caniego et al.
2006). However, the calculation of the MF involves a number
of complications. Chiefly, we are required to make a noisy es-
timate of the power spectrum from our unfiltered map, and
integrate it. In constructing the PCCS, it was found that the
Mexican Hat wavelet of the second kind (MHW2) achieved
similar performance to the MF (L ́opez-Caniego et al. 2006;
Planck Collaboration et al. 2014, 2016c). For the present
analysis we adapt the
Planck
algorithm, using instead the
SMHW2 in place of the flat-space MHW2. The SMHW2 is
straightforward to calculate, enables us to filter the entire
sky at once, and allows us to optimize a few free parame-
ters as opposed to a noisy estimate of the full noise power
spectrum.
3.1 Source Detection Algorithm
Given a sky map consisting of point sources, diffuse emis-
sion and instrumental noise, we can enhance the SNR of
point sources in the map by filtering with the SMHW2. We
perform this filtering at a range of filter scales
R
, to maxi-
mize the number of sources we extract from the sky maps.
In changing
R
, we effectively change the extent to which
we down-weight large-scale
`
-modes that are dominated by
diffuse emission, and also small scales dominated by instru-
mental noise. This is particularly important in regions close
to the Galactic plane where diffuse emission is very strong,
meaning we must down-weight large scale modes harshly
in order to extract point sources. Conversely, in regions
with little diffuse emission we may wish to be less extreme
in our down-weighting, so that we do not excessively re-
duce point-source power and subsequently miss detection of
fainter sources in these regions. We discuss the form of the
MNRAS
000
, 1–21 (2019)