A southern-sky total intensity source catalogue at 2.3 GHz from S-band Polarisation All-Sky Survey data

4.6 MB·

by  B. W. Meyers

Publications of the Astronomical Society of Australia(PASA)
(cid:13)c AstronomicalSocietyofAustralia2017;publishedbyCambridgeUniversityPress.
doi:10.1017/pas.2017.xxx.
A southern-sky total intensity source catalogue at 2.3 GHz
from S-band Polarisation All-Sky Survey data
7
1 B. W. MeyersA,B,C,∗, N. Hurley-WalkerA, P. J. HancockA,B, T. M. O. FranzenA, E. CarrettiC,D, L.
0 Staveley-SmithE,B, B. M. GaenslerF,B, M. HaverkornG,H, S. PoppiD
2 AInternational CentreforRadioAstronomyResearch(ICRAR),CurtinUniversity,Bentley,WA6102, Australia
b BARCCentreofExcellenceforAll-SkyAstrophysics(CAASTRO)
CCSIROAstronomyandSpaceScience, P.O.Box76,Epping,NewSouthWales 1710,Australia
e
DINAF/OsservatorioAstronomicodiCagliari,ViadellaScienza5,I-09047Selargius,Italy
F
EInternational CentreforRadioAstronomyResearch(ICRAR),TheUniversityofWesternAustralia,Crawley,WA6009,Australia
2 FDunlapInstituteforAstronomyandAstrophysics,50St.GeorgeSt,UniversityofToronto,ONM5S3H4,Canada
GDepartment ofAstrophysics/IMAPP,RadboudUniversityNijmegen,POBox9010,6500GLNijmegen,theNetherlands
2
HLeidenObservatory,LeidenUniversity,POBox9513,2300RALeiden,theNetherlands

]
A
G Abstract
TheS-bandPolarisation All-SkySurvey(S-PASS)hasobserved theentiresouthern skyusingthe64-metre
.
h Parkesradiotelescopeat2.3GHzwithaneffectivebandwidthof184MHz.Thesurveyedskyareacoversall
p declinations δ≤0◦.Toanalysecompact sourcesthesurveydatahavebeenre-processed toproduceasetof
- 107StokesI mapswith10.75arcminresolutionandthelargescaleemissioncontributionfilteredout.Inthis
o
paper we use these Stokes I images to create a total intensity southern-sky extragalactic source catalogue
r
t at 2.3GHz. The source catalogue contains 23,389 sources and covers a sky area of 16,600deg2, excluding
s
a the Galactic plane for latitudes |b|<10◦. Approximately 8% of catalogued sources are resolved. S-PASS
[ source positions are typically accurate to within 35arcsec. At a flux density of 225mJy theS-PASSsource
catalogue is morethan 95% complete, and ∼94% ofS-PASSsources brighterthan 500mJybeam−1 havea
2
counterpart at lower frequencies.
v
7
8 Keywords: catalogs – surveys– radio continuum:general
8
8
0
1 INTRODUCTION S-PASS is a project to map the diffuse emission of
.
1 the entire southern sky at 2.3GHz. The main survey
0 Radiosourcecataloguesthatcoverwideareasofskyare
goals areto investigate the polarisedsynchrotronemis-
7 important tools for exploring the properties and evolu-
sion, Galactic and extragalactic magnetism, and Cos-
1 tion of a large range of source populations. Combining
: mic Microwave Background polarised foregrounds. A
v multiple source catalogues allows the determination of
more detailed description of the S-PASS survey strat-
i source spectral index information and statistical stud-
X egy andscience goalsis givenbyCarretti(2010)andin
ies with large samples of different radio galaxy popu-
r the upcoming survey description paper (Carretti et al.,
a lations, such as active galactic nuclei (AGN) and star-
in prep.).
burst galaxies. Measuring how the relative fractions of
One S-PASS data product is a collection of all-
different populations change and, ultimately, how the
southern-sky total intensity maps, containing more
differential source counts evolve with frequency (see
than 104 extragalactic radio sources. Most of the ra-
de Zotti et al. 2010 for a review) provides essential in-
dio sources in the S-PASS images will be distant radio
sight into the co-evolution of galaxies and their central
galaxies.
super-massive black holes through cosmic time.
In this paper, we present the construction and ver-
The S-band Polarisation All-Sky Survey (S-PASS)
ification of the S-PASS Stokes I source catalogue. We
has mapped the southern sky for declinations δ ≤0◦
compare the S-PASS source catalogue to several other
in total intensity and polarisation with the 64-metre
radio source catalogues to assess its quality (see Ta-
Parkes radio telescope at a frequency of 2300MHz.
ble 1): the Sydney University Molonglo Sky Survey
(SUMSS; Mauch et al. 2003); the NRAO VLA Sky
Survey (NVSS; Condon et al. 1998); the Parkes-MIT-
∗email:[email protected]
1
2 B. W. Meyers et al.
Table 1Radiosourcecatalogues usedinthecomparisonandverificationoftheS-PASSsourcecatalogue.
Survey catalogue Frequency Resolution Flux density limit Epoch Overlap area N†
sources
[GHz] [arcmin] 5σ [mJybeam−1] [×1000deg2]
rms
S-PASS 2.3 10.75 65 2007–2010 16.6 23,389
SUMSS 0.843 ∼0.75 8–18 1997–2003 7 209,186
NVSS 1.4 0.75 11 1993–1996 11 567,556
PMNa 4.85 4.2 20–45 1990 8.6 17,297
PKSCAT90 2.7 ∼6 ∼50 1990 16.3 5,884
ATCA calibratorsb 2.1 ∼0.1 – 2016 – 363
†Thisisthenumberofsourcesintheoverlapregiononlyandexcludessourcesinthecomparisoncataloguesthatfallwithinthe|b|<10◦
cutimposedontheS-PASSsourcecatalogue.Ifdeclinationcutsareimposedduringverification,theyareexplicitlystatedinthetext.
aThefullPMNcatalogueiscomprisedoffoursub-catalogues.Forthispaper,weuseonlythe“Southern”and“Zenith”sub-catalogues.
bSelected sourcesabove500mJywithinthenominalS-PASSdeclinationrange.
NRAO survey (PMN; Griffith & Wright 1993 and cor- set. That way, two full sets of scans are realised with
respondingpaperseries);the AustraliaTelescopeCom- different directions in the sky, that, combined, provide
pactArray(ATCA)calibratorlist1 and;theParkesRa- an effective basket weaving.
dio Source Catalogue (PKSCAT90; Bolton et al. 1979; The Parkesobservatorystaff performedpointing cal-
Wright & Otrupcek 1990). ibrations at the beginning of each session, delivering
The paper is structured as follows. In Section 2 the thetelescopewithpointingoffsetsbetterthan10arcsec
observation strategy and image processing is outlined. in both RA and Dec (more than sufficient for 9arcmin
In Section 3 we describe the procedures used to con- beam-width observations).Scans to check pointing cal-
structthesourcecatalogue.Section4containstheanal- ibrations were performed at each session by the ob-
ysis and verification of the catalogue and in Section 5 serving team to check that no residual offset along the
we outline the catalogueformat. Finally, we review our scan direction was present. More details can be found
conclusions in Section 6. Throughout, spectral indices, in Carretti (2010), while a full description will be in-
α, are defined using the convention S ∝να. cluded in the forthcoming S-PASS survey description
ν
paper (Carretti et al., in prep.).
DatawerecollectedwiththeDigitalFilterBankmark
2 DATA COLLECTION & REDUCTION
3 (DFB3) using a configuration with 256 MHz band-
2.1 Observations width and 512 frequency channels (0.5MHz channel
width). This configuration also provides full Stokes in-
Observations were carried out over the period October
formation(autocorrelationproductsforthetwocircular
2007toJanuary2010usingthe ParkesS-bandreceiver.
polarisations RR* and LL*, and their complex cross-
The S-band receiver is a package with: a system tem-
product RL*).
peratureT =20K,a beamFull Width atHalfMaxi-
sys The primaryflux density calibratorwas PKSB1934-
mum(FWHM)of8.9arcmin,andacircularpolarisation
638, using the model from Reynolds (1994), with PKS
front-endidealforlinear polarisationobservationswith
B0407-658 as the secondary calibrator. The resulting
single-dish telescopes.
absolute flux calibration is accurate to within 5–10%.
Observing was carried out in long azimuth scans
taken at the elevation of the south celestial pole as
viewed from Parkes covering the entire declination
range (δ ≤0◦) in each scan. Specifically, a scan length 2.2 Data reduction
in azimuth of 115◦ and a scan rate of 15degmin−1 is Asoftwarepipeline developedbythe S-PASSteamwas
requiredtorealisethis.Earthrotationwasusedtospan employed to reduce and calibrate the data given the
the whole RA range. complexobservationstrategyandsciencegoals.Output
As described in Carretti (2010), each night a zig- data were binned into 8MHz channels for calibration
zag in the sky is realised (see Figure 8 of Carretti and radio frequency interference (RFI) flagging pur-
2010). Combining the different zig-zags taken on dif- poses.Thecalibratorfluxdensitymodelwasusedtocal-
ferent nights, all of the RA range can be observedwith ibrate each individual channel, giving a flat calibrated
the appropriate sampling. The azimuthal scans are ob- bandpass (see Carretti et al. 2013a) without need for
served either eastward at sky-rise, or westward at sky- further corrections.After RFI flagging,the useful band
coveredtheranges2176–2216MHzand2256–2400MHz.
1http://www.narrabri.atnf.csiro.au/calibrators/ All useful 8MHz bands were binned together in one
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The S-PASS total intensity source catalogue 3
channel for an effective central frequency of 2307MHz
and 184MHz bandwidth.
The maps are arranged in rings of declination such
that the entire S-PASS observed sky is captured. Map
centres are −7.5◦, −22.5◦, −37.5◦, −52.5◦, −67.5◦ and
−82.5◦ – with 24, 24, 21, 18, 13 and 7 maps respec-
tively in each declination range. Each map is a grid of
3×3arcmin2 inzenithalequidistant(ARC)projection.
For analysisfocusedoncompactsources,S-PASSscans
arespatiallyhigh-passfilteredtoremovethelargescale
spatial structure (Lamee et al. 2016). A median filter
with a 45arcmin window was used to achieve this. A
window size of 5× the intrinsic resolution (9arcmin)
was chosen in order to give the best trade-off between
ineffectively removing large scale structure and not af-
fectingthesourcefluxestimates.Thefilteredscanswere
then spatially convolvedwith a 6arcminGaussianwin-
dow. All data points within the Gaussian window were
binned and weighted based on the window function
value at that pixel coordinate. This generated the final
set of 107 15×15deg2 maps, with an effective beam
width of θ =10.75arcmin.
fwhm
The mean RMS noise in the StokesI maps is σ ≈
rms
12.9mJybeam−1.Sincethethermalnoiseisanorderof
magnitude lower (σ ≈1mJybeam−1, Carretti et al.
th
2013b), the sensitivity is limited by the confusion
noise which we estimate to be σ = σ2 −σ2 ≈
c rms th
12.9mJybeam−1. This is consistent witph the estimate
fromascaledapproximationofequation(14)inCondon
(1974),
ν −0.7 θ 2
σ ≈0.2 fwhm ≈12.9mJybeam−1
c
(cid:16)GHz(cid:17) (cid:18)arcmin(cid:19)
(1)
which is appropriate for synthesised beams larger than
θ =0.17arcmin.
fwhm
3 CATALOGUE CONSTRUCTION
The final S-PASS source catalogue was constructed by
combiningthesourcecatalogueforeachofthe107total
intensity maps. Here we detail the catalogue creation
process for one tile and then how the individual tile
catalogues were combined to create the final S-PASS
source catalogue.
3.1 Source finding for one tile
We used the source finding algorithm aegean2
(Hancock et al.2012)andits associatedtoolsetto cre-
ate a source catalogue from the raw images. aegean
fits one or more elliptical Gaussians to each source and
produces a set of characterising source parameters. A
signal-to-noisecut of5σ , whereσ is the localRMS Figure 1. Top: A typical S-PASS image, centred on J2000 co-
src src
ordinates(α,δ)=(05:08:42,-37:31:30).Middle:Thebackground
estimation forthe imageproduced by bane.Values arenegative
2v2.0b-81-g6b1142c-(2016-09-08), seehttp://ascl.net/1212.009 duetothemedianfilteringapplied(seeSection2.2).Bottom:The
RMSnoisemapproducedbybane.
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4 B. W. Meyers et al.
foreachsourceascalculatedduringthenoiseestimation 3.2.1 Resolved S-PASS sources
step, was imposed on the tile catalogues. Giventhe S-PASSbeamsize,wewouldexpectthatfew
sources outside the Galactic plane will be partially or
fully resolved with angular size &11arcmin.
3.1.1 Background and noise estimation
Theability todeterminewhether asourceisresolved
The Background And Noise Estimation tool (bane; typically depends on the signal-to-noise of the source,
part of the aegean tool set) was used to create back- where low signal-to-noise sources are much more diffi-
ground and RMS noise maps, evaluated on angular cult to constrain with an elliptical Gaussian. Using the
scalesof∼3◦,forindividualimages.SeeFigure1foran fitted major and minor axes (a and b) to estimate the
example of an S-PASS tile image and the correspond- sourceextentandwecandeterminewhetherthe source
ing backgroundand RMS noise maps. Backgroundval- is truly resolved.This assumes that all sources are well
ues for the individual maps are expected to be close fit,thusanyspuriousfittingerrorswillproducenonsen-
to zero, if slightly negative, due to the median filter- sical results.
ing applied to the images. Typical background values To assess how many sources are resolved, we define
measured by bane are ≈−2.3mJybeam−1, but range the extent of a source as
from−3.7mJybeam−1 to 0.5mJybeam−1.The combi-
ab
nation of the median filtering, described in Section 2.2, ζ = , (2)
a b
and the background subtraction eradicates any signifi- psf psf
cant diffuse structure away from the Galactic plane. where apsf and bpsf are the major and minor axis for
the local point spread function. In the case of S-PASS
a ≡b =645arcsec. The error in the extent is cal-
psf psf
culated by summing the fractional errors in a and b in
3.2 Catalogue combination and filtering
quadrature, i.e. (∆ζ/ζ)2 ≈(∆a/a)2+(∆b/b)2.
Thesourcetablesforeachimagewereconcatenatedinto A source is resolved at the 3σ level (≈99.7% con-
oneall-southern-skysourcecatalogue,coveringdeclina- fidence assuming Gaussian statistics) if (ζ−3∆ζ)≥1,
tions δ ≤0◦ for all right ascensions. For some right as- otherwise the source is unresolved. Figure 3 identifies
censions,sourcesarefoundoutsidethenominaldeclina- three source categories: resolved, unresolved and un-
tion boundary. There are 118 such sources with δ >0◦ constrained.Unconstrained sources are those for which
that are included in the final catalogue and are used aegean has been unable to determine errors in the
throughout the catalogue verification. semi-major (a) or semi-minor (b) axes. Resolved and
The tile images at each declination strip overlap the unresolvedS-PASSsource numbers arecalculatedfrom
next lowestdeclination strip by ∼50arcmin. The over- thetotalsourcecatalogueminusthosesourceswithun-
lap in right ascension varies with declination, ranging constrained source size errors.
from ∼1◦ at the equator to ∼10◦ at δ ≈−82◦. Due Resolved sources comprise ∼8% of the total num-
to the overlap, the combined table contained multiple ber of cataloguedsources,while unresolvedand uncon-
detections of several thousand sources. For each source strainedsourcescontribute∼73%and∼19%.Wecon-
that was detected multiple times, only the detection sider all 23,389 sources, regardless of whether they are
with the lowest RMS noise was retained in the final resolved or not, for the verification analysis.
source catalogue.
Sources near the Galactic plane (|b|<10◦) were re-
4 VERIFICATION
moved. This conservative exclusion region was chosen
because even though a median filter was applied to the TheS-PASSsourcecatalogueconsistsprimarilyofcom-
scans,theGalacticplanewouldrequireadifferentanal- pact sources. In order to assess the quality of the final
ysis and catalogue creation pipeline due to the high catalogue, a number of tests have been performed.
source density and incomplete removal of large scale We analysethe internal catalogueflux density distri-
structures. This region will be examined in an upcom- bution and the average source spectra with respect to
ing paper. PMNcounterpartsat4.8GHzandPKSCAT90counter-
Thefinalsourcecataloguecontains23,389extragalac- parts at 2.7GHz. The catalogue astrometry, complete-
tic sources and covers a sky area of approximately ness and reliability are also examined in this section.
16,600deg2 (see Figure 2 for the RMS noise map). No-
tableexceptionstotheotherwiseuniformskynoiselevel
4.1 Flux density scale
are:CentaurusA,theLargeMagellanicCloudandareas
near the Galactic plane. The 16cm (2.1GHz) ATCA calibrator catalogue has
The Stokes I source catalogue format is outlined in high resolution (∼6arcsec, assuming 6km array con-
Section 5 and an example selection of sources can be figuration), with sources selected to be compact
found in Tables 2 and 3. and (mostly) have no other nearby source within
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The S-PASS total intensity source catalogue 5
Figure 2.AnAitoffprojectionRMSnoisemapoftheskyareacoveredbyS-PASS,includingthe|b|<10◦ cut.ThemeanlocalRMS
noise for sources inthe catalogue is ≈12.9mJybeam−1 with notable exceptions being Centaurus A and the Large Magellanic Cloud
whichhavelocalRMSvalues∼6timesthemean.
10 The S-PASS catalogue was cross-matchedwith a list
ofcalibrators3forATCA.Thefluxdensitylimitforboth
source lists was restricted to S >500mJybeam−1.
peak
The two catalogues were cross-matched symmetri-
)
∆ζ cally based on sky position with a 300arcsec cross-
3 1 matching radius, taking only the best matches. The
−
ζ cross-matching included all S-PASS sources, including
(
t, those outside the nominal δ ≤0◦ boundary. This pro-
n
e
t duced a matched list containing 363 sources.
x
e e 0.1 After scaling the ATCA flux density to 2.3GHz as-
c suming a spectral index of −0.7, we calculate the ratio
r
u
o oftheS-PASStoATCAsourcefluxdensity.Themedian
S
fluxdensityratiois1.04±0.01,whichisconsistentwith
unresolved (16961)
resolved (1947) unity giventhe S-PASSabsoluteflux calibrationuncer-
0.01
unconstrained (4481) tainty (see Section 2.1) and that errors in the ATCA
10 100 1000 flux measurements are not included.
Signal-to-noise ratio The same analysis was also conducted using the
PKSCAT90 2.7GHz fluxes. This comparison has the
Figure 3. S-PASS source extents (ζ−3∆ζ) as a function of benefit that both surveys were produced with the
signal-to-noise ratio. Magenta triangles represent sources with
same instrument at similar frequencies and therefore
unconstrained source size errors (i.e. ∆a=−1arcsec or ∆b=
with comparable resolution elements, reducing cross-
−1arcsecinTable3).Resolvedsourcesaredepictedasbluecircles
and unresolved sources are shown as grey circles. The catalogue matching confusion. The cross-matched list contains
consistsof8%resolvedsourcesand73%unresolvedsources,with 1,232sources.ScalingthePKSCAT90fluxesto2.3GHz,
theremaining19%havingunconstrainedsourcesizeerrors. the median ratio of S-PASS to PKSCAT90 flux densi-
ties is0.967±0.003.Thisisagainconsistentwithinthe
S-PASS absolute flux calibration uncertainty of 10%.
Thedistributionofratiosisexpectedtocentrearound
∼11arcmin. ATCA calibrators were chosen for com- unity.Theresultsfromcross-matchingtobothreference
parison with S-PASS sources because they provide an
independent and accurate measurement of source flux 3The compiled list of sources was created from accessing
densities. http://www.narrabri.atnf.csiro.au/calibrators/on02/02/2016.
PASA(2017)
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6 B. W. Meyers et al.
60 70
50 Median (1.038) Median (-0.69)
urces3400 60 1σ confidence
Nso20
10 50
0
0 0.5 1 1.5 2
220500 Sspass/Satca Median (0.967) Nsources40
30
urces150
Nso100
20
50
0
0 0.5 1 1.5 2 10
S /S
spass pkscat90
0
Figure 4. Flux density ratio distributions for sources brighter −2 −1 0 1 2
than Speak>500mJybeam−1. Top: ATCA (2.1GHz, extrapo- αsppmasns
lated to 2.3GHz) and S-PASS source flux ratios. The median
ratio, with standard error is 1.04±0.01. Bottom: PKSCAT90
Figure 5. Spectral index distribution between 2.3GHz and
(2.7GHz, extrapolated to 2.3GHz) and S-PASS source flux ra-
tios.Themedianratio,withstandarderroris0.967±0.003.The 4.8GHz for all S-PASS sources with Speak>500mJybeam−1
andacounterpartinPMN.Thesolidblacklineidentifiestheme-
mbyedthiaendisasdhisepdlalyineed,aasnadstohlieddbolattcekdlilnine,esarraeptiroesoefnutntithyei1sσmcaorknefid- dianspectralindex(αpspmanss=−0.69±0.02)andtheblackdashed
linesrepresentthe1σ confidenceinterval(−0.93to−0.26).Note
dencelevels.
theextended tailofflatandinvertedspectralindices.
cataloguesareplottedinFigure4.Thedashedlineindi-
cates a ratioof 1,the solidline is the measuredmedian The tail of sources with spectral indices α&−0.5, vis-
ratio value and the dotted lines are the 1σ confidence ible in Figure 5, is therefore not unexpected. A similar
levels.Giventhat bothdistributions havepeaks consis- distribution is observed independently by Lamee et al.
tent with unity, we assertthat the S-PASS flux density (2016) using only a sample of ∼500 S-PASS Stokes I
scale is reliable to within the 10% uncertainty. sources and cross-matching with NVSS.
The extended tail could be evidence for two source
populations being partially resolved. In comparison
4.2 Spectral index distribution
to the Australia Telescope 20GHz Survey (AT20G;
Murphy et al. 2010) spectral index distribution, where
To furthertest the accuracyofthe S-PASSflux density
thereisnocleandistinctionbetweensourcepopulations,
scale, we examine the spectral index distribution be-
it seems more likely that S-PASS is observing a single
tween S-PASS at 2.3GHz and PMN at 4.8GHz. PMN
population with an extended “flat” spectrum tail.
waschosenasitsresolution(5arcmin)iscomparableto
thatof S-PASS(∼11arcmin),reducing cross-matching
issues. Spectral indices for 772 sources were calculated
4.3 Astrometry
by cross-matching the S-PASS and PMN catalogues
with a search radius of 300arcsec and selecting only The median signal-to-noise ratio for an S-PASS source
S-PASS sources brighter than 500mJybeam−1. isSNR∼10.ForsourceswithSNR∼10,the meanpo-
Caution should be taken when interpreting any in- sitionerror,whichaccountsforthebackgroundnoise,is
dividual source spectral index information for S-PASS ∆θ ∼35arcsec(usingequations20and21fromCondon
and PMN. The surveys are separated by decades and 1997). The Gaussian fitting errors in RA and Dec
source time-variability may result in drastic changes in (columns 4 and 5) that aegean calculates are con-
observedspectral index properties,misrepresentingthe sistent with the description given by Condon (1997),
true source spectral index. assuming a synthesised beam of 645arcsec and a pixel
Figure 5 shows the distribution of spectral indices, spacingof0.05◦.Weexpectthemeanerrorsintheright
with a median value (solid black line) of α = ascension(RA)anddeclination(Dec)fortheentirecat-
med
−0.69±0.02.The 1σ confidence interval(dashed black alogue to be approximately this value.
lines) spans spectral index values of −0.93 to −0.26. We cross-matched the S-PASS catalogue with the
There are veryfew sourcepopulations that canachieve SUMSS and NVSS catalogues, chosen for their ex-
a spectral index of α<−2, however a spectral index cellent astrometry. Bright source (S &5mJybeam−1)
ν
of −0.5.α is not uncommon (e.g. blazars and QSOs). positions in NVSS are accurate to within (ǫ ,ǫ )=
α δ
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The S-PASS total intensity source catalogue 7
1000 Cross-matching between catalogues with vastly dif-
Mean (4.7) ferent angular resolutions and much higher source den-
800
Std. dev. (24.7) sities, false matches can become an issue. Given the
urces 600 source densities of SUMSS and NVSS (∼21deg−2, ∼
Nso 400 52deg−2),andthe sizeoftheS-PASSbeam(FWHM=
200 10.75arcmin), we could expect ∼0.5 SUMSS sources,
and ∼1.3 NVSS sources per S-PASS beam. Conse-
0
−300 −200 −100 0 100 200 300
quently, this could lead to spurious cross-matches be-
RA offset [arcsec]
1000 tween unrelated sources and would increase the spread
Mean (3.1) of astrometric offsets.
800
Std. dev. (22.4)
Overall the astrometry for S-PASS sources has no
urces 600 net systematic offset and the errors are in agreement
Nso 400 withtheestimated∆θ ∼35arcsec.Notethatindividual
200 source position errors are a function of signal-to-noise.
0
−300 −200 −100 0 100 200 300
Dec offset [arcsec] 4.4 Completeness
To calculate the catalogue completeness only S-PASS
Figure 6. Astrometric offset distributions from cross-matching
S-PASS with SUMSS. The mean offset in RA is 4.7arcsec and images that do not contain the Galactic plane were se-
3.1arcsecinDec.Thesolidblacklinerepresentsthedistribution lected. The selection criterion was that the image cen-
meanandthedashedlinesidentifythestandarddeviation. tre Galactic latitude be more than 15◦ away from the
Galactic plane (i.e. |b |≥15◦). This resulted in 80
centre
of the original 107 images being used for this analysis.
1000
Mean (-8.5) To estimate the completeness, 200 simulated sources
800
Std. dev. (24.1) were injected, over a flux density range of 0.01–10Jy,
urces 600 into each of the selected S-PASS images. The back-
Nso 400 ground and noise maps from the original images (i.e.
before simulated source injection) were used with the
200
simulated maps and processed by aegean in the same
0
−300 −200 −100 0 100 200 300 manneraswhencreatingthe sourcecatalogue(seeSec-
RA offset [arcsec]
tion 3). This ensured that the background and noise
1000
Mean (-2.8) properties of each simulated image were identical to
800
Std. dev. (22.6) those of the original maps.
urces 600 The completeness (Ci) for each image, i, was calcu-
Nso 400 lated by counting the number of simulated sources de-
tected (D ) versus the number injected into each im-
200 i
ageateachfluxdensitybin(i.e.C (S )=D (S )/200).
0 i ν i ν
−300 −200 −100 0 100 200 300 These completeness values were then combined to cal-
Dec offset [arcsec]
culate the completeness for the entire catalogue.
In Figure 8, the median completeness has been plot-
Figure 7. Astrometric offset distributions from cross-matching
tedforthecatalogue,withtheshadedregionrepresent-
S-PASS with NVSS. The mean offset in RA is −8.5arcsec and
−2.8arcsecinDec.Thesolidblacklinerepresentsthedistribution ing the 1σ confidence interval. The catalogueSNR cut-
meanandthedashedlinesidentifythestandarddeviation. off, 5σ is plotted as a dashed black line for refer-
rms
ence. The catalogue achieves a completeness of >95%
at 0.225Jy and is more than 99% complete at 0.5Jy.
(0.45,0.56)arcsec (Condon et al. 1998). The SUMSS The catalogue is 100% complete above flux densities of
catalogued sources have mean offsets from their cross- 1Jy and far from the Galactic plane.
match with NVSS of h∆αi=−0.59±0.07arcsec and
h∆δi=−0.30±0.08arcsec (Mauch et al. 2003). Using
4.5 Reliability estimate
across-matchingradiusof10.75arcmin,weretrievethe
average astrometric offsets for S-PASS sources. Thereliabilitywasestimatedbymeasuringthe fraction
Cross-matching with SUMSS we find that the off- of S-PASS sources that have a counterpart in SUMSS
sets are h∆αi=4.7±24.7arcsec and h∆δi=3.1± for declinations −89◦ ≤δ ≤−40◦, and NVSS for decli-
22.4arcsec (see Figure 6). Cross-matching with NVSS nations −40◦ <δ ≤−1◦. As when examining the flux
we find that the offsets are h∆αi=−8.5±24.1arcsec scaleandspectralindexdistributionofS-PASSsources,
and h∆δi=−2.8±22.6arcsec (see Figure 7). we select only those sources with a peak flux above
PASA(2017)
doi:10.1017/pas.2017.xxx
8 B. W. Meyers et al.
5σ disentangle whether sources are truly matched to the
rms
100 appropriatecounterpart.OneS-PASSbeamcancontain
many SUMSS or NVSS sources,which results in a mis-
leading cross-match.Also,some sourceswithin SUMSS
80
andNVSSmaynotbematchedcorrectlyduetoimaging
] artefacts,highlocalnoiselevelsorcomplexsourcestruc-
%
s [ 60 ture,sothecataloguereliabilitymaywellbehigherthan
es calculatedhere.Inordertoprovideacomprehensivere-
n
e liability estimate, the issue of cross-matching with dif-
t
ple 40 ferent resolution catalogues should be addressed. This
m wouldrequireasophisticatedalgorithm,takingintoac-
o
C count more than just simple distance between sources,
20 suchasthe PositionalUpdate andMatchingAlgorithm
(PUMA4; Line et al. 2017).
median
1σ confidence
0
0.01 0.1 1 10 5 CATALOGUE FORMAT
Flux Density [Jy]
An example ofthe first 25sourceshas been included in
Figure 8. The S-PASS catalogue median completeness (solid Tables 2 and 3. A description of each column in the
black line) and the 67% confidence interval (shaded grey). The catalogue is as follows.
5σrms cut-offisindicatedbytheverticaldashedline. Column (1): the S-PASS source name, formatted as
SPASS Jhhmmss±ddmmss.
Columns (2)& (3): the J2000RA in hh:mm:ss andthe
500mJybeam−1, which is approximately the S-PASS J2000 Dec in dd:mm:ss.
99% completeness limit. If we assume that the sources Columns (4)&(5):the errorsinRAandDec inarcsec-
areself-absorbed(Sν ∝ν2.5, i.e.a worstcasescenario), onds as quoted by aegean.
this corresponds to SUMSS and NVSS flux densities of Columns (6) & (7): the peak flux density and associ-
41mJybeam−1 and145mJybeam−1respectively–well ated error in Jybeam−1. Uncertainties do not include
above the 99% completeness limit for each survey. flux scaling errors.
IntheS-PASSsourcecatalogue,thereare550sources Columns (8) & (9): the integrated flux density and as-
inthe SUMSS regionand1003sourcesin the NVSS re- sociated error in Jy. Uncertainties do not include flux
gionwithS-PASSfluxdensitiesabove500mJybeam−1. scaling errors.
Cross-matchingSUMSStoS-PASSwithamatchingra- Columns (10) & (11): the background level and local
dius of 300arcsec we find that there are 517 sources RMS value, as calculated by bane, in Jybeam−1.
above the defined flux density limit with an S-PASS Columns (12)& (13): the major axis of the fitted ellip-
source above 500mJybeam−1. Using the same cross- tical Gaussian and associated error in arcseconds.
matching criteria, we find there are 945 NVSS sources Columns (14) & (15): the minor axis of the fitted ellip-
above the defined flux density limit with an S-PASS tical Gaussian and associated error in arcseconds.
source above 500mJybeam−1. Columns (16) & (17): the position angle of the fitted
The ratio of sources detected in the cross-match to ellipticalGaussian(measuredEastfromNorth)andas-
the number of suitable sources inS-PASS givesan esti- sociated error in degrees.
mateforthereliabilityat500mJybeam−1.ForSUMSS Columns (18) & (19): the residual mean and resid-
and NVSS, this corresponds to ∼94% reliability. ual standard deviation from the fitting process in
As a baseline, a mock catalogue was created from Jybeam−1.
the S-PASS source catalogue by shifting each source
RA and Dec by +0.5◦. Cross-matchingthis mock cata-
logue in the same way as above we find that there are 6 SUMMARY
58 matches between SUMSS and S-PASS and 14 be-
Using S-PASS total intensity data, the first southern-
tween NVSS and S-PASS. This corresponds to a false
sky extragalactic source catalogue at 2.3GHz has been
matching rate between ∼2–9%, implying that the ac-
created, containing 23,389 radio sources.
tual sourcecatalogue reliability couldbe as low as 85% The S-PASS source catalogue covers 16,600deg2 of
at 500mJybeam−1.
sky.Theinternalfluxscaleisreliabletowithinthe10%
We note the discussion in Section 4.3 about source
calibration uncertainty estimate. The S-PASS source
density considerations when cross-matching S-PASS
with SUMSS and NVSS. The resolution difference be-
tween S-PASS, SUMSS and NVSS makes it difficult to 4https://github.com/JLBLine/PUMA
PASA(2017)
doi:10.1017/pas.2017.xxx
The S-PASS total intensity source catalogue 9
spectral index distribution is consistent with a popu- Carretti E., et al., 2013b, Nature, 493, 66
lation with a median spectral index of α≈−0.7 and a Condon J. J., 1974, ApJ, 188, 279
tail of flat and inverted spectrum sources. Condon J. J., 1997, PASP, 109, 166
Typical astrometric offsets are consistent with ap- CondonJ. J.,CottonW. D., GreisenE.W., Yin Q. F.,
proximately 35arcsec, though individual source astro- PerleyR.A.,TaylorG.B.,BroderickJ.J.,1998,AJ,
metric errors vary as a function of signal-to-noise. The 115, 1693
catalogueis95%completeat225mJyandis100%com-
Griffith M. R., Wright A. E., 1993, AJ, 105, 1666
pleteabove1Jy.Approximately94%ofS-PASSsources
HancockP.J.,MurphyT.,GaenslerB.M.,HopkinsA.,
with a peak flux density above 500mJybeam−1 have
Curran J. R., 2012, MNRAS, 422, 1812
a lower-frequency counterpart. Given the difference in
Hurley-Walker N., et al., 2017, MNRAS, 464, 1146
sourcedensitiesbetweenS-PASSandthecomparedcat-
Lamee M., Rudnick L., Farnes J. S., Carretti E.,
alogues, this number is difficult to correctly estimate
Gaensler B. M., HaverkornM., Poppi S., 2016, ApJ,
and could be as low as 85%.
829, 5
Avarietyofscienceapplicationsarepossibleusingthe
Line J.L.B.,Webster R.L.,PindorB.,MitchellD.A.,
S-PASS catalogue, including source spectrum studies
Trott C. M., 2017, PASA, 34, e003
by cross-matching with similar all-sky surveys, such as
Mauch T., Murphy T., Buttery H. J., Curran J., Hun-
the newly released GaLactic and Extragalactic All-sky
steadR.W.,PiestrzynskiB.,RobertsonJ.G.,Sadler
MWA (GLEAM) survey catalogue (Wayth et al. 2015,
E. M., 2003, MNRAS, 342, 1117
Hurley-Walker et al. 2017) or the Planck Catalogue of
Murphy T., et al., 2010, MNRAS, 402, 2403
Compact Sources (PCCS; Planck Collaborationet al.
2014).WithacentrefrequencyintherangewhereGiga- O’Dea C. P., Baum S. A., Stanghellini C., 1991, ApJ,
hertz PeakedSpectrum sources (O’Dea et al. 1991) are 380, 66
expected to exhibita spectralturnover,S-PASSwould Planck Collaborationet al., 2014, A&A, 571, A28
be a valuable addition to wide-band studies of these ReynoldsJ.E.,1994,TechnicalReportSeries39.3/040,
objects (e.g. Callingham et al. 2015). A Revised Flux Scale For The AT Compact Array.
Australia Telescope National Facility
Wayth R. B., et al., 2015, PASA, 32, e025
7 ACKNOWLEDGEMENTS
WrightA.,OtrupcekR.,1990,inParkesCatalog,1990,
This work has been carried out in the framework of the S- Australia Telescope National Facility.
band Polarisation All Sky Survey (S-PASS) collaboration. de Zotti G., Massardi M., Negrello M., Wall J., 2010,
The Parkes radio telescope is part of the Australia Tele- A&A Rev., 18, 1
scope National Facility, which is funded by the Common-
wealthofAustraliaforoperationasaNationalFacilityman-
aged by CSIRO. Parts of this research were conducted by
the Australian Research Council Centre of Excellence for
All-skyAstrophysics(CAASTRO),throughprojectnumber
CE110001020.ThisresearchhasmadeuseoftheVizieRcat-
alogue access tool, CDS, Strasbourg, France. The original
description of the VizieR service was published in A&AS
143, 23. The Dunlap Institute is funded through an en-
dowment established by the David Dunlap family and the
University of Toronto. B.M.G. acknowledges thesupport of
the Natural Sciences and Engineering Research Council of
Canada(NSERC)throughgrantRGPIN-2015-05948,andof
the Canada Research Chairs program.
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doi:10.1017/pas.2017.xxx
dP 1
oA 0
i:10SA Table 2Thefirst25sourcesfromtheS-PASScatalogue, orderedbyincreasingDec(column3).ThecolumnsaredefinedinSection5.ContinuedinTable3.
.10(2 (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
10
7/p17) S-PASS name RA (J2000) Dec (J2000) ∆RA ∆Dec Speak ∆Sp⋆eak Sint ∆Si⋆nt background local rms
as [h m s] [◦ ′ ′′] [′′] [′′] [Jybeam−1] [Jybeam−1] [Jy] [Jy] [Jybeam−1] [Jybeam−1]
.2
0
1 SPASS J051336-302741 05:13:36 -30:27:41 3 10 1.46 0.02 1.80 0.03 -0.0022 0.0149
7
.xx SPASS J051450-301711 05:14:50 -30:17:11 3 10 0.11 0.02 0.12 0.02 -0.0022 0.0148
x
SPASS J052257-295758 05:22:57 -29:57:58 12 12 0.29 0.01 0.26 0.01 -0.0022 0.0126
SPASS J060043-293520 06:00:43 -29:35:20 55 54 0.10 0.01 0.10 0.01 -0.0031 0.0165
SPASS J052632-294358 05:26:32 -29:43:58 58 59 0.07 0.01 0.07 0.01 -0.0022 0.0122
SPASS J054618-293123 05:46:18 -29:31:23 17 17 0.26 0.01 0.28 0.02 -0.0035 0.0145
SPASS J054523-294020 05:45:23 -29:40:20 17 17 0.09 0.01 0.06 0.01 -0.0034 0.0145
SPASS J050557-293104 05:05:57 -29:31:04 13 16 0.29 0.02 0.26 0.02 -0.0023 0.0151
SPASS J052250-293313 05:22:50 -29:33:13 47 53 0.09 0.01 0.10 0.02 -0.0023 0.0126
SPASS J051544-292649 05:15:44 -29:26:49 67 69 0.07 0.01 0.07 0.01 -0.0023 0.0132
SPASS J053126-292511 05:31:26 -29:25:11 55 41 0.09 0.01 0.10 0.02 -0.0025 0.0129
SPASS J054953-291618 05:49:53 -29:16:18 39 30 0.15 0.02 0.16 0.02 -0.0034 0.0157
SPASS J053755-291853 05:37:55 -29:18:53 34 17 0.17 0.01 0.24 0.02 -0.0028 0.0133
SPASS J052539-291724 05:25:39 -29:17:24 55 56 0.07 0.01 0.07 0.01 -0.0022 0.0122
SPASS J050150-290913 05:01:50 -29:09:13 27 42 0.12 0.02 0.10 0.02 -0.0025 0.0149
SPASS J051138-290700 05:11:38 -29:07:00 32 55 0.11 0.01 0.15 0.02 -0.0023 0.0129
SPASS J050740-290837 05:07:40 -29:08:37 61 67 0.08 0.01 0.08 0.01 -0.0023 0.0138
SPASS J050244-290357 05:02:44 -29:03:57 59 65 0.09 0.01 0.09 0.01 -0.0025 0.0148
SPASS J050537-285603 05:05:37 -28:56:03 5 7 0.69 0.01 0.73 0.02 -0.0024 0.0141
SPASS J052156-285618 05:21:56 -28:56:18 9 10 0.37 0.01 0.37 0.01 -0.0023 0.0127
SPASS J052045-284853 05:20:45 -28:48:53 9 10 0.19 0.01 0.16 0.01 -0.0022 0.0126
SPASS J054318-285226 05:43:18 -28:52:26 17 16 0.24 0.01 0.22 0.02 -0.0030 0.0141
SPASS J051543-285400 05:15:43 -28:54:00 21 26 0.15 0.01 0.12 0.01 -0.0022 0.0129
SPASS J053955-283959 05:39:55 -28:39:59 3 3 1.20 0.01 1.08 0.01 -0.0029 0.0134
SPASS J050122-283450 05:01:22 -28:34:50 56 64 0.08 0.01 0.08 0.01 -0.0026 0.0145
aWestressthattheuncertainties inpeakandintegrated fluxdensitiesdonotincludeanycorrectionforfluxscalingerrors.
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